dhcp-options(5) 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 spec-
ify 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 ip6-address data specifies an IPv6 address, like ::1 or
3ffe:bbbb:aaaa:aaaa::1.
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 be
enclosed in double quotes. The domain name is stored just as if it
were a text option.
The domain-list data type specifies a list of domain names, enclosed in
double quotes and separated by commas ("example.com", "foo.exam-
ple.com").
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, sepa-
rated 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 destination-descriptor describe the IP subnet number and subnet
mask of a particular destination using a compact encoding. This encod-
ing consists of one octet describing the width of the subnet mask, fol-
lowed by all the significant octets of the subnet number. The follow-
ing table contains some examples of how various subnet number/mask com-
binations can be encoded:
Subnet number Subnet mask Destination descriptor
0 0 0
10.0.0.0 255.0.0.0 8.10
10.0.0.0 255.255.255.0 24.10.0.0
10.17.0.0 255.255.0.0 16.10.17
10.27.129.0 255.255.255.0 24.10.27.129
10.229.0.128 255.255.255.128 25.10.229.0.128
10.198.122.47 255.255.255.255 32.10.198.122.47
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));
INCLUDING OPTION DEFINITIONS
Starting with 4.3.0 when ISC adds new option definitions those defini-
tions will be included in the code based on the definition of an argu-
ment for the RFC that defines the option in includes/site.h. This pro-
vides you with a method for over-riding the ISC definitions if neces-
sary - for example if you have previously defined the option with a
different format using the mechanism from DEFINING NEW OPTIONS below.
By default all of the options are enabled. In order to disable an
option you would edit the includes/site.h file and comment out the def-
inition for the proper RFC.
STANDARD DHCPV4 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.
Some of the options documented here are automatically generated by the
DHCP server or by clients, and cannot be configured by the user. The
value of such an option can be used in the configuration file of the
receiving DHCP protocol agent (server or client), for example in condi-
tional expressions. However, the value of the option cannot be used in
the configuration file of the sending agent, because the value is
determined only after the configuration file has been processed. In the
following documentation, such options will be shown as "not user con-
figurable"
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 associated-ip ip-address [, ip-address... ];
This option is part of lease query. It is used to return all of the
IP addresses associated with a given DHCP client.
This option is not user configurable.
option bcms-controller-address ip-address [, ip-address... ];
This option configures a list of IPv4 addresses for use as Broadcast
and Multicast Controller Servers ("BCMS").
option bcms-controller-names domain-list;
This option contains the domain names of local Broadcast and Multi-
cast Controller Servers ("BCMS") controllers which the client may
use.
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 capwap-ac-v4 ip-address [, ip-address ... ] ;
A list of IPv4 addresses of CAPWAP ACs that the WTP may use. The
addresses are listed in preference order.
This option is included based on RFC 5417.
option client-last-transaction-time uint32;
This option is part of lease query. It allows the receiver to deter-
mine the time of the most recent access by the client. The value is
a duration in seconds from when the client last communicated with the
DHCP server.
This option is not user configurable.
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 default-url string;
The format and meaning of this option is not described in any stan-
dards document, but is claimed to be in use by Apple Computer. It is
not known what clients may reasonably do if supplied with this
option. Use at your own risk.
option dhcp-client-identifier string;
This option can be used to specify 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-lease-time uint32;
This option is used in a client request (DHCPDISCOVER or DHCPREQUEST)
to allow the client to request a lease time for the IP address. In a
server reply (DHCPOFFER), a DHCP server uses this option to specify
the lease time it is willing to offer.
This option is not directly user configurable in the server; refer to
the max-lease-time and default-lease-time server options in
dhcpd.conf(5).
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-message text;
This option is used by a DHCP server to provide an error message to a
DHCP client in a DHCPNAK message in the event of a failure. A client
may use this option in a DHCPDECLINE message to indicate why the
client declined the offered parameters.
