NAME
openvpn - secure IP tunnel daemon.
SYNOPSIS
openvpn [ options ... ]
INTRODUCTION
OpenVPN is an open source VPN daemon by James Yonan. Because OpenVPN
tries to be a universal VPN tool offering a great deal of flexibility,
there are a lot of options on this manual page. If you're new to Open‐
VPN, you might want to skip ahead to the examples section where you
will see how to construct simple VPNs on the command line without even
needing a configuration file.
Also note that there's more documentation and examples on the OpenVPN
web site: http://openvpn.net/
And if you would like to see a shorter version of this manual, see the
openvpn usage message which can be obtained by running openvpn without
any parameters.
DESCRIPTION
OpenVPN is a robust and highly flexible VPN daemon. OpenVPN supports SSL/TLS security, ethernet bridging, TCP or UDP tunnel transport through proxies or NAT, support for dynamic IP addresses and DHCP, scalability to hundreds or thousands of users, and portability to most major OS platforms.
OpenVPN is tightly bound to the OpenSSL library, and derives much of its crypto capabilities from it.
OpenVPN supports conventional encryption using a pre-shared secret key (Static Key mode) or public key security (SSL/TLS mode) using client & server certificates. OpenVPN also supports non-encrypted TCP/UDP tun‐ nels.
OpenVPN is designed to work with the TUN/TAP virtual networking inter‐ face that exists on most platforms.
Overall, OpenVPN aims to offer many of the key features of IPSec but with a relatively lightweight footprint.
OPTIONS
OpenVPN allows any option to be placed either on the command line or in
a configuration file. Though all command line options are preceded by
a double-leading-dash ("--"), this prefix can be removed when an option
is placed in a configuration file.
--help Show options.
--config file
Load additional config options from file where each line corre‐
sponds to one command line option, but with the leading '--'
removed.
If --config file is the only option to the openvpn command, the
--config can be removed, and the command can be given as openvpn
file
Note that configuration files can be nested to a reasonable
depth.
Double quotation or single quotation characters ("", '') can be
used to enclose single parameters containing whitespace, and "#"
or ";" characters in the first column can be used to denote com‐
ments.
Note that OpenVPN 2.0 and higher performs backslash-based shell
escaping for characters not in single quotations, so the follow‐
ing mappings should be observed:
\\ Maps to a single backslash character (\).
\" Pass a literal doublequote character ("), don't
interpret it as enclosing a parameter.
\[SPACE] Pass a literal space or tab character, don't
interpret it as a parameter delimiter.
For example on Windows, use double backslashes to represent
pathnames:
secret "c:\\OpenVPN\\secret.key"
For examples of configuration files, see http://open‐
vpn.net/examples.html
Here is an example configuration file:
#
# Sample OpenVPN configuration file for
# using a pre-shared static key.
#
# '#' or ';' may be used to delimit comments.
# Use a dynamic tun device.
dev tun
# Our remote peer
remote mypeer.mydomain
# 10.1.0.1 is our local VPN endpoint
# 10.1.0.2 is our remote VPN endpoint
ifconfig 10.1.0.1 10.1.0.2
# Our pre-shared static key
secret static.key
Tunnel Options:
--mode m
Set OpenVPN major mode. By default, OpenVPN runs in
point-to-point mode ("p2p"). OpenVPN 2.0 introduces a new mode
("server") which implements a multi-client server capability.
--local host
Local host name or IP address for bind. If specified, OpenVPN
will bind to this address only. If unspecified, OpenVPN will
bind to all interfaces.
--remote host [port] [proto]
Remote host name or IP address. On the client, multiple
--remote options may be specified for redundancy, each referring
to a different OpenVPN server. Specifying multiple --remote
options for this purpose is a special case of the more general
connection-profile feature. See the <connection> documentation
below.
The OpenVPN client will try to connect to a server at host:port
in the order specified by the list of --remote options.
proto indicates the protocol to use when connecting with the
remote, and may be "tcp" or "udp".
For forcing IPv4 or IPv6 connection suffix tcp or udp with 4/6
like udp4/udp6/tcp4/tcp6.
The client will move on to the next host in the list, in the
event of connection failure. Note that at any given time, the
OpenVPN client will at most be connected to one server.
Note that since UDP is connectionless, connection failure is
defined by the --ping and --ping-restart options.
Note the following corner case: If you use multiple --remote
options, AND you are dropping root privileges on the client with
--user and/or --group, AND the client is running a non-Windows
OS, if the client needs to switch to a different server, and
that server pushes back different TUN/TAP or route settings, the
client may lack the necessary privileges to close and reopen the
TUN/TAP interface. This could cause the client to exit with a
fatal error.
If --remote is unspecified, OpenVPN will listen for packets from
any IP address, but will not act on those packets unless they
pass all authentication tests. This requirement for authentica‐
tion is binding on all potential peers, even those from known
and supposedly trusted IP addresses (it is very easy to forge a
source IP address on a UDP packet).
When used in TCP mode, --remote will act as a filter, rejecting
connections from any host which does not match host.
If host is a DNS name which resolves to multiple IP addresses,
OpenVPN will try them in the order that the system getaddrinfo()
presents them, so priorization and DNS randomization is done by
the system library. Unless an IP version is forced by the pro‐
tocol specification (4/6 suffix), OpenVPN will try both IPv4 and
IPv6 addresses, in the order getaddrinfo() returns them.
--remote-random-hostname
Prepend a random string (6 bytes, 12 hex characters) to hostname
to prevent DNS caching. For example, "foo.bar.gov" would be
modified to "<random-chars>.foo.bar.gov".
<connection>
Define a client connection profile. Client connection profiles
are groups of OpenVPN options that describe how to connect to a
given OpenVPN server. Client connection profiles are specified
within an OpenVPN configuration file, and each profile is brack‐
eted by <connection> and </connection>.
An OpenVPN client will try each connection profile sequentially
until it achieves a successful connection.
--remote-random can be used to initially "scramble" the connec‐
tion list.
Here is an example of connection profile usage:
client
dev tun
<connection>
remote 198.19.34.56 1194 udp
</connection>
<connection>
remote 198.19.34.56 443 tcp
</connection>
<connection>
remote 198.19.34.56 443 tcp
http-proxy 192.168.0.8 8080
</connection>
<connection>
remote 198.19.36.99 443 tcp
http-proxy 192.168.0.8 8080
</connection>
persist-key
persist-tun
pkcs12 client.p12
remote-cert-tls server
verb 3
First we try to connect to a server at 198.19.34.56:1194 using
UDP. If that fails, we then try to connect to 198.19.34.56:443
using TCP. If that also fails, then try connecting through an
HTTP proxy at 192.168.0.8:8080 to 198.19.34.56:443 using TCP.
Finally, try to connect through the same proxy to a server at
198.19.36.99:443 using TCP.
The following OpenVPN options may be used inside of a <connec‐
tion> block:
bind, connect-retry, connect-retry-max, connect-timeout,
explicit-exit-notify, float, fragment, http-proxy,
http-proxy-option, link-mtu, local, lport, mssfix, mtu-disc,
nobind, port, proto, remote, rport, socks-proxy, tun-mtu and
tun-mtu-extra.
A defaulting mechanism exists for specifying options to apply to
all <connection> profiles. If any of the above options (with
the exception of remote ) appear outside of a <connection>
block, but in a configuration file which has one or more <con‐
nection> blocks, the option setting will be used as a default
for <connection> blocks which follow it in the configuration
file.
For example, suppose the nobind option were placed in the sample
configuration file above, near the top of the file, before the
first <connection> block. The effect would be as if nobind were
declared in all <connection> blocks below it.
--proto-force p
When iterating through connection profiles, only consider pro‐
files using protocol p ('tcp'|'udp').
--remote-random
When multiple --remote address/ports are specified, or if con‐
nection profiles are being used, initially randomize the order
of the list as a kind of basic load-balancing measure.
--proto p
Use protocol p for communicating with remote host. p can be
udp, tcp-client, or tcp-server.
The default protocol is udp when --proto is not specified.
For UDP operation, --proto udp should be specified on both
peers.
For TCP operation, one peer must use --proto tcp-server and the
other must use --proto tcp-client. A peer started with
tcp-server will wait indefinitely for an incoming connection. A
peer started with tcp-client will attempt to connect, and if
that fails, will sleep for 5 seconds (adjustable via the --con‐
nect-retry option) and try again infinite or up to N retries
(adjustable via the --connect-retry-max option). Both TCP
client and server will simulate a SIGUSR1 restart signal if
either side resets the connection.
OpenVPN is designed to operate optimally over UDP, but TCP capa‐
bility is provided for situations where UDP cannot be used. In
comparison with UDP, TCP will usually be somewhat less efficient
and less robust when used over unreliable or congested networks.
This article outlines some of problems with tunneling IP over
TCP:
http://sites.inka.de/sites/bigred/devel/tcp-tcp.html
There are certain cases, however, where using TCP may be advan‐
tageous from a security and robustness perspective, such as tun‐
neling non-IP or application-level UDP protocols, or tunneling
protocols which don't possess a built-in reliability layer.
--connect-retry n [max]
Wait n seconds between connection attempts (default=5).
Repeated reconnection attempts are slowed down after 5 retries
per remote by doubling the wait time after each unsuccessful
attempt. The optional argument max specifies the maximum value
of wait time in seconds at which it gets capped (default=300).
--connect-retry-max n
n specifies the number of times each --remote or <connection>
entry is tried. Specifying n as one would try each entry exactly
once. A successful connection resets the counter.
(default=unlimited).
--show-proxy-settings
Show sensed HTTP or SOCKS proxy settings. Currently, only Win‐
dows clients support this option.
--http-proxy server port [authfile|'auto'|'auto-nct'] [auth-method]
Connect to remote host through an HTTP proxy at address server
and port port. If HTTP Proxy-Authenticate is required, authfile
is a file containing a username and password on 2 lines, or
"stdin" to prompt from console. Its content can also be speci‐
fied in the config file with the --http-proxy-user-pass option.
(See section on inline files)
auth-method should be one of "none", "basic", or "ntlm".
HTTP Digest authentication is supported as well, but only via
the auto or auto-nct flags (below).
The auto flag causes OpenVPN to automatically determine the
auth-method and query stdin or the management interface for
username/password credentials, if required. This flag exists on
OpenVPN 2.1 or higher.
The auto-nct flag (no clear-text auth) instructs OpenVPN to
automatically determine the authentication method, but to reject
weak authentication protocols such as HTTP Basic Authentication.
--http-proxy-option type [parm]
Set extended HTTP proxy options. Repeat to set multiple
options.
VERSION version -- Set HTTP version number to version
(default=1.0).
AGENT user-agent -- Set HTTP "User-Agent" string to user-agent.
CUSTOM-HEADER name content -- Adds the custom Header with name
as name and content as the content of the custom HTTP header.
--socks-proxy server [port] [authfile]
Connect to remote host through a Socks5 proxy at address server
and port port (default=1080). authfile (optional) is a file
containing a username and password on 2 lines, or "stdin" to
prompt from console.
--resolv-retry n
If hostname resolve fails for --remote, retry resolve for n sec‐
onds before failing.
Set n to "infinite" to retry indefinitely.
By default, --resolv-retry infinite is enabled. You can disable
by setting n=0.
--float
Allow remote peer to change its IP address and/or port number,
such as due to DHCP (this is the default if --remote is not
used). --float when specified with --remote allows an OpenVPN
session to initially connect to a peer at a known address, how‐
ever if packets arrive from a new address and pass all authenti‐
cation tests, the new address will take control of the session.
This is useful when you are connecting to a peer which holds a
dynamic address such as a dial-in user or DHCP client.
Essentially, --float tells OpenVPN to accept authenticated pack‐
ets from any address, not only the address which was specified
in the --remote option.
--ipchange cmd
Run command cmd when our remote ip-address is initially authen‐
ticated or changes.
cmd consists of a path to script (or executable program),
optionally followed by arguments. The path and arguments may be
single- or double-quoted and/or escaped using a backslash, and
should be separated by one or more spaces.
When cmd is executed two arguments are appended after any argu‐
ments specified in cmd , as follows:
cmd ip_address port_number
Don't use --ipchange in --mode server mode. Use a --client-con‐
nect script instead.
See the "Environmental Variables" section below for additional
parameters passed as environmental variables.
If you are running in a dynamic IP address environment where the
IP addresses of either peer could change without notice, you can
use this script, for example, to edit the /etc/hosts file with
the current address of the peer. The script will be run every
time the remote peer changes its IP address.
Similarly if our IP address changes due to DHCP, we should con‐
figure our IP address change script (see man page for dhcpcd(8)
) to deliver a SIGHUP or SIGUSR1 signal to OpenVPN. OpenVPN
will then reestablish a connection with its most recently
authenticated peer on its new IP address.
--port port
TCP/UDP port number or port name for both local and remote (sets
both --lport and --rport options to given port). The current
default of 1194 represents the official IANA port number assign‐
ment for OpenVPN and has been used since version 2.0-beta17.
Previous versions used port 5000 as the default.
--lport port
Set local TCP/UDP port number or name. Cannot be used together
with --nobind option.
--rport port
Set TCP/UDP port number or name used by the --remote option. The
port can also be set directly using the --remote option.
--bind [ipv6only]
Bind to local address and port. This is the default unless any
of --proto tcp-client , --http-proxy or --socks-proxy are used.
If the ipv6only keyword is present OpenVPN will bind only to
IPv6 (as oposed to IPv6 and IPv4) when a IPv6 socket is opened.
--nobind
Do not bind to local address and port. The IP stack will allo‐
cate a dynamic port for returning packets. Since the value of
the dynamic port could not be known in advance by a peer, this
option is only suitable for peers which will be initiating con‐
nections by using the --remote option.
--dev tunX | tapX | null
TUN/TAP virtual network device ( X can be omitted for a dynamic
device.)
See examples section below for an example on setting up a TUN
device.
You must use either tun devices on both ends of the connection
or tap devices on both ends. You cannot mix them, as they rep‐
resent different underlying network layers.
tun devices encapsulate IPv4 or IPv6 (OSI Layer 3) while tap
devices encapsulate Ethernet 802.3 (OSI Layer 2).
--dev-type device-type
Which device type are we using? device-type should be tun (OSI
Layer 3) or tap (OSI Layer 2). Use this option only if the
TUN/TAP device used with --dev does not begin with tun or tap.
--topology mode
Configure virtual addressing topology when running in --dev tun
mode. This directive has no meaning in --dev tap mode, which
always uses a subnet topology.
If you set this directive on the server, the --server and
--server-bridge directives will automatically push your chosen
topology setting to clients as well. This directive can also be
manually pushed to clients. Like the --dev directive, this
directive must always be compatible between client and server.
mode can be one of:
net30 -- Use a point-to-point topology, by allocating one /30
subnet per client. This is designed to allow point-to-point
semantics when some or all of the connecting clients might be
Windows systems. This is the default on OpenVPN 2.0.
p2p -- Use a point-to-point topology where the remote endpoint
of the client's tun interface always points to the local end‐
point of the server's tun interface. This mode allocates a sin‐
gle IP address per connecting client. Only use when none of the
connecting clients are Windows systems. This mode is function‐
ally equivalent to the --ifconfig-pool-linear directive which is
available in OpenVPN 2.0, is deprecated and will be removed in
OpenVPN 2.5
subnet -- Use a subnet rather than a point-to-point topology by
configuring the tun interface with a local IP address and subnet
mask, similar to the topology used in --dev tap and ethernet
bridging mode. This mode allocates a single IP address per con‐
necting client and works on Windows as well. Only available
when server and clients are OpenVPN 2.1 or higher, or OpenVPN
2.0.x which has been manually patched with the --topology direc‐
tive code. When used on Windows, requires version 8.2 or higher
of the TAP-Win32 driver. When used on *nix, requires that the
tun driver supports an ifconfig(8) command which sets a subnet
instead of a remote endpoint IP address.
This option exists in OpenVPN 2.1 or higher.
Note: Using --topology subnet changes the interpretation of the
arguments of --ifconfig to mean "address netmask", no longer
"local remote".
--dev-node node
Explicitly set the device node rather than using /dev/net/tun,
/dev/tun, /dev/tap, etc. If OpenVPN cannot figure out whether
node is a TUN or TAP device based on the name, you should also
specify --dev-type tun or --dev-type tap.
Under Mac OS X this option can be used to specify the default
tun implementation. Using --dev-node utun forces usage of the
native Darwin tun kernel support. Use --dev-node utunN to select
a specific utun instance. To force using the tun.kext
(/dev/tunX) use --dev-node tun. When not specifying a
--dev-node option openvpn will first try to open utun, and fall
back to tun.kext.
On Windows systems, select the TAP-Win32 adapter which is named
node in the Network Connections Control Panel or the raw GUID of
the adapter enclosed by braces. The --show-adapters option
under Windows can also be used to enumerate all available
TAP-Win32 adapters and will show both the network connections
control panel name and the GUID for each TAP-Win32 adapter.
--lladdr address
Specify the link layer address, more commonly known as the MAC
address. Only applied to TAP devices.
--iproute cmd
Set alternate command to execute instead of default iproute2
command. May be used in order to execute OpenVPN in unprivi‐
leged environment.
--ifconfig l rn
Set TUN/TAP adapter parameters. l is the IP address of the
local VPN endpoint. For TUN devices in point-to-point mode, rn
is the IP address of the remote VPN endpoint. For TAP devices,
or TUN devices used with --topology subnet, rn is the subnet
mask of the virtual network segment which is being created or
connected to.
For TUN devices, which facilitate virtual point-to-point IP con‐
nections (when used in --topology net30 or p2p mode), the proper
usage of --ifconfig is to use two private IP addresses which are
not a member of any existing subnet which is in use. The IP
addresses may be consecutive and should have their order
reversed on the remote peer. After the VPN is established, by
pinging rn, you will be pinging across the VPN.
For TAP devices, which provide the ability to create virtual
ethernet segments, or TUN devices in --topology subnet mode
(which create virtual "multipoint networks"), --ifconfig is used
to set an IP address and subnet mask just as a physical ethernet
adapter would be similarly configured. If you are attempting to
connect to a remote ethernet bridge, the IP address and subnet
should be set to values which would be valid on the the bridged
ethernet segment (note also that DHCP can be used for the same
purpose).
This option, while primarily a proxy for the ifconfig(8) com‐
mand, is designed to simplify TUN/TAP tunnel configuration by
providing a standard interface to the different ifconfig imple‐
mentations on different platforms.
--ifconfig parameters which are IP addresses can also be speci‐
fied as a DNS or /etc/hosts file resolvable name.
For TAP devices, --ifconfig should not be used if the TAP inter‐
face will be getting an IP address lease from a DHCP server.
--ifconfig-noexec
Don't actually execute ifconfig/netsh commands, instead pass
--ifconfig parameters to scripts using environmental variables.
--ifconfig-nowarn
Don't output an options consistency check warning if the
--ifconfig option on this side of the connection doesn't match
the remote side. This is useful when you want to retain the
overall benefits of the options consistency check (also see
--disable-occ option) while only disabling the ifconfig compo‐
nent of the check.
For example, if you have a configuration where the local host
uses --ifconfig but the remote host does not, use --ifcon‐
fig-nowarn on the local host.
This option will also silence warnings about potential address
conflicts which occasionally annoy more experienced users by
triggering "false positive" warnings.
--route network/IP [netmask] [gateway] [metric]
Add route to routing table after connection is established.
Multiple routes can be specified. Routes will be automatically
torn down in reverse order prior to TUN/TAP device close.
This option is intended as a convenience proxy for the route(8)
shell command, while at the same time providing portable seman‐
tics across OpenVPN's platform space.
netmask default -- 255.255.255.255
gateway default -- taken from --route-gateway or the second
parameter to --ifconfig when --dev tun is specified.
metric default -- taken from --route-metric otherwise 0.
The default can be specified by leaving an option blank or set‐
ting it to "default".
The network and gateway parameters can also be specified as a
DNS or /etc/hosts file resolvable name, or as one of three spe‐
cial keywords:
vpn_gateway -- The remote VPN endpoint address (derived either
from --route-gateway or the second parameter to --ifconfig when
--dev tun is specified).
net_gateway -- The pre-existing IP default gateway, read from
the routing table (not supported on all OSes).
remote_host -- The --remote address if OpenVPN is being run in
client mode, and is undefined in server mode.
--route-gateway gw|'dhcp'
Specify a default gateway gw for use with --route.
If dhcp is specified as the parameter, the gateway address will
be extracted from a DHCP negotiation with the OpenVPN
server-side LAN.
--route-metric m
Specify a default metric m for use with --route.
--route-delay [n] [w]
Delay n seconds (default=0) after connection establishment,
before adding routes. If n is 0, routes will be added immedi‐
ately upon connection establishment. If --route-delay is omit‐
ted, routes will be added immediately after TUN/TAP device open
and --up script execution, before any --user or --group privi‐
lege downgrade (or --chroot execution.)
