NAME
dhcpd.conf - dhcpd configuration file
DESCRIPTION
The dhcpd.conf file contains configuration information for dhcpd, the Internet Systems Consortium DHCP Server.
The dhcpd.conf file is a free-form ASCII text file. It is parsed by the recursive-descent parser built into dhcpd. The file may contain extra tabs and newlines for formatting purposes. Keywords in the file are case-insensitive. Comments may be placed anywhere within the file (except within quotes). Comments begin with the # character and end at the end of the line.
The file essentially consists of a list of statements. Statements fall into two broad categories - parameters and declarations.
Parameter statements either say how to do something (e.g., how long a lease to offer), whether to do something (e.g., should dhcpd provide addresses to unknown clients), or what parameters to provide to the client (e.g., use gateway 220.177.244.7).
Declarations are used to describe the topology of the network, to describe clients on the network, to provide addresses that can be assigned to clients, or to apply a group of parameters to a group of declarations. In any group of parameters and declarations, all parame‐ ters must be specified before any declarations which depend on those parameters may be specified.
Declarations about network topology include the shared-network and the subnet declarations. If clients on a subnet are to be assigned addresses dynamically, a range declaration must appear within the sub‐ net declaration. For clients with statically assigned addresses, or for installations where only known clients will be served, each such client must have a host declaration. If parameters are to be applied to a group of declarations which are not related strictly on a per-sub‐ net basis, the group declaration can be used.
For every subnet which will be served, and for every subnet to which the dhcp server is connected, there must be one subnet declaration, which tells dhcpd how to recognize that an address is on that subnet. A subnet declaration is required for each subnet even if no addresses will be dynamically allocated on that subnet.
Some installations have physical networks on which more than one IP subnet operates. For example, if there is a site-wide requirement that 8-bit subnet masks be used, but a department with a single physical ethernet network expands to the point where it has more than 254 nodes, it may be necessary to run two 8-bit subnets on the same ethernet until such time as a new physical network can be added. In this case, the subnet declarations for these two networks must be enclosed in a shared-network declaration.
Note that even when the shared-network declaration is absent, an empty one is created by the server to contain the subnet (and any scoped parameters included in the subnet). For practical purposes, this means that “stateless” DHCP clients, which are not tied to addresses (and therefore subnets) will receive the same configuration as stateful ones.
Some sites may have departments which have clients on more than one subnet, but it may be desirable to offer those clients a uniform set of parameters which are different than what would be offered to clients from other departments on the same subnet. For clients which will be declared explicitly with host declarations, these declarations can be enclosed in a group declaration along with the parameters which are common to that department. For clients whose addresses will be dynami‐ cally assigned, class declarations and conditional declarations may be used to group parameter assignments based on information the client sends.
When a client is to be booted, its boot parameters are determined by consulting that client’s host declaration (if any), and then consulting any class declarations matching the client, followed by the pool, sub‐ net and shared-network declarations for the IP address assigned to the client. Each of these declarations itself appears within a lexical scope, and all declarations at less specific lexical scopes are also consulted for client option declarations. Scopes are never considered twice, and if parameters are declared in more than one scope, the parameter declared in the most specific scope is the one that is used.
When dhcpd tries to find a host declaration for a client, it first looks for a host declaration which has a fixed-address declaration that lists an IP address that is valid for the subnet or shared network on which the client is booting. If it doesn’t find any such entry, it tries to find an entry which has no fixed-address declaration.
EXAMPLES
A typical dhcpd.conf file will look something like this:
global parameters...
subnet 204.254.239.0 netmask 255.255.255.224 {
subnet-specific parameters...
range 204.254.239.10 204.254.239.30;
}
subnet 204.254.239.32 netmask 255.255.255.224 {
subnet-specific parameters...
range 204.254.239.42 204.254.239.62;
}
subnet 204.254.239.64 netmask 255.255.255.224 {
subnet-specific parameters...
range 204.254.239.74 204.254.239.94;
}
group {
group-specific parameters...
host zappo.test.isc.org {
host-specific parameters...
}
host beppo.test.isc.org {
host-specific parameters...
}
host harpo.test.isc.org {
host-specific parameters...
}
}
Figure 1
Notice that at the beginning of the file, there's a place for global
parameters. These might be things like the organization's domain name,
the addresses of the name servers (if they are common to the entire
organization), and so on. So, for example:
option domain-name "isc.org";
option domain-name-servers ns1.isc.org, ns2.isc.org;
Figure 2
As you can see in Figure 2, you can specify host addresses in parame‐
ters using their domain names rather than their numeric IP addresses.
If a given hostname resolves to more than one IP address (for example,
if that host has two ethernet interfaces), then where possible, both
addresses are supplied to the client.
The most obvious reason for having subnet-specific parameters as shown
in Figure 1 is that each subnet, of necessity, has its own router. So
for the first subnet, for example, there should be something like:
option routers 204.254.239.1;
Note that the address here is specified numerically. This is not
required - if you have a different domain name for each interface on
your router, it's perfectly legitimate to use the domain name for that
interface instead of the numeric address. However, in many cases there
may be only one domain name for all of a router's IP addresses, and it
would not be appropriate to use that name here.
In Figure 1 there is also a group statement, which provides common
parameters for a set of three hosts - zappo, beppo and harpo. As you
can see, these hosts are all in the test.isc.org domain, so it might
make sense for a group-specific parameter to override the domain name
supplied to these hosts:
option domain-name "test.isc.org";
Also, given the domain they're in, these are probably test machines.
If we wanted to test the DHCP leasing mechanism, we might set the lease
timeout somewhat shorter than the default:
max-lease-time 120;
default-lease-time 120;
You may have noticed that while some parameters start with the option
keyword, some do not. Parameters starting with the option keyword cor‐
respond to actual DHCP options, while parameters that do not start with
the option keyword either control the behavior of the DHCP server
(e.g., how long a lease dhcpd will give out), or specify client parame‐
ters that are not optional in the DHCP protocol (for example, server-
name and filename).
In Figure 1, each host had host-specific parameters. These could
include such things as the hostname option, the name of a file to
upload (the filename parameter) and the address of the server from
which to upload the file (the next-server parameter). In general, any
parameter can appear anywhere that parameters are allowed, and will be
applied according to the scope in which the parameter appears.
Imagine that you have a site with a lot of NCD X-Terminals. These ter‐
minals come in a variety of models, and you want to specify the boot
files for each model. One way to do this would be to have host decla‐
rations for each server and group them by model:
group {
filename "Xncd19r";
next-server ncd-booter;
host ncd1 { hardware ethernet 0:c0:c3:49:2b:57; }
host ncd4 { hardware ethernet 0:c0:c3:80:fc:32; }
host ncd8 { hardware ethernet 0:c0:c3:22:46:81; }
}
group {
filename "Xncd19c";
next-server ncd-booter;
host ncd2 { hardware ethernet 0:c0:c3:88:2d:81; }
host ncd3 { hardware ethernet 0:c0:c3:00:14:11; }
}
group {
filename "XncdHMX";
next-server ncd-booter;
host ncd1 { hardware ethernet 0:c0:c3:11:90:23; }
host ncd4 { hardware ethernet 0:c0:c3:91:a7:8; }
host ncd8 { hardware ethernet 0:c0:c3:cc:a:8f; }
}
ADDRESS POOLS
The pool and pool6 declarations can be used to specify a pool of
addresses that will be treated differently than another pool of
addresses, even on the same network segment or subnet. For example,
you may want to provide a large set of addresses that can be assigned
to DHCP clients that are registered to your DHCP server, while provid‐
ing a smaller set of addresses, possibly with short lease times, that
are available for unknown clients. If you have a firewall, you may be
able to arrange for addresses from one pool to be allowed access to the
Internet, while addresses in another pool are not, thus encouraging
users to register their DHCP clients. To do this, you would set up a
pair of pool declarations:
subnet 10.0.0.0 netmask 255.255.255.0 {
option routers 10.0.0.254;
# Unknown clients get this pool.
pool {
option domain-name-servers bogus.example.com;
max-lease-time 300;
range 10.0.0.200 10.0.0.253;
allow unknown-clients;
}
# Known clients get this pool.
pool {
option domain-name-servers ns1.example.com, ns2.example.com;
max-lease-time 28800;
range 10.0.0.5 10.0.0.199;
deny unknown-clients;
}
}
It is also possible to set up entirely different subnets for known and
unknown clients - address pools exist at the level of shared networks,
so address ranges within pool declarations can be on different subnets.
As you can see in the preceding example, pools can have permit lists
that control which clients are allowed access to the pool and which
aren't. Each entry in a pool's permit list is introduced with the
allow or deny keyword. If a pool has a permit list, then only those
clients that match specific entries on the permit list will be eligible
to be assigned addresses from the pool. If a pool has a deny list,
then only those clients that do not match any entries on the deny list
will be eligible. If both permit and deny lists exist for a pool,
then only clients that match the permit list and do not match the deny
list will be allowed access.
The pool6 declaration is similar to the pool declaration. Currently it
is only allowed within a subnet6 declaration, and may not be included
directly in a shared network declaration. In addition to the range6
statement it allows the prefix6 statement to be included. You may
include range6 statements for both NA and TA and prefixy6 statements in
a single pool6 statement.
DYNAMIC ADDRESS ALLOCATION
Address allocation is actually only done when a client is in the INIT
state and has sent a DHCPDISCOVER message. If the client thinks it has
a valid lease and sends a DHCPREQUEST to initiate or renew that lease,
the server has only three choices - it can ignore the DHCPREQUEST, send
a DHCPNAK to tell the client it should stop using the address, or send
a DHCPACK, telling the client to go ahead and use the address for a
while.
If the server finds the address the client is requesting, and that
address is available to the client, the server will send a DHCPACK. If
the address is no longer available, or the client isn't permitted to
have it, the server will send a DHCPNAK. If the server knows nothing
about the address, it will remain silent, unless the address is incor‐
rect for the network segment to which the client has been attached and
the server is authoritative for that network segment, in which case the
server will send a DHCPNAK even though it doesn't know about the
address.
There may be a host declaration matching the client's identification.
If that host declaration contains a fixed-address declaration that
lists an IP address that is valid for the network segment to which the
client is connected. In this case, the DHCP server will never do
dynamic address allocation. In this case, the client is required to
take the address specified in the host declaration. If the client
sends a DHCPREQUEST for some other address, the server will respond
with a DHCPNAK.
When the DHCP server allocates a new address for a client (remember,
this only happens if the client has sent a DHCPDISCOVER), it first
looks to see if the client already has a valid lease on an IP address,
or if there is an old IP address the client had before that hasn't yet
been reassigned. In that case, the server will take that address and
check it to see if the client is still permitted to use it. If the
client is no longer permitted to use it, the lease is freed if the
server thought it was still in use - the fact that the client has sent
a DHCPDISCOVER proves to the server that the client is no longer using
the lease.
If no existing lease is found, or if the client is forbidden to receive
the existing lease, then the server will look in the list of address
pools for the network segment to which the client is attached for a
lease that is not in use and that the client is permitted to have. It
looks through each pool declaration in sequence (all range declarations
that appear outside of pool declarations are grouped into a single pool
with no permit list). If the permit list for the pool allows the
client to be allocated an address from that pool, the pool is examined
to see if there is an address available. If so, then the client is
tentatively assigned that address. Otherwise, the next pool is tested.
If no addresses are found that can be assigned to the client, no
response is sent to the client.
If an address is found that the client is permitted to have, and that
has never been assigned to any client before, the address is immedi‐
ately allocated to the client. If the address is available for alloca‐
tion but has been previously assigned to a different client, the server
will keep looking in hopes of finding an address that has never before
been assigned to a client.
The DHCP server generates the list of available IP addresses from a
hash table. This means that the addresses are not sorted in any par‐
ticular order, and so it is not possible to predict the order in which
the DHCP server will allocate IP addresses. Users of previous versions
of the ISC DHCP server may have become accustomed to the DHCP server
allocating IP addresses in ascending order, but this is no longer pos‐
sible, and there is no way to configure this behavior with version 3 of
the ISC DHCP server.
