path: root/usr.sbin/dhcpd/dhcpd.conf.5

.\"	$OpenBSD: dhcpd.conf.5,v 1.24 2018/07/03 12:51:44 krw Exp $
.\"
.\" Copyright (c) 1995, 1996, 1997, 1998, 1998, 1999
.\" The Internet Software Consortium.    All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\"
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. Neither the name of The Internet Software Consortium nor the names
.\"    of its contributors may be used to endorse or promote products derived
.\"    from this software without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE INTERNET SOFTWARE CONSORTIUM AND
.\" CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
.\" INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
.\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
.\" DISCLAIMED.  IN NO EVENT SHALL THE INTERNET SOFTWARE CONSORTIUM OR
.\" CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
.\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
.\" LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
.\" USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
.\" ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
.\" OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
.\" OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" This software has been written for the Internet Software Consortium
.\" by Ted Lemon <mellon@fugue.com> in cooperation with Vixie
.\" Enterprises.  To learn more about the Internet Software Consortium,
.\" see ``http://www.isc.org/isc''.  To learn more about Vixie
.\" Enterprises, see ``http://www.vix.com''.
.\"
.Dd $Mdocdate: July 3 2018 $
.Dt DHCPD.CONF 5
.Os
.Sh NAME
.Nm dhcpd.conf
.Nd DHCP server configuration file
.Sh DESCRIPTION
The
.Nm
file contains configuration information for
.Xr dhcpd 8 ,
the Internet Software Consortium DHCP Server.
.Pp
The
.Nm
file is a free-form ASCII text file.
It is parsed by the recursive-descent parser built into
.Xr dhcpd 8 .
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
.Sq #
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 say how to do something (e.g., how long a
lease to offer), whether to do something (e.g., should
.Xr dhcpd 8
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
.Ic shared-network
and the
.Ic subnet
declarations.
If clients on a subnet are to be assigned addresses dynamically, a
.Ic range
declaration must appear within the
.Ic subnet
declaration.
For clients with statically assigned addresses, or for installations where
only known clients will be served, each such client must have a
.Ic host
declaration.
If parameters are to be applied to a group of declarations which are not
related strictly on a per-subnet basis, the
.Ic group
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
.Ic subnet
declaration, which tells
.Xr dhcpd 8
how to recognize that an address is on that subnet.
A
.Ic subnet
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
.Ic subnet
declarations for these two networks may be enclosed in a
.Ic shared-network
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
.Ic host
declarations, these declarations can be enclosed in a
.Ic group
declaration along with the parameters which are common to that department.
For clients whose addresses will be dynamically assigned, there is currently no
way to group parameter assignments other than by network topology.
.Pp
When a client is to be booted, its boot parameters are determined by
first consulting that client's
.Ic host
declaration (if any), then consulting the
.Ic group
declaration (if any) which enclosed that
.Ic host
declaration, then consulting the
.Ic subnet
declaration for the subnet on which the client is booting, then consulting the
.Ic shared-network
declaration (if any) containing that subnet, and finally consulting the
top-level parameters which may be specified outside of any declaration.
.Pp
When
.Xr dhcpd 8
tries to find a
.Ic host
declaration for a client, it first looks for a
.Ic host
declaration which has a
.Ar fixed-address
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
.Ar fixed-address
parameter.
If no such entry is found, then
.Xr dhcpd 8
acts as if there is no entry in the
.Nm
file for that client, even if there is an entry for that client on a
different subnet or shared network.
.Sh EXAMPLES
A typical
.Nm
file will look something like this:
.Pp
Example 1
.Bd -unfilled -offset indent
.Ar global parameters...

shared-network ISC-BIGGIE {
.Pf "  " Ar shared-network-specific parameters ...
  subnet 204.254.239.0 netmask 255.255.255.224 {
.Pf "    " Ar subnet-specific parameters ...
    range 204.254.239.10 204.254.239.30;
  }
  subnet 204.254.239.32 netmask 255.255.255.224 {
.Pf "    " Ar subnet-specific parameters ...
    range 204.254.239.42 204.254.239.62;
  }
}

subnet 204.254.239.64 netmask 255.255.255.224 {
.Pf "  " Ar subnet-specific parameters ...
  range 204.254.239.74 204.254.239.94;
}

group {
.Pf "  " Ar group-specific parameters ...
  host zappo.test.isc.org {
.Pf "    " Ar host-specific parameters ...
