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.\" dhcpd.conf.5
.\"
.\" Copyright (c) 1996-1999 Internet Software Consortium.
.\" Use is subject to license terms which appear in the file named
.\" ISC-LICENSE that should have accompanied this file when you
.\" received it. If a file named ISC-LICENSE did not accompany this
.\" file, or you are not sure the one you have is correct, you may
.\" obtain an applicable copy of the license at:
.\"
.\" http://www.isc.org/isc-license-1.0.html.
.\"
.\" This file is part of the ISC DHCP distribution. The documentation
.\" associated with this file is listed in the file DOCUMENTATION,
.\" included in the top-level directory of this release.
.\"
.\" Support and other services are available for ISC products - see
.\" http://www.isc.org for more information.
.TH dhcpd.conf 5
.SH NAME
dhcpd.conf - dhcpd configuration file
.SH DESCRIPTION
The dhcpd.conf file contains configuration information for
.IR dhcpd,
the Internet Software Consortium DHCP Server.
.PP
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.
.PP
The file essentially consists of a list of statements. Statements
fall into two broad categories - parameters and declarations.
.PP
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).
.PP
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.
.PP
Declarations about network topology include the
\fIshared-network\fR and the \fIsubnet\fR
declarations. If clients on a subnet are to be assigned addresses
dynamically, a \fIrange\fR declaration must appear within the
\fIsubnet\fR declaration. For clients with statically assigned
addresses, or for installations where only known clients will be
served, each such client must have a \fIhost\fR declaration. If
parameters are to be applied to a group of declarations which are not
related strictly on a per-subnet basis, the \fIgroup\fR declaration
can be used.
.PP
For every subnet which will be served, and for every subnet
to which the dhcp server is connected, there must be one \fIsubnet\fR
declaration, which tells dhcpd how to recognize that an address is on
that subnet. A \fIsubnet\fR declaration is required for each subnet
even if no addresses will be dynamically allocated on that subnet.
.PP
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 \fIsubnet\fR declarations for these two networks must be
enclosed in a \fIshared-network\fR declaration.
.PP
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 \fIhost\fR declarations, these
declarations can be enclosed in a \fIgroup\fR 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.
.PP
When a client is to be booted, its boot parameters are determined by
consulting that client's \fIhost\fR declaration (if any), and then
consulting the any \fIclass\fR declarations matching the client,
followed by the \fIpool\fR, \fIsubnet\fR and \fIshared-network\fR
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 as well. 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.
.PP
When dhcpd tries to find a \fIhost\fR declaration for a client, it
first looks for a \fIhost\fR declaration which has a
\fIfixed-address\fR parameter which matches the subnet or shared
network on which the client is booting. If it doesn't find any such
entry, it then tries to find an entry which has no \fIfixed-address\fR
parameter.
.SH EXAMPLES
.PP
A typical dhcpd.conf file will look something like this:
.nf
.I global parameters...
subnet 204.254.239.0 netmask 255.255.255.224 {
\fIsubnet-specific parameters...\fR
range 204.254.239.10 204.254.239.30;
}
subnet 204.254.239.32 netmask 255.255.255.224 {
\fIsubnet-specific parameters...\fR
range 204.254.239.42 204.254.239.62;
}
subnet 204.254.239.64 netmask 255.255.255.224 {
\fIsubnet-specific parameters...\fR
range 204.254.239.74 204.254.239.94;
}
group {
\fIgroup-specific parameters...\fR
host zappo.test.isc.org {
\fIhost-specific parameters...\fR
}
host beppo.test.isc.org {
\fIhost-specific parameters...\fR
}
host harpo.test.isc.org {
\fIhost-specific parameters...\fR
}
}
.ce 1
Figure 1
.fi
.PP
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:
.nf
option domain-name "isc.org";
option domain-name-servers ns1.isc.org, ns2.isc.org;
.ce 1
Figure 2
.fi
.PP
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.
.PP
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:
.nf
option routers 204.254.239.1;
.fi
.PP
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.
.PP
In Figure 1 there is also a \fIgroup\fR 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:
.nf
option domain-name "test.isc.org";
.fi
.PP
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:
.nf
max-lease-time 120;
default-lease-time 120;
.fi
.PP
You may have noticed that while some parameters start with the
\fIoption\fR keyword, some do not. Parameters starting with the
\fIoption\fR keyword correspond to actual DHCP options, while
parameters that do not start with the option keyword either control
the behaviour 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).