IP ADDRESS CONFLICT PREVENTION
The DHCP server checks IP addresses to see if they are in use before
allocating them to clients. It does this by sending an ICMP Echo
request message to the IP address being allocated. If no ICMP Echo
reply is received within a second, the address is assumed to be free.
This is only done for leases that have been specified in range state‐
ments, and only when the lease is thought by the DHCP server to be free
- i.e., the DHCP server or its failover peer has not listed the lease
as in use.
If a response is received to an ICMP Echo request, the DHCP server
assumes that there is a configuration error - the IP address is in use
by some host on the network that is not a DHCP client. It marks the
address as abandoned, and will not assign it to clients. The lease will
remain abandoned for a minimum of abandon-lease-time seconds.
If a DHCP client tries to get an IP address, but none are available,
but there are abandoned IP addresses, then the DHCP server will attempt
to reclaim an abandoned IP address. It marks one IP address as free,
and then does the same ICMP Echo request check described previously.
If there is no answer to the ICMP Echo request, the address is assigned
to the client.
The DHCP server does not cycle through abandoned IP addresses if the
first IP address it tries to reclaim is free. Rather, when the next
DHCPDISCOVER comes in from the client, it will attempt a new allocation
using the same method described here, and will typically try a new IP
address.
DHCP FAILOVER
This version of the ISC DHCP server supports the DHCP failover protocol
as documented in draft-ietf-dhc-failover-12.txt. This is not a final
protocol document, and we have not done interoperability testing with
other vendors' implementations of this protocol, so you must not assume
that this implementation conforms to the standard. If you wish to use
the failover protocol, make sure that both failover peers are running
the same version of the ISC DHCP server.
The failover protocol allows two DHCP servers (and no more than two) to
share a common address pool. Each server will have about half of the
available IP addresses in the pool at any given time for allocation.
If one server fails, the other server will continue to renew leases out
of the pool, and will allocate new addresses out of the roughly half of
available addresses that it had when communications with the other
server were lost.
It is possible during a prolonged failure to tell the remaining server
that the other server is down, in which case the remaining server will
(over time) reclaim all the addresses the other server had available
for allocation, and begin to reuse them. This is called putting the
server into the PARTNER-DOWN state.
You can put the server into the PARTNER-DOWN state either by using the
omshell (1) command or by stopping the server, editing the last
failover state declaration in the lease file, and restarting the
server. If you use this last method, change the "my state" line to:
failover peer name state {
my state partner-down;.
peer state state at date;
}
It is only required to change "my state" as shown above.
When the other server comes back online, it should automatically detect
that it has been offline and request a complete update from the server
that was running in the PARTNER-DOWN state, and then both servers will
resume processing together.
It is possible to get into a dangerous situation: if you put one server
into the PARTNER-DOWN state, and then *that* server goes down, and the
other server comes back up, the other server will not know that the
first server was in the PARTNER-DOWN state, and may issue addresses
previously issued by the other server to different clients, resulting
in IP address conflicts. Before putting a server into PARTNER-DOWN
state, therefore, make sure that the other server will not restart
automatically.
The failover protocol defines a primary server role and a secondary
server role. There are some differences in how primaries and secon‐
daries act, but most of the differences simply have to do with provid‐
ing a way for each peer to behave in the opposite way from the other.
So one server must be configured as primary, and the other must be con‐
figured as secondary, and it doesn't matter too much which one is
which.
FAILOVER STARTUP
When a server starts that has not previously communicated with its
failover peer, it must establish communications with its failover peer
and synchronize with it before it can serve clients. This can happen
either because you have just configured your DHCP servers to perform
failover for the first time, or because one of your failover servers
has failed catastrophically and lost its database.
The initial recovery process is designed to ensure that when one
failover peer loses its database and then resynchronizes, any leases
that the failed server gave out before it failed will be honored. When
the failed server starts up, it notices that it has no saved failover
state, and attempts to contact its peer.
When it has established contact, it asks the peer for a complete copy
its peer's lease database. The peer then sends its complete database,
and sends a message indicating that it is done. The failed server then
waits until MCLT has passed, and once MCLT has passed both servers make
the transition back into normal operation. This waiting period ensures
that any leases the failed server may have given out while out of con‐
tact with its partner will have expired.
While the failed server is recovering, its partner remains in the part‐
ner-down state, which means that it is serving all clients. The failed
server provides no service at all to DHCP clients until it has made the
transition into normal operation.
In the case where both servers detect that they have never before com‐
municated with their partner, they both come up in this recovery state
and follow the procedure we have just described. In this case, no ser‐
vice will be provided to DHCP clients until MCLT has expired.
CONFIGURING FAILOVER
In order to configure failover, you need to write a peer declaration
that configures the failover protocol, and you need to write peer ref‐
erences in each pool declaration for which you want to do failover.
You do not have to do failover for all pools on a given network seg‐
ment. You must not tell one server it's doing failover on a particu‐
lar address pool and tell the other it is not. You must not have any
common address pools on which you are not doing failover. A pool dec‐
laration that utilizes failover would look like this:
pool {
failover peer "foo";
pool specific parameters
};
The server currently does very little sanity checking, so if you
configure it wrong, it will just fail in odd ways. I would recommend
therefore that you either do failover or don't do failover, but don't
do any mixed pools. Also, use the same master configuration file for
both servers, and have a separate file that contains the peer
declaration and includes the master file. This will help you to avoid
configuration mismatches. As our implementation evolves, this will
become less of a problem. A basic sample dhcpd.conf file for a
primary server might look like this:
failover peer "foo" {
primary;
address anthrax.rc.example.com;
port 519;
peer address trantor.rc.example.com;
peer port 520;
max-response-delay 60;
max-unacked-updates 10;
mclt 3600;
split 128;
load balance max seconds 3;
}
include "/etc/dhcpd.master";
The statements in the peer declaration are as follows:
The primary and secondary statements
[ primary | secondary ];
This determines whether the server is primary or secondary, as
described earlier under DHCP FAILOVER.
The address statement
address address;
The address statement declares the IP address or DNS name on which
the server should listen for connections from its failover peer, and
also the value to use for the DHCP Failover Protocol server identi‐
fier. Because this value is used as an identifier, it may not be
omitted.
The peer address statement
peer address address;
The peer address statement declares the IP address or DNS name to
which the server should connect to reach its failover peer for
failover messages.
The port statement
port port-number;
The port statement declares the TCP port on which the server should
listen for connections from its failover peer. This statement may be
omitted, in which case the IANA assigned port number 647 will be used
by default.
The peer port statement
peer port port-number;
The peer port statement declares the TCP port to which the server
should connect to reach its failover peer for failover messages.
This statement may be omitted, in which case the IANA assigned port
number 647 will be used by default.
The max-response-delay statement
max-response-delay seconds;
The max-response-delay statement tells the DHCP server how many sec‐
onds may pass without receiving a message from its failover peer
before it assumes that connection has failed. This number should be
small enough that a transient network failure that breaks the connec‐
tion will not result in the servers being out of communication for a
long time, but large enough that the server isn't constantly making
and breaking connections. This parameter must be specified.
The max-unacked-updates statement
max-unacked-updates count;
The max-unacked-updates statement tells the remote DHCP server how
many BNDUPD messages it can send before it receives a BNDACK from the
local system. We don't have enough operational experience to say
what a good value for this is, but 10 seems to work. This parameter
must be specified.
The mclt statement
mclt seconds;
The mclt statement defines the Maximum Client Lead Time. It must be
specified on the primary, and may not be specified on the secondary.
This is the length of time for which a lease may be renewed by either
failover peer without contacting the other. The longer you set this,
the longer it will take for the running server to recover IP
addresses after moving into PARTNER-DOWN state. The shorter you set
it, the more load your servers will experience when they are not com‐
municating. A value of something like 3600 is probably reasonable,
but again bear in mind that we have no real operational experience
with this.
The split statement
split bits;
The split statement specifies the split between the primary and sec‐
ondary for the purposes of load balancing. Whenever a client makes a
DHCP request, the DHCP server runs a hash on the client identifica‐
tion, resulting in value from 0 to 255. This is used as an index
into a 256 bit field. If the bit at that index is set, the primary
is responsible. If the bit at that index is not set, the secondary
is responsible. The split value determines how many of the leading
bits are set to one. So, in practice, higher split values will cause
the primary to serve more clients than the secondary. Lower split
values, the converse. Legal values are between 0 and 256 inclusive,
of which the most reasonable is 128. Note that a value of 0 makes
the secondary responsible for all clients and a value of 256 makes
the primary responsible for all clients.
The hba statement
hba colon-separated-hex-list;
The hba statement specifies the split between the primary and sec‐
ondary as a bitmap rather than a cutoff, which theoretically allows
for finer-grained control. In practice, there is probably no need
for such fine-grained control, however. An example hba statement:
hba ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:
00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00;
This is equivalent to a split 128; statement, and identical. The
following two examples are also equivalent to a split of 128, but are
not identical:
hba aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:
aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa;
hba 55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:
55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:55;
They are equivalent, because half the bits are set to 0, half are set
to 1 (0xa and 0x5 are 1010 and 0101 binary respectively) and conse‐
quently this would roughly divide the clients equally between the
servers. They are not identical, because the actual peers this would
load balance to each server are different for each example.
You must only have split or hba defined, never both. For most cases,
the fine-grained control that hba offers isn't necessary, and split
should be used.
The load balance max seconds statement
load balance max seconds seconds;
This statement allows you to configure a cutoff after which load bal‐
ancing is disabled. The cutoff is based on the number of seconds
since the client sent its first DHCPDISCOVER or DHCPREQUEST message,
and only works with clients that correctly implement the secs field -
fortunately most clients do. We recommend setting this to something
like 3 or 5. The effect of this is that if one of the failover peers
gets into a state where it is responding to failover messages but not
responding to some client requests, the other failover peer will take
over its client load automatically as the clients retry.
The auto-partner-down statement
auto-partner-down seconds;
This statement instructs the server to initiate a timed delay upon
entering the communications-interrupted state (any situation of being
out-of-contact with the remote failover peer). At the conclusion of
the timer, the server will automatically enter the partner-down
state. This permits the server to allocate leases from the partner's
free lease pool after an STOS+MCLT timer expires, which can be dan‐
gerous if the partner is in fact operating at the time (the two
servers will give conflicting bindings).
Think very carefully before enabling this feature. The partner-down
and communications-interrupted states are intentionally segregated
because there do exist situations where a failover server can fail to
communicate with its peer, but still has the ability to receive and
reply to requests from DHCP clients. In general, this feature should
only be used in those deployments where the failover servers are
directly connected to one another, such as by a dedicated hardwired
link ("a heartbeat cable").
A zero value disables the auto-partner-down feature (also the
default), and any positive value indicates the time in seconds to
wait before automatically entering partner-down.
The Failover pool balance statements.
max-lease-misbalance percentage;
max-lease-ownership percentage;
min-balance seconds;
max-balance seconds;
This version of the DHCP Server evaluates pool balance on a schedule,
rather than on demand as leases are allocated. The latter approach
proved to be slightly klunky when pool misbalanced reach total satu‐
ration — when any server ran out of leases to assign, it also lost
its ability to notice it had run dry.
In order to understand pool balance, some elements of its operation
first need to be defined. First, there are ´free´ and ´backup´
leases. Both of these are referred to as ´free state leases´.
´free´ and ´backup´ are ´the free states´ for the purpose of this
document. The difference is that only the primary may allocate from
´free´ leases unless under special circumstances, and only the sec‐
ondary may allocate ´backup´ leases.
When pool balance is performed, the only plausible expectation is to
provide a 50/50 split of the free state leases between the two
servers. This is because no one can predict which server will fail,
regardless of the relative load placed upon the two servers, so giv‐
ing each server half the leases gives both servers the same amount of
´failure endurance´. Therefore, there is no way to configure any
different behaviour, outside of some very small windows we will
describe shortly.
