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dhcp-options.5NAME
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 avail
able 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 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
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). 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 unsigned 8-bit integers. Unsigned
8-bit integers are also sometimes referred to as octets.
The string data type specifies an NVT ASCII string, which
must be enclosed in double quotes - for example, to spec
ify 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, seperated by colons. For exam
ple:
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 seper
ated 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 correct
ness 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 pro
vided anywhere 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. How
ever, 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 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 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-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 listed 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 listed 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 charac
ter 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 disable 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 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 (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 mini
mum 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 mini
mum 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 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 dis
covery. A value of 1 means that the client should per
form 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 discov
ery.
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 multi
ple 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 sec
ond 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 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.
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 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 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 charac
ters 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 1035) 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 order 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 avail
able 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+
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 declaration. 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 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 nor
mally 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 preference.
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 preference.
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
option specifies a list of STDA servers available to
the client. Servers should be listed in order of pref
erence.
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. Informa
tion about the Internet Software Consortium can be found
at http://www.isc.org/isc.
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