DHCP-OPTIONS(5) BSD File Formats Manual DHCP-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 network. 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 documented here.
REFERENCE
: OPTION STATEMENTS
DHCP option statements always start with the option 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.
Option data comes in a variety of formats, as defined below:
The 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).
A domain name must resolve 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 unsigned 8-bit integers. Unsigned 8-bit
integers are also sometimes referred to as octets.
The string data type specifies an NVT (Network Virtual Terminal) ASCII string, which must be enclosed in double quotes - for example, to
specify a domain-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 IETF draft document on DHCP options, RFC 2132. 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 exam-
ple:
option option-133 "my-option-133-text";
option option-129 1:54:c9:2b:47;
Because dhcpd(8) 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:
RFC 1497 Vendor Extensions
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 anywhere in scope, as
a last resort dhcpd(8) 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 order of prefer-
ence.
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 order of pref-
erence.
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 Domain 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-servers 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-servers 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-servers option specifies a list of RFC 1179 line printer servers available to the client. Servers should be listed in order
of preference.
option impress-servers ip-address [, ip-address ...];
The impress-servers option specifies a list of Imagen Impress servers available to the client. Servers should be listed in order of
preference.
option resource-location-servers ip-address [, ip-address ...];
This option specifies a list of RFC 887 Resource Location 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 pathname 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 the client should use when resolving hostnames via the Domain Name System.
option domain-search string;
This option specifies a list of domain names that the client should use when resolving hostnames via the Domain Name System. This
option is defined in RFC 3397.
option swap-server ip-address;
This specifies the IP address of the client's swap server.
option root-path string;
This option specifies the pathname that contains the client's root disk. The path is formatted as a character string consisting of
characters from the NVT ASCII character set.
IP Layer Parameters per Host
option ip-forwarding flag;
This option specifies whether the client should configure its IP layer for packet forwarding. A value of 0 means disable IP forward-
ing, and a value of 1 means enable IP forwarding.
option non-local-source-routing flag;
This option specifies whether the client should configure 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 datagrams, 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 filter 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 (RFC 1122) 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 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.
IP Layer Parameters per Interface
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 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 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 (RFC 1122).
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 indicates 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 perform 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 descending 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.
Link Layer Parameters per Interface
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 attempt 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) encapsulation 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.
TCP Parameters
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 connec-
tion. The time is specified as a 32-bit unsigned integer. A value of zero indicates that the client should not generate keepalive
messages on connections unless specifically requested by an application.
option tcp-keepalive-garbage flag;
This option specifies whether or not the client should send TCP keepalive messages with an 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.
Application and Service Parameters
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 RFC
1001, RFC 1002, and RFC 1035 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 order of prefer-
ence.
option x-display-manager ip-address [, ip-address ...];
This option specifies a list of systems that are running 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 a DHCP client identifier in a host declaration, so that dhcpd(8) 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+ domain. 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 decla-
ration. BOOTP clients are unlikely to support this option. Some DHCP clients will support it, and others actually 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 decla-
ration. 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 pop-server option specifies a list of POP3 servers available to the client. Servers should be listed in order of preference.
option nntp-server ip-address [, ip-address ...];
The nntp-server option specifies a list of NNTP servers available to the client. Servers should be listed in order of preference.
option www-server ip-address [, ip-address ...];
The www-server option specifies a list of WWW servers 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(1) servers available to the client. Servers should be listed in order of prefer-
ence.
option irc-server ip-address [, ip-address ...];
The irc-server option specifies a list of IRC servers 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 streettalk-directory-assistance-server ip-address [, ip-address ...];
The StreetTalk Directory Assistance (STDA) server option specifies a list of STDA servers available to the client. Servers should be
listed in order of preference.
SEE ALSO
dhclient.conf(5), dhcpd.conf(5), dhcpd.leases(5), dhclient(8), dhcpd(8)
RFC 2131, RFC 2132.
AUTHORS
The dhcpd(8) utility was written by Ted Lemon <mellon@vix.com> under a contract with Vixie Labs.
The current implementation was reworked by Henning Brauer <henning@openbsd.org>.
BSD
January 1, 1995 BSD