.PP
In Figure 1, each host had \fIhost-specific parameters\fR. These
could include such things as the \fIhostname\fR option, the name of a
file to upload (the \fIfilename parameter) and the address of the
server from which to upload the file (the \fInext-server\fR
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.
.PP
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 models. One way to do this would be to have host
declarations for each server and group them by model:
.nf
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; }
}
.fi
.SH ADDRESS POOLS
.PP
The
.B pool
declaration 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:
.PP
.nf
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;
}
}
.fi
.PP
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.
.PP
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
.I allow
or \fIdeny\fR keyword. If a pool has a permit list, then only those
clients that match specific entries on the permit list will be
elegible 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 elegible. 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.
.SH 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, 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.
.PP
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.
.PP
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
.I 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.
.PP
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.
.SH DHCP FAILOVER
This version of the ISC DHCP server supports the DHCP failover
protocol as documented in draft-ietf-dhc-failover-07.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.
.PP
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.
.PP
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.
.PP
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.
.PP
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
.I sure
that the other server will not restart automatically.
.PP
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.
.SH 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.
.PP
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 seperate 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:
.PP
.nf
failover peer "foo" {
primary;
address anthrax.rc.vix.com;
port 519;
peer address trantor.rc.vix.com;
peer port 520;
max-response-delay 60;
max-unacked-updates 10;
mclt 3600;
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;
load balance max seconds 3;
}
include "/etc/dhcpd.master";
.fi
.PP
The statements in the peer declaration are as follows:
.PP
.B The
.I primary
.B and
.I secondary
.B statements
.PP
[ \fBprimary\fR | \fBsecondary\fR ]
.PP
This determines whether the server is primary or secondary, as
described earlier under DHCP FAILOVER.
.PP
.B The
.I address
.B statement
.PP
.B address
.I address
.PP
The \fBaddress\fR statement declares the IP address 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.
.PP
.B The
.I peer address
.B statement
.PP
.B peer address
.I address
.PP
The \fBpeer address\fR statement declares the IP address to which the
server should connect to reach its failover peer for failover
messages.
.PP
.B The
.I port
.B statement
.PP
.B port
.I port-number
.PP
The \fBport\fR statement declares the TCP port on which the server
should listen for connections from its failover peer. This statement
may not currently be omitted, because the failover protocol does not
yet have a reserved TCP port number.
.PP
.B The
.I peer port
.B statement
.PP
.B peer port
.I port-number
.PP
The \fBpeer port\fR statement declares the TCP port to which the
server should connect to reach its failover peer for failover
messages. This statement may not be omitted because the failover
protocol does not yet have a reserved TCP port number. The port
number declared in the \fBpeer port\fR statement may be the same as
the port number declared in the \fBport\fR statement.
.PP
.B The
.I max-response-delay
.B statement
.PP
.nf
.B max-response-delay
.I seconds
.fi
.PP
The \fBmax-response-delay\fR 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.
.PP
.B The
.I max-unacked-updates
.B statement
.PP
.B max-unacked-updates
.I count
.PP
The \fBmax-unacked-updates\fR statement tells the DHCP server how
many many BINDUPD messages it can send before it receives a BNDACK
from the failover peer. 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.
.PP
.B The
.I mclt
.B statement
.PP
.B mclt
.I seconds
.PP
The \fBmclt\fR 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.
.PP
.B The
.I split
.B statement
.PP
.B split
.I index
.PP
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. If the hash comes out to less than the split value,
the primary answers. If it comes out to equal to or more than the
split, the secondary answers. This value should generally be set to
128, and can only be configured on the primary.
.PP
.B The
.I hba
.B statement
.PP
.B hba
.I colon-seperated-hex-list
.PP
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:
.PP
.nf
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;
.fi
.PP
.B The
.I load balance max seconds
.B statement
.PP
.B load balance max seconds
.I seconds
.PP
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 \fIsecs\fR
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.
.SH CLIENT CLASSING
Clients can be seperated into classes, and treated differently
depending on what class they are in. This seperation 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.
