path: root/server/dhcpd.conf.5

.\"	dhcpd.conf.5
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.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 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.   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 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).
.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 model.   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
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.
.SH 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.
.PP
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.
.PP
There may be a host declaration matching the client's identification,
and that host declaration contains a fixed-address declaration 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 \fIrequired\fB to take the address
specified in the host declaration.   If the client is requesting some
other address, the server will respond with a DHCPNAK.
.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 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.
.PP
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.
.PP
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.
.PP
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.
.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
You can put the server into the PARTNER-DOWN state either by using the
.B omshell (8)
command or by stopping the server, editing the last peer state
declaration in the lease file, and restarting the server.   If you use
this last method, be sure to leave the date and time of the start of
the state blank:
.PP
.nf
.B failover peer "\fIname\fB" state {
.B   my   state partner-down;
.B   peer state \fIstate\fB at \fIdate\fB;
.B }
.fi
.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
The 
.I primary
and
.I secondary
statements
.RS 0.25i
.PP
[ \fBprimary\fR | \fBsecondary\fR ]\fB;\fR
.PP
This determines whether the server is primary or secondary, as
described earlier under DHCP FAILOVER.
.RE
.PP
The 
.I address
statement
.RS 0.25i
.PP
.B address \fIaddress\fR\fB;\fR
.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.
.RE
.PP
The 
.I peer address
statement
.RS 0.25i
.PP
.B peer address \fIaddress\fR\fB;\fR
.PP
The \fBpeer address\fR statement declares the IP address to which the
server should connect to reach its failover peer for failover
messages.
.RE
.PP
The 
.I port
statement
.RS 0.25i
.PP
.B port \fIport-number\fR\fB;\fR
.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.
.RE
.PP
The 
.I peer port
statement
.RS 0.25i
.PP
.B peer port \fIport-number\fR\fB;\fR
.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.
.RE
.PP
The 
.I max-response-delay
statement
.RS 0.25i
.PP
.B max-response-delay \fIseconds\fR\fB;\fR
.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.
.RE
.PP
The 
.I max-unacked-updates
statement
.RS 0.25i
.PP
.B max-unacked-updates \fIcount\fR\fB;\fR
.PP
The \fBmax-unacked-updates\fR statement tells the DHCP server how
many BNDUPD 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.
.RE
.PP
The 
.I mclt
statement
.RS 0.25i
.PP
.B mclt \fIseconds\fR\fB;\fR
.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.
.RE
.PP
The 
.I split
statement
.RS 0.25i
.PP
.B split \fIindex\fR\fB;\fR
.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.
.RE
.PP
The 
.I hba
statement
.RS 0.25i
.PP
.B hba \fIcolon-seperated-hex-list\fB;\fR
.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
.RE
.PP
The 
.I load balance max seconds
statement
.RS 0.25i
.PP
.B load balance max seconds \fIseconds\fR\fB;\fR
.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.
.RE
.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 a 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, 1, 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, 1, 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 is important to be aware of it at least 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 in the
.B dhcp-options(5)
manual page.
.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
Two DNS update schemes are currently implemented, and another is
planned.   The two that are currently available are the ad-hoc DNS
update mode and the interim DHCP-DNS interaction draft update mode.
If and when the DHCP-DNS interaction draft and the DHCID draft make it
through the IETF standards process, there will be a third mode, which
will be the standard DNS update method.   The DHCP server must be
configured to use one of the two currently-supported methods, or not
to do dns updates.   This can be done with the
.I ddns-update-style
configuration parameter.
.SH THE AD-HOC DNS UPDATE SCHEME
The ad-hoc Dynamic DNS update scheme implemented in this version of
the ISC DHCP server is a prototype design, which does not
have much to do with the standard update method that is being
standardized in the IETF DHC working group, but rather implements some
very basic, yet useful, update capabilities.   This mode
.B does not work
with the
.I failover protocol
because it does not account for the possibility of two different DHCP
servers updating the same set of DNS records.
.PP
For the ad-hoc DNS update method, the client's FQDN is derived in two
parts.   First, the hostname is determined.   Then, the domain name is
determined, and appended to the hostname.