  }
  host beppo.test.isc.org {
.Pf "    " Ar host-specific parameters ...
  }
  host harpo.test.isc.org {
.Pf "    " Ar host-specific parameters ...
  }
}
.Ed
.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:
.Pp
Example 2
.Bd -literal -offset indent
option domain-name \&"isc.org\&";
option domain-name-servers ns1.isc.org, ns2.isc.org;
.Ed
.Pp
As you can see in Example 2, it's legal to specify host addresses in
parameters as hostnames rather than as numeric IP addresses.
.Pp
In Example 1, you can see that both the shared-network statement and
the subnet statements can have parameters.
Let us say that the shared network ISC-BIGGIE supports an entire department \-
perhaps the accounting department.
If accounting has its own domain, then a shared-network-specific parameter
might be:
.Pp
.Dl option domain-name \&"accounting.isc.org\&";
.Pp
All subnet declarations appearing in the shared-network declaration
would then have the domain-name option set to
.Dq accounting.isc.org
instead of just
.Dq isc.org .
.Pp
The most obvious reason for having subnet-specific parameters as
shown in Example 1 is that each subnet, of necessity, has its own router.
So for the first subnet, for example, there should be something like:
.Pp
.Dl option routers 204.254.239.1;
.Pp
Note that the address here is specified numerically.
This is not required \- if you have a different hostname for each
interface on your router, it's perfectly legitimate to use the hostname
for that interface instead of the numeric address.
However, in many cases there may be only one hostname for all of a router's
IP addresses, and it would not be appropriate to use that name here.
.Pp
In Example 1 there is also a
.Ic group
statement, which provides common parameters for a set of three hosts \- zappo,
beppo and harpo.
As you can see, these hosts are all in the test.isc.org domain, so it
might make sense for a group-specific parameter to override the domain
name supplied to these hosts:
.Pp
.Dl option domain-name \&"test.isc.org\&";
.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:
.Bd -literal -offset indent
max-lease-time 120;
default-lease-time 120;
.Ed
.Pp
You may have noticed that while some parameters start with the
.Ic option
keyword, some do not.
Parameters starting with the
.Ic option
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
.Xr dhcpd 8
will give out), or specify client parameters that are not optional in the
DHCP protocol (for example, server-name and filename).
.Pp
In Example 1, each host had
.Ar host-specific parameters .
These could include such things as the
.Ic hostname
option, the name of a file to download (the
.Ar filename
parameter) and the address of the server from which to download the file (the
.Ar next-server
parameter).
In general, any parameter can appear anywhere that parameters are allowed,
and will be applied according to the scope in which the parameter appears.
.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:
.Bd -literal -offset indent
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 ncd5 { hardware ethernet 0:c0:c3:11:90:23; }
  host ncd6 { hardware ethernet 0:c0:c3:91:a7:8; }
  host ncd7 { hardware ethernet 0:c0:c3:cc:a:8f; }
}
.Ed
.Sh REFERENCE: DECLARATIONS
The
.Ic shared-network
statement informs the DHCP server that some IP subnets actually
share the same physical network:
.Bd -unfilled -offset indent
.Ic shared-network Ar name No {
.Pf "  " Op Ar parameters
.Pf "  " Op Ar declarations
}
.Ed
.Pp
Any subnets in a shared network should be declared within a
.Ic shared-network
statement.
Parameters specified in the
.Ic shared-network
statement will be used when booting clients on those subnets unless
parameters provided at the subnet or host level override them.
If any subnet in a shared network has addresses available for dynamic
allocation, those addresses are collected into a common pool for that
shared network and assigned to clients as needed.
There is no way to distinguish on which subnet of a shared network a
client should boot.
.Pp
.Ar 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 hostname
(although it will never be used as such), or it may be any arbitrary
name, enclosed in quotes.
.Pp
The
.Ic subnet
statement provides
.Xr dhcpd 8
with enough information to tell whether or not an IP address is on that subnet:
.Bd -unfilled -offset indent
.Ic subnet Ar subnet-number Ic netmask Ar netmask No {
.Pf "  " Op Ar parameters
.Pf "  " Op Ar declarations
}
.Ed
.Pp
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
.Ic range
declaration.
.Pp
The
.Ar subnet-number
and
.Ar netmask
should be specified as numeric IP addresses.