.PP
To add clients to classes based on conditional evaluation, you would
write an conditional statement to match the clients you wanted in the
class, and then put an
.B add
statement in the conditional's list of statements:
.PP
.nf
if substring (option dhcp-client-identifier, 0, 3) = "RAS" {
add "ras-clients";
}
.fi
.PP
A nearly equivalent way to do this is to simply specify the conditional
expression as a matching expression in the class statement:
.PP
.nf
class "ras-clients" {
match if substring (option dhcp-client-identifier, 0, 3) = "RAS";
}
.fi
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:
.nf
class "ras-clients" {
}
.fi
.PP
Also, the
.B add
statement adds the client to the class as the client's scopes are being
evaluated - after any address assignment decision has been made. This means
that a client that's a member of a class due to an add statement will not
be affected by pool permits related to that class - when the pool permit list
is computed, the client will not yet be a member of the pool. This is an
inconsistency that will probably be addressed in later versions of the DHCP
server, but it important to be aware of it at lease for the time being.
.SH SUBCLASSES
.PP
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:
.PP
.nf
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;
}
}
.fi
.PP
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.
.PP
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.
.PP
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:
.PP
.nf
subclass "allocation-class-2" 1:08:00:2b:a1:11:31 {
option root-path "samsara:/var/diskless/alphapc";
filename "/tftpboot/netbsd.alphapc-diskless";
}
.fi
.PP
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 later on
in this document.
.SH PER-CLASS LIMITS ON DYNAMIC ADDRESS ALLOCATION
.PP
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:
.PP
.nf
class "limited-1" {
lease limit 4;
}
.fi
.PP
This will produce a class in which a maximum of four members may hold
a lease at one time.
.SH SPAWNING CLASSES
.PP
It is possible to declare a
.I spawning class\fR.
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.
.PP
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:
.PP
.nf
class "customer" {
spawn with option agent.circuit-id;
lease limit 4;
}
.fi
.PP
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
.B 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.
.PP
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.
.SH COMBINING MATCH, MATCH IF AND SPAWN WITH
.PP
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 \fBmatch
if\fR and \fBspawn with\fR statements, or the \fBmatch if\fR and
\fBmatch\fR statements. For example:
.PP
.nf
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 opton dhcp-vendor-identifier = "dvdsl";
spawn with option agent.circuit-id;
lease limit 16;
}
.fi
.PP
This allows you to have two classes that both have the same \fBspawn
with\fR expression without getting the clients in the two classes
confused with each other.
.SH DYNAMIC DNS UPDATES
.PP
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.
.PP
The Dynamic DNS update scheme implemented in this version of the ISC
DHCP server is an interim implementation, which does not implement any
of the standard update methods that have been discussed in the working
group, but rather implements some very basic, yet useful, update
capabilities.
.PP
There are three parameters, which may vary according to the scope,
that control how DDNS updates will be done. The first two are the
.I ddns-domainname
and
.I ddns-rev-domainname
statements. The
.I ddns-domainname
parameter sets the domain name that will be appended to the client's
hostname to form a fully-qualified domain-name (FQDN). For example,
if the client's hostname is "hutson" and the
.I ddns-domainname
is set to "sneedville.edu", then the client's FQDN will be
"hutson.sneedville.edu".
.PP
The
.I ddns-rev-domainname
parameter sets 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. Normally, you would set this to "in-addr.arpa", but this is
not required.
.PP
A third parameter,
.I ddns-hostname
can be used to specify the hostname that will be used as the client's
hostname. If no ddns-hostname is specified in scope, then the server
will use a host-name option sent by the client. If the client did
not send a host-name option, then if there is a host declaration that
applies to the client, the name from that declaration will be used.
If none of these applies, the server will not have a hostname for the
client, and will not be able to do a DDNS update.
.SH HOW DNS UPDATES WORK
.PP
The client's FQDN, derived as we have described, is used as the name
on which an "A" record will be stored. The A record will contain the
IP address that the client was assigned in its lease. If there is
already an A record with the same name in the DNS server, no update of
either the A or PTR records will occur - this prevents a client from
claiming that its hostname is the name of some network server. For
example, if you have a fileserver called "fs.sneedville.edu", and the
client claims its hostname is "fs", no DNS update will be done for
that client, and an error message will be logged.
.PP
If the A record update succeeds, a PTR record update for the assigned
IP address will be done, pointing to the A record. This update is
unconditional - it will be done even if another PTR record of the same
name exists. Since the IP address has been assigned to the DHCP
server, this should be safe.