.PP
The DHCP server determines the client's hostname by first looking for
a \fIddns-hostname\fR configuration option, and using that if it is
present.  If no such option is present, the server looks for a
valid hostname in the FQDN option sent by the client.  If one is
found, it is used; otherwise, if the client sent a host-name option,
that is used.  Otherwise, 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 DNS update.
.PP
The domain name is determined based strictly on the server
configuration, not on what the client sends.   First, if there is a 
.I ddns-domainname
configuration option, it is used.   Second, if there is a
\fIdomain-name\fR option configured, that is used.  Otherwise, the
server will not do the DNS update.
.PP
The client's fully-qualified domain name, 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 behavior.   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 THE INTERIM DNS UPDATE SCHEME
The interim DNS update scheme operates mostly according to several
drafts that are being considered by the IETF and are expected to
become standards, but are not yet standards, and may not be
standardized exactly as currently proposed.   These are:
.PP
.nf
.ce 3
draft-ietf-dhc-ddns-resolution-??.txt
draft-ietf-dhc-fqdn-option-??.txt
draft-ietf-dnsext-dhcid-rr-??.txt
.fi
.PP
Because our implementation is slightly different than the standard, we
will briefly document the operation of this update style here.
.PP
The first point to understand about this style of DNS update is that
unlike the ad-hoc style, 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 \fIallow client-updates;\fR or
the statement \fIignore client-updates;\fR.   By default, client
updates are allowed.
.PP
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.
.PP
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, possibly using the hostname supplied by
the client ("jschmoe" in the previous example).   It will use its own
domain name for the client, just as in the ad-hoc update scheme.
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".
.PP
The other difference between the ad-hoc scheme and the interim
scheme is that with the interim scheme, a method is used that
allows 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.   The scheme works as follows:
.PP
When the DHCP server issues a client a new lease, it creates a text
string that is an MD5 hash over the DHCP client's identification (see
draft-ietf-dnsext-dhcid-rr-??.txt for details).   The update adds an A
record with the name the server chose and a TXT record containing the
hashed identifier string (hashid).   If this update succeeds, the
server is done.
.PP
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 TXT record in the same name as the new A record, and that
TXT 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.
.PP
The interim DNS update scheme is called interim for two reasons.
First, it does not quite follow the drafts.   The current versions of
the drafts call for a new DHCID RRtype, but this is not yet
available.   The interim DNS update scheme uses a TXT record
instead.   Also, the existing ddns-resolution draft calls for the DHCP
server to put a DHCID RR on the PTR record, but the \fIinterim\fR
update method does not do this.   It is our position that this is not
useful, and we are working with the author in hopes of removing it
from the next version of the draft, or better understanding why it is
considered useful.
.PP
In addition to these differences, 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.
.PP
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.
.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
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 subdoman "foo.example.org" with no seperate
SOA, you could not write a zone declaration for "foo.example.org."
.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 updates 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 DNS
updates, so you should not define your own event handlers if you are
using the built-in DNS update mechanism.
.PP
The built-in version of the 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
behavior 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.
An unknown client is simply a client that has no host declaration.
.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.
.B The
.I client-updates
.B keyword
.PP
 \fBallow client-updates;\fR
 \fBdeny client-updates;\fR
.PP
The \fBclient-updates\fR flag tells the DHCP server whether or not to
honor the client's intention to do its own update of its A record.
This is only relevant when doing \fIinterim\fR DNS updates.   See the
documentation under the heading THE INTERIM DNS UPDATE SCHEME for
details.
.SH ALLOW AND DENY WITHIN POOL DECLARATIONS
.PP
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.
.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 keywords:
.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
The
.I always-broadcast
statement
.RS 0.25i
.PP
.B always-broadcast \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.
.RE
.PP
The
.I always-reply-rfc1048
statement
.RS 0.25i
.PP
.B always-reply-rfc1048 \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.
.RE
.PP
The
.I authoritative
statement
.RS 0.25i
.PP
.B authoritative;
.PP
.B not authoritative;
.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.