The first thing calculated on any pool balance run is a value
referred to as ´lts´, or "Leases To Send". This, simply, is the dif‐
ference in the count of free and backup leases, divided by two. For
the secondary, it is the difference in the backup and free leases,
divided by two. The resulting value is signed: if it is positive,
the local server is expected to hand out leases to retain a 50/50
balance. If it is negative, the remote server would need to send
leases to balance the pool. Once the lts value reaches zero, the
pool is perfectly balanced (give or take one lease in the case of an
odd number of total free state leases).
The current approach is still something of a hybrid of the old
approach, marked by the presence of the max-lease-misbalance state‐
ment. This parameter configures what used to be a 10% fixed value in
previous versions: if lts is less than free+backup * max-lease-mis‐
balance percent, then the server will skip balancing a given pool (it
won't bother moving any leases, even if some leases "should" be
moved). The meaning of this value is also somewhat overloaded, how‐
ever, in that it also governs the estimation of when to attempt to
balance the pool (which may then also be skipped over). The oldest
leases in the free and backup states are examined. The time they
have resided in their respective queues is used as an estimate to
indicate how much time it is probable it would take before the leases
at the top of the list would be consumed (and thus, how long it would
take to use all leases in that state). This percentage is directly
multiplied by this time, and fit into the schedule if it falls within
the min-balance and max-balance configured values. The scheduled
pool check time is only moved in a downwards direction, it is never
increased. Lastly, if the lts is more than double this number in the
negative direction, the local server will ´panic´ and transmit a
Failover protocol POOLREQ message, in the hopes that the remote sys‐
tem will be woken up into action.
Once the lts value exceeds the max-lease-misbalance percentage of
total free state leases as described above, leases are moved to the
remote server. This is done in two passes.
In the first pass, only leases whose most recent bound client would
have been served by the remote server - according to the Load Balance
Algorithm (see above split and hba configuration statements) - are
given away to the peer. This first pass will happily continue to
give away leases, decrementing the lts value by one for each, until
the lts value has reached the negative of the total number of leases
multiplied by the max-lease-ownership percentage. So it is through
this value that you can permit a small misbalance of the lease pools
- for the purpose of giving the peer more than a 50/50 share of
leases in the hopes that their clients might some day return and be
allocated by the peer (operating normally). This process is referred
to as ´MAC Address Affinity´, but this is somewhat misnamed: it
applies equally to DHCP Client Identifier options. Note also that
affinity is applied to leases when they enter the state ´free´ from
´expired´ or ´released´. In this case also, leases will not be moved
from free to backup if the secondary already has more than its share.
The second pass is only entered into if the first pass fails to
reduce the lts underneath the total number of free state leases mul‐
tiplied by the max-lease-ownership percentage. In this pass, the
oldest leases are given over to the peer without second thought about
the Load Balance Algorithm, and this continues until the lts falls
under this value. In this way, the local server will also happily
keep a small percentage of the leases that would normally load bal‐
ance to itself.
So, the max-lease-misbalance value acts as a behavioural gate.
Smaller values will cause more leases to transition states to balance
the pools over time, higher values will decrease the amount of change
(but may lead to pool starvation if there's a run on leases).
The max-lease-ownership value permits a small (percentage) skew in
the lease balance of a percentage of the total number of free state
leases.
Finally, the min-balance and max-balance make certain that a sched‐
uled rebalance event happens within a reasonable timeframe (not to be
thrown off by, for example, a 7 year old free lease).
Plausible values for the percentages lie between 0 and 100, inclu‐
sive, but values over 50 are indistinguishable from one another (once
lts exceeds 50% of the free state leases, one server must therefore
have 100% of the leases in its respective free state). It is recom‐
mended to select a max-lease-ownership value that is lower than the
value selected for the max-lease-misbalance value. max-lease-owner‐
ship defaults to 10, and max-lease-misbalance defaults to 15.
Plausible values for the min-balance and max-balance times also range
from 0 to (2^32)-1 (or the limit of your local time_t value), but
default to values 60 and 3600 respectively (to place balance events
between 1 minute and 1 hour).
CLIENT CLASSING
Clients can be separated into classes, and treated differently depend‐
ing on what class they are in. This separation can be done either with
a conditional statement, or with a match statement within the class
declaration. It is possible to specify a limit on the total number of
clients within a particular class or subclass that may hold leases at
one time, and it is possible to specify automatic subclassing based on
the contents of the client packet.
Classing support for DHCPv6 clients was added in 4.3.0. It follows the
same rules as for DHCPv4 except that support for billing classes has
not been added yet.
To add clients to classes based on conditional evaluation, you can
specify a matching expression in the class statement:
class "ras-clients" {
match if substring (option dhcp-client-identifier, 1, 3) = "RAS";
}
Note that whether you use matching expressions or add statements (or
both) to classify clients, you must always write a class declaration
for any class that you use. If there will be no match statement and no
in-scope statements for a class, the declaration should look like this:
class "ras-clients" {
}
SUBCLASSES
In addition to classes, it is possible to declare subclasses. A sub‐
class is a class with the same name as a regular class, but with a spe‐
cific submatch expression which is hashed for quick matching. This is
essentially a speed hack - the main difference between five classes
with match expressions and one class with five subclasses is that it
will be quicker to find the subclasses. Subclasses work as follows:
class "allocation-class-1" {
match pick-first-value (option dhcp-client-identifier, hardware);
}
class "allocation-class-2" {
match pick-first-value (option dhcp-client-identifier, hardware);
}
subclass "allocation-class-1" 1:8:0:2b:4c:39:ad;
subclass "allocation-class-2" 1:8:0:2b:a9:cc:e3;
subclass "allocation-class-1" 1:0:0:c4:aa:29:44;
subnet 10.0.0.0 netmask 255.255.255.0 {
pool {
allow members of "allocation-class-1";
range 10.0.0.11 10.0.0.50;
}
pool {
allow members of "allocation-class-2";
range 10.0.0.51 10.0.0.100;
}
}
The data following the class name in the subclass declaration is a con‐
stant value to use in matching the match expression for the class.
When class matching is done, the server will evaluate the match expres‐
sion and then look the result up in the hash table. If it finds a
match, the client is considered a member of both the class and the sub‐
class.
Subclasses can be declared with or without scope. In the above exam‐
ple, the sole purpose of the subclass is to allow some clients access
to one address pool, while other clients are given access to the other
pool, so these subclasses are declared without scopes. If part of the
purpose of the subclass were to define different parameter values for
some clients, you might want to declare some subclasses with scopes.
In the above example, if you had a single client that needed some con‐
figuration parameters, while most didn't, you might write the following
subclass declaration for that client:
subclass "allocation-class-2" 1:08:00:2b:a1:11:31 {
option root-path "samsara:/var/diskless/alphapc";
filename "/tftpboot/netbsd.alphapc-diskless";
}
In this example, we've used subclassing as a way to control address
allocation on a per-client basis. However, it's also possible to use
subclassing in ways that are not specific to clients - for example, to
use the value of the vendor-class-identifier option to determine what
values to send in the vendor-encapsulated-options option. An example
of this is shown under the VENDOR ENCAPSULATED OPTIONS head in the
dhcp-options(5) manual page.
PER-CLASS LIMITS ON DYNAMIC ADDRESS ALLOCATION
You may specify a limit to the number of clients in a class that can be
assigned leases. The effect of this will be to make it difficult for a
new client in a class to get an address. Once a class with such a
limit has reached its limit, the only way a new client in that class
can get a lease is for an existing client to relinquish its lease,
either by letting it expire, or by sending a DHCPRELEASE packet.
Classes with lease limits are specified as follows:
class "limited-1" {
lease limit 4;
}
This will produce a class in which a maximum of four members may hold a
lease at one time.
SPAWNING CLASSES
It is possible to declare a spawning class. A spawning class is a
class that automatically produces subclasses based on what the client
sends. The reason that spawning classes were created was to make it
possible to create lease-limited classes on the fly. The envisioned
application is a cable-modem environment where the ISP wishes to pro‐
vide clients at a particular site with more than one IP address, but
does not wish to provide such clients with their own subnet, nor give
them an unlimited number of IP addresses from the network segment to
which they are connected.
Many cable modem head-end systems can be configured to add a Relay
Agent Information option to DHCP packets when relaying them to the DHCP
server. These systems typically add a circuit ID or remote ID option
that uniquely identifies the customer site. To take advantage of this,
you can write a class declaration as follows:
class "customer" {
spawn with option agent.circuit-id;
lease limit 4;
}
Now whenever a request comes in from a customer site, the circuit ID
option will be checked against the class´s hash table. If a subclass
is found that matches the circuit ID, the client will be classified in
that subclass and treated accordingly. If no subclass is found match‐
ing the circuit ID, a new one will be created and logged in the
dhcpd.leases file, and the client will be classified in this new class.
Once the client has been classified, it will be treated according to
the rules of the class, including, in this case, being subject to the
per-site limit of four leases.
The use of the subclass spawning mechanism is not restricted to relay
agent options - this particular example is given only because it is a
fairly straightforward one.
COMBINING MATCH, MATCH IF AND SPAWN WITH
In some cases, it may be useful to use one expression to assign a
client to a particular class, and a second expression to put it into a
subclass of that class. This can be done by combining the match if and
spawn with statements, or the match if and match statements. For exam‐
ple:
class "jr-cable-modems" {
match if option dhcp-vendor-identifier = "jrcm";
spawn with option agent.circuit-id;
lease limit 4;
}
class "dv-dsl-modems" {
match if option dhcp-vendor-identifier = "dvdsl";
spawn with option agent.circuit-id;
lease limit 16;
}
This allows you to have two classes that both have the same spawn with
expression without getting the clients in the two classes confused with
each other.
DYNAMIC DNS UPDATES
The DHCP server has the ability to dynamically update the Domain Name
System. Within the configuration files, you can define how you want
the Domain Name System to be updated. These updates are RFC 2136 com‐
pliant so any DNS server supporting RFC 2136 should be able to accept
updates from the DHCP server.
There are two DNS schemes implemented. The interim option is based on
draft revisions of the DDNS documents while the standard option is
based on the RFCs for DHCP-DNS interaction and DHCIDs. A third option,
ad-hoc, was deprecated and has now been removed from the code base.
The DHCP server must be configured to use one of the two currently-sup‐
ported methods, or not to do DNS updates.
New installations should use the standard option. Older installations
may want to continue using the interim option for backwards compatibil‐
ity with the DNS database until the database can be updated. This can
be done with the ddns-update-style configuration parameter.
THE DNS UPDATE SCHEME
the interim and standard DNS update schemes operate mostly according to
work from the IETF. The interim version was based on the drafts in
progress at the time while the standard is based on the completed RFCs.
The standard RFCs are:
RFC 4701 (updated by RF5494)
RFC 4702
RFC 4703
And the corresponding drafts were:
draft-ietf-dnsext-dhcid-rr-??.txt
draft-ietf-dhc-fqdn-option-??.txt
draft-ietf-dhc-ddns-resolution-??.txt
The basic framework for the two schemes is similar with the main mate‐
rial difference being that a DHCID RR is used in the standard version
while the interim versions uses a TXT RR. The format of the TXT record
bears a resemblance to the DHCID RR but it is not equivalent (MD5 vs
SHA2, field length differences etc).
In these two schemes the DHCP server does not necessarily always update
both the A and the PTR records. The FQDN option includes a flag which,
when sent by the client, indicates that the client wishes to update its
own A record. In that case, the server can be configured either to
honor the client´s intentions or ignore them. This is done with the
statement allow client-updates; or the statement ignore client-
updates;. By default, client updates are allowed.
If the server is configured to allow client updates, then if the client
sends a fully-qualified domain name in the FQDN option, the server will
use that name the client sent in the FQDN option to update the PTR
record. For example, let us say that the client is a visitor from the
"radish.org" domain, whose hostname is "jschmoe". The server is for
the "example.org" domain. The DHCP client indicates in the FQDN option
that its FQDN is "jschmoe.radish.org.". It also indicates that it
wants to update its own A record. The DHCP server therefore does not
attempt to set up an A record for the client, but does set up a PTR
record for the IP address that it assigns the client, pointing at
jschmoe.radish.org. Once the DHCP client has an IP address, it can
update its own A record, assuming that the "radish.org" DNS server will
allow it to do so.