This option is designed to be useful in scenarios where DHCP is
used to set tap adapter addresses. The delay will give the DHCP
handshake time to complete before routes are added.
On Windows, --route-delay tries to be more intelligent by wait‐
ing w seconds (w=30 by default) for the TAP-Win32 adapter to
come up before adding routes.
--route-up cmd
Run command cmd after routes are added, subject to
--route-delay.
cmd consists of a path to script (or executable program),
optionally followed by arguments. The path and arguments may be
single- or double-quoted and/or escaped using a backslash, and
should be separated by one or more spaces.
See the "Environmental Variables" section below for additional
parameters passed as environmental variables.
--route-pre-down cmd
Run command cmd before routes are removed upon disconnection.
cmd consists of a path to script (or executable program),
optionally followed by arguments. The path and arguments may be
single- or double-quoted and/or escaped using a backslash, and
should be separated by one or more spaces.
See the "Environmental Variables" section below for additional
parameters passed as environmental variables.
--route-noexec
Don't add or remove routes automatically. Instead pass routes
to --route-up script using environmental variables.
--route-nopull
When used with --client or --pull, accept options pushed by
server EXCEPT for routes, block-outside-dns and dhcp options
like DNS servers.
When used on the client, this option effectively bars the server
from adding routes to the client's routing table, however note
that this option still allows the server to set the TCP/IP prop‐
erties of the client's TUN/TAP interface.
--allow-pull-fqdn
Allow client to pull DNS names from server (rather than being
limited to IP address) for --ifconfig, --route, and
--route-gateway.
--client-nat snat|dnat network netmask alias
This pushable client option sets up a stateless one-to-one NAT
rule on packet addresses (not ports), and is useful in cases
where routes or ifconfig settings pushed to the client would
create an IP numbering conflict.
network/netmask (for example 192.168.0.0/255.255.0.0) defines
the local view of a resource from the client perspective, while
alias/netmask (for example 10.64.0.0/255.255.0.0) defines the
remote view from the server perspective.
Use snat (source NAT) for resources owned by the client and dnat
(destination NAT) for remote resources.
Set --verb 6 for debugging info showing the transformation of
src/dest addresses in packets.
--redirect-gateway flags...
Automatically execute routing commands to cause all outgoing IP
traffic to be redirected over the VPN. This is a client-side
option.
This option performs three steps:
(1) Create a static route for the --remote address which for‐
wards to the pre-existing default gateway. This is done so that
(3) will not create a routing loop.
(2) Delete the default gateway route.
(3) Set the new default gateway to be the VPN endpoint address
(derived either from --route-gateway or the second parameter to
--ifconfig when --dev tun is specified).
When the tunnel is torn down, all of the above steps are
reversed so that the original default route is restored.
Option flags:
local -- Add the local flag if both OpenVPN servers are directly
connected via a common subnet, such as with wireless. The local
flag will cause step 1 above to be omitted.
autolocal -- Try to automatically determine whether to enable
local flag above.
def1 -- Use this flag to override the default gateway by using
0.0.0.0/1 and 128.0.0.0/1 rather than 0.0.0.0/0. This has the
benefit of overriding but not wiping out the original default
gateway.
bypass-dhcp -- Add a direct route to the DHCP server (if it is
non-local) which bypasses the tunnel (Available on Windows
clients, may not be available on non-Windows clients).
bypass-dns -- Add a direct route to the DNS server(s) (if they
are non-local) which bypasses the tunnel (Available on Windows
clients, may not be available on non-Windows clients).
block-local -- Block access to local LAN when the tunnel is
active, except for the LAN gateway itself. This is accomplished
by routing the local LAN (except for the LAN gateway address)
into the tunnel.
ipv6 -- Redirect IPv6 routing into the tunnel. This works simi‐
lar to the def1 flag, that is, more specific IPv6 routes are
added (2000::/4, 3000::/4), covering the whole IPv6 unicast
space.
!ipv4 -- Do not redirect IPv4 traffic - typically used in the
flag pair ipv6 !ipv4 to redirect IPv6-only.
--link-mtu n
Sets an upper bound on the size of UDP packets which are sent
between OpenVPN peers. It's best not to set this parameter
unless you know what you're doing.
--redirect-private [flags]
Like --redirect-gateway, but omit actually changing the default
gateway. Useful when pushing private subnets.
--tun-mtu n
Take the TUN device MTU to be n and derive the link MTU from it
(default=1500). In most cases, you will probably want to leave
this parameter set to its default value.
The MTU (Maximum Transmission Units) is the maximum datagram
size in bytes that can be sent unfragmented over a particular
network path. OpenVPN requires that packets on the control or
data channels be sent unfragmented.
MTU problems often manifest themselves as connections which hang
during periods of active usage.
It's best to use the --fragment and/or --mssfix options to deal
with MTU sizing issues.
--tun-mtu-extra n
Assume that the TUN/TAP device might return as many as n bytes
more than the --tun-mtu size on read. This parameter defaults
to 0, which is sufficient for most TUN devices. TAP devices may
introduce additional overhead in excess of the MTU size, and a
setting of 32 is the default when TAP devices are used. This
parameter only controls internal OpenVPN buffer sizing, so there
is no transmission overhead associated with using a larger
value.
--mtu-disc type
Should we do Path MTU discovery on TCP/UDP channel? Only sup‐
ported on OSes such as Linux that supports the necessary system
call to set.
'no' -- Never send DF (Don't Fragment) frames
'maybe' -- Use per-route hints
'yes' -- Always DF (Don't Fragment)
--mtu-test
To empirically measure MTU on connection startup, add the
--mtu-test option to your configuration. OpenVPN will send ping
packets of various sizes to the remote peer and measure the
largest packets which were successfully received. The
--mtu-test process normally takes about 3 minutes to complete.
--fragment max
Enable internal datagram fragmentation so that no UDP datagrams
are sent which are larger than max bytes.
The max parameter is interpreted in the same way as the
--link-mtu parameter, i.e. the UDP packet size after encapsula‐
tion overhead has been added in, but not including the UDP
header itself.
The --fragment option only makes sense when you are using the
UDP protocol ( --proto udp ).
--fragment adds 4 bytes of overhead per datagram.
See the --mssfix option below for an important related option to
--fragment.
It should also be noted that this option is not meant to replace
UDP fragmentation at the IP stack level. It is only meant as a
last resort when path MTU discovery is broken. Using this
option is less efficient than fixing path MTU discovery for your
IP link and using native IP fragmentation instead.
Having said that, there are circumstances where using OpenVPN's
internal fragmentation capability may be your only option, such
as tunneling a UDP multicast stream which requires fragmenta‐
tion.
--mssfix max
Announce to TCP sessions running over the tunnel that they
should limit their send packet sizes such that after OpenVPN has
encapsulated them, the resulting UDP packet size that OpenVPN
sends to its peer will not exceed max bytes. The default value
is 1450.
The max parameter is interpreted in the same way as the
--link-mtu parameter, i.e. the UDP packet size after encapsula‐
tion overhead has been added in, but not including the UDP
header itself. Resulting packet would be at most 28 bytes larger
for IPv4 and 48 bytes for IPv6 (20/40 bytes for IP header and 8
bytes for UDP header). Default value of 1450 allows IPv4 packets
to be transmitted over a link with MTU 1473 or higher without IP
level fragmentation.
The --mssfix option only makes sense when you are using the UDP
protocol for OpenVPN peer-to-peer communication, i.e. --proto
udp.
--mssfix and --fragment can be ideally used together, where
--mssfix will try to keep TCP from needing packet fragmentation
in the first place, and if big packets come through anyhow (from
protocols other than TCP), --fragment will internally fragment
them.
Both --fragment and --mssfix are designed to work around cases
where Path MTU discovery is broken on the network path between
OpenVPN peers.
The usual symptom of such a breakdown is an OpenVPN connection
which successfully starts, but then stalls during active usage.
If --fragment and --mssfix are used together, --mssfix will take
its default max parameter from the --fragment max option.
Therefore, one could lower the maximum UDP packet size to 1300
(a good first try for solving MTU-related connection problems)
with the following options:
--tun-mtu 1500 --fragment 1300 --mssfix
--sndbuf size
Set the TCP/UDP socket send buffer size. Defaults to operation
system default.
--rcvbuf size
Set the TCP/UDP socket receive buffer size. Defaults to opera‐
tion system default.
--mark value
Mark encrypted packets being sent with value. The mark value can
be matched in policy routing and packetfilter rules. This option
is only supported in Linux and does nothing on other operating
systems.
--socket-flags flags...
Apply the given flags to the OpenVPN transport socket. Cur‐
rently, only TCP_NODELAY is supported.
The TCP_NODELAY socket flag is useful in TCP mode, and causes
the kernel to send tunnel packets immediately over the TCP con‐
nection without trying to group several smaller packets into a
larger packet. This can result in a considerably improvement in
latency.
This option is pushable from server to client, and should be
used on both client and server for maximum effect.
--txqueuelen n
(Linux only) Set the TX queue length on the TUN/TAP interface.
Currently defaults to 100.
--shaper n
Limit bandwidth of outgoing tunnel data to n bytes per second on
the TCP/UDP port. Note that this will only work if mode is set
to p2p. If you want to limit the bandwidth in both directions,
use this option on both peers.
OpenVPN uses the following algorithm to implement traffic shap‐
ing: Given a shaper rate of n bytes per second, after a datagram
write of b bytes is queued on the TCP/UDP port, wait a minimum
of (b / n) seconds before queuing the next write.
It should be noted that OpenVPN supports multiple tunnels
between the same two peers, allowing you to construct full-speed
and reduced bandwidth tunnels at the same time, routing low-pri‐
ority data such as off-site backups over the reduced bandwidth
tunnel, and other data over the full-speed tunnel.
Also note that for low bandwidth tunnels (under 1000 bytes per
second), you should probably use lower MTU values as well (see
above), otherwise the packet latency will grow so large as to
trigger timeouts in the TLS layer and TCP connections running
over the tunnel.
OpenVPN allows n to be between 100 bytes/sec and 100 Mbytes/sec.
--inactive n [bytes]
Causes OpenVPN to exit after n seconds of inactivity on the
TUN/TAP device. The time length of inactivity is measured since
the last incoming or outgoing tunnel packet. The default value
is 0 seconds, which disables this feature.
If the optional bytes parameter is included, exit if less than
bytes of combined in/out traffic are produced on the tun/tap
device in n seconds.
In any case, OpenVPN's internal ping packets (which are just
keepalives) and TLS control packets are not considered "activ‐
ity", nor are they counted as traffic, as they are used inter‐
nally by OpenVPN and are not an indication of actual user activ‐
ity.
--ping n
Ping remote over the TCP/UDP control channel if no packets have
been sent for at least n seconds (specify --ping on both peers
to cause ping packets to be sent in both directions since Open‐
VPN ping packets are not echoed like IP ping packets). When
used in one of OpenVPN's secure modes (where --secret,
--tls-server, or --tls-client is specified), the ping packet
will be cryptographically secure.
This option has two intended uses:
(1) Compatibility with stateful firewalls. The periodic ping
will ensure that a stateful firewall rule which allows OpenVPN
UDP packets to pass will not time out.
(2) To provide a basis for the remote to test the existence of
its peer using the --ping-exit option.
--ping-exit n
Causes OpenVPN to exit after n seconds pass without reception of
a ping or other packet from remote. This option can be combined
with --inactive, --ping, and --ping-exit to create a two-tiered
inactivity disconnect.
For example,
openvpn [options...] --inactive 3600 --ping 10 --ping-exit 60
when used on both peers will cause OpenVPN to exit within 60
seconds if its peer disconnects, but will exit after one hour if
no actual tunnel data is exchanged.
--ping-restart n
Similar to --ping-exit, but trigger a SIGUSR1 restart after n
seconds pass without reception of a ping or other packet from
remote.
This option is useful in cases where the remote peer has a
dynamic IP address and a low-TTL DNS name is used to track the
IP address using a service such as http://dyndns.org/ + a
dynamic DNS client such as ddclient.
If the peer cannot be reached, a restart will be triggered,
causing the hostname used with --remote to be re-resolved (if
--resolv-retry is also specified).
In server mode, --ping-restart, --inactive, or any other type of
internally generated signal will always be applied to individual
client instance objects, never to whole server itself. Note
also in server mode that any internally generated signal which
would normally cause a restart, will cause the deletion of the
client instance object instead.
In client mode, the --ping-restart parameter is set to 120 sec‐
onds by default. This default will hold until the client pulls
a replacement value from the server, based on the --keepalive
setting in the server configuration. To disable the 120 second
default, set --ping-restart 0 on the client.
See the signals section below for more information on SIGUSR1.
Note that the behavior of SIGUSR1 can be modified by the --per‐
sist-tun, --persist-key, --persist-local-ip, and --per‐
sist-remote-ip options.
Also note that --ping-exit and --ping-restart are mutually
exclusive and cannot be used together.
--keepalive interval timeout
A helper directive designed to simplify the expression of --ping
and --ping-restart.
This option can be used on both client and server side, but it
is in enough to add this on the server side as it will push
appropriate --ping and --ping-restart options to the client. If
used on both server and client, the values pushed from server
will override the client local values.
The timeout argument will be twice as long on the server side.
This ensures that a timeout is detected on client side before
the server side drops the connection.
For example, --keepalive 10 60 expands as follows:
if mode server:
ping 10 # Argument: interval
ping-restart 120 # Argument: timeout*2
push "ping 10" # Argument: interval
push "ping-restart 60" # Argument: timeout
else
ping 10 # Argument: interval
ping-restart 60 # Argument: timeout
--ping-timer-rem
Run the --ping-exit / --ping-restart timer only if we have a
remote address. Use this option if you are starting the daemon
in listen mode (i.e. without an explicit --remote peer), and you
don't want to start clocking timeouts until a remote peer con‐
nects.
--persist-tun
Don't close and reopen TUN/TAP device or run up/down scripts
across SIGUSR1 or --ping-restart restarts.
SIGUSR1 is a restart signal similar to SIGHUP, but which offers
finer-grained control over reset options.
--persist-key
Don't re-read key files across SIGUSR1 or --ping-restart.
This option can be combined with --user nobody to allow restarts
triggered by the SIGUSR1 signal. Normally if you drop root
privileges in OpenVPN, the daemon cannot be restarted since it
will now be unable to re-read protected key files.
This option solves the problem by persisting keys across SIGUSR1
resets, so they don't need to be re-read.
--persist-local-ip
Preserve initially resolved local IP address and port number
across SIGUSR1 or --ping-restart restarts.
--persist-remote-ip
Preserve most recently authenticated remote IP address and port
number across SIGUSR1 or --ping-restart restarts.
--mlock
Disable paging by calling the POSIX mlockall function. Requires
that OpenVPN be initially run as root (though OpenVPN can subse‐
quently downgrade its UID using the --user option).
Using this option ensures that key material and tunnel data are
never written to disk due to virtual memory paging operations
which occur under most modern operating systems. It ensures
that even if an attacker was able to crack the box running Open‐
VPN, he would not be able to scan the system swap file to
recover previously used ephemeral keys, which are used for a
period of time governed by the --reneg options (see below), then
are discarded.
The downside of using --mlock is that it will reduce the amount
of physical memory available to other applications.
--up cmd
Run command cmd after successful TUN/TAP device open (pre --user
UID change).
cmd consists of a path to script (or executable program),
optionally followed by arguments. The path and arguments may be
single- or double-quoted and/or escaped using a backslash, and
should be separated by one or more spaces.
The up command is useful for specifying route commands which
route IP traffic destined for private subnets which exist at the
other end of the VPN connection into the tunnel.
For --dev tun execute as:
cmd tun_dev tun_mtu link_mtu ifconfig_local_ip ifcon‐
fig_remote_ip [ init | restart ]
For --dev tap execute as:
cmd tap_dev tap_mtu link_mtu ifconfig_local_ip ifconfig_netmask
[ init | restart ]
See the "Environmental Variables" section below for additional
parameters passed as environmental variables.
Note that if cmd includes arguments, all OpenVPN-generated argu‐
ments will be appended to them to build an argument list with
which the executable will be called.
Typically, cmd will run a script to add routes to the tunnel.
Normally the up script is called after the TUN/TAP device is
opened. In this context, the last command line parameter passed
to the script will be init. If the --up-restart option is also
used, the up script will be called for restarts as well. A
restart is considered to be a partial reinitialization of Open‐
VPN where the TUN/TAP instance is preserved (the --persist-tun
option will enable such preservation). A restart can be gener‐
ated by a SIGUSR1 signal, a --ping-restart timeout, or a connec‐
tion reset when the TCP protocol is enabled with the --proto
option. If a restart occurs, and --up-restart has been speci‐
fied, the up script will be called with restart as the last
parameter.
NOTE: on restart, OpenVPN will not pass the full set of environ‐
ment variables to the script. Namely, everything related to
routing and gateways will not be passed, as nothing needs to be
done anyway - all the routing setup is already in place. Addi‐
tionally, the up-restart script will run with the downgraded
UID/GID settings (if configured).
The following standalone example shows how the --up script can
be called in both an initialization and restart context. (NOTE:
for security reasons, don't run the following example unless UDP
port 9999 is blocked by your firewall. Also, the example will
run indefinitely, so you should abort with control-c).
openvpn --dev tun --port 9999 --verb 4 --ping-restart 10 --up
'echo up' --down 'echo down' --persist-tun --up-restart
Note that OpenVPN also provides the --ifconfig option to auto‐
matically ifconfig the TUN device, eliminating the need to
define an --up script, unless you also want to configure routes
in the --up script.
If --ifconfig is also specified, OpenVPN will pass the ifconfig
local and remote endpoints on the command line to the --up
script so that they can be used to configure routes such as:
route add -net 10.0.0.0 netmask 255.255.255.0 gw $5
--up-delay
Delay TUN/TAP open and possible --up script execution until
after TCP/UDP connection establishment with peer.
In --proto udp mode, this option normally requires the use of
--ping to allow connection initiation to be sensed in the
absence of tunnel data, since UDP is a "connectionless" proto‐
col.
On Windows, this option will delay the TAP-Win32 media state
transitioning to "connected" until connection establishment,
i.e. the receipt of the first authenticated packet from the
peer.
--down cmd
Run command cmd after TUN/TAP device close (post --user UID
change and/or --chroot ). cmd consists of a path to script (or
executable program), optionally followed by arguments. The path
and arguments may be single- or double-quoted and/or escaped
using a backslash, and should be separated by one or more spa‐
ces.
Called with the same parameters and environmental variables as
the --up option above.
Note that if you reduce privileges by using --user and/or
--group, your --down script will also run at reduced privilege.
--down-pre
Call --down cmd/script before, rather than after, TUN/TAP close.
--up-restart
Enable the --up and --down scripts to be called for restarts as
well as initial program start. This option is described more
fully above in the --up option documentation.
--setenv name value
Set a custom environmental variable name=value to pass to
script.
--setenv FORWARD_COMPATIBLE 1
Relax config file syntax checking so that unknown directives
will trigger a warning but not a fatal error, on the assumption
that a given unknown directive might be valid in future OpenVPN
versions.
This option should be used with caution, as there are good secu‐
rity reasons for having OpenVPN fail if it detects problems in a
config file. Having said that, there are valid reasons for
wanting new software features to gracefully degrade when encoun‐
tered by older software versions.
It is also possible to tag a single directive so as not to trig‐
ger a fatal error if the directive isn't recognized. To do
this, prepend the following before the directive: setenv opt
Versions prior to OpenVPN 2.3.3 will always ignore options set
with the setenv opt directive.
See also --ignore-unknown-option
--setenv-safe name value
Set a custom environmental variable OPENVPN_name=value to pass
to script.
This directive is designed to be pushed by the server to
clients, and the prepending of "OPENVPN_" to the environmental
variable is a safety precaution to prevent a LD_PRELOAD style
attack from a malicious or compromised server.
--ignore-unknown-option opt1 opt2 opt3 ... optN
When one of options opt1 ... optN is encountered in the configu‐
ration file the configuration file parsing does not fail if this
OpenVPN version does not support the option. Multiple
--ignore-unknown-option options can be given to support a larger
number of options to ignore.
This option should be used with caution, as there are good secu‐
rity reasons for having OpenVPN fail if it detects problems in a
config file. Having said that, there are valid reasons for want‐
ing new software features to gracefully degrade when encountered
by older software versions.
--ignore-unknown-option is available since OpenVPN 2.3.3.
--script-security level
This directive offers policy-level control over OpenVPN's usage
of external programs and scripts. Lower level values are more
restrictive, higher values are more permissive. Settings for
level:
0 -- Strictly no calling of external programs.
1 -- (Default) Only call built-in executables such as ifconfig,
ip, route, or netsh.
2 -- Allow calling of built-in executables and user-defined
scripts.
3 -- Allow passwords to be passed to scripts via environmental
variables (potentially unsafe).
OpenVPN releases before v2.3 also supported a method flag which
indicated how OpenVPN should call external commands and scripts.