.PP
Please note that the current implementation assumes clients only have
a single network interface. A client with two network interfaces
will see unpredictable behaviour. This is considered a bug, and will
be fixed in a later release. It may be helpful to enable the
.I one-lease-per-client
parameter so that roaming clients do not trigger this same behavior.
.PP
The DHCP protocol normally involves a four-packet exchange - first the
client sends a DHCPDISCOVER message, then the server sends a
DHCPOFFER, then the client sends a DHCPREQUEST, then the server sends
a DHCPACK. In the current version of the server, the server will do
a DNS update after it has received the DHCPREQUEST, and before it has
sent the DHCPOFFER. It only sends the DNS update if it has not sent
one for the client's address before, in order to minimize the impact
on the DHCP server.
.PP
When the client's lease expires, the DHCP server (if it is operating
at the time, or when next it operates) will remove the client's A and
PTR records from the DNS database. If the client releases its lease
by sending a DHCPRELEASE message, the server will likewise remove the
A and PTR records.
.SH DYNAMIC DNS UPDATE SECURITY
.PP
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.
.PP
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:
.PP
.nf
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; };
};
.fi
.PP
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:
.PP
.nf
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;
}
.fi
.PP
You should choose your own secret key, of course. The ISC BIND 8 and
9 distributions come with a program for generating secret keys called
dnskeygen. The version that comes with BIND 9 is likely to produce a
substantially more random key, so we recommend you use that one even
if you are not using BIND 9 as your DNS server. The key above was
generated with the command:
.nf
dnskeygen -H 128 -u -c -n DHCP_UPDATER
.fi
.PP
You may wish to enable logging of DNS transactions on your DNS server.
To do so, you might write a logging statement like the following:
.PP
.nf
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; };
};
.fi
.PP
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.
.SH REFERENCE: EVENTS
.PP
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.
.PP
To declare a set of statements to execute when an event happens, you
must use the \fBon\fR statement, followed by the name of the event,
followed by a series of statements to execute when the event happens,
enclosed in braces. Events are used to implement dynamic DNS
updates, so you should not define your own event handlers if you are
using the built-in dynamic DNS update mechanism.
.PP
The built-in version of the dynamic DNS update mechanism is in a text
string towards the top of server/dhcpd.c. If you want to use events
for things other than DNS updates, and you also want DNS updates, you
will have to start out by copying this code into your dhcpd.conf file
and modifying it.
.SH REFERENCE: DECLARATIONS
.PP
.B The
.I shared-network
.B statement
.PP
.nf
\fBshared-network\fR \fIname\fR \fB{\fR
[ \fIparameters\fR ]
[ \fIdeclarations\fR ]
\fB}\fR
.fi
.PP
The \fIshared-network\fR 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
\fIshared-network\fR statement. Parameters specified in the
\fIshared-network\fR 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.
.PP
.I 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.
.PP
.B The
.I subnet
.B statement
.PP
.nf
\fBsubnet\fR \fIsubnet-number\fR \fBnetmask\fR \fInetmask\fR \fB{\fR
[ \fIparameters\fR ]
[ \fIdeclarations\fR ]
\fB}\fR
.fi
.PP
The \fIsubnet\fR 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 \fIrange\fR
declaration.
.PP
The
.I subnet-number
should be an IP address or domain name which resolves to the subnet
number of the subnet being described. The
.I 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.
.PP
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.
.PP
.B The
.I range
.B statement
.PP
.nf
.B range\fR [ \fBdynamic-bootp\fR ] \fIlow-address\fR [ \fIhigh-address\fR]\fB;\fR
.fi
.PP
For any subnet on which addresses will be assigned dynamically, there
must be at least one \fIrange\fR 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
\fIrange\fR statement is declared. The \fIdynamic-bootp\fR 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, \fIhigh-address\fR can be omitted.
.PP
.B The
.I host
.B statement
.PP
.nf
\fBhost\fR \fIhostname\fR {
[ \fIparameters\fR ]
[ \fIdeclarations\fR ]
\fB}\fR
.fi
.PP
There must be at least one
.B host
statement for every BOOTP client that is to be served.
.B host
statements may also be specified for DHCP clients, although this is
not required unless booting is only enabled for known hosts.
.PP
If it is desirable to be able to boot a DHCP or BOOTP
client on more than one subnet with fixed addresses, more than one
address may be specified in the
.I fixed-address
parameter, or more than one
.B host
statement may be specified.
.PP
If client-specific boot parameters must change based on the network
to which the client is attached, then multiple
.B host
statements should
be used.