If the server is configured not to allow client updates, or if the
client doesn´t want to do its own update, the server will simply choose
a name for the client. By default, the server will choose from the fol‐
lowing three values:
1. fqdn option (if present)
2. hostname option (if present)
3. Configured hostname option (if defined).
If these defaults for choosing the host name are not appropriate you
can write your own statement to set the ddns-hostname variable as you
wish. If none of the above are found the server will use the host dec‐
laration name (if one) and use-host-decl-names is on.
It will use its own domain name for the client. It will then update
both the A and PTR record, using the name that it chose for the client.
If the client sends a fully-qualified domain name in the fqdn option,
the server uses only the leftmost part of the domain name - in the
example above, "jschmoe" instead of "jschmoe.radish.org".
Further, if the ignore client-updates; directive is used, then the
server will in addition send a response in the DHCP packet, using the
FQDN Option, that implies to the client that it should perform its own
updates if it chooses to do so. With deny client-updates;, a response
is sent which indicates the client may not perform updates.
Both the standard and interim options also include a method to allow
more than one DHCP server to update the DNS database without acciden‐
tally deleting A records that shouldn´t be deleted nor failing to add A
records that should be added. For the standard option the method works
as follows:
When the DHCP server issues a client a new lease, it creates a text
string that is an SHA hash over the DHCP client´s identification (see
RFCs 4701 & 4702 for details). The update attempts to add an A record
with the name the server chose and a DHCID record containing the hashed
identifier string (hashid). If this update succeeds, the server is
done.
If the update fails because the A record already exists, then the DHCP
server attempts to add the A record with the prerequisite that there
must be a DHCID record in the same name as the new A record, and that
DHCID record´s contents must be equal to hashid. If this update suc‐
ceeds, then the client has its A record and PTR record. If it fails,
then the name the client has been assigned (or requested) is in use,
and can´t be used by the client. At this point the DHCP server gives
up trying to do a DNS update for the client until the client chooses a
new name.
The server also does not update very aggressively. Because each DNS
update involves a round trip to the DNS server, there is a cost associ‐
ated with doing updates even if they do not actually modify the DNS
database. So the DHCP server tracks whether or not it has updated the
record in the past (this information is stored on the lease) and does
not attempt to update records that it thinks it has already updated.
This can lead to cases where the DHCP server adds a record, and then
the record is deleted through some other mechanism, but the server
never again updates the DNS because it thinks the data is already
there. In this case the data can be removed from the lease through
operator intervention, and once this has been done, the DNS will be
updated the next time the client renews.
The interim DNS update scheme was written before the RFCs were final‐
ized and does not quite follow them. The RFCs call for a new DHCID
RRtype while the interim DNS update scheme uses a TXT record. In addi‐
tion the ddns-resolution draft called for the DHCP server to put a
DHCID RR on the PTR record, but the interim update method does not do
this. In the final RFC this requirement was relaxed such that a server
may add a DHCID RR to the PTR record.
DYNAMIC DNS UPDATE SECURITY
When you set your DNS server up to allow updates from the DHCP server,
you may be exposing it to unauthorized updates. To avoid this, you
should use TSIG signatures - a method of cryptographically signing
updates using a shared secret key. As long as you protect the secrecy
of this key, your updates should also be secure. Note, however, that
the DHCP protocol itself provides no security, and that clients can
therefore provide information to the DHCP server which the DHCP server
will then use in its updates, with the constraints described previ‐
ously.
The DNS server must be configured to allow updates for any zone that
the DHCP server will be updating. For example, let us say that clients
in the sneedville.edu domain will be assigned addresses on the
10.10.17.0/24 subnet. In that case, you will need a key declaration
for the TSIG key you will be using, and also two zone declarations -
one for the zone containing A records that will be updates and one for
the zone containing PTR records - for ISC BIND, something like this:
key DHCP_UPDATER {
algorithm HMAC-MD5.SIG-ALG.REG.INT;
secret pRP5FapFoJ95JEL06sv4PQ==;
};
zone "example.org" {
type master;
file "example.org.db";
allow-update { key DHCP_UPDATER; };
};
zone "17.10.10.in-addr.arpa" {
type master;
file "10.10.17.db";
allow-update { key DHCP_UPDATER; };
};
You will also have to configure your DHCP server to do updates to these
zones. To do so, you need to add something like this to your
dhcpd.conf file:
key DHCP_UPDATER {
algorithm HMAC-MD5.SIG-ALG.REG.INT;
secret pRP5FapFoJ95JEL06sv4PQ==;
};
zone EXAMPLE.ORG. {
primary 127.0.0.1;
key DHCP_UPDATER;
}
zone 17.127.10.in-addr.arpa. {
primary 127.0.0.1;
key DHCP_UPDATER;
}
The primary statement specifies the IP address of the name server whose
zone information is to be updated. In addition to the primary state‐
ment there are also the primary6 , secondary and secondary6 statements.
The primary6 statement specifies an IPv6 address for the name server.
The secondaries provide for additional addresses for name servers to be
used if the primary does not respond. The number of name servers the
DDNS code will attempt to use before giving up is limited and is cur‐
rently set to three.
Note that the zone declarations have to correspond to authority records
in your name server - in the above example, there must be an SOA record
for "example.org." and for "17.10.10.in-addr.arpa.". For example, if
there were a subdomain "foo.example.org" with no separate SOA, you
could not write a zone declaration for "foo.example.org." Also keep in
mind that zone names in your DHCP configuration should end in a ".";
this is the preferred syntax. If you do not end your zone name in a
".", the DHCP server will figure it out. Also note that in the DHCP
configuration, zone names are not encapsulated in quotes where there
are in the DNS configuration.
You should choose your own secret key, of course. The ISC BIND 9 dis‐
tribution comes with a program for generating secret keys called
dnssec-keygen. If you are using BIND 9´s dnssec-keygen, the above key
would be created as follows:
dnssec-keygen -a HMAC-MD5 -b 128 -n USER DHCP_UPDATER
The key name, algorithm, and secret must match that being used by the
DNS server. The DHCP server currently supports the following algo‐
rithms:
HMAC-MD5
HMAC-SHA1
HMAC-SHA224
HMAC-SHA256
HMAC-SHA384
HMAC-SHA512
You may wish to enable logging of DNS updates on your DNS server. To
do so, you might write a logging statement like the following:
logging {
channel update_debug {
file "/var/log/update-debug.log";
severity debug 3;
print-category yes;
print-severity yes;
print-time yes;
};
channel security_info {
file "/var/log/named-auth.info";
severity info;
print-category yes;
print-severity yes;
print-time yes;
};
category update { update_debug; };
category security { security_info; };
};
You must create the /var/log/named-auth.info and /var/log/update-
debug.log files before starting the name server. For more information
on configuring ISC BIND, consult the documentation that accompanies it.
REFERENCE: EVENTS
There are three kinds of events that can happen regarding a lease, and
it is possible to declare statements that occur when any of these
events happen. These events are the commit event, when the server has
made a commitment of a certain lease to a client, the release event,
when the client has released the server from its commitment, and the
expiry event, when the commitment expires.
To declare a set of statements to execute when an event happens, you
must use the on statement, followed by the name of the event, followed
by a series of statements to execute when the event happens, enclosed
in braces.
REFERENCE: DECLARATIONS
The include statement
include "filename";
The include statement is used to read in a named file, and process the
contents of that file as though it were entered in place of the include
statement.
The shared-network statement
shared-network name {
[ parameters ]
[ declarations ]
}
The shared-network statement is used to inform the DHCP server that
some IP subnets actually share the same physical network. Any subnets
in a shared network should be declared within a shared-network state‐
ment. Parameters specified in the shared-network statement will be
used when booting clients on those subnets unless parameters provided
at the subnet or host level override them. If any subnet in a shared
network has addresses available for dynamic allocation, those addresses
are collected into a common pool for that shared network and assigned
to clients as needed. There is no way to distinguish on which subnet
of a shared network a client should boot.
Name should be the name of the shared network. This name is used when
printing debugging messages, so it should be descriptive for the shared
network. The name may have the syntax of a valid domain name (although
it will never be used as such), or it may be any arbitrary name,
enclosed in quotes.
The subnet statement
subnet subnet-number netmask netmask {
[ parameters ]
[ declarations ]
}
The subnet statement is used to provide dhcpd with enough information
to tell whether or not an IP address is on that subnet. It may also be
used to provide subnet-specific parameters and to specify what
addresses may be dynamically allocated to clients booting on that sub‐
net. Such addresses are specified using the range declaration.
The subnet-number should be an IP address or domain name which resolves
to the subnet number of the subnet being described. The netmask should
be an IP address or domain name which resolves to the subnet mask of
the subnet being described. The subnet number, together with the net‐
mask, are sufficient to determine whether any given IP address is on
the specified subnet.
Although a netmask must be given with every subnet declaration, it is
recommended that if there is any variance in subnet masks at a site, a
subnet-mask option statement be used in each subnet declaration to set
the desired subnet mask, since any subnet-mask option statement will
override the subnet mask declared in the subnet statement.
The subnet6 statement
subnet6 subnet6-number {
[ parameters ]
[ declarations ]
}
The subnet6 statement is used to provide dhcpd with enough information
to tell whether or not an IPv6 address is on that subnet6. It may also
be used to provide subnet-specific parameters and to specify what
addresses may be dynamically allocated to clients booting on that sub‐
net.
The subnet6-number should be an IPv6 network identifier, specified as
ip6-address/bits.
The range statement
range [ dynamic-bootp ] low-address [ high-address];
For any subnet on which addresses will be assigned dynamically, there
must be at least one range statement. The range statement gives the
lowest and highest IP addresses in a range. All IP addresses in the
range should be in the subnet in which the range statement is declared.
The dynamic-bootp flag may be specified if addresses in the specified
range may be dynamically assigned to BOOTP clients as well as DHCP
clients. When specifying a single address, high-address can be omit‐
ted.
The range6 statement
range6 low-address high-address;
range6 subnet6-number;
range6 subnet6-number temporary;
range6 address temporary;
For any IPv6 subnet6 on which addresses will be assigned dynamically,
there must be at least one range6 statement. The range6 statement can
either be the lowest and highest IPv6 addresses in a range6, or use
CIDR notation, specified as ip6-address/bits. All IP addresses in the
range6 should be in the subnet6 in which the range6 statement is
declared.
The temporary variant makes the prefix (by default on 64 bits) avail‐
able for temporary (RFC 4941) addresses. A new address per prefix in
the shared network is computed at each request with an IA_TA option.
Release and Confirm ignores temporary addresses.
Any IPv6 addresses given to hosts with fixed-address6 are excluded from
the range6, as are IPv6 addresses on the server itself.
The prefix6 statement
prefix6 low-address high-address / bits;
The prefix6 is the range6 equivalent for Prefix Delegation (RFC 3633).
Prefixes of bits length are assigned between low-address and high-
address.
Any IPv6 prefixes given to static entries (hosts) with fixed-prefix6
are excluded from the prefix6.
This statement is currently global but it should have a shared-network
scope.
The host statement
host hostname {
[ parameters ]
[ declarations ]
}
The host declaration provides a way for the DHCP server to identify a
DHCP or BOOTP client. This allows the server to provide configuration
information including fixed addresses or, in DHCPv6, fixed prefixes for
a specific client.
If it is desirable to be able to boot a DHCP or BOOTP client on more
than one subnet with fixed v4 addresses, more than one address may be
specified in the fixed-address declaration, or more than one host
statement may be specified matching the same client.
The fixed-address6 declaration is used for v6 addresses. At this time
it only works with a single address. For multiple addresses specify
multiple host statements.
If client-specific boot parameters must change based on the network to
which the client is attached, then multiple host declarations should be
used. The host declarations will only match a client if one of their
fixed-address statements is viable on the subnet (or shared network)
where the client is attached. Conversely, for a host declaration to
match a client being allocated a dynamic address, it must not have any
fixed-address statements. You may therefore need a mixture of host
declarations for any given client...some having fixed-address state‐
ments, others without.
hostname should be a name identifying the host. If a hostname option
is not specified for the host, hostname is used.
