dhcp-options.5

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.\"	dhcp-options.5
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.TH dhcpd-options 5
.SH NAME
dhcp-options - Dynamic Host Configuration Protocol options
.SH DESCRIPTION
The Dynamic Host Configuration protocol allows the client to receive
.B options
from the DHCP server describing the network configuration and various
services that are available on the network.   When configuring
.B dhcpd(8)
or
.B dhclient(8) ,
options must often be declared.   The syntax for declaring options,
and the names and formats of the options that can be declared, are
documented here.
.SH REFERENCE: OPTION STATEMENTS
.PP
DHCP \fIoption\fR statements always start with the \fIoption\fR
keyword, followed by an option name, followed by option data.  The
option names and data formats are described below.   It is not
necessary to exhaustively specify all DHCP options - only those
options which are needed by clients must be specified.
.PP
Option data comes in a variety of formats, as defined below:
.PP
The
.B ip-address
data type can be entered either as an explicit IP
address (e.g., 239.254.197.10) or as a domain name (e.g.,
haagen.isc.org).  When entering a domain name, be sure that that
domain name resolves to a single IP address.
.PP
The
.B int32
data type specifies a signed 32-bit integer.   The 
.B uint32
data type specifies an unsigned 32-bit integer.   The 
.B int16
and
.B uint16
data types specify signed and unsigned 16-bit integers.   The 
.B int8
and
.B uint8
data types specify signed and unsigned 8-bit integers.
Unsigned 8-bit integers are also sometimes referred to as octets.
.PP
The
.B text
data type specifies an NVT ASCII string, which must be
enclosed in double quotes - for example, to specify a root-path
option, the syntax would be
.nf
.sp 1
option root-path "10.0.1.4:/var/tmp/rootfs";
.fi
.PP
The
.B domain-name
data type specifies a domain name, which must not
enclosed in double quotes.   This data type is not used for any
existing DHCP options.   The domain name is stored just as if it were
a text option.
.PP
The
.B flag
data type specifies a boolean value.   Booleans can be either true or
false (or on or off, if that makes more sense to you).
.PP
The
.B string
data type specifies either an NVT ASCII string
enclosed in double quotes, or a series of octets specified in
hexadecimal, seperated by colons.   For example:
.nf
.sp 1
  option dhcp-client-identifier "CLIENT-FOO";
or
  option dhcp-client-identifier 43:4c:49:45:54:2d:46:4f:4f;
.fi
.SH SETTING OPTION VALUES USING EXPRESSIONS
Sometimes it's helpful to be able to set the value of a DHCP option
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
  \fBoption \fImy-option \fB= \fIexpression \fB;\fR
.fi
.PP
For example:
.nf
.sp 1
  option hostname = binary-to-ascii (16, 8, "-",
                                     substring (hardware, 1, 6));
.fi
.SH STANDARD DHCP OPTIONS
The documentation for the various options mentioned below is taken
from the latest IETF draft document on DHCP options.  Options not
listed below may not yet be implemented, but it is possible to use
such options by defining them in the configuration file.  Please see
the DEFINING NEW OPTIONS heading later in this document for more
information.
.PP
The standard options are:
.PP
.B option \fBall-subnets-local\fR \fIflag\fR\fB;\fR
.RS 0.25i
.PP
This option specifies whether or not the client may assume that all
subnets of the IP network to which the client is connected use the
same MTU as the subnet of that network to which the client is
directly connected.  A value of true indicates that all subnets share
the same MTU.  A value of false means that the client should assume that
some subnets of the directly connected network may have smaller MTUs.
.RE
.PP
.B option \fBarp-cache-timeout\fR \fIuint32\fR\fB;\fR
.RS 0.25i
.PP
This option specifies the timeout in seconds for ARP cache entries.
.RE
.PP
.B option \fBbootfile-name\fR \fItext\fR\fB;\fR
.RS 0.25i
.PP
This option is used to identify a bootstrap file.  If supported by the
client, it should have the same effect as the \fBfilename\fR
declaration.  BOOTP clients are unlikely to support this option.  Some
DHCP clients will support it, and others actually require it.
.RE
.PP
.B option \fBboot-size\fR \fIuint16\fR\fB;\fR
.RS 0.25i
.PP
This option specifies the length in 512-octet blocks of the default
boot image for the client.
.RE
.PP
.B option \fBbroadcast-address\fR \fIip-address\fR\fB;\fR
.RS 0.25i
.PP
This option specifies the broadcast address in use on the client's
subnet.  Legal values for broadcast addresses are specified in
section 3.2.1.3 of STD 3 (RFC1122).