.PP
If a client is to be booted using a fixed address if it's
possible, but should be allocated a dynamic address otherwise, then a
.B host
statement must be specified without a
.B fixed-address
clause.
.I hostname
should be a name identifying the host. If a \fIhostname\fR option is
not specified for the host, \fIhostname\fR is used.
.PP
\fIHost\fR declarations are matched to actual DHCP or BOOTP clients
by matching the \fRdhcp-client-identifier\fR option specified in the
\fIhost\fR declaration to the one supplied by the client, or, if the
\fIhost\fR declaration or the client does not provide a
\fRdhcp-client-identifier\fR option, by matching the \fIhardware\fR
parameter in the \fIhost\fR declaration to the network hardware
address supplied by the client. BOOTP clients do not normally
provide a \fIdhcp-client-identifier\fR, so the hardware address must
be used for all clients that may boot using the BOOTP protocol.
.PP
.B The
.I group
.B statement
.PP
.nf
\fBgroup\fR {
[ \fIparameters\fR ]
[ \fIdeclarations\fR ]
\fB}\fR
.fi
.PP
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.
.SH REFERENCE: ALLOW AND DENY
The
.I allow
and
.I 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
behaviour with respect to clients based on scope. In a non-pool
context, the
.I ignore
keyword can be used in place of the
.I deny
keyword to prevent logging of denied requests.
.PP
.SH 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.
.PP
.B The
.I unknown-clients
.B keyword
.PP
\fBallow unknown-clients;\fR
\fBdeny unknown-clients;\fR
\fBignore unknown-clients;\fR
.PP
The \fBunknown-clients\fR flag is used to tell dhcpd whether
or not to dynamically assign addresses to unknown clients. Dynamic
address assignment to unknown clients is \fBallow\fRed by default.
.PP
.B The
.I bootp
.B keyword
.PP
\fBallow bootp;\fR
\fBdeny bootp;\fR
\fBignore bootp;\fR
.PP
The \fBbootp\fR flag is used to tell dhcpd whether
or not to respond to bootp queries. Bootp queries are \fBallow\fRed
by default.
.PP
.B The
.I booting
.B keyword
.PP
\fBallow booting;\fR
\fBdeny booting;\fR
\fBignore booting;\fR
.PP
The \fBbooting\fR 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
\fBallow\fRed, but if it is disabled for a particular client, then
that client will not be able to get and address from the DHCP server.
.B The
.I duplicates
.B keyword
.PP
\fBallow duplicates;\fR
\fBdeny duplicates;\fR
.PP
Host declarations can match client messages based on the DHCP Client
Identifer 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.
.PP
The \fBduplicates\fR 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 \fBallow\fRed.
.B The
.I declines
.B keyword
.PP
\fBallow declines;\fR
\fBdeny declines;\fR
\fBignore declines;\fR
.PP
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 reclaim these leases, but while the client is running
through the pool, it may cause serious thrashing in the DNS, and it
will also cause the DHCP server to forget old DHCP client address
allocations.
.PP
The \fBdeclines\fR flag tells the DHCP server whether or not to honor
DHCPDECLINE messages. If it is set to \fBdeny\fR or \fBignore\fR in
a particular scope, the DHCP server will not respond to DHCPDECLINE
messages.
.SH ALLOW AND DENY WITHIN POOL DECLARATIONS
.PP
The uses of the allow and deny keyword 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.
.PP
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.
.PP
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.
.PP
When declaring permit lists for address allocation pools, the
following syntaxes are recognized following the allow or deny keyword:
.PP
\fBknown clients;\fR
.PP
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 \fIany\fR scope, not
just the current scope.
.PP
\fBunknown clients;\fR
.PP
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).
.PP
\fBmembers of "\fRclass\fB";\fR
.PP
If specified, this statement either allows or prevents allocation from
this pool to any client that is a member of the named class.
.PP
\fBdynamic bootp clients;\fR
.PP
If specified, this statement either allows or prevents allocation from
this pool to any bootp client.
.PP
\fBauthenticated clients;\fR
.PP
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.
.PP
\fBunauthenticated clients;\fR
.PP
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.
.PP
\fBall clients;\fR
.PP
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.