This option is not user configurable.
option dhcp-message-type uint8;
This option, sent by both client and server, specifies the type of
DHCP message contained in the DHCP packet. Possible values (taken
directly from RFC2132) are:
1 DHCPDISCOVER
2 DHCPOFFER
3 DHCPREQUEST
4 DHCPDECLINE
5 DHCPACK
6 DHCPNAK
7 DHCPRELEASE
8 DHCPINFORM
This option is not user configurable.
option dhcp-option-overload uint8;
This option is used to indicate that the DHCP 'sname' or 'file'
fields are being overloaded by using them to carry DHCP options. A
DHCP server inserts this option if the returned parameters will
exceed the usual space allotted for options.
If this option is present, the client interprets the specified addi-
tional fields after it concludes interpretation of the standard
option fields.
Legal values for this option are:
1 the 'file' field is used to hold options
2 the 'sname' field is used to hold options
3 both fields are used to hold options
This option is not user configurable.
option dhcp-parameter-request-list uint8 [, uint8... ];
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 lim-
ited set of options than those the server would normally return.
option dhcp-rebinding-time uint32;
This option specifies the number of seconds from the time a client
gets an address until the client transitions to the REBINDING state.
This option is user configurable, but it will be ignored if the value
is greater than or equal to the lease time.
To make DHCPv4+DHCPv6 migration easier in the future, any value con-
figured in this option is also used as a DHCPv6 "T1" (renew) time.
option dhcp-renewal-time uint32;
This option specifies the number of seconds from the time a client
gets an address until the client transitions to the RENEWING state.
This option is user configurable, but it will be ignored if the value
is greater than or equal to the rebinding time, or lease time.
To make DHCPv4+DHCPv6 migration easier in the future, any value con-
figured in this option is also used as a DHCPv6 "T2" (rebind) time.
option dhcp-requested-address ip-address;
This option is used by the client in a DHCPDISCOVER to request that a
particular IP address be assigned.
This option is not user configurable.
option dhcp-server-identifier ip-address;
This option is used in DHCPOFFER and DHCPREQUEST messages, and may
optionally be included in the DHCPACK and DHCPNAK messages. DHCP
servers include this option in the DHCPOFFER in order to allow the
client to distinguish between lease offers. DHCP clients use the
contents of the 'server identifier' field as the destination address
for any DHCP messages unicast to the DHCP server. DHCP clients also
indicate which of several lease offers is being accepted by including
this option in a DHCPREQUEST message.
The value of this option is the IP address of the server.
This option is not directly user configurable. See the server-identi-
fier server option in dhcpd.conf(5).
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 System
(STD 13, RFC 1035) name servers available to the client. Servers
should be listed in order of preference.
option domain-search domain-list;
The domain-search option specifies a 'search list' of Domain Names to
be used by the client to locate not-fully-qualified domain names.
The difference between this option and historic use of the domain-
name option for the same ends is that this option is encoded in
RFC1035 compressed labels on the wire. For example:
option domain-search "example.com", "sales.example.com",
"eng.example.com";
option extensions-path 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 servers 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 geoconf-civic string;
A string to hold the geoconf civic structure.
This option is included based on RFC 4776.
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. This option is only honored by dhclient-
script(8) if the hostname for the client machine is not set.
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 minimum
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 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 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 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 name-service-search uint16 [, uint6... ];
This option specifies a list of name services in the order the client
should attempt to use them.
This option is included based on RFC 2937.
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-tree-name 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 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 netinfo-server-address ip-address [, ip-address... ];
The netinfo-server-address option has not been described in any RFC,
but has been allocated (and is claimed to be in use) by Apple Comput-
ers. It's hard to say if the above is the correct format, or what
clients might be expected to do if values were configured. Use at
your own risk.
option netinfo-server-tag text;
The netinfo-server-tag option has not been described in any RFC, but
has been allocated (and is claimed to be in use) by Apple Computers.
It's hard to say if the above is the correct format, or what clients
might be expected to do if values were configured. Use at your own
risk.
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 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 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 servers 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 false 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
5905) servers available to the client. Servers should be listed in
order of preference.
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 option-6rd uint8 uint8 ip6-address ip-address [, ip-address
...];
This option contains information about the rapid deployment option.
It is 8 bits of ipv4 mask length, 8 bits of 6rd prefix length, an
ipv6 prefix as an ipv6 address and a list of one or more ipv4
addresses.