Host declarations are matched to actual DHCP or BOOTP clients by match‐
ing the dhcp-client-identifier option specified in the host declaration
to the one supplied by the client, or, if the host declaration or the
client does not provide a dhcp-client-identifier option, by matching
the hardware parameter in the host declaration to the network hardware
address supplied by the client. BOOTP clients do not normally provide
a dhcp-client-identifier, so the hardware address must be used for all
clients that may boot using the BOOTP protocol.
DHCPv6 servers can use the host-identifier option parameter in the host
declaration, and specify any option with a fixed value to identify
hosts.
Please be aware that only the dhcp-client-identifier option and the
hardware address can be used to match a host declaration, or the host-
identifier option parameter for DHCPv6 servers. For example, it is not
possible to match a host declaration to a host-name option. This is
because the host-name option cannot be guaranteed to be unique for any
given client, whereas both the hardware address and dhcp-client-identi‐
fier option are at least theoretically guaranteed to be unique to a
given client.
The group statement
group {
[ parameters ]
[ declarations ]
}
The group statement is used simply to apply one or more parameters to a
group of declarations. It can be used to group hosts, shared networks,
subnets, or even other groups.
REFERENCE: ALLOW AND DENY
The allow and deny statements can be used to control the response of
the DHCP server to various sorts of requests. The allow and deny key‐
words actually have different meanings depending on the context. In a
pool context, these keywords can be used to set up access lists for
address allocation pools. In other contexts, the keywords simply con‐
trol general server behavior with respect to clients based on scope.
In a non-pool context, the ignore keyword can be used in place of the
deny keyword to prevent logging of denied requests.
ALLOW DENY AND IGNORE IN SCOPE
The following usages of allow and deny will work in any scope, although
it is not recommended that they be used in pool declarations.
The unknown-clients keyword
allow unknown-clients;
deny unknown-clients;
ignore unknown-clients;
The unknown-clients flag is used to tell dhcpd whether or not to dynam‐
ically assign addresses to unknown clients. Dynamic address assignment
to unknown clients is allowed by default. An unknown client is simply
a client that has no host declaration.
The use of this option is now deprecated. If you are trying to
restrict access on your network to known clients, you should use deny
unknown-clients; inside of your address pool, as described under the
heading ALLOW AND DENY WITHIN POOL DECLARATIONS.
The bootp keyword
allow bootp;
deny bootp;
ignore bootp;
The bootp flag is used to tell dhcpd whether or not to respond to bootp
queries. Bootp queries are allowed by default.
The booting keyword
allow booting;
deny booting;
ignore booting;
The booting flag is used to tell dhcpd whether or not to respond to
queries from a particular client. This keyword only has meaning when
it appears in a host declaration. By default, booting is allowed, but
if it is disabled for a particular client, then that client will not be
able to get an address from the DHCP server.
The duplicates keyword
allow duplicates;
deny duplicates;
Host declarations can match client messages based on the DHCP Client
Identifier option or based on the client's network hardware type and
MAC address. If the MAC address is used, the host declaration will
match any client with that MAC address - even clients with different
client identifiers. This doesn't normally happen, but is possible when
one computer has more than one operating system installed on it - for
example, Microsoft Windows and NetBSD or Linux.
The duplicates flag tells the DHCP server that if a request is received
from a client that matches the MAC address of a host declaration, any
other leases matching that MAC address should be discarded by the
server, even if the UID is not the same. This is a violation of the
DHCP protocol, but can prevent clients whose client identifiers change
regularly from holding many leases at the same time. By default,
duplicates are allowed.
The declines keyword
allow declines;
deny declines;
ignore declines;
The DHCPDECLINE message is used by DHCP clients to indicate that the
lease the server has offered is not valid. When the server receives a
DHCPDECLINE for a particular address, it normally abandons that
address, assuming that some unauthorized system is using it. Unfortu‐
nately, a malicious or buggy client can, using DHCPDECLINE messages,
completely exhaust the DHCP server's allocation pool. The server will
eventually reclaim these leases, but not while the client is running
through the pool. This may cause serious thrashing in the DNS, and it
will also cause the DHCP server to forget old DHCP client address allo‐
cations.
The declines flag tells the DHCP server whether or not to honor DHCPDE‐
CLINE messages. If it is set to deny or ignore in a particular scope,
the DHCP server will not respond to DHCPDECLINE messages.
The declines flag is only supported by DHCPv4 servers. Given the large
IPv6 address space and the internal limits imposed by the server's
address generation mechanism we don't think it is necessary for DHCPv6
servers at this time.
Currently, abandoned IPv6 addresses are reclaimed in one of two ways:
a) Client renews a specific address:
If a client using a given DUID submits a DHCP REQUEST containing
the last address abandoned by that DUID, the address will be
reassigned to that client.
b) Upon the second restart following an address abandonment. When
an address is abandoned it is both recorded as such in the lease
file and retained as abandoned in server memory until the server
is restarted. Upon restart, the server will process the lease file
and all addresses whose last known state is abandoned will be
retained as such in memory but not rewritten to the lease file.
This means that a subsequent restart of the server will not see the
abandoned addresses in the lease file and therefore have no record
of them as abandoned in memory and as such perceive them as free
for assignment.
The total number addresses in a pool, available for a given DUID value,
is internally limited by the server's address generation mechanism. If
through mistaken configuration, multiple clients are using the same
DUID they will competing for the same addresses causing the server to
reach this internal limit rather quickly. The internal limit isolates
this type of activity such that address range is not exhausted for
other DUID values. The appearance of the following error log, can be
an indication of this condition:
"Best match for DUID <XX> is an abandoned address, This may be a
result of multiple clients attempting to use this DUID"
where <XX> is an actual DUID value depicted as colon separated
string of bytes in hexadecimal values.
The client-updates keyword
allow client-updates;
deny client-updates;
The client-updates flag tells the DHCP server whether or not to honor
the client's intention to do its own update of its A record. See the
documentation under the heading THE DNS UPDATE SCHEME for details.
The leasequery keyword
allow leasequery;
deny leasequery;
The leasequery flag tells the DHCP server whether or not to answer DHC‐
PLEASEQUERY packets. The answer to a DHCPLEASEQUERY packet includes
information about a specific lease, such as when it was issued and when
it will expire. By default, the server will not respond to these pack‐
ets.
ALLOW AND DENY WITHIN POOL DECLARATIONS
The uses of the allow and deny keywords shown in the previous section
work pretty much the same way whether the client is sending a DHCPDIS‐
COVER or a DHCPREQUEST message - an address will be allocated to the
client (either the old address it's requesting, or a new address) and
then that address will be tested to see if it's okay to let the client
have it. If the client requested it, and it's not okay, the server
will send a DHCPNAK message. Otherwise, the server will simply not
respond to the client. If it is okay to give the address to the
client, the server will send a DHCPACK message.
The primary motivation behind pool declarations is to have address
allocation pools whose allocation policies are different. A client may
be denied access to one pool, but allowed access to another pool on the
same network segment. In order for this to work, access control has to
be done during address allocation, not after address allocation is
done.
When a DHCPREQUEST message is processed, address allocation simply con‐
sists of looking up the address the client is requesting and seeing if
it's still available for the client. If it is, then the DHCP server
checks both the address pool permit lists and the relevant in-scope
allow and deny statements to see if it's okay to give the lease to the
client. In the case of a DHCPDISCOVER message, the allocation process
is done as described previously in the ADDRESS ALLOCATION section.
When declaring permit lists for address allocation pools, the following
syntaxes are recognized following the allow or deny keywords:
known-clients;
If specified, this statement either allows or prevents allocation from
this pool to any client that has a host declaration (i.e., is known).
A client is known if it has a host declaration in any scope, not just
the current scope.
unknown-clients;
If specified, this statement either allows or prevents allocation from
this pool to any client that has no host declaration (i.e., is not
known).
members of "class";
If specified, this statement either allows or prevents allocation from
this pool to any client that is a member of the named class.
dynamic bootp clients;
If specified, this statement either allows or prevents allocation from
this pool to any bootp client.
authenticated clients;
If specified, this statement either allows or prevents allocation from
this pool to any client that has been authenticated using the DHCP
authentication protocol. This is not yet supported.
unauthenticated clients;
If specified, this statement either allows or prevents allocation from
this pool to any client that has not been authenticated using the DHCP
authentication protocol. This is not yet supported.
all clients;
If specified, this statement either allows or prevents allocation from
this pool to all clients. This can be used when you want to write a
pool declaration for some reason, but hold it in reserve, or when you
want to renumber your network quickly, and thus want the server to
force all clients that have been allocated addresses from this pool to
obtain new addresses immediately when they next renew.
after time;
If specified, this statement either allows or prevents allocation from
this pool after a given date. This can be used when you want to move
clients from one pool to another. The server adjusts the regular lease
time so that the latest expiry time is at the given time+min-lease-
time. A short min-lease-time enforces a step change, whereas a longer
min-lease-time allows for a gradual change. time is either second
since epoch, or a UTC time string e.g. 4 2007/08/24 09:14:32 or a
string with time zone offset in seconds e.g. 4 2007/08/24 11:14:32
-7200
REFERENCE: PARAMETERS
The abandon-lease-time statement
adandon-lease-time time;
Time should be the maximum amount of time (in seconds) that an aban‐
doned IPv4 lease remains unavailable for assignment to a client.
Abandoned leases will only be offered to clients if there are no free
leases. If not defined, the default abandon lease time is 86400 sec‐
onds (24 hours). Note the abandoned lease time for a given lease is
preserved across server restarts. The parameter may only be set at
the global scope and is evaluated only once during server startup.
Values less than sixty seconds are not recommended as this is below
the ping check threshold and can cause leases once abandoned but
since returned to the free state to not be pinged before being
offered. If the requested time is larger than 0x7FFFFFFF - 1 or the
sum of the current time plus the abandoned time isgreater than
0x7FFFFFFF it is treated as infinite.
The adaptive-lease-time-threshold statement
adaptive-lease-time-threshold percentage;
When the number of allocated leases within a pool rises above the
percentage given in this statement, the DHCP server decreases the
lease length for new clients within this pool to min-lease-time sec‐
onds. Clients renewing an already valid (long) leases get at least
the remaining time from the current lease. Since the leases expire
faster, the server may either recover more quickly or avoid pool
exhaustion entirely. Once the number of allocated leases drop below
the threshold, the server reverts back to normal lease times. Valid
percentages are between 1 and 99.
The always-broadcast statement
always-broadcast flag;
The DHCP and BOOTP protocols both require DHCP and BOOTP clients to
set the broadcast bit in the flags field of the BOOTP message header.
Unfortunately, some DHCP and BOOTP clients do not do this, and there‐
fore may not receive responses from the DHCP server. The DHCP server
can be made to always broadcast its responses to clients by setting
this flag to ´on´ for the relevant scope; relevant scopes would be
inside a conditional statement, as a parameter for a class, or as a
parameter for a host declaration. To avoid creating excess broadcast
traffic on your network, we recommend that you restrict the use of
this option to as few clients as possible. For example, the Micro‐
soft DHCP client is known not to have this problem, as are the Open‐
Transport and ISC DHCP clients.
The always-reply-rfc1048 statement
always-reply-rfc1048 flag;
Some BOOTP clients expect RFC1048-style responses, but do not follow
RFC1048 when sending their requests. You can tell that a client is
having this problem if it is not getting the options you have config‐
ured for it and if you see in the server log the message "(non-
rfc1048)" printed with each BOOTREQUEST that is logged.
If you want to send rfc1048 options to such a client, you can set the
always-reply-rfc1048 option in that client's host declaration, and
the DHCP server will respond with an RFC-1048-style vendor options
field. This flag can be set in any scope, and will affect all
clients covered by that scope.
The authoritative statement
authoritative;
not authoritative;
The DHCP server will normally assume that the configuration informa‐
tion about a given network segment is not known to be correct and is
not authoritative. This is so that if a naive user installs a DHCP
server not fully understanding how to configure it, it does not send
spurious DHCPNAK messages to clients that have obtained addresses
from a legitimate DHCP server on the network.