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
The
.Ic range
statement gives the lowest and highest IP addresses in a range:
.Bd -filled -offset indent
.Ic range Op Ic dynamic-bootp
.Ar low-address Oo Ar high-address Oc ;
.Ed
.Pp
For any subnet on which addresses will be assigned dynamically, there
must be at least one
.Ic range
statement.
All IP addresses in the range should be in the subnet in which the
.Ic range
statement is declared.
The
.Ic dynamic-bootp
flag may be specified if addresses in the specified range may be dynamically
assigned to BOOTP clients as well as DHCP clients.
When specifying a single address,
.Ar high-address
can be omitted.
.Pp
There must be at least one
.Ic host
statement for every BOOTP client that is to be served:
.Bd -unfilled -offset indent
.Ic host Ar hostname No {
.Pf "  " Op Ar parameters
.Pf "  " Op Ar declarations
}
.Ed
.Pp
.Ic 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
.Ar fixed-address
parameter, or more than one
.Ic 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
.Ic 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
.Ic host
statement must be specified without a
.Ar fixed-address
clause.
.Ar hostname
should be a name identifying the host.
If a
.Ar hostname
option is not specified for the host,
.Ar hostname
is used.
.Pp
.Ic host
declarations are matched to actual DHCP or BOOTP clients by matching the
.Ic dhcp-client-identifier
option specified in the
.Ic host
declaration to the one supplied by the client or, if the
.Ic host
declaration or the client does not provide a
.Ic dhcp-client-identifier
option, by matching the
.Ar hardware
parameter in the
.Ic host
declaration to the network hardware address supplied by the client.
BOOTP clients do not normally provide a
.Ar dhcp-client-identifier ,
so the hardware address must be used for all clients that may boot using
the BOOTP protocol.
.Pp
The
.Ic group
statement is used simply to apply one or more parameters to a group of
declarations:
.Bd -unfilled -offset indent
.Ic group No {
.Pf "  " Op Ar parameters
.Pf "  " Op Ar declarations
}
.Ed
.Pp
It can be used to group hosts, shared networks, subnets, or even other groups.
.Sh REFERENCE: ALLOW and DENY
The
.Ic allow
and
.Ic deny
statements can be used to control the behaviour of
.Xr dhcpd 8
to various sorts of requests.
.Pp
The
.Ar unknown-clients
flag tells
.Xr dhcpd 8
whether or not to dynamically assign addresses to unknown clients:
.Bd -literal -offset indent
allow unknown-clients;
deny unknown-clients;
.Ed
.Pp
Dynamic address assignment to unknown clients is allowed by default.
.Pp
The
.Ar bootp
flag tells
.Xr dhcpd 8
whether or not to respond to bootp queries:
.Bd -literal -offset indent
allow bootp;
deny bootp;
.Ed
.Pp
Bootp queries are allowed by default.
.Pp
The
.Ar booting
flag tells
.Xr dhcpd 8
whether or not to respond to queries from a particular client:
.Bd -literal -offset indent
allow booting;
deny booting;
.Ed
.Pp
This keyword only has meaning when it appears in a host declaration.
By default, booting is allowed, but if it is disabled for a particular client,
then that client will not be able to get an address from the DHCP server.
.Sh REFERENCE: PARAMETERS
The
.Ic default-lease-time
statement specifies the
.Ar time
in seconds that will be assigned to a lease
if the client requesting the lease does not ask for a specific expiration time:
.Pp
.D1 Ic default-lease-time Ar time ;
.Pp
The
.Ic max-lease-time
statement specifies the maximum
.Ar time
in seconds that will be assigned to a lease
if the client requesting the lease asks for a specific expiration time:
.Pp
.D1 Ic max-lease-time Ar time ;
.Pp
The
.Ic hardware
statement allows a
BOOTP client to be recognized in a
.Ic host
statement:
.Pp
.D1 Ic hardware Ar hardware-type hardware-address ;
.Pp
.Ar hardware-type
must be the name of a hardware interface type.
Currently, the
.Cm ethernet ,
.Cm token-ring
and
.Cm fddi
physical interface types are recognized,
although support for DHCP-over-IPsec virtual interface type
.Cm ipsec-tunnel
is provided.
The
.Ar hardware-address
should be a set of hexadecimal octets (numbers from 0 through ff)
separated by colons.
The
.Ic hardware
statement may also be used for DHCP clients.
