dhcpd-options(5)

NAME

dhcp-options - Dynamic Host Configuration Protocol options

DESCRIPTION

The Dynamic Host Configuration protocol allows the client

to receive options from the DHCP server describing the network

configuration and various services that are available on the net

work. When configuring dhcpd(8) or 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 docu

mented here.

REFERENCE: OPTION STATEMENTS

DHCP option statements always start with the option key

word, 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.

Option data comes in a variety of formats, as defined be

low:

The ip-address data type can be entered either as an ex

plicit 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.

The int32 data type specifies a signed 32-bit integer.

The uint32 data type specifies an unsigned 32-bit integer. The

int16 and uint16 data types specify signed and unsigned 16-bit

integers. The int8 and uint8 data types specify signed and un

signed 8-bit integers. Unsigned 8-bit integers are also some

times referred to as octets.

The string data type specifies an NVT ASCII string, which

must be enclosed in double quotes - for example, to specify a do

main-name option, the syntax would be

option domain-name "isc.org";

The 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).

The data-string data type specifies either an NVT ASCII

string enclosed in double quotes, or a series of octets specified

in hexadecimal, separated by colons. For example:

option dhcp-client-identifier "CLIENT-FOO";

or

option dhcp-client-identifier

43:4c:49:45:54:2d:46:4f:4f;

The documentation for the various options mentioned below

is taken from the latest IETF draft document on DHCP options.

Options which are not listed by name may be defined by the name

option-nnn, where nnn is the decimal number of the option code.

These options may be followed either by a string, enclosed in

quotes, or by a series of octets, expressed as two-digit

hexadecimal numbers separated by colons. For example:

option option-133 "my-option-133-text";
option option-129 1:54:c9:2b:47;

Because dhcpd does not know the format of these undefined

option codes, no checking is done to ensure the correctness of

the entered data.

The standard options are:

option subnet-mask ip-address;

The subnet mask option specifies the client's subnet

mask as per RFC 950. If no subnet mask option is provided any

where in scope, as a last resort dhcpd will use the subnet mask

from the subnet declaration for the network on which an address

is being assigned. However, any subnet-mask option declaration

that is in scope for the address being assigned will override the

subnet mask specified in the subnet declaration.

option time-offset int32;

The time-offset option specifies the offset of the

client's subnet in seconds from Coordinated Universal Time (UTC).

option routers ip-address [, ip-address... ];

The routers option specifies a list of IP addresses for

routers on the client's subnet. Routers should be listed in or

der of preference.

option time-servers ip-address [, ip-address... ];

The time-server option specifies a list of RFC 868 time

servers available to the client. Servers should be listed in or

der of preference.

option ien116-name-servers ip-address [, ip-address... ];

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.

option domain-name-servers ip-address [, ip-address... ];

The domain-name-servers option specifies a list of Do

main Name System (STD 13, RFC 1035) name servers available to the

client. Servers should be listed in order of preference.

option log-servers ip-address [, ip-address... ];

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.

option cookie-servers ip-address [, ip-address... ];

The cookie server option specifies a list of RFC 865

cookie servers available to the client. Servers should be listed

in order of preference.

option lpr-servers ip-address [, ip-address... ];

The LPR server option specifies a list of RFC 1179 line

printer servers available to the client. Servers should be list

ed in order of preference.

option impress-servers ip-address [, ip-address... ];

The impress-server option specifies a list of Imagen

Impress servers available to the client. Servers should be list

ed in order of preference.

option resource-location-servers ip-address [,

ip-address... ];

This option specifies a list of RFC 887 Resource Loca

tion servers available to the client. Servers should be listed

in order of preference.

option host-name string;

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.

option boot-size uint16;

This option specifies the length in 512-octet blocks of

the default boot image for the client.

option merit-dump string;

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.

option domain-name string;

This option specifies the domain name that client

should use when resolving hostnames via the Domain Name System.

option swap-server ip-address;

This specifies the IP address of the client's swap

server.

option root-path string;

This option specifies the path-name that contains the

client's root disk. The path is formatted as a character string

consisting of characters from the NVT ASCII character set.

option ip-forwarding flag;

This option specifies whether the client should config

ure its IP layer for packet forwarding. A value of 0 means dis

able IP forwarding, and a value of 1 means enable IP forwarding.

option non-local-source-routing flag;

This option specifies whether the client should config

ure its IP layer to allow forwarding of datagrams with non-local

source routes (see Section 3.3.5 of [4] for a discussion of this

topic). A value of 0 means disallow forwarding of such data

grams, and a value of 1 means allow forwarding.

option policy-filter ip-address ip-address [, ip-address

ip-address... ];

This option specifies policy filters for non-local

source routing. The filters consist of a list of IP addresses

and masks which specify destination/mask pairs with which to fil

ter incoming source routes.

