DHCP-OPTIONS(5) BSD Reference Manual DHCP-OPTIONS(5)
dhcp-options - Dynamic Host Configuration Protocol options
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 for-
mats 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 in-
tegers 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:49:45:4e: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 example:
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:
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 sub-
net declaration for the network on which an address is being as-
signed. 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 sub-
net 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
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 order 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 ord-
er 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 ord-
er 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 ord-
er 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 ord-
er 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 ord-
er 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 de-
fault 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 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.
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 for-
warding, 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 rout-
ing. The filters consist of a list of IP addresses and masks
which specify destination/mask pairs with which to filter incom-
ing 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.
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 speci-
fied 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 ad-
dress 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 stat-
ic 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 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 en-
tries.
option ieee802-3-encapsulation flag;
This option specifies whether or not the client should use Ether-
net 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.
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 con-
nection. 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.
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 char-
acter string consisting of characters from the NVT ASCII charac-
ter 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 ord-
er 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. Net-
BIOS Name Service is currently more commonly referred to as WINS.
WINS servers can be specified using the netbios-name-servers op-
tion.
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 prefer-
ence.
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 identi-
fies 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
preference.
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 charac-
ters 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 ord-
er 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 de-
claration. 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 de-
claration. 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
preference.
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 ord-
er of preference.
option streettalk-directory-assistance-server ip-address [, ip-address
...];
The StreetTalk Directory Assistance (STDA) server option speci-
fies a list of STDA servers available to the client. Servers
should be listed in order of preference.
dhclient.conf(5), dhcpd.conf(5), dhcpd.leases(5), dhclient(8), dhcpd(8)
RFC 2131, RFC 2132.
dhcpd(8) 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>.
MirBSD #10-current January 1, 1995 6