.RE
.PP
The \fIboot-unknown-clients\fR statement
.RS 0.25i
.PP
.B boot-unknown-clients \fIname\fB;\fR
.PP
If the \fIboot-unknown-clients\fR statement is present and has a value
of \fIfalse\fR or \fIoff\fR, then clients for which there is no
.I host
declaration will not be allowed to obtain IP addresses.   If this
statement is not present or has a value of \fItrue\fR or \fIon\fR,
then clients without host declarations will be allowed to obtain IP
addresses, as long as those addresses are not restricted by
.I allow
and \fIdeny\fR statements within their \fIpool\fR declarations.
.RE
.PP
The \fIddns-hostname\fR statement
.RS 0.25i
.PP
.B ddns-hostname \fIname\fB;\fR
.PP
The \fIname\fR 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.
.RE
.PP
The \fIddns-domainname\fR statement
.RS 0.25i
.PP
.B ddns-domainname \fIname\fB;\fR
.PP
The \fIname\fR parameter should be the domain name that will be
appended to the client's hostname to form a fully-qualified
domain-name (FQDN).
.RE
.PP
The \fIddns-rev-domainname\fR statement
.RS 0.25i
.PP
.B ddns-rev-domainname \fIname\fB;\fR
The \fIname\fR 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.
.PP
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.
.RE
.PP
The \fIddns-update-style\fR parameter
.RS 0.25i
.PP
.B ddns-update-style \fIstyle\fB;\fR
.PP
The
.I style
parameter must be one of \fBad-hoc\fR, \fBinterim\fR or \fBnone\fR.
The \fIddns-update-style\fR 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.
.RE
.PP
.B The  
.I ddns-updates
.B statement
.RS 0.25i
.PP
 \fBddns-updates \fIflag\fR\fB;\fR
.PP
The \fIddns-updates\fR parameter controls whether or not the server will
attempt to do a DNS 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.   To disable DNS
updates in all scopes, it is preferable to use the
\fIddns-update-style\fR statement, setting the style to \fInone\fR.
.RE
.PP
The
.I default-lease-time
statement
.RS 0.25i
.PP
.B default-lease-time \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.
.RE
.PP
The
.I dynamic-bootp-lease-cutoff
statement
.RS 0.25i
.PP
.B dynamic-bootp-lease-cutoff \fIdate\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
Coordinated Universal Time (UTC), not local time.
.RE
.PP
The
.I dynamic-bootp-lease-length
statement
.RS 0.25i
.PP
.B dynamic-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.
.RE
.PP
The
.I filename
statement
.RS 0.25i
.PP
.B filename\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.
.RE
.PP
The
.I fixed-address
statement
.RS 0.25i
.PP
.B fixed-address address\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.
.RE
.PP
The
.I get-lease-hostnames
statement
.RS 0.25i
.PP
.B get-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.
.RE
.PP
The 
.I hardware
statement
.RS 0.25i
.PP
.B hardware \fIhardware-type hardware-address\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.
.RE
.PP
The
.I lease-file-name
statement
.RS 0.25i
.PP
.B lease-file-name \fIname\fB;\fR
.PP
.I Name
should be the name of the DHCP server's lease file.   By default, this
is DBDIR/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.
.RE
.PP
The
.I local-port
statement
.RS 0.25i
.PP
.B local-port \fIport\fB;\fR
.PP
This statement causes the DHCP server to listen for DHCP requests on
the UDP port specified in \fIport\fR, rather than on port 67.
.RE
.PP
The
.I log-facility
statement
.RS 0.25i
.PP
.B log-facility \fIfacility\fB;\fR
.PP
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.
.PP
In addition to setting this value, you may need to modify your
.I syslog.conf
file to configure logging of the DHCP server.   For example, you might
add a line like this:
.PP
.nf
	local7.debug /var/log/dhcpd.log
.fi
.PP
The syntax of the \fIsyslog.conf\fR file may be different on some
operating systems - consult the \fIsyslog.conf\fR 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.
.PP
Because the \fIlog-facility\fR 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
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.
.RE
.PP
The
.I max-lease-time
statement
.RS 0.25i
.PP
.B max-lease-time \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.
.RE
.PP
The
.I min-lease-time
statement
.RS 0.25i
.PP
.B min-lease-time \fItime\fR\fB;\fR
.PP
.I Time
should be the minimum length in seconds that will be assigned to a
lease.
.RE
.PP
The
.I min-secs
statement
.RS 0.25i
.PP
.B min-secs \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.