.SH REFERENCE: PARAMETERS
.PP
.B The
.I lease-file-name
.B statement
.PP
.B lease-file-name
.I name\fR\fB;\fR
.PP
.I Name
should be the name of the DHCP server's lease file. By default, this
is /var/db/dhcpd.leases. This statement \fBmust\fR appear in the outer
scope of the configuration file - if it appears in some other scope,
it will have no effect.
.PP
.B The
.I pid-file-name
.B statement
.PP
.B pid-file-name
.I name\fR\fB;\fR
.PP
.I 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.
.PP
.B The
.I default-lease-time
.B statement
.PP
\fBdefault-lease-time\fR \fItime\fR\fB;\fR
.PP
.I 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.
.PP
.B The
.I max-lease-time
.B statement
.PP
\fBmax-lease-time\fR \fItime\fR\fB;\fR
.PP
.I Time
should be the maximum length in seconds that will be assigned to a
lease. The only exception to this is that Dynamic BOOTP lease
lengths, which are not specified by the client, are not limited by
this maximum.
.PP
.B The
.I min-lease-time
.B statement
.PP
\fBmin-lease-time\fR \fItime\fR\fB;\fR
.PP
.I Time
should be the minimum length in seconds that will be assigned to a
lease.
.PP
.B The
.I min-secs
.B statement
.PP
\fBmin-secs\fR \fIseconds\fR\fB;\fR
.PP
.I 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.
.PP
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.
.PP
.B The
.I hardware
.B statement
.PP
\fBhardware\fR \fIhardware-type\fR \fIhardware-address\fR\fB;\fR
.PP
In order for a BOOTP client to be recognized, its network hardware
address must be declared using a \fIhardware\fR clause in the
.I host
statement.
.I hardware-type
must be the name of a physical hardware interface type. Currently,
only the
.B ethernet
and
.B token-ring
types are recognized, although support for a
.B fddi
hardware type (and others) would also be desirable.
The
.I hardware-address
should be a set of hexadecimal octets (numbers from 0 through ff)
seperated by colons. The \fIhardware\fR statement may also be used
for DHCP clients.
.PP
.B The
.I filename
.B statement
.PP
\fBfilename\fR \fB"\fR\fIfilename\fR\fB";\fR
.PP
The \fIfilename\fR statement can be used to specify the name of the
initial boot file which is to be loaded by a client. The
.I filename
should be a filename recognizable to whatever file transfer protocol
the client can be expected to use to load the file.
.PP
.B The
.I server-name
.B statement
.PP
\fBserver-name\fR \fB"\fR\fIname\fR\fB";\fR
.PP
The \fIserver-name\fR statement can be used to inform the client of
the name of the server from which it is booting. \fIName\fR should
be the name that will be provided to the client.
.PP
.B The
.I next-server
.B statement
.PP
\fBnext-server\fR \fIserver-name\fR\fB;\fR
.PP
The \fInext-server\fR statement is used to specify the host address of
the server from which the initial boot file (specified in the
\fIfilename\fR statement) is to be loaded. \fIServer-name\fR should
be a numeric IP address or a domain name. If no \fInext-server\fR
parameter applies to a given client, the DHCP server's IP address is
used.
.PP
.B The
.I fixed-address
.B statement
.PP
\fBfixed-address\fR \fIaddress\fR [\fB,\fR \fIaddress\fR ... ]\fB;\fR
.PP
The \fIfixed-address\fR statement is used to assign one or more fixed
IP addresses to a client. It should only appear in a \fIhost\fR
declaration. If more than one address is supplied, then when the
client boots, it will be assigned the address which corresponds to the
network on which it is booting. If none of the addresses in the
\fIfixed-address\fR statement are on the network on which the client
is booting, that client will not match the \fIhost\fR declaration
containing that \fIfixed-address\fR statement. Each \fIaddress\fR
should be either an IP address or a domain name which resolves to one
or more IP addresses.
.PP
.B The
.I dynamic-bootp-lease-cutoff
.B statement
.PP
\fBdynamic-bootp-lease-cutoff\fR \fIdate\fR\fB;\fR
.PP
The \fIdynamic-bootp-lease-cutoff\fR 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 assignes 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.
.PP
.I Date
should be the date on which all assigned BOOTP leases will end. The
date is specified in the form:
.PP
.ce 1
W YYYY/MM/DD HH:MM:SS
.PP
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
Universal Coordinated Time (UTC), not local time.