This option is included based on RFC 5969.
option pana-agent ip-address [, ip-address ... ] ;
A set of IPv4 addresses of a PAA for the client to use. The
addresses are listed in preferred order.
This option is included based on RFC 5192.
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 pcode text;
This option specifies a string suitable for the TZ variable.
This option is included based on RFC 4833.
option perform-mask-discovery flag;
This option specifies whether or not the client should perform subnet
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 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 pop-server ip-address [, ip-address... ];
The POP3 server option specifies a list of POP3 servers available to
the client. Servers should be listed in order of preference.
option rdnss-selection uint8 ip-address ip-address domain-name;
The rdnss-selection option specifies an 8 bit flags field, a primary
and secondary ip address for the name server and a domainlist of
domains for which the RDNSS has special knowledge.
This option is included based on RFC 6731.
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 transmit
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 multi-
cast 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 Proto-
col 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 preference to
the list provided in this option.
The text string should be a comma-separated 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 virtu-
ally useless, and is not implemented by any of the popular DHCP
clients, for example the Microsoft DHCP client.
NOTE to Fedora dhclient users:
dhclient-script interprets trailing 0 octets of the target as indi-
cating the subnet class of the route, so for the following static-
routes value:
option static-routes 172.0.0.0 172.16.2.254,
192.168.0.0 192.168.2.254;
dhclient-script will create routes:
172/8 via 172.16.2.254 dev $interface
192.168/16 via 192.168.2.254 dev $interface
option classless-static-routes destination-descriptor ip-address
[, destination-descriptor ip-address...];
This option (see RFC3442) specifies a list of classless static routes
that the client should install in its routing cache.
This option can contain one or more static routes, each of which con-
sists of a destination descriptor and the IP address of the router
that should be used to reach that destination.
Many clients may not implement the Classless Static Routes option.
DHCP server administrators should therefore configure their DHCP
servers to send both a Router option and a Classless Static Routes
option, and should specify the default router(s) both in the Router
option and in the Classless Static Routes option.
If the DHCP server returns both a Classless Static Routes option and
a Router option, the DHCP client ignores the Router option.
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. 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 subnet-selection ip-address;
Sent by the client if an address is required in a subnet other than
the one that would normally be selected (based on the relaying
address of the connected subnet the request is obtained from). See
RFC3011. Note that the option number used by this server is 118; this
has not always been the defined number, and some clients may use a
different value. Use of this option should be regarded as slightly
experimental!
This option is not user configurable in the server.
option swap-server ip-address;
This specifies the IP address of the client's swap server.
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 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 tcode text;
This option specifies a name of a zone entry in the TZ database.
This option is included based on RFC 4833.
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 false indicates that the client should not attempt to use trail-
ers. A value of true means that the client should attempt to use
trailers.
option uap-servers text;
This option specifies a list of URLs, each pointing to a user authen-
tication 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 normal
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 com-
ponent, the path /uap is assumed. If more than one URL is specified
in this list, the URLs are separated 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 v4-access-domain domain-name;
The domain name associated with the access network for use with LIS
Discovery.
This option is included based on RFC 5986.
option v4-lost domain-name;
The domain name of the LoST server for the client to use.
This option is included based on RFC 5223.
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 information 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
clients are sending, you can write the following in your DHCP server
configuration file:
set vendor-string = 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-string = "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 later in this manual page for further information.
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 configura-
tion file - instead, a vendor class is defined for each vendor, ven-
dor 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 auto-
matically, but otherwise this must be configured manually - see the
VENDOR ENCAPSULATED OPTIONS section later in this manual page for
details.
option vivso string;
The vivso option can contain multiple separate options, one for each
32-bit Enterprise ID. Each Enterprise-ID discriminated option then
contains additional options whose format is defined by the vendor who
holds that ID. This option is usually not configured manually, but
rather is configured via intervening option definitions. Please also
see the VENDOR ENCAPSULATED OPTIONS section later in this manual page
for details.
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 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.