.RE
.PP
The
.I next-server
statement
.RS 0.25i
.PP
.B next-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.
.RE
.PP
The
.I omapi-port
statement
.RS 0.25i
.PP
.B omapi-port\fR \fIport\fR\fB;\fR
.PP
The \fIomapi-port\fR 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.
.RE
.PP
The
.I one-lease-per-client
statement
.RS 0.25i
.PP
.B one-lease-per-client \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.
.RE
.PP
The
.I pid-file-name
statement
.RS 0.25i
.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 RUNDIR/dhcpd.pid.   Like the
lease-file-name statement, this statement must appear in the outer scope
of the configuration file.
.RE
.PP
The
.I ping-checks
statement
.RS 0.25i
.PP
.B ping-checks
.I flag\fR\fB;\fR
.PP
When the DHCP server is considering dynamically allocating an IP
address to a client, it first sends an ICMP Echo request (a \fIping\fR)
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 \fIis\fR heard, the lease is abandoned, and the server does
not respond to the client.
.PP
This \fIping check\fR introduces a one-second delay in responding to
DHCPDISCOVER messages, which can be a problem for some clients.   The
ping-checks configuration parameter can be used to control checking -
if its value is false, no ping check is done.
.RE
.PP
The
.I server-identifier
statement
.RS 0.25i
.PP
.B server-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.
.RE
.PP
The
.I server-name
statement
.RS 0.25i
.PP
.B server-name "\fIname\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.
.RE
.PP
The
.I site-option-space
statement
.RS 0.25i
.PP
.B site-option-space "\fIname\fB";\fR
.PP
The \fIsite-option-space\fR 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 \fIvendor-option-space\fR statement.
Site-local options in DHCP are those options whose numeric codes are
greater than 128.   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 \fIsite-option-space\fR option can be used
to assign a different set of site-specific options for each such
vendor, using conditional evaluation (see \fIdhcp-eval.5\fR for
details).
.RE
.PP
The
.I stash-agent-options
statement
.RS 0.25i
.PP
.B stash-agent-options \fIflag\fB;\fR
.PP
If the \fIstash-agent-options\fR 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.
.PP
The
.I update-optimization
statement
.RS 0.25i
.PP
.B update-optimization \fIflag\fB;\fR
.PP
If the \fIupdate-optimization\fR 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 do not recommend enabling
this on large networks with the current DHCP server.   This option
only affects the behavior of the interim DNS update scheme,
and has no effect on the ad-hoc DNS update scheme.   If this
parameter is not specified, or is false, the DHCP server will only
update when the client information changes, the client gets a
different lease, or the client's lease expires.
.PP
The
.I update-static-leases
statement
.RS 0.25i
.PP
.B update-static-leases \fIflag\fB;\fR
.PP
The \fIupdate-static-leases\fR 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 \fIfixed-address\fR statement - that
is, the client is being given a static assignment.   This can only
work with the \fIinterim\fR DNS update scheme.   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.
.PP
The
.I use-host-decl-names
statement
.RS 0.25i
.PP
.B use-host-decl-names \fIflag\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
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.
.RE
.PP
The
.I use-lease-addr-for-default-route
statement
.RS 0.25i
.PP
.B use-lease-addr-for-default-route \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.   The use of this feature is
not recommended, because it won't work for many DHCP clients.
.RE
.PP
The
.I vendor-option-space
statement
.RS 0.25i
.PP
.B vendor-option-space \fIstring\fR\fB;\fR
.PP
The \fIvendor-option-space\fR parameter determines from what option
space vendor options are taken.   The use of this configuration
parameter is illustrated in the \fIdhcp-options(5)\fR manual page, in
the \fIVENDOR ENCAPSULATED OPTIONS\fR section.
.RE
.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
DHCP option statements are documented in the
.B dhcp-options(5)
manual page.
.SH REFERENCE: EXPRESSIONS
Expressions used in DHCP option statements and elsewhere are
documented in the
.B dhcp-eval(5)
manual page.
.SH SEE ALSO
dhcpd(8), dhcpd.leases(5), dhcp-options(5), dhcp-eval(5), RFC2132, RFC2131.
.SH AUTHOR
.B dhcpd.conf(5)
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.