This could be either execve or system. As of OpenVPN 2.3, this
flag is no longer accepted. In most *nix environments the
execve() approach has been used without any issues.
Some directives such as --up allow options to be passed to the
external script. In these cases make sure the script name does
not contain any spaces or the configuration parser will choke
because it can't determine where the script name ends and script
options start.
To run scripts in Windows in earlier OpenVPN versions you needed
to either add a full path to the script interpreter which can
parse the script or use the system flag to run these scripts.
As of OpenVPN 2.3 it is now a strict requirement to have full
path to the script interpreter when running non-executables
files. This is not needed for executable files, such as .exe,
.com, .bat or .cmd files. For example, if you have a Visual
Basic script, you must use this syntax now:
--up 'C:\\Windows\\System32\\wscript.exe C:\\Program\ Files\\OpenVPN\\config\\my-up-script.vbs'
Please note the single quote marks and the escaping of the back‐
slashes (\) and the space character.
The reason the support for the system flag was removed is due to
the security implications with shell expansions when executing
scripts via the system() call.
--disable-occ
Don't output a warning message if option inconsistencies are
detected between peers. An example of an option inconsistency
would be where one peer uses --dev tun while the other peer uses
--dev tap.
Use of this option is discouraged, but is provided as a tempo‐
rary fix in situations where a recent version of OpenVPN must
connect to an old version.
--user user
Change the user ID of the OpenVPN process to user after initial‐
ization, dropping privileges in the process. This option is
useful to protect the system in the event that some hostile
party was able to gain control of an OpenVPN session. Though
OpenVPN's security features make this unlikely, it is provided
as a second line of defense.
By setting user to nobody or somebody similarly unprivileged,
the hostile party would be limited in what damage they could
cause. Of course once you take away privileges, you cannot
return them to an OpenVPN session. This means, for example,
that if you want to reset an OpenVPN daemon with a SIGUSR1 sig‐
nal (for example in response to a DHCP reset), you should make
use of one or more of the --persist options to ensure that Open‐
VPN doesn't need to execute any privileged operations in order
to restart (such as re-reading key files or running ifconfig on
the TUN device).
--group group
Similar to the --user option, this option changes the group ID
of the OpenVPN process to group after initialization.
--cd dir
Change directory to dir prior to reading any files such as con‐
figuration files, key files, scripts, etc. dir should be an
absolute path, with a leading "/", and without any references to
the current directory such as "." or "..".
This option is useful when you are running OpenVPN in --daemon
mode, and you want to consolidate all of your OpenVPN control
files in one location.
--chroot dir
Chroot to dir after initialization. --chroot essentially rede‐
fines dir as being the top level directory tree (/). OpenVPN
will therefore be unable to access any files outside this tree.
This can be desirable from a security standpoint.
Since the chroot operation is delayed until after initializa‐
tion, most OpenVPN options that reference files will operate in
a pre-chroot context.
In many cases, the dir parameter can point to an empty direc‐
tory, however complications can result when scripts or restarts
are executed after the chroot operation.
Note: The SSL library will probably need /dev/urandom to be
available inside the chroot directory dir. This is because SSL
libraries occasionally need to collect fresh random. Newer
linux kernels and some BSDs implement a getrandom() or geten‐
tropy() syscall that removes the need for /dev/urandom to be
available.
--setcon context
Apply SELinux context after initialization. This essentially
provides the ability to restrict OpenVPN's rights to only net‐
work I/O operations, thanks to SELinux. This goes further than
--user and --chroot in that those two, while being great secu‐
rity features, unfortunately do not protect against privilege
escalation by exploitation of a vulnerable system call. You can
of course combine all three, but please note that since setcon
requires access to /proc you will have to provide it inside the
chroot directory (e.g. with mount --bind).
Since the setcon operation is delayed until after initializa‐
tion, OpenVPN can be restricted to just network-related system
calls, whereas by applying the context before startup (such as
the OpenVPN one provided in the SELinux Reference Policies) you
will have to allow many things required only during initializa‐
tion.
Like with chroot, complications can result when scripts or
restarts are executed after the setcon operation, which is why
you should really consider using the --persist-key and --per‐
sist-tun options.
--daemon [progname]
Become a daemon after all initialization functions are com‐
pleted. This option will cause all message and error output to
be sent to the syslog file (such as /var/log/messages), except
for the output of scripts and ifconfig commands, which will go
to /dev/null unless otherwise redirected. The syslog redirect‐
ion occurs immediately at the point that --daemon is parsed on
the command line even though the daemonization point occurs
later. If one of the --log options is present, it will
supercede syslog redirection.
The optional progname parameter will cause OpenVPN to report its
program name to the system logger as progname. This can be use‐
ful in linking OpenVPN messages in the syslog file with specific
tunnels. When unspecified, progname defaults to "openvpn".
When OpenVPN is run with the --daemon option, it will try to
delay daemonization until the majority of initialization func‐
tions which are capable of generating fatal errors are complete.
This means that initialization scripts can test the return sta‐
tus of the openvpn command for a fairly reliable indication of
whether the command has correctly initialized and entered the
packet forwarding event loop.
In OpenVPN, the vast majority of errors which occur after ini‐
tialization are non-fatal.
Note: as soon as OpenVPN has daemonized, it can not ask for
usernames, passwords, or key pass phrases anymore. This has
certain consequences, namely that using a password-protected
private key will fail unless the --askpass option is used to
tell OpenVPN to ask for the pass phrase (this requirement is new
in v2.3.7, and is a consequence of calling daemon() before ini‐
tializing the crypto layer).
Further, using --daemon together with --auth-user-pass (entered
on console) and --auth-nocache will fail as soon as key renego‐
tiation (and reauthentication) occurs.
--syslog [progname]
Direct log output to system logger, but do not become a daemon.
See --daemon directive above for description of progname parame‐
ter.
--errors-to-stderr
Output errors to stderr instead of stdout unless log output is
redirected by one of the --log options.
--passtos
Set the TOS field of the tunnel packet to what the payload's TOS
is.
--inetd [wait|nowait] [progname]
Use this option when OpenVPN is being run from the inetd or
xinetd(8) server.
The wait/nowait option must match what is specified in the
inetd/xinetd config file. The nowait mode can only be used with
--proto tcp-server. The default is wait. The nowait mode can
be used to instantiate the OpenVPN daemon as a classic TCP
server, where client connection requests are serviced on a sin‐
gle port number. For additional information on this kind of
configuration, see the OpenVPN FAQ: http://open‐
vpn.net/faq.html#oneport
This option precludes the use of --daemon, --local, or --remote.
Note that this option causes message and error output to be han‐
dled in the same way as the --daemon option. The optional prog‐
name parameter is also handled exactly as in --daemon.
Also note that in wait mode, each OpenVPN tunnel requires a sep‐
arate TCP/UDP port and a separate inetd or xinetd entry. See
the OpenVPN 1.x HOWTO for an example on using OpenVPN with
xinetd: http://openvpn.net/1xhowto.html
--log file
Output logging messages to file, including output to std‐
out/stderr which is generated by called scripts. If file
already exists it will be truncated. This option takes effect
immediately when it is parsed in the command line and will
supercede syslog output if --daemon or --inetd is also speci‐
fied. This option is persistent over the entire course of an
OpenVPN instantiation and will not be reset by SIGHUP, SIGUSR1,
or --ping-restart.
Note that on Windows, when OpenVPN is started as a service, log‐
ging occurs by default without the need to specify this option.
--log-append file
Append logging messages to file. If file does not exist, it
will be created. This option behaves exactly like --log except
that it appends to rather than truncating the log file.
--suppress-timestamps
Avoid writing timestamps to log messages, even when they other‐
wise would be prepended. In particular, this applies to log mes‐
sages sent to stdout.
--machine-readable-output
Always write timestamps and message flags to log messages, even
when they otherwise would not be prefixed. In particular, this
applies to log messages sent to stdout.
--writepid file
Write OpenVPN's main process ID to file.
--nice n
Change process priority after initialization ( n greater than 0
is lower priority, n less than zero is higher priority).
--fast-io
(Experimental) Optimize TUN/TAP/UDP I/O writes by avoiding a
call to poll/epoll/select prior to the write operation. The
purpose of such a call would normally be to block until the
device or socket is ready to accept the write. Such blocking is
unnecessary on some platforms which don't support write blocking
on UDP sockets or TUN/TAP devices. In such cases, one can opti‐
mize the event loop by avoiding the poll/epoll/select call,
improving CPU efficiency by 5% to 10%.
This option can only be used on non-Windows systems, when
--proto udp is specified, and when --shaper is NOT specified.
--multihome
Configure a multi-homed UDP server. This option needs to be
used when a server has more than one IP address (e.g. multiple
interfaces, or secondary IP addresses), and is not using --local
to force binding to one specific address only. This option will
add some extra lookups to the packet path to ensure that the UDP
reply packets are always sent from the address that the client
is talking to. This is not supported on all platforms, and it
adds more processing, so it's not enabled by default.
Note: this option is only relevant for UDP servers.
Note 2: if you do an IPv6+IPv4 dual-stack bind on a Linux
machine with multiple IPv4 address, connections to IPv4
addresses will not work right on kernels before 3.15, due to
missing kernel support for the IPv4-mapped case (some distribu‐
tions have ported this to earlier kernel versions, though).
--echo [parms...]
Echo parms to log output.
Designed to be used to send messages to a controlling applica‐
tion which is receiving the OpenVPN log output.
--remap-usr1 signal
Control whether internally or externally generated SIGUSR1 sig‐
nals are remapped to SIGHUP (restart without persisting state)
or SIGTERM (exit).
signal can be set to "SIGHUP" or "SIGTERM". By default, no
remapping occurs.
--verb n
Set output verbosity to n (default=1). Each level shows all
info from the previous levels. Level 3 is recommended if you
want a good summary of what's happening without being swamped by
output.
0 -- No output except fatal errors.
1 to 4 -- Normal usage range.
5 -- Output R and W characters to the console for each packet
read and write, uppercase is used for TCP/UDP packets and lower‐
case is used for TUN/TAP packets.
6 to 11 -- Debug info range (see errlevel.h for additional
information on debug levels).
--status file [n]
Write operational status to file every n seconds.
Status can also be written to the syslog by sending a SIGUSR2
signal.
--status-version [n]
Choose the status file format version number. Currently n can
be 1, 2, or 3 and defaults to 1.
--mute n
Log at most n consecutive messages in the same category. This
is useful to limit repetitive logging of similar message types.
--compress [algorithm]
Enable a compression algorithm.
The algorithm parameter may be "lzo", "lz4", or empty. LZO and
LZ4 are different compression algorithms, with LZ4 generally
offering the best performance with least CPU usage. For back‐
wards compatibility with OpenVPN versions before v2.4, use "lzo"
(which is identical to the older option "--comp-lzo yes").
If the algorithm parameter is empty, compression will be turned
off, but the packet framing for compression will still be
enabled, allowing a different setting to be pushed later.
--comp-lzo [mode]
DEPRECATED This option will be removed in a future OpenVPN
release. Use the newer --compress instead.
Use LZO compression -- may add up to 1 byte per packet for
incompressible data. mode may be "yes", "no", or "adaptive"
(default).
In a server mode setup, it is possible to selectively turn com‐
pression on or off for individual clients.
First, make sure the client-side config file enables selective
compression by having at least one --comp-lzo directive, such as
--comp-lzo no. This will turn off compression by default, but
allow a future directive push from the server to dynamically
change the on/off/adaptive setting.
Next in a --client-config-dir file, specify the compression set‐
ting for the client, for example:
comp-lzo yes
push "comp-lzo yes"
The first line sets the comp-lzo setting for the server side of
the link, the second sets the client side.
--comp-noadapt
When used in conjunction with --comp-lzo, this option will dis‐
able OpenVPN's adaptive compression algorithm. Normally, adap‐
tive compression is enabled with --comp-lzo.
Adaptive compression tries to optimize the case where you have
compression enabled, but you are sending predominantly incom‐
pressible (or pre-compressed) packets over the tunnel, such as
an FTP or rsync transfer of a large, compressed file. With
adaptive compression, OpenVPN will periodically sample the com‐
pression process to measure its efficiency. If the data being
sent over the tunnel is already compressed, the compression
efficiency will be very low, triggering openvpn to disable com‐
pression for a period of time until the next re-sample test.
--management IP port [pw-file]
Enable a TCP server on IP:port to handle daemon management func‐
tions. pw-file, if specified, is a password file (password on
first line) or "stdin" to prompt from standard input. The pass‐
word provided will set the password which TCP clients will need
to provide in order to access management functions.
The management interface can also listen on a unix domain
socket, for those platforms that support it. To use a unix
domain socket, specify the unix socket pathname in place of IP
and set port to 'unix'. While the default behavior is to create
a unix domain socket that may be connected to by any process,
the --management-client-user and --management-client-group
directives can be used to restrict access.
The management interface provides a special mode where the TCP
management link can operate over the tunnel itself. To enable
this mode, set IP = "tunnel". Tunnel mode will cause the man‐
agement interface to listen for a TCP connection on the local
VPN address of the TUN/TAP interface.
While the management port is designed for programmatic control
of OpenVPN by other applications, it is possible to telnet to
the port, using a telnet client in "raw" mode. Once connected,
type "help" for a list of commands.
For detailed documentation on the management interface, see the
management-notes.txt file in the management folder of the Open‐
VPN source distribution.
It is strongly recommended that IP be set to 127.0.0.1 (local‐
host) to restrict accessibility of the management server to
local clients.
--management-client
Management interface will connect as a TCP/unix domain client to
IP:port specified by --management rather than listen as a TCP
server or on a unix domain socket.
If the client connection fails to connect or is disconnected, a
SIGTERM signal will be generated causing OpenVPN to quit.
--management-query-passwords
Query management channel for private key password and
--auth-user-pass username/password. Only query the management
channel for inputs which ordinarily would have been queried from
the console.
--management-query-proxy
Query management channel for proxy server information for a spe‐
cific --remote (client-only).
--management-query-remote
Allow management interface to override --remote directives
(client-only).
--management-external-key
Allows usage for external private key file instead of --key
option (client-only).
--management-external-cert certificate-hint
Allows usage for external certificate instead of --cert option
(client-only). certificate-hint is an arbitrary string which is
passed to a management interface client as an argument of
NEED-CERTIFICATE notification. Requires --management-exter‐
nal-key.
--management-forget-disconnect
Make OpenVPN forget passwords when management session discon‐
nects.
This directive does not affect the --http-proxy username/pass‐
word. It is always cached.
--management-hold
Start OpenVPN in a hibernating state, until a client of the man‐
agement interface explicitly starts it with the hold release
command.
--management-signal
Send SIGUSR1 signal to OpenVPN if management session discon‐
nects. This is useful when you wish to disconnect an OpenVPN
session on user logoff. For --management-client this option is
not needed since a disconnect will always generate a SIGTERM.
--management-log-cache n
Cache the most recent n lines of log file history for usage by
the management channel.
--management-up-down
Report tunnel up/down events to management interface.
--management-client-auth
Gives management interface client the responsibility to authen‐
ticate clients after their client certificate has been verified.
See management-notes.txt in OpenVPN distribution for detailed
notes.
--management-client-pf
Management interface clients must specify a packet filter file
for each connecting client. See management-notes.txt in OpenVPN
distribution for detailed notes.
--management-client-user u
When the management interface is listening on a unix domain
socket, only allow connections from user u.
--management-client-group g
When the management interface is listening on a unix domain
socket, only allow connections from group g.
--plugin module-pathname [init-string]
Load plug-in module from the file module-pathname, passing
init-string as an argument to the module initialization func‐
tion. Multiple plugin modules may be loaded into one OpenVPN
process.
The module-pathname argument can be just a filename or a file‐
name with a relative or absolute path. The format of the file‐
name and path defines if the plug-in will be loaded from a
default plug-in directory or outside this directory.
--plugin path Effective directory used
====================================================
myplug.so DEFAULT_DIR/myplug.so
subdir/myplug.so DEFAULT_DIR/subdir/myplug.so
./subdir/myplug.so CWD/subdir/myplug.so
/usr/lib/my/plug.so /usr/lib/my/plug.so
DEFAULT_DIR is replaced by the default plug-in directory, which
is configured at the build time of OpenVPN. CWD is the current
directory where OpenVPN was started or the directory OpenVPN
have swithed into via the --cd option before the --plugin
option.
For more information and examples on how to build OpenVPN
plug-in modules, see the README file in the plugin folder of the
OpenVPN source distribution.
If you are using an RPM install of OpenVPN, see /usr/share/open‐
vpn/plugin. The documentation is in doc and the actual plugin
modules are in lib.
Multiple plugin modules can be cascaded, and modules can be used
in tandem with scripts. The modules will be called by OpenVPN
in the order that they are declared in the config file. If both
a plugin and script are configured for the same callback, the
script will be called last. If the return code of the mod‐
ule/script controls an authentication function (such as tls-ver‐
ify, auth-user-pass-verify, or client-connect), then every mod‐
ule and script must return success (0) in order for the connec‐
tion to be authenticated.
--keying-material-exporter label len
Save Exported Keying Material [RFC5705] of len bytes (must be
between 16 and 4095 bytes) using label in environment
(exported_keying_material) for use by plugins in OPENVPN_PLUG‐
IN_TLS_FINAL callback.
Note that exporter labels have the potential to collide with
existing PRF labels. In order to prevent this, labels MUST begin
with "EXPORTER".
This option requires OpenSSL 1.0.1 or newer.
Server Mode
Starting with OpenVPN 2.0, a multi-client TCP/UDP server mode is sup‐
ported, and can be enabled with the --mode server option. In server
mode, OpenVPN will listen on a single port for incoming client connec‐
tions. All client connections will be routed through a single tun or
tap interface. This mode is designed for scalability and should be
able to support hundreds or even thousands of clients on sufficiently
fast hardware. SSL/TLS authentication must be used in this mode.
--server network netmask ['nopool']
A helper directive designed to simplify the configuration of
OpenVPN's server mode. This directive will set up an OpenVPN
server which will allocate addresses to clients out of the given
network/netmask. The server itself will take the ".1" address
of the given network for use as the server-side endpoint of the
local TUN/TAP interface.
For example, --server 10.8.0.0 255.255.255.0 expands as follows:
mode server
tls-server
push "topology [topology]"
if dev tun AND (topology == net30 OR topology == p2p):
ifconfig 10.8.0.1 10.8.0.2
if !nopool:
ifconfig-pool 10.8.0.4 10.8.0.251
route 10.8.0.0 255.255.255.0
if client-to-client:
push "route 10.8.0.0 255.255.255.0"
else if topology == net30:
push "route 10.8.0.1"
if dev tap OR (dev tun AND topology == subnet):
ifconfig 10.8.0.1 255.255.255.0
if !nopool:
ifconfig-pool 10.8.0.2 10.8.0.253 255.255.255.0
push "route-gateway 10.8.0.1"
if route-gateway unset:
route-gateway 10.8.0.2
Don't use --server if you are ethernet bridging. Use
--server-bridge instead.
--server-bridge gateway netmask pool-start-IP pool-end-IP
--server-bridge ['nogw']
A helper directive similar to --server which is designed to sim‐
plify the configuration of OpenVPN's server mode in ethernet
bridging configurations.
If --server-bridge is used without any parameters, it will
enable a DHCP-proxy mode, where connecting OpenVPN clients will
receive an IP address for their TAP adapter from the DHCP server
running on the OpenVPN server-side LAN. Note that only clients
that support the binding of a DHCP client with the TAP adapter
(such as Windows) can support this mode. The optional nogw flag
(advanced) indicates that gateway information should not be
pushed to the client.
To configure ethernet bridging, you must first use your OS's
bridging capability to bridge the TAP interface with the ether‐
net NIC interface. For example, on Linux this is done with the
brctl tool, and with Windows XP it is done in the Network Con‐
nections Panel by selecting the ethernet and TAP adapters and
right-clicking on "Bridge Connections".
Next you you must manually set the IP/netmask on the bridge
interface. The gateway and netmask parameters to
--server-bridge can be set to either the IP/netmask of the
bridge interface, or the IP/netmask of the default gate‐
way/router on the bridged subnet.
Finally, set aside a IP range in the bridged subnet, denoted by
pool-start-IP and pool-end-IP, for OpenVPN to allocate to con‐
necting clients.
For example, server-bridge 10.8.0.4 255.255.255.0 10.8.0.128
10.8.0.254 expands as follows:
mode server
tls-server
ifconfig-pool 10.8.0.128 10.8.0.254 255.255.255.0
push "route-gateway 10.8.0.4"
In another example, --server-bridge (without parameters) expands
as follows:
mode server
tls-server
push "route-gateway dhcp"
Or --server-bridge nogw expands as follows:
mode server
tls-server
--push option
Push a config file option back to the client for remote execu‐
tion. Note that option must be enclosed in double quotes ("").