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 allo-
cation 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 informa-
tion 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 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-speci-
fied, and it should generally be assumed to be an opaque object that
is administratively guaranteed to be unique to a particular remote
end of a circuit.
option agent.DOCSIS-device-class uint32;
The DOCSIS-device-class suboption is intended to convey information
about the host endpoint, hardware, and software, that either the host
operating system or the DHCP server may not otherwise be aware of
(but the relay is able to distinguish). This is implemented as a
32-bit field (4 octets), each bit representing a flag describing the
host in one of these ways. So far, only bit zero (being the least
significant bit) is defined in RFC3256. If this bit is set to one,
the host is considered a CPE Controlled Cable Modem (CCCM). All
other bits are reserved.
option agent.link-selection ip-address;
The link-selection suboption is provided by relay agents to inform
servers what subnet the client is actually attached to. This is use-
ful in those cases where the giaddr (where responses must be sent to
the relay agent) is not on the same subnet as the client. When this
option is present in a packet from a relay agent, the DHCP server
will use its contents to find a subnet declared in configuration, and
from here take one step further backwards to any shared-network the
subnet may be defined within; the client may be given any address
within that shared network, as normally appropriate.
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 con-
figuration side, it controls the format in which the fqdn.fqdn subop-
tion is encoded.
option fqdn.rcode1 flag;
option fqdn.rcode2 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 '.' character in the name, it is not fully-qualified, and
the server will generally update that name in some locally-defined
domain.
option fqdn.hostname --never set--;
This option should never be set, but it can be read back using the
option and config-option operators in an expression, in which case it
returns the first label in the fqdn.fqdn suboption - for example, if
the value of fqdn.fqdn is "foo.example.com.", then fqdn.hostname will
be "foo".
option fqdn.domainname --never set--;
This option should never be set, but it can be read back using the
option and config-option operators in an expression, in which case it
returns all labels after the first label in the fqdn.fqdn suboption -
for example, if the value of fqdn.fqdn is "foo.example.com.", then
fqdn.domainname will be "example.com.". If this suboption value is
not set, it means that an unqualified name was sent in the fqdn
option, or that no fqdn option was sent at all.
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 contacting
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 administration
utility uses this value as Primary DSS server when configuring a sec-
ondary DSS server.
STANDARD DHCPV6 OPTIONS
DHCPv6 options differ from DHCPv4 options partially due to using 16-bit
code and length tags, but semantically zero-length options are legal in
DHCPv6, and multiple options are treated differently. Whereas in
DHCPv4 multiple options would be concatenated to form one option, in
DHCPv6 they are expected to be individual instantiations. Understand-
ably, many options are not "allowed" to have multiple instances in a
packet - normally these are options which are digested by the DHCP pro-
tocol software, and not by users or applications.
option dhcp6.client-id string;
This option specifies the client's DUID identifier. DUIDs are simi-
lar but different from DHCPv4 client identifiers - there are docu-
mented duid types:
duid-llt
duid-en
duid-ll
This value should not be configured, but rather is provided by
clients and treated as an opaque identifier key blob by servers.
option dhcp6.server-id string;
This option specifies the server's DUID identifier. One may use this
option to configure an opaque binary blob for your server's identi-
fier.
option dhcp6.ia-na string;
The Identity Association for Non-temporary Addresses (ia-na) carries
assigned addresses that are not temporary addresses for use by the
DHCPv6 client. This option is produced by the DHCPv6 server soft-
ware, and should not be configured.
option dhcp6.ia-ta string;
The Identity Association for Temporary Addresses (ia-ta) carries tem-
porary addresses, which may change upon every renewal. There is no
support for this in the current DHCPv6 software.
option dhcp6.ia-addr string;
The Identity Association Address option is encapsulated inside ia-na
or ia-ta options in order to represent addresses associated with
those IA's. These options are manufactured by the software, so
should not be configured.
option dhcp6.oro uint16 [ , uint16, ... ];
The Option Request Option ("ORO") is the DHCPv6 equivalent of the
parameter-request-list. Clients supply this option to ask servers to
reply with options relevant to their needs and use. This option must
not be directly configured, the request syntax in dhclient.conf (5)
should be used instead.
