dhcp-options
dhcpd-options(5) 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 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., haa-
gen.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 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
option root-path "10.0.1.4:/var/tmp/rootfs";
The domain-name data type specifies a domain name, which must not
enclosed in double quotes. This data type is not used for any exist-
ing DHCP options. The domain name is stored just as if it were a text
option.
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 string data type specifies either an NVT ASCII string enclosed in
double quotes, or a series of octets specified in hexadecimal, seper-
ated by colons. For example:
option dhcp-client-identifier "CLIENT-FOO";
or
option dhcp-client-identifier 43:4c:49:45:54:2d:46:4f:4f;
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 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:
option my-option = expression ;
For example:
option hostname = binary-to-ascii (16, 8, "-",
substring (hardware, 1, 6));
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 informa-
tion.
The standard options are:
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 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.
option arp-cache-timeout uint32;
This option specifies the timeout in seconds for ARP cache entries.
option bootfile-name text;
This option is used to identify a bootstrap file. If supported by
the client, it should have the same effect as the filename declara-
tion. BOOTP clients are unlikely to support this option. Some DHCP
clients will support it, and others actually require it.
option boot-size uint16;
This option specifies the length in 512-octet blocks of the default
boot image for the client.
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 sec-
tion 3.2.1.3 of STD 3 (RFC1122).
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 pref-
erence.
option default-ip-ttl uint8;
This option specifies the default time-to-live that the client
should use on outgoing datagrams.
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 dhcp-client-identifier 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.
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:
option dhcp-client-identifier "\0foo";
rather than:
option dhcp-client-identifier "foo";
option dhcp-max-message-size uint16;
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.
option dhcp-parameter-request-list uint16;
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 request 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.
option domain-name text;
This option specifies the domain name that client should use when
resolving hostnames via the Domain Name System.
option domain-name-servers ip-address [, ip-address... ];
The domain-name-servers option specifies a list of Domain Name Sys-
tem (STD 13, RFC 1035) name servers available to the client.
Servers should be listed in order of preference.
option extensions-path-name text;
This option specifies the name of a file containing additional
options to be interpreted according to the DHCP option format as
specified in RFC2132.
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 font-servers ip-address [, ip-address... ];
This option specifies a list of X Window System Font servers avail-
able 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 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 false indicates that the
client should use RFC 894 encapsulation. A value of true means that
the client should use RFC 1042 encapsulation.
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 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 pref-
erence.
option interface-mtu uint16;
This option specifies the MTU to use on this interface. The mini-
mum legal value for the MTU is 68.
option ip-forwarding flag;
This option specifies whether the client should configure its IP
layer for packet forwarding. A value of false means disable IP for-
warding, and a value of true means enable IP forwarding.
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 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 pref-
erence.
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 mask-supplier flag;
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.
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 merit-dump text;
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 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 nds-context string;
The nds-context option specifies the name of the initial Netware
Directory Service for an NDS client.
option nds-servers ip-address [, ip-address... ];
The nds-servers option specifies a list of IP addresses of NDS
servers.
option nds-tree-name string;
The nds-context option specifies NDS tree name that the NDS client
should use.
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-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-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 RFC1001,
RFC1002, and RFC1035 for character-set restrictions.
option nwip-domain string;
The name of the NetWare/IP domain that a NetWare/IP client should
use.
option nwip-suboptions string;
A sequence of suboptions for NetWare/IP clients - see RFC2242 for
details. Normally this option is set by specifying specific Net-
Ware/IP suboptions - see the NETWARE/IP SUBOPTIONS section for more
information.
option nis-domain text;
This option specifies the name of the client's NIS (Sun Network
Information Services) domain. The domain is formatted as a charac-
ter 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 pref-
erence.
option nisplus-domain text;
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 pref-
erence.
option nntp-server ip-address [, ip-address... ];
The NNTP server option specifies a list of NNTP available to the
client. Servers should be listed in order of preference.
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
true means allow forwarding.
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 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 perform-mask-discovery flag;
This option specifies whether or not the client should perform sub-
net mask discovery using ICMP. A value of false indicates that the
client should not perform mask discovery. A value of true means
that the client should perform mask discovery.