Network administrators setting up authoritative DHCP servers for
their networks should always write authoritative; at the top of their
configuration file to indicate that the DHCP server should send DHCP‐
NAK messages to misconfigured clients. If this is not done, clients
will be unable to get a correct IP address after changing subnets
until their old lease has expired, which could take quite a long
time.
Usually, writing authoritative; at the top level of the file should
be sufficient. However, if a DHCP server is to be set up so that it
is aware of some networks for which it is authoritative and some net‐
works for which it is not, it may be more appropriate to declare
authority on a per-network-segment basis.
Note that the most specific scope for which the concept of authority
makes any sense is the physical network segment - either a shared-
network statement or a subnet statement that is not contained within
a shared-network statement. It is not meaningful to specify that the
server is authoritative for some subnets within a shared network, but
not authoritative for others, nor is it meaningful to specify that
the server is authoritative for some host declarations and not oth‐
ers.
The boot-unknown-clients statement
boot-unknown-clients flag;
If the boot-unknown-clients statement is present and has a value of
false or off, then clients for which there is no host declaration
will not be allowed to obtain IP addresses. If this statement is not
present or has a value of true or on, then clients without host dec‐
larations will be allowed to obtain IP addresses, as long as those
addresses are not restricted by allow and deny statements within
their pool declarations.
The db-time-format statement
db-time-format [ default | local ] ;
The DHCP server software outputs several timestamps when writing
leases to persistent storage. This configuration parameter selects
one of two output formats. The default format prints the day, date,
and time in UTC, while the local format prints the system seconds-
since-epoch, and helpfully provides the day and time in the system
timezone in a comment. The time formats are described in detail in
the dhcpd.leases(5) manpage.
The ddns-hostname statement
ddns-hostname name;
The name parameter should be the hostname that will be used in set‐
ting up the client's A and PTR records. If no ddns-hostname is spec‐
ified in scope, then the server will derive the hostname automati‐
cally, using an algorithm that varies for each of the different
update methods.
The ddns-domainname statement
ddns-domainname name;
The name parameter should be the domain name that will be appended to
the client's hostname to form a fully-qualified domain-name (FQDN).
The dns-local-address4 and dns-local-address6 statements
ddns-local-address4 address;
ddns-local-address6 address;
The address parameter should be the local IPv4 or IPv6 address the
server should use as the from address when sending DDNS update
requests.
The ddns-rev-domainname statement
ddns-rev-domainname name;
The name parameter should be the domain name that will be appended to
the client's reversed IP address to produce a name for use in the
client's PTR record. By default, this is "in-addr.arpa.", but the
default can be overridden here.
The reversed IP address to which this domain name is appended is
always the IP address of the client, in dotted quad notation,
reversed - for example, if the IP address assigned to the client is
10.17.92.74, then the reversed IP address is 74.92.17.10. So a
client with that IP address would, by default, be given a PTR record
of 10.17.92.74.in-addr.arpa.
The ddns-update-style parameter
ddns-update-style style;
The style parameter must be one of standard, interim or none. The
ddns-update-style statement is only meaningful in the outer scope -
it is evaluated once after reading the dhcpd.conf file, rather than
each time a client is assigned an IP address, so there is no way to
use different DNS update styles for different clients. The default is
none.
The ddns-updates statement
ddns-updates flag;
The ddns-updates parameter controls whether or not the server will
attempt to do a DNS update when a lease is confirmed. Set this to
off if the server should not attempt to do updates within a certain
scope. The ddns-updates parameter is on by default. To disable DNS
updates in all scopes, it is preferable to use the ddns-update-style
statement, setting the style to none.
The default-lease-time statement
default-lease-time time;
Time should be the length in seconds that will be assigned to a lease
if the client requesting the lease does not ask for a specific expi‐
ration time. This is used for both DHCPv4 and DHCPv6 leases (it is
also known as the "valid lifetime" in DHCPv6). The default is 43200
seconds.
The delayed-ack and max-ack-delay statements
delayed-ack count;
max-ack-delay microseconds;
Count should be an integer value from zero to 2^16-1, and defaults to
28. The count represents how many DHCPv4 replies maximum will be
queued pending transmission until after a database commit event. If
this number is reached, a database commit event (commonly resulting
in fsync() and representing a performance penalty) will be made, and
the reply packets will be transmitted in a batch afterwards. This
preserves the RFC2131 direction that "stable storage" be updated
prior to replying to clients. Should the DHCPv4 sockets "go dry"
(select() returns immediately with no read sockets), the commit is
made and any queued packets are transmitted.
Similarly, microseconds indicates how many microseconds are permitted
to pass inbetween queuing a packet pending an fsync, and performing
the fsync. Valid values range from 0 to 2^32-1, and defaults to
250,000 (1/4 of a second).
The delayed-ack feature is not compiled in by default, but must be
enabled at compile time with ´./configure --enable-delayed-ack´.
While we no longer consider it experimental and we don't know of any
issues with it, in order to minimize problems with existing configu‐
ration files we have left it disabled by default.
The dhcp-cache-threshold statement
dhcp-cache-threshold percentage;
The dhcp-cache-threshold statement takes one integer parameter with
allowed values between 0 and 100. The default value is 25 (25% of the
lease time). This parameter expresses the percentage of the total
lease time, measured from the beginning, during which a client's
attempt to renew its lease will result in getting the already
assigned lease, rather than an extended lease.
Clients that attempt renewal frequently can cause the server to
update and write the database frequently resulting in a performance
impact on the server. The dhcp-cache-threshold statement instructs
the DHCP server to avoid updating leases too frequently thus avoiding
this behavior. Instead the server assigns the same lease (i.e.
reuses it) with no modifications except for CLTT (Client Last Trans‐
mission Time) which does not require disk operations. This feature
applies to IPv4 only.
When an existing lease is matched to a renewing client, it will be
reused if all of the following conditions are true:
1. The dhcp-cache-threshold is larger than zero
2. The current lease is active
3. The percentage of the lease time that has elapsed is less than
dhcp-cache-threshold
4. The client information provided in the renewal does not alter
any of the following:
a. DNS information and DNS updates are enabled
b. Billing class to which the lease is associated
c. The host declaration associated with the lease
d. The client id - this may happen if a client boots without
a client id and then starts using one in subsequent requests.
Note that the lease can be reused if the options the client or relay
agent sends are changed. These changes will not be recorded in the
in-memory or on-disk databases until the client renews after the
threshold time is reached.
The do-forward-updates statement
do-forward-updates flag;
The do-forward-updates statement instructs the DHCP server as to
whether it should attempt to update a DHCP client´s A record when the
client acquires or renews a lease. This statement has no effect
unless DNS updates are enabled. Forward updates are enabled by
default. If this statement is used to disable forward updates, the
DHCP server will never attempt to update the client´s A record, and
will only ever attempt to update the client´s PTR record if the
client supplies an FQDN that should be placed in the PTR record using
the fqdn option. If forward updates are enabled, the DHCP server
will still honor the setting of the client-updates flag.
The dont-use-fsync statement
dont-use-fsync flag;
The dont-use-fsync statement instructs the DHCP server if it should
call fsync() when writing leases to the lease file. By default and
if the flag is set to false the server will call fsync(). Suppress‐
ing the call to fsync() may increase the performance of the server
but it also adds a risk that a lease will not be properly written to
the disk after it has been issued to a client and before the server
stops. This can lead to duplicate leases being issued to different
clients. Using this option is not recommended.
The dynamic-bootp-lease-cutoff statement
dynamic-bootp-lease-cutoff date;
The dynamic-bootp-lease-cutoff statement sets the ending time for all
leases assigned dynamically to BOOTP clients. Because BOOTP clients
do not have any way of renewing leases, and don't know that their
leases could expire, by default dhcpd assigns infinite leases to all
BOOTP clients. However, it may make sense in some situations to set
a cutoff date for all BOOTP leases - for example, the end of a school
term, or the time at night when a facility is closed and all machines
are required to be powered off.
Date should be the date on which all assigned BOOTP leases will end.
The date is specified in the form:
W YYYY/MM/DD HH:MM:SS
W is the day of the week expressed as a number from zero (Sunday) to
six (Saturday). YYYY is the year, including the century. MM is the
month expressed as a number from 1 to 12. DD is the day of the
month, counting from 1. HH is the hour, from zero to 23. MM is the
minute and SS is the second. The time is always in Coordinated Uni‐
versal Time (UTC), not local time.
The dynamic-bootp-lease-length statement
dynamic-bootp-lease-length length;
The dynamic-bootp-lease-length statement is used to set the length of
leases dynamically assigned to BOOTP clients. At some sites, it may
be possible to assume that a lease is no longer in use if its holder
has not used BOOTP or DHCP to get its address within a certain time
period. The period is specified in length as a number of seconds.
If a client reboots using BOOTP during the timeout period, the lease
duration is reset to length, so a BOOTP client that boots frequently
enough will never lose its lease. Needless to say, this parameter
should be adjusted with extreme caution.
The echo-client-id statement
echo-client-id flag;
The echo-client-id statement is used to enable or disable RFC 6842
compliant behavior. If the echo-client-id statement is present and
has a value of true or on, and a DHCP DISCOVER or REQUEST is received
which contains the client identifier option (Option code 61), the
server will copy the option into its response (DHCP ACK or NAK) per
RFC 6842. In other words if the client sends the option it will
receive it back. By default, this flag is off and client identifiers
will not echoed back to the client.
The filename statement
filename "filename";
The filename statement can be used to specify the name of the initial
boot file which is to be loaded by a client. The filename should be
a filename recognizable to whatever file transfer protocol the client
can be expected to use to load the file.
The fixed-address declaration
fixed-address address [, address ... ];
The fixed-address declaration is used to assign one or more fixed IP
addresses to a client. It should only appear in a host declaration.
If more than one address is supplied, then when the client boots, it
will be assigned the address that corresponds to the network on which
it is booting. If none of the addresses in the fixed-address state‐
ment are valid for the network to which the client is connected, that
client will not match the host declaration containing that fixed-
address declaration. Each address in the fixed-address declaration
should be either an IP address or a domain name that resolves to one
or more IP addresses.
The fixed-address6 declaration
fixed-address6 ip6-address ;
The fixed-address6 declaration is used to assign a fixed IPv6
addresses to a client. It should only appear in a host declaration.
The fixed-prefix6 declaration
fixed-prefix6 low-address / bits;
The fixed-prefix6 declaration is used to assign a fixed IPv6 prefix
to a client. It should only appear in a host declaration, but multi‐
ple fixed-prefix6 statements may appear in a single host declaration.
The low-address specifies the start of the prefix and the bits speci‐
fies the size of the prefix in bits.
If there are multiple prefixes for a given host entry the server will
choose one that matches the requested prefix size or, if none match,
the first one.
If there are multiple host declarations the server will try to choose
a declaration where the fixed-address6 matches the client's subnet.
If none match it will choose one that doesn't have a fixed-address6
statement.
Note Well: Unlike the fixed address the fixed prefix does not need to
match a subnet in order to be served. This allows you to provide a
prefix to a client that is outside of the subnet on which the client
makes the request to the the server.
The get-lease-hostnames statement
get-lease-hostnames flag;
The get-lease-hostnames statement is used to tell dhcpd whether or
not to look up the domain name corresponding to the IP address of
each address in the lease pool and use that address for the DHCP
hostname option. If flag is true, then this lookup is done for all
addresses in the current scope. By default, or if flag is false, no
lookups are done.
The hardware statement
hardware hardware-type hardware-address;
In order for a BOOTP client to be recognized, its network hardware
address must be declared using a hardware clause in the host state‐
ment. hardware-type must be the name of a physical hardware inter‐
face type. Currently, only the ethernet and token-ring types are
recognized, although support for a fddi hardware type (and others)
would also be desirable. The hardware-address should be a set of
hexadecimal octets (numbers from 0 through ff) separated by colons.
The hardware statement may also be used for DHCP clients.