The client must specify --pull in its config file. The set of
options which can be pushed is limited by both feasibility and
security. Some options such as those which would execute
scripts are banned, since they would effectively allow a compro‐
mised server to execute arbitrary code on the client. Other
options such as TLS or MTU parameters cannot be pushed because
the client needs to know them before the connection to the
server can be initiated.
This is a partial list of options which can currently be pushed:
--route, --route-gateway, --route-delay, --redirect-gateway,
--ip-win32, --dhcp-option, --inactive, --ping, --ping-exit,
--ping-restart, --setenv, --auth-token, --persist-key, --per‐
sist-tun, --echo, --comp-lzo, --socket-flags, --sndbuf, --rcvbuf
--push-reset
Don't inherit the global push list for a specific client
instance. Specify this option in a client-specific context such
as with a --client-config-dir configuration file. This option
will ignore --push options at the global config file level.
--push-remove opt
selectively remove all --push options matching "opt" from the
option list for a client. "opt" is matched as a substring
against the whole option string to-be-pushed to the client, so
--push-remove route would remove all --push route ... and
--push route-ipv6 ... statements, while --push-remove
'route-ipv6 2001:' would only remove IPv6 routes for 2001:...
networks.
--push-remove can only be used in a client-specific context,
like in a --client-config-dir file, or --client-connect script
or plugin -- similar to --push-reset, just more selective.
NOTE: to change an option, --push-remove can be used to first
remove the old value, and then add a new --push option with the
new value.
--push-peer-info
Push additional information about the client to server. The
following data is always pushed to the server:
IV_VER=<version> -- the client OpenVPN version
IV_PLAT=[linux|solaris|openbsd|mac|netbsd|freebsd|win] -- the
client OS platform
IV_LZO_STUB=1 -- if client was built with LZO stub capability
IV_LZ4=1 -- if the client supports LZ4 compressions.
IV_PROTO=2 -- if the client supports peer-id floating mechansim
IV_NCP=2 -- negotiable ciphers, client supports --cipher pushed
by the server, a value of 2 or greater indicates client supports
AES-GCM-128 and AES-GCM-256.
IV_UI_VER=<gui_id> <version> -- the UI version of a UI if one is
running, for example "de.blinkt.openvpn 0.5.47" for the Android
app.
When --push-peer-info is enabled the additional information con‐
sists of the following data:
IV_HWADDR=<mac address> -- the MAC address of clients default
gateway
IV_SSL=<version string> -- the ssl version used by the client,
e.g. "OpenSSL 1.0.2f 28 Jan 2016".
IV_PLAT_VER=x.y - the version of the operating system, e.g. 6.1
for Windows 7.
UV_<name>=<value> -- client environment variables whose names
start with "UV_"
--disable
Disable a particular client (based on the common name) from con‐
necting. Don't use this option to disable a client due to key
or password compromise. Use a CRL (certificate revocation list)
instead (see the --crl-verify option).
This option must be associated with a specific client instance,
which means that it must be specified either in a client
instance config file using --client-config-dir or dynamically
generated using a --client-connect script.
--ifconfig-pool start-IP end-IP [netmask]
Set aside a pool of subnets to be dynamically allocated to con‐
necting clients, similar to a DHCP server. For tun-style tun‐
nels, each client will be given a /30 subnet (for interoperabil‐
ity with Windows clients). For tap-style tunnels, individual
addresses will be allocated, and the optional netmask parameter
will also be pushed to clients.
--ifconfig-pool-persist file [seconds]
Persist/unpersist ifconfig-pool data to file, at seconds inter‐
vals (default=600), as well as on program startup and shutdown.
The goal of this option is to provide a long-term association
between clients (denoted by their common name) and the virtual
IP address assigned to them from the ifconfig-pool. Maintaining
a long-term association is good for clients because it allows
them to effectively use the --persist-tun option.
file is a comma-delimited ASCII file, formatted as <Com‐
mon-Name>,<IP-address>.
If seconds = 0, file will be treated as read-only. This is use‐
ful if you would like to treat file as a configuration file.
Note that the entries in this file are treated by OpenVPN as
suggestions only, based on past associations between a common
name and IP address. They do not guarantee that the given com‐
mon name will always receive the given IP address. If you want
guaranteed assignment, use --ifconfig-push
--ifconfig-pool-linear
DEPRECATED This option will be removed in OpenVPN 2.5
Modifies the --ifconfig-pool directive to allocate individual
TUN interface addresses for clients rather than /30 subnets.
NOTE: This option is incompatible with Windows clients.
This option is deprecated, and should be replaced with --topol‐
ogy p2p which is functionally equivalent.
--ifconfig-push local remote-netmask [alias]
Push virtual IP endpoints for client tunnel, overriding the
--ifconfig-pool dynamic allocation.
The parameters local and remote-netmask are set according to the
--ifconfig directive which you want to execute on the client
machine to configure the remote end of the tunnel. Note that
the parameters local and remote-netmask are from the perspective
of the client, not the server. They may be DNS names rather
than IP addresses, in which case they will be resolved on the
server at the time of client connection.
The optional alias parameter may be used in cases where NAT
causes the client view of its local endpoint to differ from the
server view. In this case local/remote-netmask will refer to
the server view while alias/remote-netmask will refer to the
client view.
This option must be associated with a specific client instance,
which means that it must be specified either in a client
instance config file using --client-config-dir or dynamically
generated using a --client-connect script.
Remember also to include a --route directive in the main OpenVPN
config file which encloses local, so that the kernel will know
to route it to the server's TUN/TAP interface.
OpenVPN's internal client IP address selection algorithm works
as follows:
1 -- Use --client-connect script generated file for static IP
(first choice).
2 -- Use --client-config-dir file for static IP (next choice).
3 -- Use --ifconfig-pool allocation for dynamic IP (last
choice).
--iroute network [netmask]
Generate an internal route to a specific client. The netmask
parameter, if omitted, defaults to 255.255.255.255.
This directive can be used to route a fixed subnet from the
server to a particular client, regardless of where the client is
connecting from. Remember that you must also add the route to
the system routing table as well (such as by using the --route
directive). The reason why two routes are needed is that the
--route directive routes the packet from the kernel to OpenVPN.
Once in OpenVPN, the --iroute directive routes to the specific
client.
This option must be specified either in a client instance config
file using --client-config-dir or dynamically generated using a
--client-connect script.
The --iroute directive also has an important interaction with
--push "route ...". --iroute essentially defines a subnet which
is owned by a particular client (we will call this client A).
If you would like other clients to be able to reach A's subnet,
you can use --push "route ..." together with --client-to-client
to effect this. In order for all clients to see A's subnet,
OpenVPN must push this route to all clients EXCEPT for A, since
the subnet is already owned by A. OpenVPN accomplishes this by
not not pushing a route to a client if it matches one of the
client's iroutes.
--client-to-client
Because the OpenVPN server mode handles multiple clients through
a single tun or tap interface, it is effectively a router. The
--client-to-client flag tells OpenVPN to internally route
client-to-client traffic rather than pushing all client-origi‐
nating traffic to the TUN/TAP interface.
When this option is used, each client will "see" the other
clients which are currently connected. Otherwise, each client
will only see the server. Don't use this option if you want to
firewall tunnel traffic using custom, per-client rules.
--duplicate-cn
Allow multiple clients with the same common name to concurrently
connect. In the absence of this option, OpenVPN will disconnect
a client instance upon connection of a new client having the
same common name.
--client-connect cmd
Run command cmd on client connection.
cmd consists of a path to script (or executable program),
optionally followed by arguments. The path and arguments may be
single- or double-quoted and/or escaped using a backslash, and
should be separated by one or more spaces.
The command is passed the common name and IP address of the
just-authenticated client as environmental variables (see envi‐
ronmental variable section below). The command is also passed
the pathname of a freshly created temporary file as the last
argument (after any arguments specified in cmd ), to be used by
the command to pass dynamically generated config file directives
back to OpenVPN.
If the script wants to generate a dynamic config file to be
applied on the server when the client connects, it should write
it to the file named by the last argument.
See the --client-config-dir option below for options which can
be legally used in a dynamically generated config file.
Note that the return value of script is significant. If script
returns a non-zero error status, it will cause the client to be
disconnected.
--client-disconnect cmd
Like --client-connect but called on client instance shutdown.
Will not be called unless the --client-connect script and plug‐
ins (if defined) were previously called on this instance with
successful (0) status returns.
The exception to this rule is if the --client-disconnect command
or plugins are cascaded, and at least one client-connect func‐
tion succeeded, then ALL of the client-disconnect functions for
scripts and plugins will be called on client instance object
deletion, even in cases where some of the related client-connect
functions returned an error status.
The --client-disconnect command is passed the same pathname as
the corresponding --client-connect command as its last argument.
(after any arguments specified in cmd ).
--client-config-dir dir
Specify a directory dir for custom client config files. After a
connecting client has been authenticated, OpenVPN will look in
this directory for a file having the same name as the client's
X509 common name. If a matching file exists, it will be opened
and parsed for client-specific configuration options. If no
matching file is found, OpenVPN will instead try to open and
parse a default file called "DEFAULT", which may be provided but
is not required. Note that the configuration files must be read‐
able by the OpenVPN process after it has dropped it's root priv‐
ileges.
This file can specify a fixed IP address for a given client
using --ifconfig-push, as well as fixed subnets owned by the
client using --iroute.
One of the useful properties of this option is that it allows
client configuration files to be conveniently created, edited,
or removed while the server is live, without needing to restart
the server.
The following options are legal in a client-specific context:
--push, --push-reset, --push-remove, --iroute, --ifconfig-push,
and --config.
--ccd-exclusive
Require, as a condition of authentication, that a connecting
client has a --client-config-dir file.
--tmp-dir dir
Specify a directory dir for temporary files. This directory
will be used by openvpn processes and script to communicate tem‐
porary data with openvpn main process. Note that the directory
must be writable by the OpenVPN process after it has dropped
it's root privileges.
This directory will be used by in the following cases:
* --client-connect scripts to dynamically generate client-spe‐
cific configuration files.
* OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY plugin hook to return
success/failure via auth_control_file when using deferred auth
method
* OPENVPN_PLUGIN_ENABLE_PF plugin hook to pass filtering rules
via pf_file
--hash-size r v
Set the size of the real address hash table to r and the virtual
address table to v. By default, both tables are sized at 256
buckets.
--bcast-buffers n
Allocate n buffers for broadcast datagrams (default=256).
--tcp-queue-limit n
Maximum number of output packets queued before TCP (default=64).
When OpenVPN is tunneling data from a TUN/TAP device to a remote
client over a TCP connection, it is possible that the TUN/TAP
device might produce data at a faster rate than the TCP connec‐
tion can support. When the number of output packets queued
before sending to the TCP socket reaches this limit for a given
client connection, OpenVPN will start to drop outgoing packets
directed at this client.
--tcp-nodelay
This macro sets the TCP_NODELAY socket flag on the server as
well as pushes it to connecting clients. The TCP_NODELAY flag
disables the Nagle algorithm on TCP sockets causing packets to
be transmitted immediately with low latency, rather than waiting
a short period of time in order to aggregate several packets
into a larger containing packet. In VPN applications over TCP,
TCP_NODELAY is generally a good latency optimization.
The macro expands as follows:
if mode server:
socket-flags TCP_NODELAY
push "socket-flags TCP_NODELAY"
--max-clients n
Limit server to a maximum of n concurrent clients.
--max-routes-per-client n
Allow a maximum of n internal routes per client (default=256).
This is designed to help contain DoS attacks where an authenti‐
cated client floods the server with packets appearing to come
from many unique MAC addresses, forcing the server to deplete
virtual memory as its internal routing table expands. This
directive can be used in a --client-config-dir file or auto-gen‐
erated by a --client-connect script to override the global value
for a particular client.
Note that this directive affects OpenVPN's internal routing ta‐
ble, not the kernel routing table.
--stale-routes-check n [t]
Remove routes haven't had activity for n seconds (i.e. the age‐
ing time).
This check is ran every t seconds (i.e. check interval).
If t is not present it defaults to n
This option helps to keep the dynamic routing table small. See
also --max-routes-per-client
--connect-freq n sec
Allow a maximum of n new connections per sec seconds from
clients. This is designed to contain DoS attacks which flood
the server with connection requests using certificates which
will ultimately fail to authenticate.
This is an imperfect solution however, because in a real DoS
scenario, legitimate connections might also be refused.
For the best protection against DoS attacks in server mode, use
--proto udp and either --tls-auth or --tls-crypt.
--learn-address cmd
Run command cmd to validate client virtual addresses or routes.
cmd consists of a path to script (or executable program),
optionally followed by arguments. The path and arguments may be
single- or double-quoted and/or escaped using a backslash, and
should be separated by one or more spaces.
Three arguments will be appended to any arguments in cmd as fol‐
lows:
[1] operation -- "add", "update", or "delete" based on whether
or not the address is being added to, modified, or deleted from
OpenVPN's internal routing table.
[2] address -- The address being learned or unlearned. This can
be an IPv4 address such as "198.162.10.14", an IPv4 subnet such
as "198.162.10.0/24", or an ethernet MAC address (when --dev tap
is being used) such as "00:FF:01:02:03:04".
[3] common name -- The common name on the certificate associated
with the client linked to this address. Only present for "add"
or "update" operations, not "delete".
On "add" or "update" methods, if the script returns a failure
code (non-zero), OpenVPN will reject the address and will not
modify its internal routing table.
Normally, the cmd script will use the information provided above
to set appropriate firewall entries on the VPN TUN/TAP inter‐
face. Since OpenVPN provides the association between virtual IP
or MAC address and the client's authenticated common name, it
allows a user-defined script to configure firewall access poli‐
cies with regard to the client's high-level common name, rather
than the low level client virtual addresses.
--auth-user-pass-verify cmd method
Require the client to provide a username/password (possibly in
addition to a client certificate) for authentication.
OpenVPN will run command cmd to validate the username/password
provided by the client.
cmd consists of a path to script (or executable program),
optionally followed by arguments. The path and arguments may be
single- or double-quoted and/or escaped using a backslash, and
should be separated by one or more spaces.
If method is set to "via-env", OpenVPN will call script with the
environmental variables username and password set to the user‐
name/password strings provided by the client. Be aware that
this method is insecure on some platforms which make the envi‐
ronment of a process publicly visible to other unprivileged pro‐
cesses.
If method is set to "via-file", OpenVPN will write the username
and password to the first two lines of a temporary file. The
filename will be passed as an argument to script, and the file
will be automatically deleted by OpenVPN after the script
returns. The location of the temporary file is controlled by
the --tmp-dir option, and will default to the current directory
if unspecified. For security, consider setting --tmp-dir to a
volatile storage medium such as /dev/shm (if available) to pre‐
vent the username/password file from touching the hard drive.
The script should examine the username and password, returning a
success exit code (0) if the client's authentication request is
to be accepted, or a failure code (1) to reject the client.
This directive is designed to enable a plugin-style interface
for extending OpenVPN's authentication capabilities.
To protect against a client passing a maliciously formed user‐
name or password string, the username string must consist only
of these characters: alphanumeric, underbar ('_'), dash ('-'),
dot ('.'), or at ('@'). The password string can consist of any
printable characters except for CR or LF. Any illegal charac‐
ters in either the username or password string will be converted
to underbar ('_').
Care must be taken by any user-defined scripts to avoid creating
a security vulnerability in the way that these strings are han‐
dled. Never use these strings in such a way that they might be
escaped or evaluated by a shell interpreter.
For a sample script that performs PAM authentication, see sam‐
ple-scripts/auth-pam.pl in the OpenVPN source distribution.
--auth-gen-token [lifetime]
After successful user/password authentication, the OpenVPN
server will with this option generate a temporary authentication
token and push that to client. On the following renegotiations,
the OpenVPN client will pass this token instead of the users
password. On the server side the server will do the token
authentication internally and it will NOT do any additional
authentications against configured external user/password
authentication mechanisms.
The lifetime argument defines how long the generated token is
valid. The lifetime is defined in seconds. If lifetime is not
set or it is set to 0, the token will never expire.
This feature is useful for environments which is configured to
use One Time Passwords (OTP) as part of the user/password
authentications and that authentication mechanism does not
implement any auth-token support.
--opt-verify
Clients that connect with options that are incompatible with
those of the server will be disconnected.
Options that will be compared for compatibility include
dev-type, link-mtu, tun-mtu, proto, ifconfig, comp-lzo, frag‐
ment, keydir, cipher, auth, keysize, secret, no-replay, no-iv,
tls-auth, key-method, tls-server, and tls-client.
This option requires that --disable-occ NOT be used.
--auth-user-pass-optional
Allow connections by clients that do not specify a user‐
name/password. Normally, when --auth-user-pass-verify or --man‐
agement-client-auth is specified (or an authentication plugin
module), the OpenVPN server daemon will require connecting
clients to specify a username and password. This option makes
the submission of a username/password by clients optional, pass‐
ing the responsibility to the user-defined authentication mod‐
ule/script to accept or deny the client based on other factors
(such as the setting of X509 certificate fields). When this
option is used, and a connecting client does not submit a user‐
name/password, the user-defined authentication module/script
will see the username and password as being set to empty strings
(""). The authentication module/script MUST have logic to
detect this condition and respond accordingly.
--client-cert-not-required
DEPRECATED This option will be removed in OpenVPN 2.5
Don't require client certificate, client will authenticate using
username/password only. Be aware that using this directive is
less secure than requiring certificates from all clients.
Please note: This is replaced by --verify-client-cert which
allows for more flexibility. The option --verify-client-cert
none is functionally equivalent to --client-cert-not-required
--verify-client-cert none|optional|require
Specify whether the client is required to supply a valid cer‐
tificate.
Possible options are
none : a client certificate is not required. the client need to
authenticate using username/password only. Be aware that using
this directive is less secure than requiring certificates from
all clients.
If you use this directive, the entire responsibility of authen‐
tication will rest on your --auth-user-pass-verify script, so
keep in mind that bugs in your script could potentially compro‐
mise the security of your VPN.
--verify-client-cert none is functionally equivalent to
--client-cert-not-required.
optional : a client may present a certificate but it is not
required to do so. When using this directive, you should also
use a --auth-user-pass-verify script to ensure that clients are
authenticated using a certificate, a username and password, or
possibly even both.
Again, the entire responsibility of authentication will rest on
your --auth-user-pass-verify script, so keep in mind that bugs
in your script could potentially compromise the security of your
VPN.
require : this is the default option. A client is required to
present a certificate, otherwise VPN access is refused.
If you don't use this directive (or use --verify-client-cert
require ) but you also specify an --auth-user-pass-verify
script, then OpenVPN will perform double authentication. The
client certificate verification AND the --auth-user-pass-verify
script will need to succeed in order for a client to be authen‐
ticated and accepted onto the VPN.
--username-as-common-name
For --auth-user-pass-verify authentication, use the authenti‐
cated username as the common name, rather than the common name
from the client cert.
--compat-names [no-remapping]
DEPRECATED This option will be removed in OpenVPN 2.5
Until OpenVPN v2.3 the format of the X.509 Subject fields was
formatted like this:
/C=US/L=Somewhere/CN=John Doe/emailAddress=john@example.com
In addition the old behaviour was to remap any character other
than alphanumeric, underscore ('_'), dash ('-'), dot ('.'), and
slash ('/') to underscore ('_'). The X.509 Subject string as
returned by the tls_id environmental variable, could addition‐
ally contain colon (':') or equal ('=').
When using the --compat-names option, this old formatting and
remapping will be re-enabled again. This is purely implemented
for compatibility reasons when using older plug-ins or scripts
which does not handle the new formatting or UTF-8 characters.
In OpenVPN 2.3 the formatting of these fields changed into a
more standardised format. It now looks like:
C=US, L=Somewhere, CN=John Doe, emailAddress=john@example.com
The new default format in OpenVPN 2.3 also does not do the char‐
acter remapping which happened earlier. This new format enables
proper support for UTF-8 characters in the usernames, X.509 Sub‐
ject fields and Common Name variables and it complies to the RFC
2253, UTF-8 String Representation of Distinguished Names.
The no-remapping mode flag can be used with the --compat-names
option to be compatible with the now deprecated --no-name-remap‐
ping option. It is only available at the server. When this mode
flag is used, the Common Name, Subject, and username strings are
allowed to include any printable character including space, but
excluding control characters such as tab, newline, and car‐
riage-return. no-remapping is only available on the server side.
Please note: This option is immediately deprecated. It is only
implemented to make the transition to the new formatting less
intrusive. It will be removed in OpenVPN 2.5. So please update
your scripts/plug-ins where necessary.
--no-name-remapping
DEPRECATED This option will be removed in OpenVPN 2.5
The --no-name-remapping option is an alias for --com‐
pat-names no-remapping. It ensures compatibility with server
configurations using the --no-name-remapping option.
Please note: This option is now deprecated. It will be removed
in OpenVPN 2.5. So please make sure you support the new X.509
name formatting described with the --compat-names option as soon
as possible.