Any source routed datagram whose next-hop address does

not match one of the filters should be discarded by the client.

See STD 3 (RFC1122) for further information.

option max-dgram-reassembly uint16;

This option specifies the maximum size datagram that

the client should be prepared to reassemble. The minimum value

legal value is 576.

option default-ip-ttl uint8;

This option specifies the default time-to-live that the

client should use on outgoing datagrams.

option path-mtu-aging-timeout uint32;

This option specifies the timeout (in seconds) to use

when aging Path MTU values discovered by the mechanism defined in

RFC 1191.

option path-mtu-plateau-table uint16 [, uint16... ];

This option specifies a table of MTU sizes to use when

performing Path MTU Discovery as defined in RFC 1191. The table

is formatted as a list of 16-bit unsigned integers, ordered from

smallest to largest. The minimum MTU value cannot be smaller

than 68.

option interface-mtu uint16;

This option specifies the MTU to use on this interface.

The minimum legal value for the MTU is 68.

option all-subnets-local flag;

This option specifies whether or not the client may as

sume 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 1 indicates that

all subnets share the same MTU. A value of 0 means that the

client should assume that some subnets of the directly connected

network may have smaller MTUs.

option broadcast-address ip-address;

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).

option perform-mask-discovery flag;

This option specifies whether or not the client should

perform subnet mask discovery using ICMP. A value of 0 indicates

that the client should not perform mask discovery. A value of 1

means that the client should perform mask discovery.

option mask-supplier flag;

This option specifies whether or not the client should

respond to subnet mask requests using ICMP. A value of 0 indi

cates that the client should not respond. A value of 1 means

that the client should respond.

option router-discovery flag;

This option specifies whether or not the client should

solicit routers using the Router Discovery mechanism defined in

RFC 1256. A value of 0 indicates that the client should not per

form router discovery. A value of 1 means that the client should

perform router discovery.

option router-solicitation-address ip-address;

This option specifies the address to which the client

should transmit router solicitation requests.

option static-routes ip-address ip-address [, ip-address

ip-address... ];

This option specifies a list of static routes that the

client should install in its routing cache. If multiple routes

to the same destination are specified, they are listed in de

scending order of priority.

The routes consist of a list of IP address pairs. The

first address is the destination address, and the second address

is the router for the destination.

The default route (0.0.0.0) is an illegal destination

for a static route. To specify the default route, use the

routers option.

option trailer-encapsulation flag;

This option specifies whether or not the client should

negotiate the use of trailers (RFC 893 [14]) when using the ARP

protocol. A value of 0 indicates that the client should not at

tempt to use trailers. A value of 1 means that the client should

attempt to use trailers.

option arp-cache-timeout uint32;

This option specifies the timeout in seconds for ARP

cache entries.

option ieee802-3-encapsulation flag;

This option specifies whether or not the client should

use Ethernet Version 2 (RFC 894) or IEEE 802.3 (RFC 1042) encap

sulation if the interface is an Ethernet. A value of 0 indicates

that the client should use RFC 894 encapsulation. A value of 1

means that the client should use RFC 1042 encapsulation.

option default-tcp-ttl uint8;

This option specifies the default TTL that the client

should use when sending TCP segments. The minimum value is 1.

option tcp-keepalive-interval uint32;

This option specifies the interval (in seconds) that

the client TCP should wait before sending a keepalive message on

a TCP connection. The time is specified as a 32-bit unsigned in

teger. A value of zero indicates that the client should not gen

erate keepalive messages on connections unless specifically re

quested by an application.

option tcp-keepalive-garbage flag;

This option specifies the whether or not the client

should send TCP keepalive messages with a octet of garbage for

compatibility with older implementations. A value of 0 indicates

that a garbage octet should not be sent. A value of 1 indicates

that a garbage octet should be sent.