The host-identifier option statement
host-identifier option option-name option-data;
or
host-identifier v6relopt number option-name option-data;
This identifies a DHCPv6 client in a host statement. option-name is
any option, and option-data is the value for the option that the
client will send. The option-data must be a constant value. In the
v6relopts case the additional number is the relay to examine for the
specified option name and value. The values are the same as for the
v6relay option. 0 is a no-op, 1 is the relay closest to the client,
2 the next one in and so on. Values that are larger than the maximum
number of relays (currently 32) indicate the relay closest to the
server independent of number.
The ignore-client-uids statement
ignore-client-uids flag;
If the ignore-client-uids statement is present and has a value of
true or on, the UID for clients will not be recorded. If this state‐
ment is not present or has a value of false or off, then client UIDs
will be recorded.
The infinite-is-reserved statement
infinite-is-reserved flag;
ISC DHCP now supports ´reserved´ leases. See the section on RESERVED
LEASES below. If this flag is on, the server will automatically
reserve leases allocated to clients which requested an infinite
(0xffffffff) lease-time.
The default is off.
The lease-file-name statement
lease-file-name name;
Name should be the name of the DHCP server's lease file. By default,
this is /var/lib/dhcp/dhcpd.leases. This statement must appear in
the outer scope of the configuration file - if it appears in some
other scope, it will have no effect. Furthermore, it has no effect
if overridden by the -lf flag or the PATH_DHCPD_DB environment vari‐
able.
The limit-addrs-per-ia statement
limit-addrs-per-ia number;
By default, the DHCPv6 server will limit clients to one IAADDR per IA
option, meaning one address. If you wish to permit clients to hang
onto multiple addresses at a time, configure a larger number here.
Note that there is no present method to configure the server to
forcibly configure the client with one IP address per each subnet on
a shared network. This is left to future work.
The dhcpv6-lease-file-name statement
dhcpv6-lease-file-name name;
Name is the name of the lease file to use if and only if the server
is running in DHCPv6 mode. By default, this is
/var/lib/dhcp/dhcpd6.leases. This statement, like lease-file-name,
must appear in the outer scope of the configuration file. It has no
effect if overridden by the -lf flag or the PATH_DHCPD6_DB environ‐
ment variable. If dhcpv6-lease-file-name is not specified, but
lease-file-name is, the latter value will be used.
The lease-id-format parameter
lease-id-format format;
The format parameter must be either octal or hex. This parameter
governs the format used to write certain values to lease files. With
the default format, octal, values are written as quoted strings in
which non-printable characters are represented as octal escapes - a
backslash character followed by three octal digits. When the hex
format is specified, values are written as an unquoted series of
pairs of hexadecimal digits, separated by colons.
Currently, the values written out based on lease-id-format are the
server-duid, the uid (DHCPv4 leases), and the IAID_DUID (DHCPv6
leases). Note the server automatically reads the values in either
format.
The local-port statement
local-port port;
This statement causes the DHCP server to listen for DHCP requests on
the UDP port specified in port, rather than on port 67.
The local-address statement
local-address address;
This statement causes the DHCP server to listen for DHCP requests
sent to the specified address, rather than requests sent to all
addresses. Since serving directly attached DHCP clients implies that
the server must respond to requests sent to the all-ones IP address,
this option cannot be used if clients are on directly attached net‐
works; it is only realistically useful for a server whose only
clients are reached via unicasts, such as via DHCP relay agents.
Note: This statement is only effective if the server was compiled
using the USE_SOCKETS #define statement, which is default on a small
number of operating systems, and must be explicitly chosen at com‐
pile-time for all others. You can be sure if your server is compiled
with USE_SOCKETS if you see lines of this format at startup:
Listening on Socket/eth0
Note also that since this bind()s all DHCP sockets to the specified
address, that only one address may be supported in a daemon at a
given time.
The log-facility statement
log-facility facility;
This statement causes the DHCP server to do all of its logging on the
specified log facility once the dhcpd.conf file has been read. By
default the DHCP server logs to the daemon facility. Possible log
facilities include auth, authpriv, cron, daemon, ftp, kern, lpr,
mail, mark, news, ntp, security, syslog, user, uucp, and local0
through local7. Not all of these facilities are available on all
systems, and there may be other facilities available on other sys‐
tems.
In addition to setting this value, you may need to modify your sys‐
log.conf file to configure logging of the DHCP server. For example,
you might add a line like this:
local7.debug /var/log/dhcpd.log
The syntax of the syslog.conf file may be different on some operating
systems - consult the syslog.conf manual page to be sure. To get
syslog to start logging to the new file, you must first create the
file with correct ownership and permissions (usually, the same owner
and permissions of your /var/log/messages or /usr/adm/messages file
should be fine) and send a SIGHUP to syslogd. Some systems support
log rollover using a shell script or program called newsyslog or
logrotate, and you may be able to configure this as well so that your
log file doesn't grow uncontrollably.
Because the log-facility setting is controlled by the dhcpd.conf
file, log messages printed while parsing the dhcpd.conf file or
before parsing it are logged to the default log facility. To prevent
this, see the README file included with this distribution, which
describes BUG: where is that mentioned in README? how to change the
default log facility. When this parameter is used, the DHCP server
prints its startup message a second time after parsing the configura‐
tion file, so that the log will be as complete as possible.
The log-threshold-high and log-threshold-low statements
log-threshold-high percentage;
log-threshold-low percentage;
The log-threshold-low and log-threshold-high statements are used to
control when a message is output about pool usage. The value for
both of them is the percentage of the pool in use. If the high
threshold is 0 or has not been specified, no messages will be pro‐
duced. If a high threshold is given, a message is output once the
pool usage passes that level. After that, no more messages will be
output until the pool usage falls below the low threshold. If the
low threshold is not given, it default to a value of zero.
A special case occurs when the low threshold is set to be higher than
the high threshold. In this case, a message will be generated each
time a lease is acknowledged when the pool usage is above the high
threshold.
Note that threshold logging will be automatically disabled for shared
subnets whose total number of addresses is larger than (2^64)-1. The
server will emit a log statement at startup when threshold logging is
disabled as shown below:
"Threshold logging disabled for shared subnet of ranges:
<addresses>"
This is likely to have no practical runtime effect as CPUs are
unlikely to support a server actually reaching such a large number of
leases.
The max-lease-time statement
max-lease-time time;
Time should be the maximum length in seconds that will be assigned to
a lease. If not defined, the default maximum lease time is 86400.
The only exception to this is that Dynamic BOOTP lease lengths, which
are not specified by the client, are not limited by this maximum.
The min-lease-time statement
min-lease-time time;
Time should be the minimum length in seconds that will be assigned to
a lease. The default is the minimum of 300 seconds or max-lease-
time.
The min-secs statement
min-secs seconds;
Seconds should be the minimum number of seconds since a client began
trying to acquire a new lease before the DHCP server will respond to
its request. The number of seconds is based on what the client
reports, and the maximum value that the client can report is 255 sec‐
onds. Generally, setting this to one will result in the DHCP server
not responding to the client's first request, but always responding
to its second request.
This can be used to set up a secondary DHCP server which never offers
an address to a client until the primary server has been given a
chance to do so. If the primary server is down, the client will bind
to the secondary server, but otherwise clients should always bind to
the primary. Note that this does not, by itself, permit a primary
server and a secondary server to share a pool of dynamically-allocat‐
able addresses.
The next-server statement
next-server server-name;
The next-server statement is used to specify the host address of the
server from which the initial boot file (specified in the filename
statement) is to be loaded. Server-name should be a numeric IP
address or a domain name.
The omapi-port statement
omapi-port port;
The omapi-port statement causes the DHCP server to listen for OMAPI
connections on the specified port. This statement is required to
enable the OMAPI protocol, which is used to examine and modify the
state of the DHCP server as it is running.
The one-lease-per-client statement
one-lease-per-client flag;
If this flag is enabled, whenever a client sends a DHCPREQUEST for a
particular lease, the server will automatically free any other leases
the client holds. This presumes that when the client sends a DHCPRE‐
QUEST, it has forgotten any lease not mentioned in the DHCPREQUEST -
i.e., the client has only a single network interface and it does not
remember leases it's holding on networks to which it is not currently
attached. Neither of these assumptions are guaranteed or provable,
so we urge caution in the use of this statement.
The pid-file-name statement
pid-file-name name;
Name should be the name of the DHCP server's process ID file. This
is the file in which the DHCP server's process ID is stored when the
server starts. By default, this is /var/run/dhcpd.pid. Like the
lease-file-name statement, this statement must appear in the outer
scope of the configuration file. It has no effect if overridden by
the -pf flag or the PATH_DHCPD_PID environment variable.
The dhcpv6-pid-file-name statement
dhcpv6-pid-file-name name;
Name is the name of the pid file to use if and only if the server
is running in DHCPv6 mode. By default, this is
/var/lib/dhcp/dhcpd6.pid. This statement, like pid-file-name,
must appear in the outer scope of the configuration file. It has
no effect if overridden by the -pf flag or the PATH_DHCPD6_PID
environment variable. If dhcpv6-pid-file-name is not specified,
but pid-file-name is, the latter value will be used.
The ping-check statement
ping-check flag;
When the DHCP server is considering dynamically allocating an IP
address to a client, it first sends an ICMP Echo request (a ping)
to the address being assigned. It waits for a second, and if no
ICMP Echo response has been heard, it assigns the address. If a
response is heard, the lease is abandoned, and the server does not
respond to the client. The lease will remain abandoned for a min‐
imum of abandon-lease-time seconds.
If a there are no free addresses but there are abandoned IP
addresses, the DHCP server will attempt to reclaim an abandoned IP
address regardless of the value of abandon-lease-time.
This ping check introduces a default one-second delay in respond‐
ing to DHCPDISCOVER messages, which can be a problem for some
clients. The default delay of one second may be configured using
the ping-timeout parameter. The ping-check configuration parame‐
ter can be used to control checking - if its value is false, no
ping check is done.
The ping-timeout statement
ping-timeout seconds;
If the DHCP server determined it should send an ICMP echo request
(a ping) because the ping-check statement is true, ping-timeout
allows you to configure how many seconds the DHCP server should
wait for an ICMP Echo response to be heard, if no ICMP Echo
response has been received before the timeout expires, it assigns
the address. If a response is heard, the lease is abandoned, and
the server does not respond to the client. If no value is set,
ping-timeout defaults to 1 second.
The preferred-lifetime statement
preferred-lifetime seconds;
IPv6 addresses have ´valid´ and ´preferred´ lifetimes. The valid
lifetime determines at what point at lease might be said to have
expired, and is no longer useable. A preferred lifetime is an
advisory condition to help applications move off of the address
and onto currently valid addresses (should there still be any open
TCP sockets or similar).
The preferred lifetime defaults to 5/8 the default lease time.
The prefix-length-mode statement
prefix-length-mode mode;
According to RFC 3633, DHCPv6 clients may specify preferences when
soliciting prefixes by including an IA_PD Prefix option within the
IA_PD option. Among the preferences that may be conveyed is the
"prefix-length". When non-zero it indicates a client's desired
length for offered prefixes. The RFC states that servers "MAY
choose to use the information...to select prefix(es)" but does not
specify any particular rules for doing so. The prefix-length-mode
statement can be used to set the prefix selection rules employed
by the server, when clients send a non-zero prefix-length value.
The mode parameter must be one of ignore, prefer, exact, minimum,
or maximum where:
1. ignore - The requested length is ignored. The server will offer
the first available prefix.
2. prefer - The server will offer the first available prefix with
the same length as the requested length. If none are found then
it will offer the first available prefix of any length.
3. exact - The server will offer the first available prefix with
the same length as the requested length. If none are found, it
will return a status indicating no prefixes available. This is
the default behavior.
4. minimum - The server will offer the first available prefix with
the same length as the requested length. If none are found, it
will return the first available prefix whose length is greater
than (e.g. longer than), the requested value. If none of those
are found, it will return a status indicating no prefixes avail‐
able. For example, if client requests a length of /60, and the
server has available prefixes of lengths /56 and /64, it will
offer prefix of length /64.