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 spec-
ify 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 pop-server ip-address [, ip-address... ];
The POP3 server option specifies a list of POP3 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 pref-
erence.
option root-path text;
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 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 false indicates that the client should not perform router
discovery. A value of true means that the client should perform
router discovery.
option router-solicitation-address ip-address;
This option specifies the address to which the client should trans-
mit router solicitation requests.
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 slp-directory-agent boolean ip-address [, ip-address... ];
This option specifies two things: the IP addresses of one or more
Service Location Protocol Directory Agents, and whether the use of
these addresses is mandatory. If the initial boolean value is
true, the SLP agent should just use the IP addresses given. If the
value is false, the SLP agent may additionally do active or passive
multicast discovery of SLP agents (see RFC2165 for details).
Please note that in this option and the slp-service-scope option,
the term "SLP Agent" is being used to refer to a Service Location
Protocol agent running on a machine that is being configured using
the DHCP protocol.
Also, please be aware that some companies may refer to SLP as NDS.
If you have an NDS directory agent whose address you need to config-
ure, the slp-directory-agent option should work.
option slp-service-scope boolean text;
The Service Location Protocol Service Scope Option specifies two
things: a list of service scopes for SLP, and whether the use of
this list is mandatory. If the initial boolean value is true, the
SLP agent should only use the list of scopes provided in this
option; otherwise, it may use its own static configuration in pref-
erence to the list provided in this option.
The text string should be a comma-seperated list of scopes that the
SLP agent should use. It may be omitted, in which case the SLP
Agent will use the aggregated list of scopes of all directory agents
known to the SLP 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 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 desti-
nation are specified, they are listed in descending order of prior-
ity.
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.
Also, please note that this option is not intended for classless IP
routing - it does not include a subnet mask. Since classless IP
routing is now the most widely deployed routing standard, this
option is virtually useless, and is not implemented by any of the
popular DHCP clients, for example the Microsoft DHCP client.
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.
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 pref-
erence.
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 will use the subnet mask from the subnet declara-
tion 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 swap-server ip-address;
This specifies the IP address of the client's swap server.
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 false indicates that a garbage
octet should not be sent. A value of true indicates that a garbage
octet should be sent.
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 tftp-server-name text;
This option is used to identify a TFTP server and, if supported by
the client, should have the same effect as the server-name declara-
tion. BOOTP clients are unlikely to support this option. Some
DHCP clients will support it, and others actually require it.
option time-offset int32;
The time-offset option specifies the offset of the client's subnet
in seconds from Coordinated Universal Time (UTC).
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 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 true means that the client should attempt to use trail-
ers.
option uap-servers text;
This option specifies a list of URLs, each pointing to a user
authentication service that is capable of processing authentication
requests encapsulated in the User Authentication Protocol (UAP).
UAP servers can accept either HTTP 1.1 or SSLv3 connections. If the
list includes a URL that does not contain a port component, the nor-
mal default port is assumed (i.e., port 80 for http and port 443 for
https). If the list includes a URL that does not contain a path
component, the path /uap is assumed. If more than one URL is spec-
ified in this list, the URLs are seperated by spaces.
option user-class string;
This option is used by some DHCP clients as a way for users to spec-
ify identifying information to the client. This can be used in a
similar way to the vendor-class-identifier option, but the value of
the option is specified by the user, not the vendor. Most recent
DHCP clients have a way in the user interface to specify the value
for this identifier, usually as a text string.
option vendor-class-identifier string;
This option is used by some DHCP clients to identify the vendor
type and possibly the configuration of a DHCP client. The infor-
mation is a string of bytes whose contents are specific to the
vendor and are not specified in a standard. To see what vendor
class identifier a clients are sending, you can write the follow-
ing in your DHCP server configuration file:
set vendor-class option vendor-class-identifier;
This will result in all entries in the DHCP server lease database
file for clients that sent vendor-class-identifier options having
a set statement that looks something like this:
set vendor-class "SUNW.Ultra-5_10";
The vendor-class-identifier option is normally used by the DHCP
server to determine the options that are returned in the vendor-
encapsulated-options option. Please see the VENDOR ENCAPSULATED
OPTIONS section of the dhcpd.conf manual page for further informa-
tion.
option vendor-encapsulated-options string;
The vendor-encapsulated-options option can contain either a single
vendor-specific value or one or more vendor-specific suboptions.