.PP
.B The
.I dynamic-bootp-lease-length
.B statement
.PP
\fBdynamic-bootp-lease-length\fR \fIlength\fR\fB;\fR
.PP
The \fIdynamic-bootp-lease-length\fR 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 \fIlength\fR as a
number of seconds. If a client reboots using BOOTP during the
timeout period, the lease duration is reset to \fIlength\fR, so a
BOOTP client that boots frequently enough will never lose its lease.
Needless to say, this parameter should be adjusted with extreme
caution.
.PP
.B The
.I get-lease-hostnames
.B statement
.PP
\fBget-lease-hostnames\fR \fIflag\fR\fB;\fR
.PP
The \fIget-lease-hostnames\fR 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
\fIhostname\fR option. If \fIflag\fR is true, then this lookup is
done for all addresses in the current scope. By default, or if
\fIflag\fR is false, no lookups are done.
.PP
.B The
.I use-host-decl-names
.B statement
.PP
\fBuse-host-decl-names\fR \fIflag\fR\fB;\fR
.PP
If the \fIuse-host-decl-names\fR 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,
.PP
.nf
group {
use-host-decl-names on;
host joe {
hardware ethernet 08:00:2b:4c:29:32;
fixed-address joe.fugue.com;
}
}
is equivalent to
host joe {
hardware ethernet 08:00:2b:4c:29:32;
fixed-address joe.fugue.com;
option host-name "joe";
}
.fi
.PP
An \fIoption host-name\fR statement within a host declaration will
override the use of the name in the host declaration.
.PP
.B The
.I authoritative
.B statement
.PP
\fBauthoritative;\fR
.PP
\fBnot authoritative;\fR
.PP
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.
.PP
Network administrators setting up authoritative DHCP servers for their
networks should always write \fBauthoritative;\fR at the top of their
configuration file to indicate that the DHCP server \fIshould\fR 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.
.PP
Usually, writing \fBauthoritative;\fR 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.
.PP
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.
.PP
.B The
.I always-reply-rfc1048
.B statement
.PP
\fBalways-reply-rfc1048\fR \fIflag\fR\fB;\fR
.PP
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.
.PP
If you want to send rfc1048 options to such a client, you can set the
.B 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.
.PP
.B The
.I always-broadcast
.B statement
.PP
\fBalways-broadcast\fR \fIflag\fR\fB;\fR
.PP
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. 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.
.PP
.B The
.I one-lease-per-client
.B statement
.PP
\fBone-lease-per-client\fR \fIflag\fR\fB;\fR
.PP
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
.I 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.
.PP
.B The
.I use-lease-addr-for-default-route
.B statement
.PP
\fBuse-lease-addr-for-default-route\fR \fIflag\fR\fB;\fR
.PP
If the \fIuse-lease-addr-for-default-route\fR 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.
.PP
.B The
.I server-identifier
.B statement
.PP
\fBserver-identifier \fIhostname\fR\fB;\fR
.PP
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 \fBmust\fR be an IP address for the DHCP server, and
must be reachable by all clients served by a particular scope.
.PP
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.
.PP
The usual case where the
\fIserver-identifier\fR 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.
.PP
Supplying a value for the dhcp-server-identifier option is equivalent
to using the server-identifier statement.
.PP
.B The
.I ddns-updates
.B statement
.PP
\fBddns-updates \fIflag\fR\fB;\fR
.PP
The \fIddns-updates\fR parameter controls whether or not the server will
attempt to do a ddns update when a lease is confirmed. Set this to \fIoff\fR
if the server should not attempt to do updates within a certain scope.
The \fIddns-updates\fR parameter is on by default.
.SH 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
.B 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:
.nf
.sp 1
\fImy-parameter \fB= \fIexpression \fB;\fR
.fi
.PP
For example:
.nf
.sp 1
ddns-hostname = binary-to-ascii (16, 8, "-",
substring (hardware, 1, 6));
.fi
.SH REFERENCE: OPTION STATEMENTS
.PP
DHCP option statements are documented in the
.B dhcp-options(5)
manual page.
.SH SEE ALSO
dhcpd(8), dhcpd.leases(5), RFC2132, RFC2131.
.SH AUTHOR
.B dhcpd(8)
was written by Ted Lemon <mellon@vix.com>
under a contract with Vixie Labs. Funding
for this project was provided by the Internet Software Consortium.
Information about the Internet Software Consortium can be found at
.B http://www.isc.org/isc.
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