.RE
.PP
.B option \fBcookie-servers\fR \fIip-address\fR [\fB,\fR \fIip-address\fR...
]\fB;\fR
.RS 0.25i
.PP
The cookie server option specifies a list of RFC 865 cookie
servers available to the client.  Servers should be listed in order
of preference.
.RE
.PP
.B option \fBdefault-ip-ttl\fR \fIuint8;\fR
.RS 0.25i
.PP
This option specifies the default time-to-live that the client should
use on outgoing datagrams.
.RE
.PP
.B option \fBdefault-tcp-ttl\fR \fIuint8\fR\fB;\fR
.RS 0.25i
.PP
This option specifies the default TTL that the client should use when
sending TCP segments.  The minimum value is 1.
.RE
.PP
.B option \fBdhcp-client-identifier\fR \fIstring\fR\fB;\fR
.RS 0.25i
.PP
This option can be used to specify the a DHCP client identifier in a
host declaration, so that dhcpd can find the host record by matching
against the client identifier.
.PP
Please be aware that some DHCP clients, when configured with client
identifiers that are ASCII text, will prepend a zero to the ASCII
text.   So you may need to write:
.nf

	option dhcp-client-identifier "\\0foo";

rather than:

	option dhcp-client-identifier "foo";
.fi
.RE
.PP
.B option \fBdhcp-max-message-size\fR \fIuint16\fR\fB;\fR
.RS 0.25i
.PP
This option, when sent by the client, specifies the maximum size of
any response that the server sends to the client.   When specified on
the server, if the client did not send a dhcp-max-message-size option,
the size specified on the server is used.   This works for BOOTP as
well as DHCP responses.
.RE
.PP
.B option \fBdhcp-parameter-request-list\fR \fIuint16\fR\fB;\fR
.RS 0.25i
.PP
This option, when sent by the client, specifies which options the
client wishes the server to return.   Normally, in the ISC DHCP
client, this is done using the \fIrequest\fR statement.   If this
option is not specified by the client, the DHCP server will normally
return every option that is valid in scope and that fits into the
reply.   When this option is specified on the server, the server
returns the specified options.   This can be used to force a client to
take options that it hasn't requested, and it can also be used to
tailor the response of the DHCP server for clients that may need a
more limited set of options than those the server would normally
return.
.RE
.PP
.B option \fBdomain-name\fR \fItext\fR\fB;\fR
.RS 0.25i
.PP
This option specifies the domain name that client should use when
resolving hostnames via the Domain Name System.
.RE
.PP
.B option \fBdomain-name-servers\fR \fIip-address\fR [\fB,\fR \fIip-address\fR...
]\fB;\fR
.RS 0.25i
.PP
The domain-name-servers option specifies a list of Domain Name System
(STD 13, RFC 1035) name servers available to the client.  Servers
should be listed in order of preference.
.RE
.PP
.B option \fBextensions-path-name\fR \fItext\fR\fB;\fR
.RS 0.25i
.PP
This option specifies the name of a file containing additional options
to be interpreted according to the DHCP option format as specified in
RFC2132.
.RE
.PP
.B option \fBfinger-server\fR \fIip-address\fR [\fB,\fR
\fIip-address\fR... ]\fB;\fR
.RS 0.25i
.PP
The Finger server option specifies a list of Finger available to the
client.  Servers should be listed in order of preference.
.RE
.PP
.B option \fBfont-servers\fR \fIip-address\fR [\fB,\fR \fIip-address\fR...
]\fB;\fR
.RS 0.25i
.PP
This option specifies a list of X Window System Font servers available
to the client. Servers should be listed in order of preference.
.RE
.PP
.B option \fBhost-name\fR \fIstring\fR\fB;\fR
.RS 0.25i
.PP
This option specifies the name of the client.  The name may or may
not be qualified with the local domain name (it is preferable to use
the domain-name option to specify the domain name).  See RFC 1035 for
character set restrictions.
.RE
.PP
.B option \fBieee802-3-encapsulation\fR \fIflag\fR\fB;\fR
.RS 0.25i
.PP
This option specifies whether or not the client should use Ethernet
Version 2 (RFC 894) or IEEE 802.3 (RFC 1042) encapsulation if the
interface is an Ethernet.  A value of false indicates that the client
should use RFC 894 encapsulation.  A value of true means that the client
should use RFC 1042 encapsulation.
.RE
.PP
.B option \fBien116-name-servers\fR \fIip-address\fR [\fB,\fR \fIip-address\fR...
];
.RS 0.25i
.PP
The ien116-name-servers option specifies a list of IEN 116 name servers
available to the client.  Servers should be listed in order of
preference.