This option is not normally specified in the DHCP server configu-
ration file - instead, a vendor class is defined for each vendor,
vendor class suboptions are defined, values for those suboptions
are defined, and the DHCP server makes up a response on that
basis.
Some default behaviours for well-known DHCP client vendors (cur-
rently, the Microsoft Windows 2000 DHCP client) are configured
automatically, but otherwise this must be configured manually -
see the VENDOR ENCAPSULATED OPTIONS section of the dhcpd.conf man-
ual page for details.
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 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.
RELAY AGENT INFORMATION OPTION
An IETF draft, draft-ietf-dhc-agent-options-11.txt, defines a series of
encapsulated options that a relay agent can add to a DHCP packet when
relaying it to the DHCP server. The server can then make address
allocation decisions (or whatever other decisions it wants) based on
these options. The server also returns these options in any replies
it sends through the relay agent, so that the relay agent can use the
information in these options for delivery or accounting purposes.
The current draft defines two options. To reference these options in
the dhcp server, specify the option space name, "agent", followed by a
period, followed by the option name. It is not normally useful to
define values for these options in the server, although it is permissi-
ble. These options are not supported in the client.
option agent.circuit-id string;
The circuit-id suboption encodes an agent-local identifier of the
circuit from which a DHCP client-to-server packet was received. It
is intended for use by agents in relaying DHCP responses back to the
proper circuit. The format of this option is currently defined to
be vendor-dependent, and will probably remain that way, although the
current draft allows for for the possibility of standardizing the
format in the future.
option agent.remote-id string;
The remote-id suboption encodes information about the remote host
end of a circuit. Examples of what it might contain include caller
ID information, username information, remote ATM address, cable
modem ID, and similar things. In principal, the meaning is not
well-specified, and it should generally be assumed to be an opaque
object that is administratively guaranteed to be unique to a partic-
ular remote end of a circuit.
THE CLIENT FQDN SUBOPTIONS
The Client FQDN option, currently defined in the Internet Draft draft-
ietf-dhc-fqdn-option-00.txt is not a standard yet, but is in suffi-
ciently wide use already that we have implemented it. Due to the com-
plexity of the option format, we have implemented it as a suboption
space rather than a single option. In general this option should not
be configured by the user - instead it should be used as part of an
automatic DNS update system.
option fqdn.no-client-update flag;
When the client sends this, if it is true, it means the client will
not attempt to update its A record. When sent by the server to the
client, it means that the client should not update its own A record.
option fqdn.server-update flag;
When the client sends this to the server, it is requesting that the
server update its A record. When sent by the server, it means that
the server has updated (or is about to update) the client's A
record.
option fqdn.encoded flag;
If true, this indicates that the domain name included in the option
is encoded in DNS wire format, rather than as plain ASCII text.
The client normally sets this to false if it doesn't support DNS
wire format in the FQDN option. The server should always send back
the same value that the client sent. When this value is set on the
configuration side, it controls the format in which the fqdn.fqdn
suboption is encoded.
option fqdn.rcode1 flag;
option fqdn.rcode1 flag;
These options specify the result of the updates of the A and PTR
records, respectively, and are only sent by the DHCP server to the
DHCP client. The values of these fields are those defined in the
DNS protocol specification.
option fqdn.fqdn text;
Specifies the domain name that the client wishes to use. This can
be a fully-qualified domain name, or a single label. If there is
no trailing generally update that name in some locally-defined
domain.
If you wish to use any of these suboptions, we strongly recommend that
you refer to the Client FQDN option draft (or standard, when it becomes
a standard) - the documentation here is sketchy and incomplete in com-
parison, and is just intended for reference by people who already
understand the Client FQDN option specification.
THE NETWARE/IP SUBOPTIONS
RFC2242 defines a set of encapsulated options for Novell NetWare/IP
clients. To use these options in the dhcp server, specify the option
space name, "nwip", followed by a period, followed by the option name.