--port-share host port [dir]
When run in TCP server mode, share the OpenVPN port with another
application, such as an HTTPS server. If OpenVPN senses a con‐
nection to its port which is using a non-OpenVPN protocol, it
will proxy the connection to the server at host:port. Currently
only designed to work with HTTP/HTTPS, though it would be theo‐
retically possible to extend to other protocols such as ssh.
dir specifies an optional directory where a temporary file with
name N containing content C will be dynamically generated for
each proxy connection, where N is the source IP:port of the
client connection and C is the source IP:port of the connection
to the proxy receiver. This directory can be used as a dictio‐
nary by the proxy receiver to determine the origin of the con‐
nection. Each generated file will be automatically deleted when
the proxied connection is torn down.
Not implemented on Windows.
Client Mode
Use client mode when connecting to an OpenVPN server which has
--server, --server-bridge, or --mode server in it's configuration.
--client
A helper directive designed to simplify the configuration of
OpenVPN's client mode. This directive is equivalent to:
pull
tls-client
--pull This option must be used on a client which is connecting to a
multi-client server. It indicates to OpenVPN that it should
accept options pushed by the server, provided they are part of
the legal set of pushable options (note that the --pull option
is implied by --client ).
In particular, --pull allows the server to push routes to the
client, so you should not use --pull or --client in situations
where you don't trust the server to have control over the
client's routing table.
--pull-filter accept|ignore|reject text
Filter options received from the server if the option starts
with text. Runs on client. The action flag accept allows the
option, ignore removes it and reject flags an error and triggers
a SIGUSR1 restart. The filters may be specified multiple times,
and each filter is applied in the order it is specified. The
filtering of each option stops as soon as a match is found.
Unmatched options are accepted by default.
Prefix comparison is used to match text against the received
option so that
--pull-filter ignore "route"
would remove all pushed options starting with route which would
include, for example, route-gateway. Enclose text in quotes to
embed spaces.
--pull-filter accept "route 192.168.1."
--pull-filter ignore "route "
would remove all routes that do not start with 192.168.1.
This option may be used only on clients. Note that reject may
result in a repeated cycle of failure and reconnect, unless mul‐
tiple remotes are specified and connection to the next remote
succeeds. To silently ignore an option pushed by the server, use
ignore.
--auth-user-pass [up]
Authenticate with server using username/password. up is a file
containing username/password on 2 lines. If the password line is
missing, OpenVPN will prompt for one.
If up is omitted, username/password will be prompted from the
console.
The server configuration must specify an --auth-user-pass-verify
script to verify the username/password provided by the client.
--auth-retry type
Controls how OpenVPN responds to username/password verification
errors such as the client-side response to an AUTH_FAILED mes‐
sage from the server or verification failure of the private key
password.
Normally used to prevent auth errors from being fatal on the
client side, and to permit username/password requeries in case
of error.
An AUTH_FAILED message is generated by the server if the client
fails --auth-user-pass authentication, or if the server-side
--client-connect script returns an error status when the client
tries to connect.
type can be one of:
none -- Client will exit with a fatal error (this is the
default).
nointeract -- Client will retry the connection without requery‐
ing for an --auth-user-pass username/password. Use this option
for unattended clients.
interact -- Client will requery for an --auth-user-pass user‐
name/password and/or private key password before attempting a
reconnection.
Note that while this option cannot be pushed, it can be con‐
trolled from the management interface.
--static-challenge t e
Enable static challenge/response protocol using challenge text
t, with echo flag given by e (0|1).
The echo flag indicates whether or not the user's response to
the challenge should be echoed.
See management-notes.txt in the OpenVPN distribution for a
description of the OpenVPN challenge/response protocol.
--server-poll-timeout n, --connect-timeout n
When connecting to a remote server do not wait for more than n
seconds waiting for a response before trying the next server.
The default value is 120s. This timeout includes proxy and TCP
connect timeouts.
--explicit-exit-notify [n]
In UDP client mode or point-to-point mode, send server/peer an
exit notification if tunnel is restarted or OpenVPN process is
exited. In client mode, on exit/restart, this option will tell
the server to immediately close its client instance object
rather than waiting for a timeout. The n parameter (default=1)
controls the maximum number of attempts that the client will try
to resend the exit notification message.
In UDP server mode, send RESTART control channel command to con‐
nected clients. The n parameter (default=1) controls client
behavior. With n = 1 client will attempt to reconnect to the
same server, with n = 2 client will advance to the next server.
OpenVPN will not send any exit notifications unless this option
is enabled.
--allow-recursive-routing
When this option is set, OpenVPN will not drop incoming tun
packets with same destination as host.
Data Channel Encryption Options:
These options are meaningful for both Static & TLS-negotiated key modes
(must be compatible between peers).
--secret file [direction]
Enable Static Key encryption mode (non-TLS). Use pre-shared
secret file which was generated with --genkey.
The optional direction parameter enables the use of 4 distinct
keys (HMAC-send, cipher-encrypt, HMAC-receive, cipher-decrypt),
so that each data flow direction has a different set of HMAC and
cipher keys. This has a number of desirable security properties
including eliminating certain kinds of DoS and message replay
attacks.
When the direction parameter is omitted, 2 keys are used bidi‐
rectionally, one for HMAC and the other for encryption/decryp‐
tion.
The direction parameter should always be complementary on either
side of the connection, i.e. one side should use "0" and the
other should use "1", or both sides should omit it altogether.
The direction parameter requires that file contains a 2048 bit
key. While pre-1.5 versions of OpenVPN generate 1024 bit key
files, any version of OpenVPN which supports the direction
parameter, will also support 2048 bit key file generation using
the --genkey option.
Static key encryption mode has certain advantages, the primary
being ease of configuration.
There are no certificates or certificate authorities or compli‐
cated negotiation handshakes and protocols. The only require‐
ment is that you have a pre-existing secure channel with your
peer (such as ssh ) to initially copy the key. This require‐
ment, along with the fact that your key never changes unless you
manually generate a new one, makes it somewhat less secure than
TLS mode (see below). If an attacker manages to steal your key,
everything that was ever encrypted with it is compromised. Con‐
trast that to the perfect forward secrecy features of TLS mode
(using Diffie Hellman key exchange), where even if an attacker
was able to steal your private key, he would gain no information
to help him decrypt past sessions.
Another advantageous aspect of Static Key encryption mode is
that it is a handshake-free protocol without any distinguishing
signature or feature (such as a header or protocol handshake
sequence) that would mark the ciphertext packets as being gener‐
ated by OpenVPN. Anyone eavesdropping on the wire would see
nothing but random-looking data.
--key-direction
Alternative way of specifying the optional direction parameter
for the --tls-auth and --secret options. Useful when using
inline files (See section on inline files).
--auth alg
Authenticate data channel packets and (if enabled) tls-auth con‐
trol channel packets with HMAC using message digest algorithm
alg. (The default is SHA1 ). HMAC is a commonly used message
authentication algorithm (MAC) that uses a data string, a secure
hash algorithm, and a key, to produce a digital signature.
The OpenVPN data channel protocol uses encrypt-then-mac (i.e.
first encrypt a packet, then HMAC the resulting ciphertext),
which prevents padding oracle attacks.
If an AEAD cipher mode (e.g. GCM) is chosen, the specified
--auth algorithm is ignored for the data channel, and the
authentication method of the AEAD cipher is used instead. Note
that alg still specifies the digest used for tls-auth.
In static-key encryption mode, the HMAC key is included in the
key file generated by --genkey. In TLS mode, the HMAC key is
dynamically generated and shared between peers via the TLS con‐
trol channel. If OpenVPN receives a packet with a bad HMAC it
will drop the packet. HMAC usually adds 16 or 20 bytes per
packet. Set alg=none to disable authentication.
For more information on HMAC see
http://www.cs.ucsd.edu/users/mihir/papers/hmac.html
--cipher alg
Encrypt data channel packets with cipher algorithm alg.
The default is BF-CBC, an abbreviation for Blowfish in Cipher
Block Chaining mode. When cipher negotiation (NCP) is allowed,
OpenVPN 2.4 and newer on both client and server side will auto‐
matically upgrade to AES-256-GCM. See --ncp-ciphers and
--ncp-disable for more details on NCP.
Using BF-CBC is no longer recommended, because of its 64-bit
block size. This small block size allows attacks based on col‐
lisions, as demonstrated by SWEET32. See https://commu‐
nity.openvpn.net/openvpn/wiki/SWEET32 for details. Due to this,
support for BF-CBC, DES, CAST5, IDEA and RC2 ciphers will be
removed in OpenVPN 2.6.
To see other ciphers that are available with OpenVPN, use the
--show-ciphers option.
Set alg=none to disable encryption.
--ncp-ciphers cipher_list
Restrict the allowed ciphers to be negotiated to the ciphers in
cipher_list. cipher_list is a colon-separated list of ciphers,
and defaults to "AES-256-GCM:AES-128-GCM".
For servers, the first cipher from cipher_list will be pushed to
clients that support cipher negotiation.
Cipher negotiation is enabled in client-server mode only. I.e.
if --mode is set to 'server' (server-side, implied by setting
--server ), or if --pull is specified (client-side, implied by
setting --client).
If both peers support and do not disable NCP, the negotiated
cipher will override the cipher specified by --cipher.
Additionally, to allow for more smooth transition, if NCP is
enabled, OpenVPN will inherit the cipher of the peer if that
cipher is different from the local --cipher setting, but the
peer cipher is one of the ciphers specified in --ncp-ciphers.
E.g. a non-NCP client (<=v2.3, or with --ncp-disabled set) con‐
necting to a NCP server (v2.4+) with "--cipher BF-CBC" and
"--ncp-ciphers AES-256-GCM:AES-256-CBC" set can either specify
"--cipher BF-CBC" or "--cipher AES-256-CBC" and both will work.
--ncp-disable
Disable "negotiable crypto parameters". This completely dis‐
ables cipher negotiation.
--keysize n
DEPRECATED This option will be removed in OpenVPN 2.6.
Size of cipher key in bits (optional). If unspecified, defaults
to cipher-specific default. The --show-ciphers option (see
below) shows all available OpenSSL ciphers, their default key
sizes, and whether the key size can be changed. Use care in
changing a cipher's default key size. Many ciphers have not
been extensively cryptanalyzed with non-standard key lengths,
and a larger key may offer no real guarantee of greater secu‐
rity, or may even reduce security.
--prng alg [nsl]
(Advanced) For PRNG (Pseudo-random number generator), use digest
algorithm alg (default=sha1), and set nsl (default=16) to the
size in bytes of the nonce secret length (between 16 and 64).
Set alg=none to disable the PRNG and use the OpenSSL RAND_bytes
function instead for all of OpenVPN's pseudo-random number
needs.
--engine [engine-name]
Enable OpenSSL hardware-based crypto engine functionality.
If engine-name is specified, use a specific crypto engine. Use
the --show-engines standalone option to list the crypto engines
which are supported by OpenSSL.
--no-replay
DEPRECATED This option will be removed in OpenVPN 2.5.
(Advanced) Disable OpenVPN's protection against replay attacks.
Don't use this option unless you are prepared to make a tradeoff
of greater efficiency in exchange for less security.
OpenVPN provides datagram replay protection by default.
Replay protection is accomplished by tagging each outgoing data‐
gram with an identifier that is guaranteed to be unique for the
key being used. The peer that receives the datagram will check
for the uniqueness of the identifier. If the identifier was
already received in a previous datagram, OpenVPN will drop the
packet. Replay protection is important to defeat attacks such
as a SYN flood attack, where the attacker listens in the wire,
intercepts a TCP SYN packet (identifying it by the context in
which it occurs in relation to other packets), then floods the
receiving peer with copies of this packet.
OpenVPN's replay protection is implemented in slightly different
ways, depending on the key management mode you have selected.
In Static Key mode or when using an CFB or OFB mode cipher,
OpenVPN uses a 64 bit unique identifier that combines a time
stamp with an incrementing sequence number.
When using TLS mode for key exchange and a CBC cipher mode,
OpenVPN uses only a 32 bit sequence number without a time stamp,
since OpenVPN can guarantee the uniqueness of this value for
each key. As in IPSec, if the sequence number is close to wrap‐
ping back to zero, OpenVPN will trigger a new key exchange.
To check for replays, OpenVPN uses the sliding window algorithm
used by IPSec.
--replay-window n [t]
Use a replay protection sliding-window of size n and a time win‐
dow of t seconds.
By default n is 64 (the IPSec default) and t is 15 seconds.
This option is only relevant in UDP mode, i.e. when either
--proto udp is specified, or no --proto option is specified.
When OpenVPN tunnels IP packets over UDP, there is the possibil‐
ity that packets might be dropped or delivered out of order.
Because OpenVPN, like IPSec, is emulating the physical network
layer, it will accept an out-of-order packet sequence, and will
deliver such packets in the same order they were received to the
TCP/IP protocol stack, provided they satisfy several con‐
straints.
(a) The packet cannot be a replay (unless --no-replay is speci‐
fied, which disables replay protection altogether).
(b) If a packet arrives out of order, it will only be accepted
if the difference between its sequence number and the highest
sequence number received so far is less than n.
(c) If a packet arrives out of order, it will only be accepted
if it arrives no later than t seconds after any packet contain‐
ing a higher sequence number.
If you are using a network link with a large pipeline (meaning
that the product of bandwidth and latency is high), you may want
to use a larger value for n. Satellite links in particular
often require this.
If you run OpenVPN at --verb 4, you will see the message
"Replay-window backtrack occurred [x]" every time the maximum
sequence number backtrack seen thus far increases. This can be
used to calibrate n.
There is some controversy on the appropriate method of handling
packet reordering at the security layer.
Namely, to what extent should the security layer protect the
encapsulated protocol from attacks which masquerade as the kinds
of normal packet loss and reordering that occur over IP net‐
works?
The IPSec and OpenVPN approach is to allow packet reordering
within a certain fixed sequence number window.
OpenVPN adds to the IPSec model by limiting the window size in
time as well as sequence space.
OpenVPN also adds TCP transport as an option (not offered by
IPSec) in which case OpenVPN can adopt a very strict attitude
towards message deletion and reordering: Don't allow it. Since
TCP guarantees reliability, any packet loss or reordering event
can be assumed to be an attack.
In this sense, it could be argued that TCP tunnel transport is
preferred when tunneling non-IP or UDP application protocols
which might be vulnerable to a message deletion or reordering
attack which falls within the normal operational parameters of
IP networks.
So I would make the statement that one should never tunnel a
non-IP protocol or UDP application protocol over UDP, if the
protocol might be vulnerable to a message deletion or reordering
attack that falls within the normal operating parameters of what
is to be expected from the physical IP layer. The problem is
easily fixed by simply using TCP as the VPN transport layer.
--mute-replay-warnings
Silence the output of replay warnings, which are a common false
alarm on WiFi networks. This option preserves the security of
the replay protection code without the verbosity associated with
warnings about duplicate packets.
--replay-persist file
Persist replay-protection state across sessions using file to
save and reload the state.
This option will strengthen protection against replay attacks,
especially when you are using OpenVPN in a dynamic context (such
as with --inetd) when OpenVPN sessions are frequently started
and stopped.
This option will keep a disk copy of the current replay protec‐
tion state (i.e. the most recent packet timestamp and sequence
number received from the remote peer), so that if an OpenVPN
session is stopped and restarted, it will reject any replays of
packets which were already received by the prior session.
This option only makes sense when replay protection is enabled
(the default) and you are using either --secret (shared-secret
key mode) or TLS mode with --tls-auth.
--no-iv
DEPRECATED This option will be removed in OpenVPN 2.5.
(Advanced) Disable OpenVPN's use of IV (cipher initialization
vector). Don't use this option unless you are prepared to make
a tradeoff of greater efficiency in exchange for less security.
OpenVPN uses an IV by default, and requires it for CFB and OFB
cipher modes (which are totally insecure without it). Using an
IV is important for security when multiple messages are being
encrypted/decrypted with the same key.
IV is implemented differently depending on the cipher mode used.
In CBC mode, OpenVPN uses a pseudo-random IV for each packet.
In CFB/OFB mode, OpenVPN uses a unique sequence number and time
stamp as the IV. In fact, in CFB/OFB mode, OpenVPN uses a data‐
gram space-saving optimization that uses the unique identifier
for datagram replay protection as the IV.
--use-prediction-resistance
Enable prediction resistance on mbed TLS's RNG.
Enabling prediction resistance causes the RNG to reseed in each
call for random. Reseeding this often can quickly deplete the
kernel entropy pool.
If you need this option, please consider running a daemon that
adds entropy to the kernel pool.
--test-crypto
Do a self-test of OpenVPN's crypto options by encrypting and
decrypting test packets using the data channel encryption
options specified above. This option does not require a peer to
function, and therefore can be specified without --dev or
--remote.
The typical usage of --test-crypto would be something like this:
openvpn --test-crypto --secret key
or
openvpn --test-crypto --secret key --verb 9
This option is very useful to test OpenVPN after it has been
ported to a new platform, or to isolate problems in the com‐
piler, OpenSSL crypto library, or OpenVPN's crypto code. Since
it is a self-test mode, problems with encryption and authentica‐
tion can be debugged independently of network and tunnel issues.
TLS Mode Options:
TLS mode is the most powerful crypto mode of OpenVPN in both security
and flexibility. TLS mode works by establishing control and data chan‐
nels which are multiplexed over a single TCP/UDP port. OpenVPN initi‐
ates a TLS session over the control channel and uses it to exchange
cipher and HMAC keys to protect the data channel. TLS mode uses a
robust reliability layer over the UDP connection for all control chan‐
nel communication, while the data channel, over which encrypted tunnel
data passes, is forwarded without any mediation. The result is the
best of both worlds: a fast data channel that forwards over UDP with
only the overhead of encrypt, decrypt, and HMAC functions, and a con‐
trol channel that provides all of the security features of TLS, includ‐
ing certificate-based authentication and Diffie Hellman forward
secrecy.
To use TLS mode, each peer that runs OpenVPN should have its own local
certificate/key pair ( --cert and --key ), signed by the root certifi‐
cate which is specified in --ca.
When two OpenVPN peers connect, each presents its local certificate to
the other. Each peer will then check that its partner peer presented a
certificate which was signed by the master root certificate as speci‐
fied in --ca.
If that check on both peers succeeds, then the TLS negotiation will
succeed, both OpenVPN peers will exchange temporary session keys, and
the tunnel will begin passing data.
The OpenVPN project provides a set of scripts for managing RSA certifi‐
cates & keys: https://github.com/OpenVPN/easy-rsa
--tls-server
Enable TLS and assume server role during TLS handshake. Note
that OpenVPN is designed as a peer-to-peer application. The
designation of client or server is only for the purpose of nego‐
tiating the TLS control channel.
--tls-client
Enable TLS and assume client role during TLS handshake.
--ca file
Certificate authority (CA) file in .pem format, also referred to
as the root certificate. This file can have multiple certifi‐
cates in .pem format, concatenated together. You can construct
your own certificate authority certificate and private key by
using a command such as:
openssl req -nodes -new -x509 -keyout ca.key -out ca.crt
Then edit your openssl.cnf file and edit the certificate vari‐
able to point to your new root certificate ca.crt.
For testing purposes only, the OpenVPN distribution includes a
sample CA certificate (ca.crt). Of course you should never use
the test certificates and test keys distributed with OpenVPN in
a production environment, since by virtue of the fact that they
are distributed with OpenVPN, they are totally insecure.
--capath dir
Directory containing trusted certificates (CAs and CRLs). Not
available with mbed TLS.
When using the --capath option, you are required to supply valid
CRLs for the CAs too. CAs in the capath directory are expected
to be named <hash>.<n>. CRLs are expected to be named
<hash>.r<n>. See the -CApath option of openssl verify , and the
-hash option of openssl x509 and openssl crl for more informa‐
tion.
--dh file
File containing Diffie Hellman parameters in .pem format
(required for --tls-server only).
Set file=none to disable Diffie Hellman key exchange (and use
ECDH only). Note that this requires peers to be using an SSL
library that supports ECDH TLS cipher suites (e.g. OpenSSL
1.0.1+, or mbed TLS 2.0+).
Use openssl dhparam -out dh2048.pem 2048 to generate 2048-bit DH
parameters. Diffie Hellman parameters may be considered public.
--ecdh-curve name
Specify the curve to use for elliptic curve Diffie Hellman.
Available curves can be listed with --show-curves. The speci‐
fied curve will only be used for ECDH TLS-ciphers.
This option is not supported in mbed TLS builds of OpenVPN.
--cert file
Local peer's signed certificate in .pem format -- must be signed
by a certificate authority whose certificate is in --ca file.