.RE
.PP
.B option \fBimpress-servers\fR \fIip-address\fR [\fB,\fR \fIip-address\fR...
]\fB;\fR
.RS 0.25i
.PP
The impress-server option specifies a list of Imagen Impress servers
available to the client.  Servers should be listed in order of
preference.
.RE
.PP
.B option \fBinterface-mtu\fR \fIuint16\fR\fB;\fR
.RS 0.25i
.PP
This option specifies the MTU to use on this interface.   The minimum
legal value for the MTU is 68.
.RE
.PP
.B option \fBip-forwarding\fR \fIflag\fR\fB;\fR
.RS 0.25i
.PP
This option specifies whether the client should configure its IP
layer for packet forwarding.  A value of false means disable IP
forwarding, and a value of true means enable IP forwarding.
.RE
.PP
.B option \fBirc-server\fR \fIip-address\fR [\fB,\fR
\fIip-address\fR... ]\fB;\fR
.RS 0.25i
.PP
The IRC server option specifies a list of IRC available to the
client.  Servers should be listed in order of preference.
.RE
.PP
.B option \fBlog-servers\fR \fIip-address\fR [\fB,\fR \fIip-address\fR...
]\fB;\fR
.RS 0.25i
.PP
The log-server option specifies a list of MIT-LCS UDP log servers
available to the client.  Servers should be listed in order of
preference.
.RE
.PP
.B option \fBlpr-servers\fR \fIip-address \fR [\fB,\fR \fIip-address\fR...
]\fB;\fR
.RS 0.25i
.PP
The LPR server option specifies a list of RFC 1179 line printer
servers available to the client.  Servers should be listed in order
of preference.
.RE
.PP
.B option \fBmask-supplier\fR \fIflag\fR\fB;\fR
.RS 0.25i
.PP
This option specifies whether or not the client should respond to
subnet mask requests using ICMP.  A value of false indicates that the
client should not respond.  A value of true means that the client should
respond.
.RE
.PP
.B option \fBmax-dgram-reassembly\fR \fIuint16\fR\fB;\fR
.RS 0.25i
.PP
This option specifies the maximum size datagram that the client
should be prepared to reassemble.  The minimum value legal value is
576.
.RE
.PP
.B option \fBmerit-dump\fR \fItext\fR\fB;\fR
.RS 0.25i
.PP
This option specifies the path-name of a file to which the client's
core image should be dumped in the event the client crashes.  The
path is formatted as a character string consisting of characters from
the NVT ASCII character set.
.RE
.PP
.B option \fBmobile-ip-home-agent\fR \fIip-address\fR [\fB,\fR \fIip-address\fR... ]\fB;\fR
.RS 0.25i
.PP
This option specifies a list of IP addresses indicating mobile IP
home agents available to the client.  Agents should be listed in
order of preference, although normally there will be only one such
agent.
.RE
.PP
.B option \fBnds-context\fR \fIstring\fR\fB;\fR
.RS 0.25i
.PP
The nds-context option specifies the name of the initial Netware
Directory Service for an NDS client.
.RE
.PP
.B option \fBnds-servers\fR \fIip-address\fR [\fB,\fR \fIip-address\fR... ]\fB;\fR
.RS 0.25i
.PP
The nds-servers option specifies a list of IP addresses of NDS servers.
.RE
.PP
.B option \fBnds-tree-name\fR \fIstring\fR\fB;\fR
.RS 0.25i
.PP
The nds-context option specifies NDS tree name that the NDS client
should use.
.RE
.PP
.B option \fBnetbios-dd-server\fR \fIip-address\fR [\fB,\fR \fIip-address\fR...
]\fB;\fR
.RS 0.25i
.PP
The NetBIOS datagram distribution server (NBDD) option specifies a
list of RFC 1001/1002 NBDD servers listed in order of preference.
.RE
.PP
.B option \fBnetbios-name-servers\fR \fIip-address\fR [\fB,\fR \fIip-address\fR...]\fB;\fR
.RS 0.25i
.PP
The NetBIOS name server (NBNS) option specifies a list of RFC
1001/1002 NBNS name servers listed in order of preference.   NetBIOS
Name Service is currently more commonly referred to as WINS.   WINS
servers can be specified using the netbios-name-servers option.
.RE
.PP
.B option \fBnetbios-node-type\fR \fIuint8\fR\fB;\fR
.RS 0.25i
.PP
The NetBIOS node type option allows NetBIOS over TCP/IP clients which
are configurable to be configured as described in RFC 1001/1002.  The
value is specified as a single octet which identifies the client type.
.PP
Possible node types are:
.PP
.TP 5