Provided by: isc-dhcp-server_4.3.5-3ubuntu7.4_amd64 
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
parameters 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 subnet 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-subnet 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 dynamically 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, subnet 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 parameters 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 correspond 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 parameters
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 terminals 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 declarations 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 providing 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 incorrect 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 immediately allocated to the client. If the
address is available for allocation 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 particular 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 possible, 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 statements, 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 secondaries act, but most of the differences
simply have to do with providing 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 configured
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 contact with its partner
will have expired.
While the failed server is recovering, its partner remains in the partner-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 communicated with their
partner, they both come up in this recovery state and follow the procedure we have just
described. In this case, no service 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 references 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 segment. You must not tell one server it's doing failover on a particular
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 declaration 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 identifier. 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 seconds 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
connection 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 communicating. 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 secondary for the
purposes of load balancing. Whenever a client makes a DHCP request, the DHCP server
runs a hash on the client identification, 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 secondary 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 consequently 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 balancing 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 dangerous 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 saturation — 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 secondary 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 giving 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 difference 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 statement. This parameter configures what used to
be a 10% fixed value in previous versions: if lts is less than free+backup * max-lease-
misbalance 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, however, 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 system 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 multiplied 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 balance 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 scheduled 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, inclusive, 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 recommended to select a max-lease-ownership value that is lower than the value
selected for the max-lease-misbalance value. max-lease-ownership 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 depending 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 subclass is a class with
the same name as a regular class, but with a specific 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 constant value to use
in matching the match expression for the class. When class matching is done, the server
will evaluate the match expression 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 subclass.
Subclasses can be declared with or without scope. In the above example, 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 configuration
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
provide 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 matching 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 example:
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 compliant 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-supported
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 compatibility 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 material 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 following 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 declaration 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 accidentally 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 succeeds, 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 associated 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 finalized 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 addition 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 previously.
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 statement 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 currently 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 distribution 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 algorithms:
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 statement. 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 subnet. 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 netmask, 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 subnet.
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 omitted.
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) available 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 statements, 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 matching 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-identifier 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 keywords 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 control
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 dynamically 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.
Unfortunately, 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 allocations.
The declines flag tells the DHCP server whether or not to honor DHCPDECLINE 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 DHCPLEASEQUERY 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 packets.
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 DHCPDISCOVER 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 consists 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 abandoned 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 seconds (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 seconds. 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 therefore 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 Microsoft DHCP
client is known not to have this problem, as are the OpenTransport 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 configured 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 information 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 DHCPNAK 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 networks 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 others.
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 declarations 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 setting up the client's A
and PTR records. If no ddns-hostname is specified in scope, then the server will derive
the hostname automatically, 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 expiration 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 configuration 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 Transmission 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(). Suppressing 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 Universal 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
statement 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 multiple fixed-prefix6 statements may
appear in a single host declaration.
The low-address specifies the start of the prefix and the bits specifies 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 statement. hardware-type must be the name
of a physical hardware interface 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 statement 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 variable.
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 environment 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 networks; 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 compile-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 systems.
In addition to setting this value, you may need to modify your syslog.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 configuration 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
produced. 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 seconds. 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-allocatable 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 DHCPREQUEST, 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 minimum 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 responding 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
parameter 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 available. 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
configuration 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 generally 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
particular 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 equivalent 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 associated 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-rebinding-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 seconds, 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 calculate 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 allocated 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 RENEWING
state. This works around a problem with relay agent information 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 leaving 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 recommended 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 significant 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 provided 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 discussed in more detail under DNS updates.
An option host-name statement within a host declaration will override 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
evaluation 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)