The following options can be specified:
option nwip.nsq-broadcast flag;
If true, the client should use the NetWare Nearest Server Query to
locate a NetWare/IP server. The behaviour of the Novell client if
this suboption is false, or is not present, is not specified.
option nwip.preferred-dss ip-address [, ip-address... ];
This suboption specifies a list of up to five IP addresses, each of
which should be the IP address of a NetWare Domain SAP/RIP server
(DSS).
option nwip.nearest-nwip-server ip-address
[, ip-address...];
This suboption specifies a list of up to five IP addresses, each of
which should be the IP address of a Nearest NetWare IP server.
option nwip.autoretries uint8;
Specifies the number of times that a NetWare/IP client should
attempt to communicate with a given DSS server at startup.
option nwip.autoretry-secs uint8;
Specifies the number of seconds that a Netware/IP client should wait
between retries when attempting to establish communications with a
DSS server at startup.
option nwip.nwip-1-1 uint8;
If true, the NetWare/IP client should support NetWare/IP version 1.1
compatibility. This is only needed if the client will be contact-
ing Netware/IP version 1.1 servers.
option nwip.primary-dss ip-address;
Specifies the IP address of the Primary Domain SAP/RIP Service
server (DSS) for this NetWare/IP domain. The NetWare/IP adminis-
tration utility uses this value as Primary DSS server when configur-
ing a secondary DSS server.
DEFINING NEW OPTIONS
The Internet Software Consortium DHCP client and server provide the
capability to define new options. Each DHCP option has a name, a
code, and a structure. The name is used by you to refer to the
option. The code is a number, used by the DHCP server and client to
refer to an option. The structure describes what the contents of an
option looks like.
To define a new option, you need to choose a name for it that is not in
use for some other option - for example, you can't use "host-name"
because the DHCP protocol already defines a host-name option, which is
documented earlier in this manual page. If an option name doesn't
appear in this manual page, you can use it, but it's probably a good
idea to put some kind of unique string at the beginning so you can be
sure that future options don't take your name. For example, you might
define an option, "local-host-name", feeling some confidence that no
official DHCP option name will ever start with "local".
Once you have chosen a name, you must choose a code. For site-local
options, all codes between 128 and 254 are reserved for DHCP options,
so you can pick any one of these. In practice, some vendors have
interpreted the protocol rather loosely and have used option code val-
ues greater than 128 themselves. There's no real way to avoid this
problem, but it's not likely to cause too much trouble in practice.
The structure of an option is simply the format in which the option
data appears. The ISC DHCP server currently supports a few simple
types, like integers, booleans, strings and IP addresses, and it also
supports the ability to define arrays of single types or arrays of
fixed sequences of types.
New options are declared as follows:
option new-name code new-code = definition ;
The values of new-name and new-code should be the name you have chosen
for the new option and the code you have chosen. The definition
should be the definition of the structure of the option.
The following simple option type definitions are supported:
BOOLEAN
option new-name code new-code = boolean ;
An option of type boolean is a flag with a value of either on or off
(or true or false). So an example use of the boolean type would be:
option use-zephyr code 180 = boolean;
option use-zephyr on;
INTEGER
option new-name code new-code = sign integer width ;
The sign token should either be blank, unsigned or signed. The width
can be either 8, 16 or 32, and refers to the number of bits in the
integer. So for example, the following two lines show a definition of
the sql-connection-max option and its use:
option sql-connection-max code 192 = unsigned integer 16;
option sql-connection-max 1536;
IP-ADDRESS
option new-name code new-code = ip-address ;
An option whose structure is an IP address can be expressed either as a
domain name or as a dotted quad. So the following is an example use of
the ip-address type:
option sql-server-address code 193 = ip-address;
option sql-server-address sql.example.com;
TEXT
option new-name code new-code = text ;
An option whose type is text will encode an ASCII text string. For
example:
option sql-default-connection-name code 194 = text;
option sql-default-connection-name "PRODZA";
DATA STRING
option new-name code new-code = string ;
An option whose type is a data string is essentially just a collection
of bytes, and can be specified either as quoted text, like the text
type, or as a list of hexadecimal contents seperated by colons whose
values must be between 0 and FF. For example:
option sql-identification-token code 195 = string;
option sql-identification-token 17:23:19:a6:42:ea:99:7c:22;
ENCAPSULATION
option new-name code new-code = encapsulate identifier ;
An option whose type is encapsulate will encapsulate the contents of
the option space specified in identifier. Examples of encapsulated
options in the DHCP protocol as it currently exists include the vendor-
encapsulated-options option, the netware-suboptions option and the
relay-agent-information option.
option space local;
option local.demo code 1 = text;
option local-encapsulation code 197 = encapsulate local;
option local.demo "demo";
ARRAYS
Options can contain arrays of any of the above types except for the
text and data string types, which aren't currently supported in arrays.