option dhcp6.preference uint8;
The preference option informs a DHCPv6 client which server is 'pre-
ferred' for use on a given subnet. This preference is only applied
during the initial stages of configuration - once a client is bound
to an IA, it will remain bound to that IA until it is no longer valid
or has expired. This value may be configured on the server, and is
digested by the client software.
option dhcp6.elapsed-time uint16;
The elapsed-time option is constructed by the DHCPv6 client software,
and is potentially consumed by intermediaries. This option should
not be configured.
option dhcp6.relay-msg string;
The relay-msg option is constructed by intervening DHCPv6 relay agent
software. This option is entirely used by protocol software, and is
not meant for user configuration.
option dhcp6.unicast ip6-address;
The unicast option is provided by DHCPv6 servers which are willing
(or prefer) to receive Request, Renew, Decline, and Release packets
from their clients via unicast. Normally, DHCPv6 clients will multi-
cast these messages. Per RFC 3315, the server will reject a unicast
message received from a client unless it previously sent (or would
have sent) the unicast option to that client. This option may be
configured on the server at the global and shared network level.
When a unicast message is received, the server will check for an
applicable definition of the unicast option. If such an option is
found the message will be accepted, if not it will be rejected.
option dhcp6.status-code status-code [ string ] ;
The status-code option is provided by DHCPv6 servers to inform
clients of error conditions during protocol communication. This
option is manufactured and digested by protocol software, and should
not be configured.
option dhcp6.rapid-commit ;
The rapid-commit option is a zero-length option that clients use to
indicate their desire to enter into rapid-commit with the server.
option dhcp6.vendor-opts string;
The vendor-opts option is actually an encapsulated sub-option space,
in which each Vendor-specific Information Option (VSIO) is identified
by a 32-bit Enterprise-ID number. The encapsulated option spaces
within these options are defined by the vendors.
To make use of this option, the best way is to examine the section
titled VENDOR ENCAPSULATED OPTIONS below, in particular the bits
about the "vsio" option space.
option dhcp6.interface-id string;
The interface-id option is manufactured by relay agents, and may be
used to guide configuration differentiating clients by the interface
they are remotely attached to. It does not make sense to configure a
value for this option, but it may make sense to inspect its contents.
option dhcp6.reconf-msg dhcpv6-message;
The reconf-msg option is manufactured by servers, and sent to clients
in Reconfigure messages to inform them of what message the client
should Reconfigure using. There is no support for DHCPv6 Reconfigure
extensions, and this option is documented informationally only.
option dhcp6.reconf-accept ;
The reconf-accept option is included by DHCPv6 clients that support
the Reconfigure extensions, advertising that they will respond if the
server were to ask them to Reconfigure. There is no support for
DHCPv6 Reconfigure extensions, and this option is documented informa-
tionally only.
option dhcp6.sip-servers-names domain-list;
The sip-servers-names option allows SIP clients to locate a local SIP
server that is to be used for all outbound SIP requests, a so-
called"outbound proxy server." If you wish to use manually entered
IPv6 addresses instead, please see the sip-servers-addresses option
below.
option dhcp6.sip-servers-addresses ip6-address [, ip6-address ... ] ;
The sip-servers-addresses option allows SIP clients to locate a local
SIP server that is to be used for all outbound SIP requests, a so-
called "outbound proxy servers." If you wish to use domain names
rather than IPv6 addresses, please see the sip-servers-names option
above.
option dhcp6.name-servers ip6-address [, ip6-address ... ] ;
The name-servers option instructs clients about locally available
recursive DNS servers. It is easiest to describe this as the "name-
server" line in /etc/resolv.conf.
option dhcp6.domain-search domain-list;
The domain-search option specifies the client's domain search path to
be applied to recursive DNS queries. It is easiest to describe this
as the "search" line in /etc/resolv.conf.
option dhcp6.ia-pd string;
The ia-pd option is manufactured by clients and servers to create a
Prefix Delegation binding - to delegate an IPv6 prefix to the client.