Each peer in an OpenVPN link running in TLS mode should have its
own certificate and private key file. In addition, each cer‐
tificate should have been signed by the key of a certificate
authority whose public key resides in the --ca certificate
authority file. You can easily make your own certificate
authority (see above) or pay money to use a commercial service
such as thawte.com (in which case you will be helping to finance
the world's second space tourist :). To generate a certificate,
you can use a command such as:
openssl req -nodes -new -keyout mycert.key -out mycert.csr
If your certificate authority private key lives on another
machine, copy the certificate signing request (mycert.csr) to
this other machine (this can be done over an insecure channel
such as email). Now sign the certificate with a command such
as:
openssl ca -out mycert.crt -in mycert.csr
Now copy the certificate (mycert.crt) back to the peer which
initially generated the .csr file (this can be over a public
medium). Note that the openssl ca command reads the location of
the certificate authority key from its configuration file such
as /usr/share/ssl/openssl.cnf -- note also that for certificate
authority functions, you must set up the files index.txt (may be
empty) and serial (initialize to 01 ).
--extra-certs file
Specify a file containing one or more PEM certs (concatenated
together) that complete the local certificate chain.
This option is useful for "split" CAs, where the CA for server
certs is different than the CA for client certs. Putting certs
in this file allows them to be used to complete the local cer‐
tificate chain without trusting them to verify the peer-submit‐
ted certificate, as would be the case if the certs were placed
in the ca file.
--key file
Local peer's private key in .pem format. Use the private key
which was generated when you built your peer's certificate (see
--cert file above).
--tls-version-min version ['or-highest']
Sets the minimum TLS version we will accept from the peer
(default is "1.0"). Examples for version include "1.0", "1.1",
or "1.2". If 'or-highest' is specified and version is not rec‐
ognized, we will only accept the highest TLS version supported
by the local SSL implementation.
--tls-version-max version
Set the maximum TLS version we will use (default is the highest
version supported). Examples for version include "1.0", "1.1",
or "1.2".
--pkcs12 file
Specify a PKCS #12 file containing local private key, local cer‐
tificate, and root CA certificate. This option can be used
instead of --ca, --cert, and --key. Not available with mbed
TLS.
--verify-hash hash [algo]
Specify SHA1 or SHA256 fingerprint for level-1 cert. The
level-1 cert is the CA (or intermediate cert) that signs the
leaf certificate, and is one removed from the leaf certificate
in the direction of the root. When accepting a connection from
a peer, the level-1 cert fingerprint must match hash or certifi‐
cate verification will fail. Hash is specified as XX:XX:... For
example:
AD:B0:95:D8:09:C8:36:45:12:A9:89:C8:90:09:CB:13:72:A6:AD:16
The algo flag can be either SHA1 or SHA256. If not provided, it
defaults to SHA1.
--pkcs11-cert-private [0|1]...
Set if access to certificate object should be performed after
login. Every provider has its own setting.
--pkcs11-id name
Specify the serialized certificate id to be used. The id can be
gotten by the standalone --show-pkcs11-ids option.
--pkcs11-id-management
Acquire PKCS#11 id from management interface. In this case a
NEED-STR 'pkcs11-id-request' real-time message will be trig‐
gered, application may use pkcs11-id-count command to retrieve
available number of certificates, and pkcs11-id-get command to
retrieve certificate id and certificate body.
--pkcs11-pin-cache seconds
Specify how many seconds the PIN can be cached, the default is
until the token is removed.
--pkcs11-protected-authentication [0|1]...
Use PKCS#11 protected authentication path, useful for biometric
and external keypad devices. Every provider has its own set‐
ting.
--pkcs11-providers provider...
Specify a RSA Security Inc. PKCS #11 Cryptographic Token Inter‐
face (Cryptoki) providers to load. This option can be used
instead of --cert, --key, and --pkcs12.
If p11-kit is present on the system, its p11-kit-proxy.so module
will be loaded by default if either the --pkcs11-id or
--pkcs11-id-management options are specified without
--pkcs11-provider being given.
--pkcs11-private-mode mode...
Specify which method to use in order to perform private key
operations. A different mode can be specified for each
provider. Mode is encoded as hex number, and can be a mask one
of the following:
0 (default) -- Try to determine automatically.
1 -- Use sign.
2 -- Use sign recover.
4 -- Use decrypt.
8 -- Use unwrap.
--cryptoapicert select-string
Load the certificate and private key from the Windows Certifi‐
cate System Store (Windows/OpenSSL Only).
Use this option instead of --cert and --key.
This makes it possible to use any smart card, supported by Win‐
dows, but also any kind of certificate, residing in the Cert
Store, where you have access to the private key. This option
has been tested with a couple of different smart cards (GemSAFE,
Cryptoflex, and Swedish Post Office eID) on the client side, and
also an imported PKCS12 software certificate on the server side.
To select a certificate, based on a substring search in the cer‐
tificate's subject:
cryptoapicert "SUBJ:Peter Runestig"
To select a certificate, based on certificate's thumbprint:
cryptoapicert "THUMB:f6 49 24 41 01 b4 ..."
The thumbprint hex string can easily be copy-and-pasted from the
Windows Certificate Store GUI.
--key-method m
DEPRECATED This option will be removed in OpenVPN 2.5
Use data channel key negotiation method m. The key method must
match on both sides of the connection.
After OpenVPN negotiates a TLS session, a new set of keys for
protecting the tunnel data channel is generated and exchanged
over the TLS session.
In method 1 (the default for OpenVPN 1.x), both sides generate
random encrypt and HMAC-send keys which are forwarded to the
other host over the TLS channel. Method 1 is deprecated in Open‐
VPN 2.4 , and will be removed in OpenVPN 2.5.
In method 2, (the default for OpenVPN 2.0) the client generates
a random key. Both client and server also generate some random
seed material. All key source material is exchanged over the
TLS channel. The actual keys are generated using the TLS PRF
function, taking source entropy from both client and server.
Method 2 is designed to closely parallel the key generation
process used by TLS 1.0.
Note that in TLS mode, two separate levels of keying occur:
(1) The TLS connection is initially negotiated, with both sides
of the connection producing certificates and verifying the cer‐
tificate (or other authentication info provided) of the other
side. The --key-method parameter has no effect on this process.
(2) After the TLS connection is established, the tunnel session
keys are separately negotiated over the existing secure TLS
channel. Here, --key-method determines the derivation of the
tunnel session keys.
--tls-cipher l
A list l of allowable TLS ciphers delimited by a colon (":").
This setting can be used to ensure that certain cipher suites
are used (or not used) for the TLS connection. OpenVPN uses TLS
to secure the control channel, over which the keys that are used
to protect the actual VPN traffic are exchanged.
The supplied list of ciphers is (after potential OpenSSL/IANA
name translation) simply supplied to the crypto library. Please
see the OpenSSL and/or mbed TLS documentation for details on the
cipher list interpretation.
Use --show-tls to see a list of TLS ciphers supported by your
crypto library.
Warning! --tls-cipher is an expert feature, which - if used
correcly - can improve the security of your VPN connection. But
it is also easy to unwittingly use it to carefully align a gun
with your foot, or just break your connection. Use with care!
The default for --tls-cipher is to use mbed TLS's default cipher
list when using mbed TLS or
"DEFAULT:!EXP:!LOW:!MEDIUM:!kDH:!kECDH:!DSS:!PSK:!SRP:!kRSA"
when using OpenSSL.
--tls-timeout n
Packet retransmit timeout on TLS control channel if no acknowl‐
edgment from remote within n seconds (default=2). When OpenVPN
sends a control packet to its peer, it will expect to receive an
acknowledgement within n seconds or it will retransmit the
packet, subject to a TCP-like exponential backoff algorithm.
This parameter only applies to control channel packets. Data
channel packets (which carry encrypted tunnel data) are never
acknowledged, sequenced, or retransmitted by OpenVPN because the
higher level network protocols running on top of the tunnel such
as TCP expect this role to be left to them.
--reneg-bytes n
Renegotiate data channel key after n bytes sent or received
(disabled by default with an exception, see below). OpenVPN
allows the lifetime of a key to be expressed as a number of
bytes encrypted/decrypted, a number of packets, or a number of
seconds. A key renegotiation will be forced if any of these
three criteria are met by either peer.
If using ciphers with cipher block sizes less than 128-bits,
--reneg-bytes is set to 64MB by default, unless it is explicitly
disabled by setting the value to 0, but this is HIGHLY DISCOUR‐
AGED as this is designed to add some protection against the
SWEET32 attack vector. For more information see the --cipher
option.
--reneg-pkts n
Renegotiate data channel key after n packets sent and received
(disabled by default).
--reneg-sec n
Renegotiate data channel key after n seconds (default=3600).
When using dual-factor authentication, note that this default
value may cause the end user to be challenged to reauthorize
once per hour.
Also, keep in mind that this option can be used on both the
client and server, and whichever uses the lower value will be
the one to trigger the renegotiation. A common mistake is to
set --reneg-sec to a higher value on either the client or
server, while the other side of the connection is still using
the default value of 3600 seconds, meaning that the renegotia‐
tion will still occur once per 3600 seconds. The solution is to
increase --reneg-sec on both the client and server, or set it to
0 on one side of the connection (to disable), and to your chosen
value on the other side.
--hand-window n
Handshake Window -- the TLS-based key exchange must finalize
within n seconds of handshake initiation by any peer (default =
60 seconds). If the handshake fails we will attempt to reset
our connection with our peer and try again. Even in the event
of handshake failure we will still use our expiring key for up
to --tran-window seconds to maintain continuity of transmission
of tunnel data.
--tran-window n
Transition window -- our old key can live this many seconds
after a new a key renegotiation begins (default = 3600 seconds).
This feature allows for a graceful transition from old to new
key, and removes the key renegotiation sequence from the criti‐
cal path of tunnel data forwarding.
--single-session
After initially connecting to a remote peer, disallow any new
connections. Using this option means that a remote peer cannot
connect, disconnect, and then reconnect.
If the daemon is reset by a signal or --ping-restart, it will
allow one new connection.
--single-session can be used with --ping-exit or --inactive to
create a single dynamic session that will exit when finished.
--tls-exit
Exit on TLS negotiation failure.
--tls-auth file [direction]
Add an additional layer of HMAC authentication on top of the TLS
control channel to mitigate DoS attacks and attacks on the TLS
stack.
In a nutshell, --tls-auth enables a kind of "HMAC firewall" on
OpenVPN's TCP/UDP port, where TLS control channel packets bear‐
ing an incorrect HMAC signature can be dropped immediately with‐
out response.
file (required) is a file in OpenVPN static key format which can
be generated by --genkey
Older versions (up to OpenVPN 2.3) supported a freeform
passphrase file. This is no longer supported in newer versions
(v2.4+).
See the --secret option for more information on the optional
direction parameter.
--tls-auth is recommended when you are running OpenVPN in a mode
where it is listening for packets from any IP address, such as
when --remote is not specified, or --remote is specified with
--float.
The rationale for this feature is as follows. TLS requires a
multi-packet exchange before it is able to authenticate a peer.
During this time before authentication, OpenVPN is allocating
resources (memory and CPU) to this potential peer. The poten‐
tial peer is also exposing many parts of OpenVPN and the OpenSSL
library to the packets it is sending. Most successful network
attacks today seek to either exploit bugs in programs (such as
buffer overflow attacks) or force a program to consume so many
resources that it becomes unusable. Of course the first line of
defense is always to produce clean, well-audited code. OpenVPN
has been written with buffer overflow attack prevention as a top
priority. But as history has shown, many of the most widely
used network applications have, from time to time, fallen to
buffer overflow attacks.
So as a second line of defense, OpenVPN offers this special
layer of authentication on top of the TLS control channel so
that every packet on the control channel is authenticated by an
HMAC signature and a unique ID for replay protection. This sig‐
nature will also help protect against DoS (Denial of Service)
attacks. An important rule of thumb in reducing vulnerability
to DoS attacks is to minimize the amount of resources a poten‐
tial, but as yet unauthenticated, client is able to consume.
--tls-auth does this by signing every TLS control channel packet
with an HMAC signature, including packets which are sent before
the TLS level has had a chance to authenticate the peer. The
result is that packets without the correct signature can be
dropped immediately upon reception, before they have a chance to
consume additional system resources such as by initiating a TLS
handshake. --tls-auth can be strengthened by adding the
--replay-persist option which will keep OpenVPN's replay protec‐
tion state in a file so that it is not lost across restarts.
It should be emphasized that this feature is optional and that
the key file used with --tls-auth gives a peer nothing more than
the power to initiate a TLS handshake. It is not used to
encrypt or authenticate any tunnel data.
Use --tls-crypt instead if you want to use the key file to not
only authenticate, but also encrypt the TLS control channel.
--tls-crypt keyfile
Encrypt and authenticate all control channel packets with the
key from keyfile. (See --tls-auth for more background.)
Encrypting (and authenticating) control channel packets:
· provides more privacy by hiding the certificate used for the
TLS connection,
· makes it harder to identify OpenVPN traffic as such,
· provides "poor-man's" post-quantum security, against attackers
who will never know the pre-shared key (i.e. no forward
secrecy).
In contrast to --tls-auth, --tls-crypt does *not* require the
user to set --key-direction.
Security Considerations
All peers use the same --tls-crypt pre-shared group key to
authenticate and encrypt control channel messages. To ensure
that IV collisions remain unlikely, this key should not be used
to encrypt more than 2^48 client-to-server or 2^48
server-to-client control channel messages. A typical initial
negotiation is about 10 packets in each direction. Assuming
both initial negotiation and renegotiations are at most 2^16
(65536) packets (to be conservative), and (re)negotiations hap‐
pen each minute for each user (24/7), this limits the tls-crypt
key lifetime to 8171 years divided by the number of users. So a
setup with 1000 users should rotate the key at least once each
eight years. (And a setup with 8000 users each year.)
If IV collisions were to occur, this could result in the secu‐
rity of --tls-crypt degrading to the same security as using
--tls-auth. That is, the control channel still benefits from
the extra protection against active man-in-the-middle-attacks
and DoS attacks, but may no longer offer extra privacy and
post-quantum security on top of what TLS itself offers.
--askpass [file]
Get certificate password from console or file before we daemo‐
nize.
For the extremely security conscious, it is possible to protect
your private key with a password. Of course this means that
every time the OpenVPN daemon is started you must be there to
type the password. The --askpass option allows you to start
OpenVPN from the command line. It will query you for a password
before it daemonizes. To protect a private key with a password
you should omit the -nodes option when you use the openssl com‐
mand line tool to manage certificates and private keys.
If file is specified, read the password from the first line of
file. Keep in mind that storing your password in a file to a
certain extent invalidates the extra security provided by using
an encrypted key.
--auth-nocache
Don't cache --askpass or --auth-user-pass username/passwords in
virtual memory.
If specified, this directive will cause OpenVPN to immediately
forget username/password inputs after they are used. As a
result, when OpenVPN needs a username/password, it will prompt
for input from stdin, which may be multiple times during the
duration of an OpenVPN session.
When using --auth-nocache in combination with a user/password
file and --chroot or --daemon, make sure to use an absolute
path.
This directive does not affect the --http-proxy username/pass‐
word. It is always cached.
--auth-token token
This is not an option to be used directly in any configuration
files, but rather push this option from a --client-connect
script or a --plugin which hooks into the OPENVPN_PLUG‐
IN_CLIENT_CONNECT or OPENVPN_PLUGIN_CLIENT_CONNECT_V2 calls.
This option provides a possibility to replace the clients pass‐
word with an authentication token during the lifetime of the
OpenVPN client.
Whenever the connection is renegotiated and the
--auth-user-pass-verify script or --plugin making use of the
OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY hook is triggered, it will
pass over this token as the password instead of the password the
user provided. The authentication token can only be reset by a
full reconnect where the server can push new options to the
client. The password the user entered is never preserved once
an authentication token have been set. If the OpenVPN server
side rejects the authentication token, the client will receive
an AUTH_FAIL and disconnect.
The purpose of this is to enable two factor authentication meth‐
ods, such as HOTP or TOTP, to be used without needing to
retrieve a new OTP code each time the connection is renegoti‐
ated. Another use case is to cache authentication data on the
client without needing to have the users password cached in mem‐
ory during the life time of the session.
To make use of this feature, the --client-connect script or
--plugin needs to put
push "auth-token UNIQUE_TOKEN_VALUE"
into the file/buffer for dynamic configuration data. This will
then make the OpenVPN server to push this value to the client,
which replaces the local password with the UNIQUE_TOKEN_VALUE.
--tls-verify cmd
Run command cmd to verify the X509 name of a pending TLS connec‐
tion that has otherwise passed all other tests of certification
(except for revocation via --crl-verify directive; the revoca‐
tion test occurs after the --tls-verify test).
cmd should return 0 to allow the TLS handshake to proceed, or 1
to fail.
cmd consists of a path to script (or executable program),
optionally followed by arguments. The path and arguments may be
single- or double-quoted and/or escaped using a backslash, and
should be separated by one or more spaces.
When cmd is executed two arguments are appended after any argu‐
ments specified in cmd , as follows:
cmd certificate_depth subject
These arguments are, respectively, the current certificate depth
and the X509 common name (cn) of the peer.
This feature is useful if the peer you want to trust has a cer‐
tificate which was signed by a certificate authority who also
signed many other certificates, where you don't necessarily want
to trust all of them, but rather be selective about which peer
certificate you will accept. This feature allows you to write a
script which will test the X509 name on a certificate and decide
whether or not it should be accepted. For a simple perl script
which will test the common name field on the certificate, see
the file verify-cn in the OpenVPN distribution.
See the "Environmental Variables" section below for additional
parameters passed as environmental variables.
--tls-export-cert directory
Store the certificates the clients uses upon connection to this
directory. This will be done before --tls-verify is called. The
certificates will use a temporary name and will be deleted when
the tls-verify script returns. The file name used for the cer‐
tificate is available via the peer_cert environment variable.
--x509-username-field [ext:]fieldname
Field in the X.509 certificate subject to be used as the user‐
name (default=CN). Typically, this option is specified with
fieldname as either of the following:
--x509-username-field emailAddress
--x509-username-field ext:subjectAltName
The first example uses the value of the "emailAddress" attribute
in the certificate's Subject field as the username. The second
example uses the ext: prefix to signify that the X.509 extension
fieldname "subjectAltName" be searched for an rfc822Name (email)
field to be used as the username. In cases where there are mul‐
tiple email addresses in ext:fieldname, the last occurrence is
chosen.
When this option is used, the --verify-x509-name option will
match against the chosen fieldname instead of the Common Name.
Only the subjectAltName and issuerAltName X.509 extensions are
supported.
Please note: This option has a feature which will convert an
all-lowercase fieldname to uppercase characters, e.g., ou -> OU.
A mixed-case fieldname or one having the ext: prefix will be
left as-is. This automatic upcasing feature is deprecated and
will be removed in a future release.
--verify-x509-name name type
Accept connections only if a host's X.509 name is equal to name.
The remote host must also pass all other tests of verification.
Which X.509 name is compared to name depends on the setting of
type. type can be "subject" to match the complete subject DN
(default), "name" to match a subject RDN or "name-prefix" to
match a subject RDN prefix. Which RDN is verified as name
depends on the --x509-username-field option. But it defaults to
the common name (CN), e.g. a certificate with a subject DN
"C=KG, ST=NA, L=Bishkek, CN=Server-1" would be matched by:
--verify-x509-name 'C=KG, ST=NA, L=Bishkek, CN=Server-1' and
--verify-x509-name Server-1 name or you could use --ver‐
ify-x509-name Server- name-prefix if you want a client to only
accept connections to "Server-1", "Server-2", etc.
--verify-x509-name is a useful replacement for the --tls-verify
option to verify the remote host, because --verify-x509-name
works in a --chroot environment without any dependencies.
Using a name prefix is a useful alternative to managing a CRL
(Certificate Revocation List) on the client, since it allows the
client to refuse all certificates except for those associated
with designated servers.
NOTE: Test against a name prefix only when you are using OpenVPN
with a custom CA certificate that is under your control. Never
use this option with type "name-prefix" when your client cer‐
tificates are signed by a third party, such as a commercial web
CA.
--x509-track attribute
Save peer X509 attribute value in environment for use by plugins
and management interface. Prepend a '+' to attribute to save
values from full cert chain. Values will be encoded as
X509_<depth>_<attribute>=<value>. Multiple --x509-track options
can be defined to track multiple attributes.
--ns-cert-type client|server
DEPRECATED This option will be removed in OpenVPN 2.5. Use the
more modern equivalent --remote-cert-tls instead. This option
will be removed in OpenVPN 2.5.
Require that peer certificate was signed with an explicit
nsCertType designation of "client" or "server".
This is a useful security option for clients, to ensure that the
host they connect with is a designated server.
See the easy-rsa/build-key-server script for an example of how
to generate a certificate with the nsCertType field set to
"server".
If the server certificate's nsCertType field is set to "server",
then the clients can verify this with --ns-cert-type server.
This is an important security precaution to protect against a
man-in-the-middle attack where an authorized client attempts to
connect to another client by impersonating the server. The
attack is easily prevented by having clients verify the server
certificate using any one of --ns-cert-type, --verify-x509-name,
or --tls-verify.
--remote-cert-ku [v...]
Require that peer certificate was signed with an explicit key
usage.
If present in the certificate, the keyUsage value is validated
by the TLS library during the TLS handshake. Specifying this
option without arguments requires this extension to be present
(so the TLS library will verify it).