An example of an array definition is as follows:
option kerberos-servers code 200 = array of ip-address;
option kerberos-servers 10.20.10.1, 10.20.11.1;
RECORDS
Options can also contain data structures consisting of a sequence of
data types, which is sometimes called a record type. For example:
option contrived-001 code 201 = { boolean, integer 32, text };
option contrived-001 on 1772 "contrivance";
It's also possible to have options that are arrays of records, for
example:
option new-static-routes code 201 = array of {
ip-address, ip-address, ip-address, integer 8 };
option static-routes
10.0.0.0 255.255.255.0 net-0-rtr.example.com 1,
10.0.1.0 255.255.255.0 net-1-rtr.example.com 1,
10.2.0.0 255.255.224.0 net-2-0-rtr.example.com 3;
VENDOR ENCAPSULATED OPTIONS
The DHCP protocol defines the vendor-encapsulated-options option,
which allows vendors to define their own options that will be sent
encapsulated in a standard DHCP option. The format of the vendor-
encapsulated-options option is either a series of bytes whose format is
not specified, or a sequence of options, each of which consists of a
single-byte vendor-specific option code, followed by a single-byte
length, followed by as many bytes of data as are specified in the
length (the length does not include itself or the option code).
The value of this option can be set in one of two ways. The first way
is to simply specify the data directly, using a text string or a colon-
seperated list of hexadecimal values. For example:
option vendor-encapsulated-options
2:4:AC:11:41:1:
3:12:73:75:6e:64:68:63:70:2d:73:65:72:76:65:72:31:37:2d:31:
4:12:2f:65:78:70:6f:72:74:2f:72:6f:6f:74:2f:69:38:36:70:63;
The second way of setting the value of this option is to have the DHCP
server generate a vendor-specific option buffer. To do this, you must
do four things: define an option space, define some options in that
option space, provide values for them, and specify that that option
space should be used to generate the vendor-encapsulated-options
option.
To define a new option space in which vendor options can be stored, use
the option space statement:
option space name ;
The name can then be used in option definitions, as described earlier
in this document. For example:
option space SUNW;
option SUNW.server-address code 2 = ip-address;
option SUNW.server-name code 3 = text;
option SUNW.root-path code 4 = text;
Once you have defined an option space and the format of some options,
you can set up scopes that define values for those options, and you can
say when to use them. For example, suppose you want to handle two
different classes of clients. Using the option space definition shown
in the previous example, you can send different option values to dif-
ferent clients based on the vendor-class-identifier option that the
clients send, as follows:
class "vendor-classes" {
match option vendor-class-identifier;
}
option SUNW.server-address 172.17.65.1;
option SUNW.server-name "sundhcp-server17-1";
subclass "vendor-classes" "SUNW.Ultra-5_10" {
vendor-option-space SUNW;
option SUNW.root-path "/export/root/sparc";
}
subclass "vendor-classes" "SUNW.i86pc" {
vendor-option-space SUNW;
option SUNW.root-path "/export/root/i86pc";
}
As you can see in the preceding example, regular scoping rules apply,
so you can define values that are global in the global scope, and only
define values that are specific to a particular class in the local
scope. The vendor-option-space declaration tells the DHCP server to
use options in the SUNW option space to construct the vendor-encapsu-
lated-options option.
SEE ALSO
dhcpd.conf(5), dhcpd.leases(5), dhclient.conf(5), dhcp-eval(5),
dhcpd(8), dhclient(8), RFC2132, RFC2131, draft-ietf-dhc-agent-
options-??.txt.
AUTHOR
The Internet Software Consortium DHCP Distribution was written by Ted
Lemon under a contract with Vixie Labs. Funding for this project was
provided through the Internet Software Consortium. Information about
the Internet Software Consortium can be found at http://www.isc.org.
dhcpd-options(5)