It is not directly edited in dhcpd.conf(5) or dhclient.conf(5), but
rather is manufactured and consumed by the software.
option dhcp6.ia-prefix string;
The ia-prefix option is placed inside ia-pd options in order to iden-
tify the prefix(es) allocated to the client. It is not directly
edited in dhcpd.conf(5) or dhclient.conf(5), but rather is manufac-
tured and consumed by the software.
option dhcp6.nis-servers ip6-address [, ip6-address ... ] ;
The nis-servers option identifies, in order, NIS servers available to
the client.
option dhcp6.nisp-servers ip6-address [, ip6-address ... ] ;
The nisp-servers option identifies, in order, NIS+ servers available
to the client.
option nis-domain-name domain-list;
The nis-domain-name option specifies the NIS domain name the client
is expected to use, and is related to the nis-servers option.
option dhcp6.nis-domain-name domain-name;
The dhcp6.nis-domain-name option specifies NIS domain name the client
is expected to use, and is related to dhcp6.nis-servers option.
option nisp-domain-name domain-list;
The nisp-domain-name option specifies the NIS+ domain name the client
is expected to use, and is related to the nisp-servers option.
option dhcp6.nisp-domain-name domain-name;
The dhcp6.nis-domain-name option specifies NIS+ domain name the
client is expected to use, and is related to dhcp6.nisp-servers
option.
option dhcp6.sntp-servers ip6-address [, ip6-address ... ] ;
The sntp-servers option specifies a list of local SNTP servers avail-
able for the client to synchronize their clocks.
option dhcp6.info-refresh-time uint32;
The info-refresh-time option gives DHCPv6 clients using Information-
request messages a hint as to how long they should between refreshing
the information they were given. Note that this option will only be
delivered to the client, and be likely to affect the client's behav-
iour, if the client requested the option.
option dhcp6.bcms-server-d domain-list;
The bcms-server-d option contains the domain names of local BCMS
(Broadcast and Multicast Control Services) controllers which the
client may use.
option dhcp6.bcms-server-a ip6-address [, ip6-address ... ] ;
The bcms-server-a option contains the IPv6 addresses of local BCMS
(Broadcast and Multicast Control Services) controllers which the
client may use.
option dhcp6.geoconf-civic string;
A string to hold the geoconf civic structure.
This option is included based on RFC 4776.
option dhcp6.remote-id string;
The remote-id option is constructed by relay agents, to inform the
server of details pertaining to what the relay knows about the client
(such as what port it is attached to, and so forth). The contents of
this option have some vendor-specific structure (similar to VSIO),
but we have chosen to treat this option as an opaque field.
option dhcp6.subscriber-id string;
The subscriber-id option is an opaque field provided by the relay
agent, which provides additional information about the subscriber in
question. The exact contents of this option depend upon the vendor
and/or the operator's configuration of the remote device, and as such
is an opaque field.
option dhcp6.fqdn string;
The fqdn option is normally constructed by the client or server, and
negotiates the client's Fully Qualified Domain Name, as well as which
party is responsible for Dynamic DNS Updates. See the section on the
Client FQDN SubOptions for full details (the DHCPv4 and DHCPv6 FQDN
options use the same "fqdn." encapsulated space, so are in all ways
identical).
option dhcp6.pana-agent ip6-address [, ip6-address ... ] ;
A set of IPv6 addresses of a PAA for the client to use. The
addresses are listed in preferred order.
This option is included based on RFC 5192.
option dhcp6.new-posix-timezone text;
This option specifies a string suitable for the TZ variable.
This option is included based on RFC 4833.
option dhcp6.new-tzdb-timezone text;
This option specifies a name of a zone entry in the TZ database.
This option is included based on RFC 4833.
option dhcp6.ero uint16 [, uint16 ... ] ;
A list of the options requested by the relay agent.
This option is included based on RFC 4994.
option dhcp6.lq-query string;
The lq-query option is used internally for lease query.
option dhcp6.client-data string;
The client-data option is used internally for lease query.
option dhcp6.clt-time uint32;
The clt-time option is used internally for lease query.
option dhcp6.lq-relay-data ip6-address string;
The lq-relay-data option is used internally for lease query.
option dhcp6.lq-client-link ip6-address [, ip6-address ... ] ;
The lq-client-link option is used internally for lease query.
option dhcp6.v6-lost domain-name;
The domain name of the LoST server for the client to use.