If the list v... is also supplied, the keyUsage field must have
at least the same bits set as the bits in one of the values sup‐
plied in the list v...
The key usage values in the list must be encoded in hex, e.g.
"--remote-cert-ku a0"
--remote-cert-eku oid
Require that peer certificate was signed with an explicit
extended key usage.
This is a useful security option for clients, to ensure that the
host they connect to is a designated server.
The extended key usage should be encoded in oid notation, or
OpenSSL symbolic representation.
--remote-cert-tls client|server
Require that peer certificate was signed with an explicit key
usage and extended key usage based on RFC3280 TLS rules.
This is a useful security option for clients, to ensure that the
host they connect to is a designated server. Or the other way
around; for a server to verify that only hosts with a client
certificate can connect.
The --remote-cert-tls client option is equivalent to
--remote-cert-ku --remote-cert-eku "TLS Web Client Authentica‐
tion"
The --remote-cert-tls server option is equivalent to
--remote-cert-ku --remote-cert-eku "TLS Web Server Authentica‐
tion"
This is an important security precaution to protect against a
man-in-the-middle attack where an authorized client attempts to
connect to another client by impersonating the server. The
attack is easily prevented by having clients verify the server
certificate using any one of --remote-cert-tls, --ver‐
ify-x509-name, or --tls-verify.
--crl-verify crl ['dir']
Check peer certificate against the file crl in PEM format.
A CRL (certificate revocation list) is used when a particular
key is compromised but when the overall PKI is still intact.
Suppose you had a PKI consisting of a CA, root certificate, and
a number of client certificates. Suppose a laptop computer con‐
taining a client key and certificate was stolen. By adding the
stolen certificate to the CRL file, you could reject any connec‐
tion which attempts to use it, while preserving the overall
integrity of the PKI.
The only time when it would be necessary to rebuild the entire
PKI from scratch would be if the root certificate key itself was
compromised.
If the optional dir flag is specified, enable a different mode
where crl is a directory containing files named as revoked
serial numbers (the files may be empty, the contents are never
read). If a client requests a connection, where the client cer‐
tificate serial number (decimal string) is the name of a file
present in the directory, it will be rejected.
Note: As the crl file (or directory) is read every time a peer
connects, if you are dropping root privileges with --user, make
sure that this user has sufficient privileges to read the file.
SSL Library information:
--show-ciphers
(Standalone) Show all cipher algorithms to use with the --cipher
option.
--show-digests
(Standalone) Show all message digest algorithms to use with the
--auth option.
--show-tls
(Standalone) Show all TLS ciphers supported by the crypto
library. OpenVPN uses TLS to secure the control channel, over
which the keys that are used to protect the actual VPN traffic
are exchanged. The TLS ciphers will be sorted from highest
preference (most secure) to lowest.
Be aware that whether a cipher suite in this list can actually
work depends on the specific setup of both peers (e.g. both
peers must support the cipher, and an ECDSA cipher suite will
not work if you are using an RSA certificate, etc.).
--show-engines
(Standalone) Show currently available hardware-based crypto
acceleration engines supported by the OpenSSL library.
--show-curves
(Standalone) Show all available elliptic curves to use with the
--ecdh-curve option.
Generate a random key:
Used only for non-TLS static key encryption mode.
--genkey
(Standalone) Generate a random key to be used as a shared
secret, for use with the --secret option. This file must be
shared with the peer over a pre-existing secure channel such as
scp(1)
--secret file
Write key to file.
TUN/TAP persistent tunnel config mode:
Available with Linux 2.4.7+. These options comprise a standalone mode
of OpenVPN which can be used to create and delete persistent tunnels.
--mktun
(Standalone) Create a persistent tunnel on platforms which sup‐
port them such as Linux. Normally TUN/TAP tunnels exist only
for the period of time that an application has them open. This
option takes advantage of the TUN/TAP driver's ability to build
persistent tunnels that live through multiple instantiations of
OpenVPN and die only when they are deleted or the machine is
rebooted.
One of the advantages of persistent tunnels is that they elimi‐
nate the need for separate --up and --down scripts to run the
appropriate ifconfig(8) and route(8) commands. These commands
can be placed in the the same shell script which starts or ter‐
minates an OpenVPN session.
Another advantage is that open connections through the
TUN/TAP-based tunnel will not be reset if the OpenVPN peer
restarts. This can be useful to provide uninterrupted connec‐
tivity through the tunnel in the event of a DHCP reset of the
peer's public IP address (see the --ipchange option above).
One disadvantage of persistent tunnels is that it is harder to
automatically configure their MTU value (see --link-mtu and
--tun-mtu above).
On some platforms such as Windows, TAP-Win32 tunnels are persis‐
tent by default.
--rmtun
(Standalone) Remove a persistent tunnel.
--dev tunX | tapX
TUN/TAP device
--user user
Optional user to be owner of this tunnel.
--group group
Optional group to be owner of this tunnel.
Windows-Specific Options:
--win-sys path
Set the Windows system directory pathname to use when looking
for system executables such as route.exe and netsh.exe. By
default, if this directive is not specified, OpenVPN will use
the SystemRoot environment variable.
This option have changed behaviour in OpenVPN 2.3. Earlier you
had to define --win-sys env to use the SystemRoot environment
variable, otherwise it defaulted to C:\WINDOWS. It is not
needed to use the env keyword any more, and it will just be
ignored. A warning is logged when this is found in the configu‐
ration file.
--ip-win32 method
When using --ifconfig on Windows, set the TAP-Win32 adapter IP
address and netmask using method. Don't use this option unless
you are also using --ifconfig.
manual -- Don't set the IP address or netmask automatically.
Instead output a message to the console telling the user to con‐
figure the adapter manually and indicating the IP/netmask which
OpenVPN expects the adapter to be set to.
dynamic [offset] [lease-time] -- Automatically set the IP
address and netmask by replying to DHCP query messages generated
by the kernel. This mode is probably the "cleanest" solution
for setting the TCP/IP properties since it uses the well-known
DHCP protocol. There are, however, two prerequisites for using
this mode: (1) The TCP/IP properties for the TAP-Win32 adapter
must be set to "Obtain an IP address automatically," and (2)
OpenVPN needs to claim an IP address in the subnet for use as
the virtual DHCP server address. By default in --dev tap mode,
OpenVPN will take the normally unused first address in the sub‐
net. For example, if your subnet is 192.168.4.0 netmask
255.255.255.0, then OpenVPN will take the IP address 192.168.4.0
to use as the virtual DHCP server address. In --dev tun mode,
OpenVPN will cause the DHCP server to masquerade as if it were
coming from the remote endpoint. The optional offset parameter
is an integer which is > -256 and < 256 and which defaults to
-1. If offset is positive, the DHCP server will masquerade as
the IP address at network address + offset. If offset is nega‐
tive, the DHCP server will masquerade as the IP address at
broadcast address + offset. The Windows ipconfig /all command
can be used to show what Windows thinks the DHCP server address
is. OpenVPN will "claim" this address, so make sure to use a
free address. Having said that, different OpenVPN instantia‐
tions, including different ends of the same connection, can
share the same virtual DHCP server address. The lease-time
parameter controls the lease time of the DHCP assignment given
to the TAP-Win32 adapter, and is denoted in seconds. Normally a
very long lease time is preferred because it prevents routes
involving the TAP-Win32 adapter from being lost when the system
goes to sleep. The default lease time is one year.
netsh -- Automatically set the IP address and netmask using the
Windows command-line "netsh" command. This method appears to
work correctly on Windows XP but not Windows 2000.
ipapi -- Automatically set the IP address and netmask using the
Windows IP Helper API. This approach does not have ideal seman‐
tics, though testing has indicated that it works okay in prac‐
tice. If you use this option, it is best to leave the TCP/IP
properties for the TAP-Win32 adapter in their default state,
i.e. "Obtain an IP address automatically."
adaptive -- (Default) Try dynamic method initially and fail over
to netsh if the DHCP negotiation with the TAP-Win32 adapter does
not succeed in 20 seconds. Such failures have been known to
occur when certain third-party firewall packages installed on
the client machine block the DHCP negotiation used by the
TAP-Win32 adapter. Note that if the netsh failover occurs, the
TAP-Win32 adapter TCP/IP properties will be reset from DHCP to
static, and this will cause future OpenVPN startups using the
adaptive mode to use netsh immediately, rather than trying
dynamic first. To "unstick" the adaptive mode from using netsh,
run OpenVPN at least once using the dynamic mode to restore the
TAP-Win32 adapter TCP/IP properties to a DHCP configuration.
--route-method m
Which method m to use for adding routes on Windows?
adaptive (default) -- Try IP helper API first. If that fails,
fall back to the route.exe shell command.
ipapi -- Use IP helper API.
exe -- Call the route.exe shell command.
--dhcp-option type [parm]
Set extended TAP-Win32 TCP/IP properties, must be used with
--ip-win32 dynamic or --ip-win32 adaptive. This option can be
used to set additional TCP/IP properties on the TAP-Win32
adapter, and is particularly useful for configuring an OpenVPN
client to access a Samba server across the VPN.
DOMAIN name -- Set Connection-specific DNS Suffix.
DNS addr -- Set primary domain name server IPv4 address. Repeat
this option to set secondary DNS server addresses.
DNS6 addr -- Set primary domain name server IPv6 address.
Repeat this option to set secondary DNS server IPv6 addresses.
Note: currently this is handled using netsh (the existing DHCP
code can only do IPv4 DHCP, and that protocol only permits IPv4
addresses anywhere). The option will be put into the environ‐
ment, so an --up script could act upon it if needed.
WINS addr -- Set primary WINS server address (NetBIOS over
TCP/IP Name Server). Repeat this option to set secondary WINS
server addresses.
NBDD addr -- Set primary NBDD server address (NetBIOS over
TCP/IP Datagram Distribution Server) Repeat this option to set
secondary NBDD server addresses.
NTP addr -- Set primary NTP server address (Network Time Proto‐
col). Repeat this option to set secondary NTP server addresses.
NBT type -- Set NetBIOS over TCP/IP Node type. Possible
options: 1 = b-node (broadcasts), 2 = p-node (point-to-point
name queries to a WINS server), 4 = m-node (broadcast then query
name server), and 8 = h-node (query name server, then broad‐
cast).
NBS scope-id -- Set NetBIOS over TCP/IP Scope. A NetBIOS Scope
ID provides an extended naming service for the NetBIOS over
TCP/IP (Known as NBT) module. The primary purpose of a NetBIOS
scope ID is to isolate NetBIOS traffic on a single network to
only those nodes with the same NetBIOS scope ID. The NetBIOS
scope ID is a character string that is appended to the NetBIOS
name. The NetBIOS scope ID on two hosts must match, or the two
hosts will not be able to communicate. The NetBIOS Scope ID also
allows computers to use the same computer name, as they have
different scope IDs. The Scope ID becomes a part of the NetBIOS
name, making the name unique. (This description of NetBIOS
scopes courtesy of NeonSurge@abyss.com)
DISABLE-NBT -- Disable Netbios-over-TCP/IP.
Note that if --dhcp-option is pushed via --push to a non-windows
client, the option will be saved in the client's environment
before the up script is called, under the name "for‐
eign_option_{n}".
--tap-sleep n
Cause OpenVPN to sleep for n seconds immediately after the
TAP-Win32 adapter state is set to "connected".
This option is intended to be used to troubleshoot problems with
the --ifconfig and --ip-win32 options, and is used to give the
TAP-Win32 adapter time to come up before Windows IP Helper API
operations are applied to it.
--show-net-up
Output OpenVPN's view of the system routing table and network
adapter list to the syslog or log file after the TUN/TAP adapter
has been brought up and any routes have been added.
--block-outside-dns
Block DNS servers on other network adapters to prevent DNS
leaks. This option prevents any application from accessing TCP
or UDP port 53 except one inside the tunnel. It uses Windows
Filtering Platform (WFP) and works on Windows Vista or later.
This option is considered unknown on non-Windows platforms and
unsupported on Windows XP, resulting in fatal error. You may
want to use --setenv opt or --ignore-unknown-option (not suit‐
able for Windows XP) to ignore said error. Note that pushing
unknown options from server does not trigger fatal errors.
--dhcp-renew
Ask Windows to renew the TAP adapter lease on startup. This
option is normally unnecessary, as Windows automatically trig‐
gers a DHCP renegotiation on the TAP adapter when it comes up,
however if you set the TAP-Win32 adapter Media Status property
to "Always Connected", you may need this flag.
--dhcp-release
Ask Windows to release the TAP adapter lease on shutdown. This
option has no effect now, as it is enabled by default starting
with OpenVPN 2.4.1.
--register-dns
Run ipconfig /flushdns and ipconfig /registerdns on connection
initiation. This is known to kick Windows into recognizing
pushed DNS servers.
--pause-exit
Put up a "press any key to continue" message on the console
prior to OpenVPN program exit. This option is automatically
used by the Windows explorer when OpenVPN is run on a configura‐
tion file using the right-click explorer menu.
--service exit-event [0|1]
Should be used when OpenVPN is being automatically executed by
another program in such a context that no interaction with the
user via display or keyboard is possible. In general, end-users
should never need to explicitly use this option, as it is auto‐
matically added by the OpenVPN service wrapper when a given
OpenVPN configuration is being run as a service.
exit-event is the name of a Windows global event object, and
OpenVPN will continuously monitor the state of this event object
and exit when it becomes signaled.
The second parameter indicates the initial state of exit-event
and normally defaults to 0.
Multiple OpenVPN processes can be simultaneously executed with
the same exit-event parameter. In any case, the controlling
process can signal exit-event, causing all such OpenVPN pro‐
cesses to exit.
When executing an OpenVPN process using the --service directive,
OpenVPN will probably not have a console window to output sta‐
tus/error messages, therefore it is useful to use --log or
--log-append to write these messages to a file.
--show-adapters
(Standalone) Show available TAP-Win32 adapters which can be
selected using the --dev-node option. On non-Windows systems,
the ifconfig(8) command provides similar functionality.
--allow-nonadmin [TAP-adapter]
(Standalone) Set TAP-adapter to allow access from non-adminis‐
trative accounts. If TAP-adapter is omitted, all TAP adapters
on the system will be configured to allow non-admin access. The
non-admin access setting will only persist for the length of
time that the TAP-Win32 device object and driver remain loaded,
and will need to be re-enabled after a reboot, or if the driver
is unloaded and reloaded. This directive can only be used by an
administrator.
--show-valid-subnets
(Standalone) Show valid subnets for --dev tun emulation. Since
the TAP-Win32 driver exports an ethernet interface to Windows,
and since TUN devices are point-to-point in nature, it is neces‐
sary for the TAP-Win32 driver to impose certain constraints on
TUN endpoint address selection.
Namely, the point-to-point endpoints used in TUN device emula‐
tion must be the middle two addresses of a /30 subnet (netmask
255.255.255.252).
--show-net
(Standalone) Show OpenVPN's view of the system routing table and
network adapter list.
PKCS#11 Standalone Options:
--show-pkcs11-ids [provider] [cert_private]
(Standalone) Show PKCS#11 token object list. Specify cert_pri‐
vate as 1 if certificates are stored as private objects.
If p11-kit is present on the system, the provider argument is
optional; if omitted the default p11-kit-proxy.so module will be
queried.
--verb option can be used BEFORE this option to produce debug‐
ging information.
Standalone Debug Options:
--show-gateway [v6target]
(Standalone) Show current IPv4 and IPv6 default gateway and
interface towards the gateway (if the protocol in question is
enabled). If an IPv6 address is passed as argument, the IPv6
route for this host is reported.
IPv6 Related Options
The following options exist to support IPv6 tunneling in peer-to-peer
and client-server mode. All options are modeled after their IPv4 coun‐
terparts, so more detailed explanations given there apply here as well
(except for --topology , which has no effect on IPv6).
--ifconfig-ipv6 ipv6addr/bits ipv6remote
configure IPv6 address ipv6addr/bits on the ``tun'' device. The
second parameter is used as route target for --route-ipv6 if no
gateway is specified.
--route-ipv6 ipv6addr/bits [gateway] [metric]
setup IPv6 routing in the system to send the specified IPv6 net‐
work into OpenVPN's ``tun''. The gateway parameter is only used
for IPv6 routes across ``tap'' devices, and if missing, the
``ipv6remote'' field from --ifconfig-ipv6 is used.
--server-ipv6 ipv6addr/bits
convenience-function to enable a number of IPv6 related options
at once, namely --ifconfig-ipv6, --ifconfig-ipv6-pool and --push
tun-ipv6 Is only accepted if ``--mode server'' or ``--server''
is set. Pushing of the --tun-ipv6 directive is done for older
clients which require an explicit ``--tun-ipv6'' in their con‐
figuration.
--ifconfig-ipv6-pool ipv6addr/bits
Specify an IPv6 address pool for dynamic assignment to clients.
The pool starts at ipv6addr and matches the offset determined
from the start of the IPv4 pool.
--ifconfig-ipv6-push ipv6addr/bits ipv6remote
for ccd/ per-client static IPv6 interface configuration, see
--client-config-dir and --ifconfig-push for more details.
--iroute-ipv6 ipv6addr/bits
for ccd/ per-client static IPv6 route configuration, see
--iroute for more details how to setup and use this, and how
--iroute and --route interact.
SCRIPTING AND ENVIRONMENTAL VARIABLES
OpenVPN exports a series of environmental variables for use by
user-defined scripts.
Script Order of Execution
--up Executed after TCP/UDP socket bind and TUN/TAP open.
--tls-verify
Executed when we have a still untrusted remote peer.
--ipchange
Executed after connection authentication, or remote IP address
change.
--client-connect
Executed in --mode server mode immediately after client authen‐
tication.
--route-up
Executed after connection authentication, either immediately
after, or some number of seconds after as defined by the
--route-delay option.
--route-pre-down
Executed right before the routes are removed.
--client-disconnect
Executed in --mode server mode on client instance shutdown.
--down Executed after TCP/UDP and TUN/TAP close.
--learn-address
Executed in --mode server mode whenever an IPv4 address/route or
MAC address is added to OpenVPN's internal routing table.
--auth-user-pass-verify
Executed in --mode server mode on new client connections, when
the client is still untrusted.
String Types and Remapping
In certain cases, OpenVPN will perform remapping of characters in
strings. Essentially, any characters outside the set of permitted
characters for each string type will be converted to underbar ('_').
Q: Why is string remapping necessary?
A: It's an important security feature to prevent the malicious coding
of strings from untrusted sources to be passed as parameters to
scripts, saved in the environment, used as a common name, translated to
a filename, etc.
Q: Can string remapping be disabled?
A: Yes, by using the --no-name-remapping option, however this should be
considered an advanced option.
Here is a brief rundown of OpenVPN's current string types and the per‐
mitted character class for each string:
X509 Names: Alphanumeric, underbar ('_'), dash ('-'), dot ('.'), at
('@'), colon (':'), slash ('/'), and equal ('='). Alphanumeric is
defined as a character which will cause the C library isalnum() func‐
tion to return true.
Common Names: Alphanumeric, underbar ('_'), dash ('-'), dot ('.'), and
at ('@').
--auth-user-pass username: Same as Common Name, with one exception:
starting with OpenVPN 2.0.1, the username is passed to the OPEN‐
VPN_PLUGIN_AUTH_USER_PASS_VERIFY plugin in its raw form, without string
remapping.
--auth-user-pass password: Any "printable" character except CR or LF.
Printable is defined to be a character which will cause the C library
isprint() function to return true.
--client-config-dir filename as derived from common name or username:
Alphanumeric, underbar ('_'), dash ('-'), and dot ('.') except for "."
or ".." as standalone strings. As of v2.0.1-rc6, the at ('@') charac‐
ter has been added as well for compatibility with the common name char‐
acter class.
Environmental variable names: Alphanumeric or underbar ('_').
Environmental variable values: Any printable character.
For all cases, characters in a string which are not members of the
legal character class for that string type will be remapped to underbar
('_').
Environmental Variables
Once set, a variable is persisted indefinitely until it is reset by a
new value or a restart,
As of OpenVPN 2.0-beta12, in server mode, environmental variables set
by OpenVPN are scoped according to the client objects they are associ‐
ated with, so there should not be any issues with scripts having access
to stale, previously set variables which refer to different client
instances.
bytes_received
Total number of bytes received from client during VPN session.