.Pp
The
.Ic filename
statement can be used to specify the name of the initial boot file which
is to be loaded by a client:
.Pp
.D1 Ic filename Qq Ar filename ;
.Pp
The
.Ar filename
should be a filename recognizable to whatever file transfer protocol
the client can be expected to use to load the file.
.Pp
The
.Ic server-name
statement can be used to inform the client of the name of the server
from which it is booting:
.Pp
.D1 Ic server-name Qq Ar name ;
.Pp
.Ar name
should be the name that will be provided to the client.
.Pp
The
.Ic next-server
statement specifies the host address of
the server from which the initial boot file (specified in the
.Ic filename
statement) is to be loaded:
.Pp
.D1 Ic next-server Ar server-name ;
.Pp
.Ar server-name
should be a numeric IP address or a hostname.
If no
.Ic next-server
parameter applies to a given client, the DHCP server's IP address is used.
.Pp
The
.Ic fixed-address
statement assigns one or more fixed IP addresses to a client:
.Pp
.D1 Ic fixed-address Ar address Op , Ar address ... ;
.Pp
It should only appear in a
.Ic host
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
.Ic fixed-address
statement are on the network on which the client is booting, that client will
not match the
.Ic host
declaration containing that
.Ic fixed-address
statement.
Each
.Ar address
should be either an IP address or a hostname which resolves to one
or more IP addresses.
.Pp
Clients with fixed addresses are not assigned DHCP leases,
and may therefore not be used with the
.Fl ACL
table options of
.Xr dhcpd 8 .
.Pp
The
.Ic dynamic-bootp-lease-cutoff
statement sets the ending time for all leases assigned dynamically to
BOOTP clients:
.Pp
.D1 Ic dynamic-bootp-lease-cutoff Ar date ;
.Pp
Because BOOTP clients do not have any way of renewing leases,
and don't know that their leases could expire, by default
.Xr dhcpd 8
assigns infinite leases to all BOOTP clients.
However, it may make sense in some situations to set a cutoff date for all
BOOTP leases \- for example, the end of a school term,
or the time at night when a facility is closed and all
machines are required to be powered off.
.Pp
.Ar date
should be the date on which all assigned BOOTP leases will end.
The date is specified in the form:
.Pp
.Dl 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.
.Pp
The
.Ic dynamic-bootp-lease-length
statement sets the length of leases dynamically assigned to BOOTP clients:
.Pp
.D1 Ic dynamic-bootp-lease-length Ar length ;
.Pp
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
.Ar length
as a number of seconds.
If a client reboots using BOOTP during the timeout period, the lease
duration is reset to
.Ar length ,
so a BOOTP client that boots frequently enough will never lose its lease.
Needless to say, this parameter should be adjusted with extreme caution.
.Pp
The
.Ic get-lease-hostnames
statement tells
.Xr dhcpd 8
whether or not to look up the hostname corresponding to the IP address of
each address in the lease pool and use that address for the DHCP
.Ic hostname
option:
.Pp
.D1 Ic get-lease-hostnames Ar flag ;
.Pp
If
.Ar flag
is true, then this lookup is done for all addresses in the current scope.
By default, or if
.Ar flag
is false, no lookups are done.
.Pp
If the
.Ic use-host-decl-names
parameter is true in a given scope, then for every host declaration within
that scope, the name provided for the host declaration will be supplied to
the client as its hostname:
.Pp
.D1 Ic use-host-decl-names Ar flag ;
.Pp
So, for example:
.Bd -literal -offset indent
group {
  use-host-decl-names on;

  host joe {
    hardware ethernet 08:00:2b:4c:29:32;
    fixed-address joe.fugue.com;
  }
}
.Ed
.Pp
is equivalent to:
.Bd -literal -offset indent
host joe {
  hardware ethernet 08:00:2b:4c:29:32;
  fixed-address joe.fugue.com;
  option host-name "joe";
}
.Ed
.Pp
An
.Ic option host-name
statement within a host declaration will override the use of the name
in the host declaration.
.Pp
The
.Ic authoritative
statement:
.Pp
.D1 Ic authoritative ;
.D1 Ic not authoritative ;
.Pp
The DHCP server will normally assume that the configuration
information about a given network segment is known to be correct and
is authoritative.
So if a client requests an IP address on a given network segment that the
server knows is not valid for that segment, the server will respond with a
DHCPNAK message, causing the client to forget its IP address and try to get
a new one.