This option is included based on RFC 5223.
option dhcp6.capwap-ac-v6 ip6-address [, ip6-address ... ] ;
A list of IPv6 addresses of CAPWAP ACs that the WTP may use. The
addresses are listed in preference order.
This option is included based on RFC 5417.
option dhcp6.relay-id string;
The DUID for the relay agent.
This option is included based on RFC 5460.
option dhcp6.v6-access-domain domain-name;
The domain name associated with the access network for use with LIS
Discovery.
This option is included based on RFC5986.
option dhcp6.sip-ua-cs-list domain-list;
The list of domain names in the SIP User Agent Configuration Service
Domains.
This option is included based on RFC 6011.
option dhcp6.bootfile-url text;
The URL for a boot file.
This option is included based on RFC 5970.
option dhcp6.bootfile-param string;
A string for the parameters to the bootfile. See RFC 5970 for more
description of the layout of the parameters within the string.
This option is included based on RFC 5970.
option dhcp6.client-arch-type uint16 [, uint16 ... ] ;
A list of one or more architecture types described as 16 bit values.
This option is included based on RFC 5970.
option dhcp6.nii uint8 uint8 uint8;
The client network interface identitier option supplies information
about a client's level of UNDI support. The values are, in order,
the type, the major value and the minor value.
This option is included based on RFC5970.
option dhcp6.aftr-name domain-name;
A domain name of the AFTR tunnel endpoint.
This option is included based on RFC 6334.
option dhcp6.erp-local-domain-name domain-name;
A domain name for the ERP domain.
This option is included based on RFC 6440.
option dhcp6.rdnss-selection ip6-address uint8 domain-name;
RDNSS information consists of an IPv6 address of RDNSS, an 8 bit
flags field and a domain-list of domains for which the RDNSS has spe-
cial knowledge.
This option is included based on RFC 6731.
option dhcp6.client-linklayer-addr string;
A client link-layer address. The first two bytes must be the type of
the link-layer followed by the address itself.
This option is included based on RFC 6939.
option dhcp6.link-address ip6-address;
An IPv6 address used by a relay agent to indicate to the server the
link on which the client is located.
This option is included based on RFC 6977.
option dhcp6.solmax-rt uint32;
A value to override the default for SOL_MAX_RT. This is a 32 bit
value.
This option is included based on RFC 7083.
option dhcp6.inf-max-rt uint32;
A value to override the default for INF_MAX_RT. This is a 32 bit
value.
This option is included based on RFC 7083.
ACCESSING DHCPV6 RELAY OPTIONS
v6relay (relay-number, option) This option allows access to an option
that has been added to a packet by a relay agent. Relay-number value
selects the relay to examine and option is the option to find. In
DHCPv6 each relay encapsulates the entire previous message into an
option, adds its own options (if any) and sends the result onwards.
The RFC specifies a limit of 32 hops. A relay-number of 0 is a no-op
and means don't look at the relays. 1 is the relay that is closest to
the client, 2 would be the next in from the client and so on. Any
value greater than the max number of hops is which is closest to the
server independent of number. To use this option in a class statement
you would have something like this:
match if v6relay(1, option dhcp6.subscriber-id) = "client_1";
DEFINING NEW OPTIONS
The Internet Systems 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. All codes between
224 and 254 are reserved as 'site-local' DHCP options, so you can pick
any one of these for your site (not for your product/application). In
RFC3942, site-local space was moved from starting at 128 to starting at
224. In practice, some vendors have interpreted the protocol rather
loosely and have used option code values greater than 128 themselves.
There's no real way to avoid this problem, and it was thought to be
unlikely to cause too much trouble in practice. If you come across a
vendor-documented option code in either the new or old site-local spa-
ces, please contact your vendor and inform them about rfc3942.
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;
IP6-ADDRESS
option new-name code new-code = ip6-address ;
An option whose structure is an IPv6 address must be expressed as a
valid IPv6 address. The following is an example use of the ip6-address
type:
option dhcp6.some-server code 1234 = array of ip6-address;
option dhcp6.some-server 3ffe:bbbb:aaaa:aaaa::1, 3ffe:bbbb:aaaa:aaaa::2;
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 separated 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;
DOMAIN-LIST
option new-name code new-code = domain-list [compressed] ;
An option whose type is domain-list is an RFC1035 formatted (on the
wire, "DNS Format") list of domain names, separated by root labels.