Set prior to execution of the --client-disconnect script.
bytes_sent
Total number of bytes sent to client during VPN session. Set
prior to execution of the --client-disconnect script.
common_name
The X509 common name of an authenticated client. Set prior to
execution of --client-connect, --client-disconnect, and
--auth-user-pass-verify scripts.
config Name of first --config file. Set on program initiation and
reset on SIGHUP.
daemon Set to "1" if the --daemon directive is specified, or "0" other‐
wise. Set on program initiation and reset on SIGHUP.
daemon_log_redirect
Set to "1" if the --log or --log-append directives are speci‐
fied, or "0" otherwise. Set on program initiation and reset on
SIGHUP.
dev The actual name of the TUN/TAP device, including a unit number
if it exists. Set prior to --up or --down script execution.
dev_idx
On Windows, the device index of the TUN/TAP adapter (to be used
in netsh.exe calls which sometimes just do not work right with
interface names). Set prior to --up or --down script execution.
foreign_option_{n}
An option pushed via --push to a client which does not natively
support it, such as --dhcp-option on a non-Windows system, will
be recorded to this environmental variable sequence prior to
--up script execution.
ifconfig_broadcast
The broadcast address for the virtual ethernet segment which is
derived from the --ifconfig option when --dev tap is used. Set
prior to OpenVPN calling the ifconfig or netsh (windows version
of ifconfig) commands which normally occurs prior to --up script
execution.
ifconfig_ipv6_local
The local VPN endpoint IPv6 address specified in the --ifcon‐
fig-ipv6 option (first parameter). Set prior to OpenVPN calling
the ifconfig or netsh (windows version of ifconfig) commands
which normally occurs prior to --up script execution.
ifconfig_ipv6_netbits
The prefix length of the IPv6 network on the VPN interface.
Derived from the /nnn parameter of the IPv6 address in the
--ifconfig-ipv6 option (first parameter). Set prior to OpenVPN
calling the ifconfig or netsh (windows version of ifconfig) com‐
mands which normally occurs prior to --up script execution.
ifconfig_ipv6_remote
The remote VPN endpoint IPv6 address specified in the --ifcon‐
fig-ipv6 option (second parameter). Set prior to OpenVPN call‐
ing the ifconfig or netsh (windows version of ifconfig) commands
which normally occurs prior to --up script execution.
ifconfig_local
The local VPN endpoint IP address specified in the --ifconfig
option (first parameter). Set prior to OpenVPN calling the
ifconfig or netsh (windows version of ifconfig) commands which
normally occurs prior to --up script execution.
ifconfig_remote
The remote VPN endpoint IP address specified in the --ifconfig
option (second parameter) when --dev tun is used. Set prior to
OpenVPN calling the ifconfig or netsh (windows version of ifcon‐
fig) commands which normally occurs prior to --up script execu‐
tion.
ifconfig_netmask
The subnet mask of the virtual ethernet segment that is speci‐
fied as the second parameter to --ifconfig when --dev tap is
being used. Set prior to OpenVPN calling the ifconfig or netsh
(windows version of ifconfig) commands which normally occurs
prior to --up script execution.
ifconfig_pool_local_ip
The local virtual IP address for the TUN/TAP tunnel taken from
an --ifconfig-push directive if specified, or otherwise from the
ifconfig pool (controlled by the --ifconfig-pool config file
directive). Only set for --dev tun tunnels. This option is set
on the server prior to execution of the --client-connect and
--client-disconnect scripts.
ifconfig_pool_netmask
The virtual IP netmask for the TUN/TAP tunnel taken from an
--ifconfig-push directive if specified, or otherwise from the
ifconfig pool (controlled by the --ifconfig-pool config file
directive). Only set for --dev tap tunnels. This option is set
on the server prior to execution of the --client-connect and
--client-disconnect scripts.
ifconfig_pool_remote_ip
The remote virtual IP address for the TUN/TAP tunnel taken from
an --ifconfig-push directive if specified, or otherwise from the
ifconfig pool (controlled by the --ifconfig-pool config file
directive). This option is set on the server prior to execution
of the --client-connect and --client-disconnect scripts.
link_mtu
The maximum packet size (not including the IP header) of tunnel
data in UDP tunnel transport mode. Set prior to --up or --down
script execution.
local The --local parameter. Set on program initiation and reset on
SIGHUP.
local_port
The local port number or name, specified by --port or --lport.
Set on program initiation and reset on SIGHUP.
password
The password provided by a connecting client. Set prior to
--auth-user-pass-verify script execution only when the via-env
modifier is specified, and deleted from the environment after
the script returns.
proto The --proto parameter. Set on program initiation and reset on
SIGHUP.
remote_{n}
The --remote parameter. Set on program initiation and reset on
SIGHUP.
remote_port_{n}
The remote port number, specified by --port or --rport. Set on
program initiation and reset on SIGHUP.
route_net_gateway
The pre-existing default IP gateway in the system routing table.
Set prior to --up script execution.
route_vpn_gateway
The default gateway used by --route options, as specified in
either the --route-gateway option or the second parameter to
--ifconfig when --dev tun is specified. Set prior to --up
script execution.
route_{parm}_{n}
A set of variables which define each route to be added, and are
set prior to --up script execution.
parm will be one of "network", "netmask", "gateway", or "met‐
ric".
n is the OpenVPN route number, starting from 1.
If the network or gateway are resolvable DNS names, their IP
address translations will be recorded rather than their names as
denoted on the command line or configuration file.
route_ipv6_{parm}_{n}
A set of variables which define each IPv6 route to be added, and
are set prior to --up script execution.
parm will be one of "network" or "gateway" ("netmask" is con‐
tained as "/nnn" in the route_ipv6_network_{n}, unlike IPv4
where it is passed in a separate environment variable).
n is the OpenVPN route number, starting from 1.
If the network or gateway are resolvable DNS names, their IP
address translations will be recorded rather than their names as
denoted on the command line or configuration file.
peer_cert
Temporary file name containing the client certificate upon con‐
nection. Useful in conjunction with --tls-verify
script_context
Set to "init" or "restart" prior to up/down script execution.
For more information, see documentation for --up.
script_type
Prior to execution of any script, this variable is set to the
type of script being run. It can be one of the following: up,
down, ipchange, route-up, tls-verify, auth-user-pass-verify,
client-connect, client-disconnect, or learn-address. Set prior
to execution of any script.
signal The reason for exit or restart. Can be one of sigusr1, sighup,
sigterm, sigint, inactive (controlled by --inactive option),
ping-exit (controlled by --ping-exit option), ping-restart (con‐
trolled by --ping-restart option), connection-reset (triggered
on TCP connection reset), error, or unknown (unknown signal).
This variable is set just prior to down script execution.
time_ascii
Client connection timestamp, formatted as a human-readable time
string. Set prior to execution of the --client-connect script.
time_duration
The duration (in seconds) of the client session which is now
disconnecting. Set prior to execution of the --client-discon‐
nect script.
time_unix
Client connection timestamp, formatted as a unix integer
date/time value. Set prior to execution of the --client-connect
script.
tls_digest_{n} / tls_digest_sha256_{n}
Contains the certificate SHA1 / SHA256 fingerprint, where n is
the verification level. Only set for TLS connections. Set
prior to execution of --tls-verify script.
tls_id_{n}
A series of certificate fields from the remote peer, where n is
the verification level. Only set for TLS connections. Set
prior to execution of --tls-verify script.
tls_serial_{n}
The serial number of the certificate from the remote peer, where
n is the verification level. Only set for TLS connections. Set
prior to execution of --tls-verify script. This is in the form
of a decimal string like "933971680", which is suitable for
doing serial-based OCSP queries (with OpenSSL, do not prepend
"0x" to the string) If something goes wrong while reading the
value from the certificate it will be an empty string, so your
code should check that. See the con‐
trib/OCSP_check/OCSP_check.sh script for an example.
tls_serial_hex_{n}
Like tls_serial_{n}, but in hex form (e.g. "12:34:56:78:9A").
tun_mtu
The MTU of the TUN/TAP device. Set prior to --up or --down
script execution.
trusted_ip (or trusted_ip6)
Actual IP address of connecting client or peer which has been
authenticated. Set prior to execution of --ipchange,
--client-connect, and --client-disconnect scripts. If using
ipv6 endpoints (udp6, tcp6), trusted_ip6 will be set instead.
trusted_port
Actual port number of connecting client or peer which has been
authenticated. Set prior to execution of --ipchange,
--client-connect, and --client-disconnect scripts.
untrusted_ip (or untrusted_ip6)
Actual IP address of connecting client or peer which has not
been authenticated yet. Sometimes used to nmap the connecting
host in a --tls-verify script to ensure it is firewalled prop‐
erly. Set prior to execution of --tls-verify and
--auth-user-pass-verify scripts. If using ipv6 endpoints (udp6,
tcp6), untrusted_ip6 will be set instead.
untrusted_port
Actual port number of connecting client or peer which has not
been authenticated yet. Set prior to execution of --tls-verify
and --auth-user-pass-verify scripts.
username
The username provided by a connecting client. Set prior to
--auth-user-pass-verify script execution only when the via-env
modifier is specified.
X509_{n}_{subject_field}
An X509 subject field from the remote peer certificate, where n
is the verification level. Only set for TLS connections. Set
prior to execution of --tls-verify script. This variable is
similar to tls_id_{n} except the component X509 subject fields
are broken out, and no string remapping occurs on these field
values (except for remapping of control characters to "_"). For
example, the following variables would be set on the OpenVPN
server using the sample client certificate in sample-keys
(client.crt). Note that the verification level is 0 for the
client certificate and 1 for the CA certificate.
X509_0_emailAddress=me@myhost.mydomain
X509_0_CN=Test-Client
X509_0_O=OpenVPN-TEST
X509_0_ST=NA
X509_0_C=KG
X509_1_emailAddress=me@myhost.mydomain
X509_1_O=OpenVPN-TEST
X509_1_L=BISHKEK
X509_1_ST=NA
X509_1_C=KG
INLINE FILE SUPPORT
OpenVPN allows including files in the main configuration for the --ca,
--cert, --dh, --extra-certs, --key, --pkcs12, --secret, --crl-verify,
--http-proxy-user-pass, --tls-auth and --tls-crypt options.
Each inline file started by the line <option> and ended by the line
</option>
Here is an example of an inline file usage
<cert>
-----BEGIN CERTIFICATE-----
[...]
-----END CERTIFICATE-----
</cert>
When using the inline file feature with --pkcs12 the inline file has to
be base64 encoded. Encoding of a .p12 file into base64 can be done for
example with OpenSSL by running openssl base64 -in input.p12
SIGNALS
SIGHUP Cause OpenVPN to close all TUN/TAP and network connections,
restart, re-read the configuration file (if any), and reopen
TUN/TAP and network connections.
SIGUSR1
Like SIGHUP, except don't re-read configuration file, and possi‐
bly don't close and reopen TUN/TAP device, re-read key files,
preserve local IP address/port, or preserve most recently
authenticated remote IP address/port based on --persist-tun,
--persist-key, --persist-local-ip, and --persist-remote-ip
options respectively (see above).
This signal may also be internally generated by a timeout condi‐
tion, governed by the --ping-restart option.
This signal, when combined with --persist-remote-ip, may be sent
when the underlying parameters of the host's network interface
change such as when the host is a DHCP client and is assigned a
new IP address. See --ipchange above for more information.
SIGUSR2
Causes OpenVPN to display its current statistics (to the syslog
file if --daemon is used, or stdout otherwise).
SIGINT, SIGTERM
Causes OpenVPN to exit gracefully.
TUN/TAP DRIVER SETUP
If you are running Linux 2.4.7 or higher, you probably have the TUN/TAP
driver already installed. If so, there are still a few things you need
to do:
Make device: mknod /dev/net/tun c 10 200
Load driver: modprobe tun
EXAMPLES
Prior to running these examples, you should have OpenVPN installed on
two machines with network connectivity between them. If you have not
yet installed OpenVPN, consult the INSTALL file included in the OpenVPN
distribution.
TUN/TAP Setup:
If you are using Linux 2.4 or higher, make the tun device node and load
the tun module:
mknod /dev/net/tun c 10 200
modprobe tun
If you installed from RPM, the mknod step may be omitted, because the
RPM install does that for you.
Only Linux 2.4 and newer are supported.
For other platforms, consult the INSTALL file at http://open‐
vpn.net/install.html for more information.
Firewall Setup:
If firewalls exist between the two machines, they should be set to for‐
ward UDP port 1194 in both directions. If you do not have control over
the firewalls between the two machines, you may still be able to use
OpenVPN by adding --ping 15 to each of the openvpn commands used below
in the examples (this will cause each peer to send out a UDP ping to
its remote peer once every 15 seconds which will cause many stateful
firewalls to forward packets in both directions without an explicit
firewall rule).
If you are using a Linux iptables-based firewall, you may need to enter
the following command to allow incoming packets on the TUN device:
iptables -A INPUT -i tun+ -j ACCEPT
See the firewalls section below for more information on configuring
firewalls for use with OpenVPN.
VPN Address Setup:
For purposes of our example, our two machines will be called bob.exam‐
ple.com and alice.example.com. If you are constructing a VPN over the
internet, then replace bob.example.com and alice.example.com with the
internet hostname or IP address that each machine will use to contact
the other over the internet.
Now we will choose the tunnel endpoints. Tunnel endpoints are private
IP addresses that only have meaning in the context of the VPN. Each
machine will use the tunnel endpoint of the other machine to access it
over the VPN. In our example, the tunnel endpoint for bob.example.com
will be 10.4.0.1 and for alice.example.com, 10.4.0.2.
Once the VPN is established, you have essentially created a secure
alternate path between the two hosts which is addressed by using the
tunnel endpoints. You can control which network traffic passes between
the hosts (a) over the VPN or (b) independently of the VPN, by choosing
whether to use (a) the VPN endpoint address or (b) the public internet
address, to access the remote host. For example if you are on bob.exam‐
ple.com and you wish to connect to alice.example.com via ssh without
using the VPN (since ssh has its own built-in security) you would use
the command ssh alice.example.com. However in the same scenario, you
could also use the command telnet 10.4.0.2 to create a telnet session
with alice.example.com over the VPN, that would use the VPN to secure
the session rather than ssh.
You can use any address you wish for the tunnel endpoints but make sure
that they are private addresses (such as those that begin with 10 or
192.168) and that they are not part of any existing subnet on the net‐
works of either peer, unless you are bridging. If you use an address
that is part of your local subnet for either of the tunnel endpoints,
you will get a weird feedback loop.
Example 1: A simple tunnel without security
On bob:
openvpn --remote alice.example.com --dev tun1 --ifconfig
10.4.0.1 10.4.0.2 --verb 9
On alice:
openvpn --remote bob.example.com --dev tun1 --ifconfig 10.4.0.2
10.4.0.1 --verb 9
Now verify the tunnel is working by pinging across the tunnel.
On bob:
ping 10.4.0.2
On alice:
ping 10.4.0.1
The --verb 9 option will produce verbose output, similar to the tcp‐
dump(8) program. Omit the --verb 9 option to have OpenVPN run quietly.
Example 2: A tunnel with static-key security (i.e. using a pre-shared
secret)
First build a static key on bob.
openvpn --genkey --secret key
This command will build a random key file called key (in ascii format).
Now copy key to alice over a secure medium such as by using the scp(1)
program.
On bob:
openvpn --remote alice.example.com --dev tun1 --ifconfig
10.4.0.1 10.4.0.2 --verb 5 --secret key
On alice:
openvpn --remote bob.example.com --dev tun1 --ifconfig 10.4.0.2
10.4.0.1 --verb 5 --secret key
Now verify the tunnel is working by pinging across the tunnel.
On bob:
ping 10.4.0.2
On alice:
ping 10.4.0.1
Example 3: A tunnel with full TLS-based security
For this test, we will designate bob as the TLS client and alice as the
TLS server. Note that client or server designation only has meaning
for the TLS subsystem. It has no bearing on OpenVPN's peer-to-peer,
UDP-based communication model.
First, build a separate certificate/key pair for both bob and alice
(see above where --cert is discussed for more info). Then construct
Diffie Hellman parameters (see above where --dh is discussed for more
info). You can also use the included test files client.crt,
client.key, server.crt, server.key and ca.crt. The .crt files are cer‐
tificates/public-keys, the .key files are private keys, and ca.crt is a
certification authority who has signed both client.crt and server.crt.
For Diffie Hellman parameters you can use the included file dh1024.pem.
Note that all client, server, and certificate authority certificates
and keys included in the OpenVPN distribution are totally insecure and
should be used for testing only.
On bob:
openvpn --remote alice.example.com --dev tun1 --ifconfig
10.4.0.1 10.4.0.2 --tls-client --ca ca.crt --cert client.crt
--key client.key --reneg-sec 60 --verb 5
On alice:
openvpn --remote bob.example.com --dev tun1 --ifconfig 10.4.0.2
10.4.0.1 --tls-server --dh dh1024.pem --ca ca.crt --cert
server.crt --key server.key --reneg-sec 60 --verb 5
Now verify the tunnel is working by pinging across the tunnel.
On bob:
ping 10.4.0.2
On alice:
ping 10.4.0.1
Notice the --reneg-sec 60 option we used above. That tells OpenVPN to
renegotiate the data channel keys every minute. Since we used --verb 5
above, you will see status information on each new key negotiation.
For production operations, a key renegotiation interval of 60 seconds
is probably too frequent. Omit the --reneg-sec 60 option to use Open‐
VPN's default key renegotiation interval of one hour.
Routing:
Assuming you can ping across the tunnel, the next step is to route a
real subnet over the secure tunnel. Suppose that bob and alice have
two network interfaces each, one connected to the internet, and the
other to a private network. Our goal is to securely connect both pri‐
vate networks. We will assume that bob's private subnet is 10.0.0.0/24
and alice's is 10.0.1.0/24.
First, ensure that IP forwarding is enabled on both peers. On Linux,
enable routing:
echo 1 > /proc/sys/net/ipv4/ip_forward
and enable TUN packet forwarding through the firewall:
iptables -A FORWARD -i tun+ -j ACCEPT
On bob:
route add -net 10.0.1.0 netmask 255.255.255.0 gw 10.4.0.2
On alice:
route add -net 10.0.0.0 netmask 255.255.255.0 gw 10.4.0.1
Now any machine on the 10.0.0.0/24 subnet can access any machine on the
10.0.1.0/24 subnet over the secure tunnel (or vice versa).
In a production environment, you could put the route command(s) in a
script and execute with the --up option.
FIREWALLS
OpenVPN's usage of a single UDP port makes it fairly firewall-friendly.
You should add an entry to your firewall rules to allow incoming Open‐
VPN packets. On Linux 2.4+:
iptables -A INPUT -p udp -s 1.2.3.4 --dport 1194 -j ACCEPT
This will allow incoming packets on UDP port 1194 (OpenVPN's default
UDP port) from an OpenVPN peer at 1.2.3.4.
If you are using HMAC-based packet authentication (the default in any
of OpenVPN's secure modes), having the firewall filter on source
address can be considered optional, since HMAC packet authentication is
a much more secure method of verifying the authenticity of a packet
source. In that case:
iptables -A INPUT -p udp --dport 1194 -j ACCEPT
would be adequate and would not render the host inflexible with respect
to its peer having a dynamic IP address.
OpenVPN also works well on stateful firewalls. In some cases, you may
not need to add any static rules to the firewall list if you are using
a stateful firewall that knows how to track UDP connections. If you
specify --ping n, OpenVPN will be guaranteed to send a packet to its
peer at least once every n seconds. If n is less than the stateful
firewall connection timeout, you can maintain an OpenVPN connection
indefinitely without explicit firewall rules.
You should also add firewall rules to allow incoming IP traffic on TUN
or TAP devices such as:
iptables -A INPUT -i tun+ -j ACCEPT
to allow input packets from tun devices,
iptables -A FORWARD -i tun+ -j ACCEPT
to allow input packets from tun devices to be forwarded to other hosts
on the local network,
iptables -A INPUT -i tap+ -j ACCEPT
to allow input packets from tap devices, and
iptables -A FORWARD -i tap+ -j ACCEPT
to allow input packets from tap devices to be forwarded to other hosts
on the local network.
These rules are secure if you use packet authentication, since no
incoming packets will arrive on a TUN or TAP virtual device unless they
first pass an HMAC authentication test.
FAQ
http://openvpn.net/faq.html
HOWTO
For a more comprehensive guide to setting up OpenVPN in a production
setting, see the OpenVPN HOWTO at http://openvpn.net/howto.html
PROTOCOL
For a description of OpenVPN's underlying protocol, see http://open‐
vpn.net/security.html
WEB
OpenVPN's web site is at http://openvpn.net/
Go here to download the latest version of OpenVPN, subscribe to the
mailing lists, read the mailing list archives, or browse the SVN repos‐
itory.
BUGS
Report all bugs to the OpenVPN team <info@openvpn.net>.
SEE ALSO
dhcpcd(8), ifconfig(8), openssl(1), route(8), scp(1) ssh(1)
NOTES
This product includes software developed by the OpenSSL Project (
http://www.openssl.org/ )
For more information on the TLS protocol, see
http://www.ietf.org/rfc/rfc2246.txt
For more information on the LZO real-time compression library see
http://www.oberhumer.com/opensource/lzo/
COPYRIGHT
Copyright (C) 2002-2017 OpenVPN Technologies, Inc. This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License version 2 as published by the Free Soft‐ ware Foundation.
AUTHORS
James Yonan <jim@yonan.net>
25 August 2016 openvpn(8)