5. maximum - The server will offer the first available prefix with
the same length as the requested length. If none are found, it
will return the first available prefix whose length is less than
(e.g. shorter than), the requested value. If none of those are
found, it will return a status indicating no prefixes available.
For example, if client requests a length of /60, and the server
has available prefixes of lengths /56 and /64, it will offer a
prefix of length /56.
In general "first available" is determined by the order in which
pools are defined in the server's configuration. For example, if
a subnet is defined with three prefix pools A,B, and C:
subnet 3000::/64 {
# pool A
pool6 {
:
}
# pool B
pool6 {
:
}
# pool C
pool6 {
:
}
}
then the pools will be checked in the order A, B, C. For modes
prefer, minimum, and maximum this may mean checking the pools in
that order twice. A first pass through is made looking for an
available prefix of exactly the preferred length. If none are
found, then a second pass is performed starting with pool A but
with appropriately adjusted length criteria.
The remote-port statement
remote-port port;
This statement causes the DHCP server to transmit DHCP responses
to DHCP clients upon the UDP port specified in port, rather than
on port 68. In the event that the UDP response is transmitted to
a DHCP Relay, the server generally uses the local-port configura‐
tion value. Should the DHCP Relay happen to be addressed as
127.0.0.1, however, the DHCP Server transmits its response to the
remote-port configuration value. This is generally only useful
for testing purposes, and this configuration value should gener‐
ally not be used.
The server-identifier statement
server-identifier hostname;
The server-identifier statement can be used to define the value
that is sent in the DHCP Server Identifier option for a given
scope. The value specified must be an IP address for the DHCP
server, and must be reachable by all clients served by a particu‐
lar scope.
The use of the server-identifier statement is not recommended -
the only reason to use it is to force a value other than the
default value to be sent on occasions where the default value
would be incorrect. The default value is the first IP address
associated with the physical network interface on which the
request arrived.
The usual case where the server-identifier statement needs to be
sent is when a physical interface has more than one IP address,
and the one being sent by default isn't appropriate for some or
all clients served by that interface. Another common case is when
an alias is defined for the purpose of having a consistent IP
address for the DHCP server, and it is desired that the clients
use this IP address when contacting the server.
Supplying a value for the dhcp-server-identifier option is equiva‐
lent to using the server-identifier statement.
The server-id-check statement
server-id-check flag;
The server-id-check statement is used to control whether or not a
server, participating in failover, verifies that the value of the
dhcp-server-identifier option in received DHCP REQUESTs match the
server's id before processing the request. Server id checking is
disabled by default. Setting this flag enables id checking and
thereafter the server will only process requests that match. Note
the flag setting should be consistent between failover partners.
Unless overridden by use of the server-identifier statement, the
value the server uses as its id will be the first IP address asso‐
ciated with the physical network interface on which the request
arrived.
In order to reduce runtime overhead the server only checks for a
server id option in the global and subnet scopes. Complicated
configurations may result in different server ids for this check
and when the server id for a reply packet is determined, which
would prohibit the server from responding.
The primary use for this option is when a client broadcasts a
request but requires that the response come from a specific
failover peer. An example of this would be when a client reboots
while its lease is still active - in this case both servers will
normally respond. Most of the time the client won't check the
server id and can use either of the responses. However if the
client does check the server id it may reject the response if it
came from the wrong peer. If the timing is such that the "wrong"
peer responds first most of the time the client may not get an
address for some time.
Care should be taken before enabling this option.
The server-duid statement
server-duid LLT [ hardware-type timestamp hardware-address ] ;
server-duid EN enterprise-number enterprise-identifier ;
server-duid LL [ hardware-type hardware-address ] ;
The server-duid statement configures the server DUID. You may pick
either LLT (link local address plus time), EN (enterprise), or LL
(link local).
If you choose LLT or LL, you may specify the exact contents of the
DUID. Otherwise the server will generate a DUID of the specified
type.
If you choose EN, you must include the enterprise number and the
enterprise-identifier.
If there is a server-duid statement in the lease file it will take
precedence over the server-duid statement from the config file and
a dhcp6.server-id option in the config file will override both.
The default server-duid type is LLT.
The server-name statement
server-name name ;
The server-name statement can be used to inform the client of the
name of the server from which it is booting. Name should be the
name that will be provided to the client.
The dhcpv6-set-tee-times statement
dhcpv6-set-tee-times flag;
The dhcpv6-set-tee-times statement enables setting T1 and T2 to
the values recommended in RFC 3315 (Section 22.4). When setting
T1 and T2, the server will use dhcp-renewal-time and dhcp-rebind‐
ing-time, respectively. A value of zero tells the client it may
choose its own value.
When those options are not defined then values will be set to zero
unless the global dhcpv6-set-tee-times is enabled. When this
option is enabled the times are calculated as recommended by RFC
3315, Section 22.4:
T1 will be set to 0.5 times the shortest preferred lifetime
in the reply. If the "shortest" preferred lifetime is
0xFFFFFFFF, T1 will set to 0xFFFFFFFF.
T2 will be set to 0.8 times the shortest preferred lifetime
in the reply. If the "shortest" preferred lifetime is
0xFFFFFFFF, T2 will set to 0xFFFFFFFF.
Keep in mind that given sufficiently small lease lifetimes, the
above calculations will result in the two values being equal. For
example, a 9 second lease lifetime would yield T1 = T2 = 4 sec‐
onds, which would cause clients to issue rebinds only. In such a
case it would likely be better to explicitly define the values.
Note that dhcpv6-set-tee-times is intended to be transitional and
will likely be removed in a future release. Once removed the
behavior will be to use the configured values when present or cal‐
culate them per the RFC. If you want zeros, define them as zeros.
The site-option-space statement
site-option-space name ;
The site-option-space statement can be used to determine from what
option space site-local options will be taken. This can be used
in much the same way as the vendor-option-space statement. Site-
local options in DHCP are those options whose numeric codes are
greater than 224. These options are intended for site-specific
uses, but are frequently used by vendors of embedded hardware that
contains DHCP clients. Because site-specific options are allo‐
cated on an ad hoc basis, it is quite possible that one vendor's
DHCP client might use the same option code that another vendor's
client uses, for different purposes. The site-option-space option
can be used to assign a different set of site-specific options for
each such vendor, using conditional evaluation (see dhcp-eval (5)
for details).
The stash-agent-options statement
stash-agent-options flag;
If the stash-agent-options parameter is true for a given client,
the server will record the relay agent information options sent
during the client's initial DHCPREQUEST message when the client
was in the SELECTING state and behave as if those options are
included in all subsequent DHCPREQUEST messages sent in the RENEW‐
ING state. This works around a problem with relay agent informa‐
tion options, which is that they usually not appear in DHCPREQUEST
messages sent by the client in the RENEWING state, because such
messages are unicast directly to the server and not sent through a
relay agent.
The update-conflict-detection statement
update-conflict-detection flag;
If the update-conflict-detection parameter is true, the server
will perform standard DHCID multiple-client, one-name conflict
detection. If the parameter has been set false, the server will
skip this check and instead simply tear down any previous bindings
to install the new binding without question. The default is true.
The update-optimization statement
update-optimization flag;
If the update-optimization parameter is false for a given client,
the server will attempt a DNS update for that client each time the
client renews its lease, rather than only attempting an update
when it appears to be necessary. This will allow the DNS to heal
from database inconsistencies more easily, but the cost is that
the DHCP server must do many more DNS updates. We recommend leav‐
ing this option enabled, which is the default. If this parameter
is not specified, or is true, the DHCP server will only update
when the client information changes, the client gets a different
lease, or the client's lease expires.
The update-static-leases statement
update-static-leases flag;
The update-static-leases flag, if enabled, causes the DHCP server
to do DNS updates for clients even if those clients are being
assigned their IP address using a fixed-address statement - that
is, the client is being given a static assignment. It is not rec‐
ommended because the DHCP server has no way to tell that the
update has been done, and therefore will not delete the record
when it is not in use. Also, the server must attempt the update
each time the client renews its lease, which could have a signifi‐
cant performance impact in environments that place heavy demands
on the DHCP server.
The use-host-decl-names statement
use-host-decl-names flag;
If the use-host-decl-names parameter is true in a given scope,
then for every host declaration within that scope, the name pro‐
vided for the host declaration will be supplied to the client as
its hostname. So, for example,
group {
use-host-decl-names on;
host joe {
hardware ethernet 08:00:2b:4c:29:32;
fixed-address joe.example.com;
}
}
is equivalent to
host joe {
hardware ethernet 08:00:2b:4c:29:32;
fixed-address joe.example.com;
option host-name "joe";
}
Additionally, enabling use-host-decl-names instructs the server to
use the host declaration name in the the forward DNS name, if no
other values are available. This value selection process is dis‐
cussed in more detail under DNS updates.
An option host-name statement within a host declaration will over‐
ride the use of the name in the host declaration.
It should be noted here that most DHCP clients completely ignore
the host-name option sent by the DHCP server, and there is no way
to configure them not to do this. So you generally have a choice
of either not having any hostname to client IP address mapping
that the client will recognize, or doing DNS updates. It is
beyond the scope of this document to describe how to make this
determination.
The use-lease-addr-for-default-route statement
use-lease-addr-for-default-route flag;
If the use-lease-addr-for-default-route parameter is true in a
given scope, then instead of sending the value specified in the
routers option (or sending no value at all), the IP address of the
lease being assigned is sent to the client. This supposedly
causes Win95 machines to ARP for all IP addresses, which can be
helpful if your router is configured for proxy ARP. The use of
this feature is not recommended, because it won't work for many
DHCP clients.
The vendor-option-space statement
vendor-option-space string;
The vendor-option-space parameter determines from what option
space vendor options are taken. The use of this configuration
parameter is illustrated in the dhcp-options(5) manual page, in
the VENDOR ENCAPSULATED OPTIONS section.
SETTING PARAMETER VALUES USING EXPRESSIONS
Sometimes it's helpful to be able to set the value of a DHCP server
parameter based on some value that the client has sent. To do this,
you can use expression evaluation. The dhcp-eval(5) manual page
describes how to write expressions. To assign the result of an evalua‐
tion to an option, define the option as follows:
my-parameter = expression ;
For example:
ddns-hostname = binary-to-ascii (16, 8, "-",
substring (hardware, 1, 6));
RESERVED LEASES
It's often useful to allocate a single address to a single client, in
approximate perpetuity. Host statements with fixed-address clauses
exist to a certain extent to serve this purpose, but because host
statements are intended to approximate ´static configuration´, they
suffer from not being referenced in a littany of other Server Services,
such as dynamic DNS, failover, ´on events´ and so forth.
If a standard dynamic lease, as from any range statement, is marked
´reserved´, then the server will only allocate this lease to the client
it is identified by (be that by client identifier or hardware address).
In practice, this means that the lease follows the normal state engine,
enters ACTIVE state when the client is bound to it, expires, or is
released, and any events or services that would normally be supplied
during these events are processed normally, as with any other dynamic
lease. The only difference is that failover servers treat reserved
leases as special when they enter the FREE or BACKUP states - each
server applies the lease into the state it may allocate from - and the
leases are not placed on the queue for allocation to other clients.
Instead they may only be ´found´ by client identity. The result is
that the lease is only offered to the returning client.
Care should probably be taken to ensure that the client only has one
lease within a given subnet that it is identified by.
Leases may be set ´reserved´ either through OMAPI, or through the
´infinite-is-reserved´ configuration option (if this is applicable to
your environment and mixture of clients).
It should also be noted that leases marked ´reserved´ are effectively
treated the same as leases marked ´bootp´.
REFERENCE: OPTION STATEMENTS
DHCP option statements are documented in the dhcp-options(5) manual
page.
REFERENCE: EXPRESSIONS
Expressions used in DHCP option statements and elsewhere are documented
in the dhcp-eval(5) manual page.
SEE ALSO
dhcpd(8), dhcpd.leases(5), dhcp-options(5), dhcp-eval(5), RFC2132, RFC2131.
AUTHOR
dhcpd.conf(5) is maintained by ISC. Information about Internet Systems Consortium can be found at https://www.isc.org.
dhcpd.conf(5)