.Pp
If a DHCP server is being configured by somebody who is not the
network administrator and who therefore does not wish to assert this
level of authority, then the statement
.Dq not authoritative
should be written in the appropriate scope in the configuration file.
.Pp
Usually, writing
.Ic not authoritative;
at the top level of the file should be sufficient.
However, if a DHCP server is to be set up so that it is aware of some
networks for which it is authoritative and some networks for which it is not,
it may be more appropriate to declare authority on a per-network-segment basis.
.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
The
.Ic use-lease-addr-for-default-route
statement:
.Pp
.D1 Ic use-lease-addr-for-default-route Ar flag ;
.Pp
If the
.Ic use-lease-addr-for-default-route
parameter is true in a given scope, then instead of sending the value
specified in the routers option (or sending no value at all),
the IP address of the lease being assigned is sent to the client.
This supposedly causes Win95 machines to ARP for all IP addresses,
which can be helpful if your router is configured for proxy ARP.
.Pp
If
.Ic use-lease-addr-for-default-route
is enabled and an option routers statement are both in scope,
the routers option will be preferred.
The rationale for this is that in situations where you want to use
this feature, you probably want it enabled for a whole bunch of
Windows 95 machines, and you want to override it for a few other machines.
Unfortunately, if the opposite happens to be true for your
site, you are probably better off not trying to use this flag.
.Pp
The
.Ic always-reply-rfc1048
statement:
.Pp
.D1 Ic always-reply-rfc1048 Ar flag ;
.Pp
Some BOOTP clients expect RFC 1048-style responses, but do not follow
RFC 1048 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
.Dq (non-rfc1048)
printed with each BOOTREQUEST that is logged.
.Pp
If you want to send RFC 1048 options to such a client, you can set the
.Ic 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
The
.Ic server-identifier
statement can be used to define the value that is sent in the
DHCP Server Identifier option for a given scope:
.Pp
.D1 Ic server-identifier Ar hostname ;
.Pp
The value specified
.Em must
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
.Ic server-identifier
statement needs to be sent is when a physical interface has more than one
IP address, and the one being sent by default isn't appropriate for some
or all clients served by that interface.
Another common case is when an alias is defined for the purpose of
having a consistent IP address for the DHCP server, and it is desired
that the clients use this IP address when contacting the server.
.Pp
Supplying a value for the
.Ic dhcp-server-identifier
option is equivalent to using the
.Ic server-identifier
statement.
.Pp
The
.Ic echo-client-id
statement controls RFC 6842 compliant behavior and has a default
value of true:
.Pp
.D1 Ic echo-client-id Ar flag ;
.Pp
Some devices, especially old printers, require
.Ic echo-client-id
to be false.
.Pp
When
.Ic echo-client-id
is true
.Xr dhcpd 8
copies option dhcp-client-identifier (code 61)
from
DHCP DISCOVER or REQUEST messages into
the DHCP ACK or NAK reply sent to the client.
.Sh REFERENCE: OPTION STATEMENTS
DHCP option statements are documented in the
.Xr dhcp-options 5
manual page.
.Sh SEE ALSO
.Xr dhcp-options 5 ,
.Xr dhcpd.leases 5 ,
.Xr dhcpd 8
.Sh STANDARDS
.Rs
.%A R. Droms
.%D March 1997
.%R RFC 2131
.%T Dynamic Host Configuration Protocol
.Re
.Pp
.Rs
.%A S. Alexander
.%A R. Droms
.%D March 1997
.%R RFC 2132
.%T DHCP Options and BOOTP Vendor Extensions
.Re
.Pp
.Rs
.%A M. Patrick
.%D January 2001
.%R RFC 3046
.%T DHCP Relay Agent Information Option
.Re
.Pp
.Rs
.%A B. Patel
.%A B. Aboba
.%A S. Kelly
.%A V. Gupta
.%D January 2003
.%R RFC 3456
.%T Dynamic Host Configuration Protocol (DHCPv4) Configuration of IPsec Tunnel Mode
.Re
.Sh AUTHORS
.An -nosplit
.Xr dhcpd 8
was written by
.An Ted Lemon Aq Mt mellon@vix.com
under a contract with Vixie Labs.
.Pp
The current implementation was reworked by
.An Henning Brauer Aq Mt henning@openbsd.org .