option nis-domain string;

This option specifies the name of the client's NIS (Sun

Network Information Services) domain. The domain is formatted as

a character string consisting of characters from the NVT ASCII

character set.

option nis-servers ip-address [, ip-address... ];

This option specifies a list of IP addresses indicating

NIS servers available to the client. Servers should be listed in

order of preference.

option ntp-servers ip-address [, ip-address... ];

This option specifies a list of IP addresses indicating

NTP (RFC 1305) servers available to the client. Servers should

be listed in order of preference.

option netbios-name-servers ip-address [, ip-address...

];

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.

option netbios-dd-server ip-address [, ip-address... ];

The NetBIOS datagram distribution server (NBDD) option

specifies a list of RFC 1001/1002 NBDD servers listed in order of

preference.

option netbios-node-type uint8;

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.

Possible node types are:

1 B-node: Broadcast - no WINS

2 P-node: Peer - WINS only.

4 M-node: Mixed - broadcast, then WINS

8 H-node: Hybrid - WINS, then broadcast

option netbios-scope string;

The NetBIOS scope option specifies the NetBIOS over

TCP/IP scope parameter for the client as specified in RFC

1001/1002. See RFC1001, RFC1002, and RFC1035 for character-set

restrictions.

option font-servers ip-address [, ip-address... ];

This option specifies a list of X Window System Font

servers available to the client. Servers should be listed in or

der of preference.

option x-display-manager ip-address [, ip-address... ];

This option specifies a list of systems that are run

ning the X Window System Display Manager and are available to the

client. Addresses should be listed in order of preference.

option dhcp-client-identifier data-string;

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.

option nisplus-domain string;

This option specifies the name of the client's NIS+ do

main. The domain is formatted as a character string consisting

of characters from the NVT ASCII character set.

option nisplus-servers ip-address [, ip-address... ];

This option specifies a list of IP addresses indicating

NIS+ servers available to the client. Servers should be listed

in order of preference.

option tftp-server-name string;

This option is used to identify a TFTP server and, if

supported by the client, should have the same effect as the

server-name declaration. BOOTP clients are unlikely to support

this option. Some DHCP clients will support it, and others actu

ally require it.

option bootfile-name string;

This option is used to identify a bootstrap file. If

supported by the client, it should have the same effect as the

filename declaration. BOOTP clients are unlikely to support this

option. Some DHCP clients will support it, and others actually

require it.

option mobile-ip-home-agent ip-address [, ip-address... ];

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.

option smtp-server ip-address [, ip-address... ];

The SMTP server option specifies a list of SMTP servers

available to the client. Servers should be listed in order of

preference.

option pop-server ip-address [, ip-address... ];

The POP3 server option specifies a list of POP3 avail

able to the client. Servers should be listed in order of prefer

ence.

option nntp-server ip-address [, ip-address... ];

The NNTP server option specifies a list of NNTP avail

able to the client. Servers should be listed in order of prefer

ence.

option www-server ip-address [, ip-address... ];

The WWW server option specifies a list of WWW available

to the client. Servers should be listed in order of preference.

option finger-server ip-address [, ip-address... ];

The Finger server option specifies a list of Finger

available to the client. Servers should be listed in order of

preference.

option irc-server ip-address [, ip-address... ];

The IRC server option specifies a list of IRC available

to the client. Servers should be listed in order of preference.

option streettalk-server ip-address [, ip-address... ];

The StreetTalk server option specifies a list of

StreetTalk servers available to the client. Servers should be

listed in order of preference.

option streetalk-directory-assistance-server ip-address [,

ip-address... ];

The StreetTalk Directory Assistance (STDA) server op

tion specifies a list of STDA servers available to the client.

Servers should be listed in order of preference.

SEE ALSO

dhcpd.conf(5), dhcpd.leases(5), dhclient.conf(5),

dhcpd(8), dhclient(8), RFC2132, RFC2131.

AUTHOR

dhcpd(8) was written by Ted Lemon <mellon@vix.com> under a

contract with Vixie Labs. Funding for this project was provided

by the Internet Software Corporation. Information about the In

ternet Software Consortium can be found at

http://www.isc.org/isc.

dhcpd

options(5)