The optional compressed keyword indicates if the option should be com-
pressed relative to the start of the option contents (not the packet
contents).
When in doubt, omit the compressed keyword. When the software receives
an option that is compressed and the compressed keyword is omitted, it
will still decompress the option (relative to the option contents
field). The keyword only controls whether or not transmitted packets
are compressed.
Note that when domain-list formatted options are output as environment
variables to dhclient-script(8), the standard DNS -escape mechanism is
used: they are decimal. This is appropriate for direct use in eg
/etc/resolv.conf.
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 encapsu-
lated in a standard DHCP option. It also defines the Vendor Identified
Vendor Sub Options option ("VIVSO"), and the DHCPv6 protocol defines
the Vendor-specific Information Option ("VSIO"). The format of all of
these options is usually internally a string of options, similarly to
other normal DHCP options. The VIVSO and VSIO options differ in that
they contain options that correspond to vendor Enterprise-ID numbers
(assigned by IANA), which then contain options according to each Ven-
dor's specifications. You will need to refer to your vendor's documen-
tation in order to form options to their specification.
The value of these options 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-separated list of hexadecimal values. For help in forming these
strings, please refer to RFC2132 for the DHCPv4 Vendor Specific Infor-
mation Option, RFC3925 for the DHCPv4 Vendor Identified Vendor Sub
Options, or RFC3315 for the DHCPv6 Vendor-specific Information Option.
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;
option vivso
00:00:09:bf:0E:
01:0c:
48:65:6c:6c:6f:20:77:6f:72:6c:64:21;
option dhcp6.vendor-opts
00:00:09:bf:
00:01:00:0c:
48:65:6c:6c:6f:20:77:6f:72:6c:64:21;
The second way of setting the value of these options 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 relevant option.
To define a new option space in which vendor options can be stored, use
the option space statement:
option space name [ [ code width number ] [ length width number ] [
hash size number ] ] ;
Where the numbers following code width, length width, and hash size
respectively identify the number of bytes used to describe option
codes, option lengths, and the size in buckets of the hash tables to
hold options in this space (most DHCPv4 option spaces use 1 byte codes
and lengths, which is the default, whereas most DHCPv6 option spaces
use 2 byte codes and lengths).
The code and length widths are used in DHCP protocol - you must config-
ure these numbers to match the applicable option space you are config-
uring. They each default to 1. Valid values for code widths are 1, 2
or 4. Valid values for length widths are 0, 1 or 2. Most DHCPv4
option spaces use 1 byte codes and lengths, which is the default,
whereas most DHCPv6 option spaces use 2 byte codes and lengths. A
zero-byte length produces options similar to the DHCPv6 Vendor-specific
Information Option - but not their contents!
The hash size defaults depend upon the code width selected, and may be
254 or 1009. Valid values range between 1 and 65535. Note that the
higher you configure this value, the more memory will be used. It is
considered good practice to configure a value that is slightly larger
than the estimated number of options you plan to configure within the
space. Previous versions of ISC DHCP (up to and including DHCP 3.0.*),
this value was fixed at 9973.
The name can then be used in option definitions, as described earlier
in this document. For example:
option space SUNW code width 1 length width 1 hash size 3;
option SUNW.server-address code 2 = ip-address;
option SUNW.server-name code 3 = text;
option SUNW.root-path code 4 = text;
option space ISC code width 1 length width 1 hash size 3;
option ISC.sample code 1 = text;
option vendor.ISC code 2495 = encapsulate vivso-sample;
option vendor-class.ISC code 2495 = text;
option ISC.sample "configuration text here";
option vendor-class.ISC "vendor class here";
option space docsis code width 2 length width 2 hash size 17;
option docsis.tftp-servers code 32 = array of ip6-address;
option docsis.cablelabs-configuration-file code 33 = text;
option docsis.cablelabs-syslog-servers code 34 = array of ip6-address;
option docsis.device-id code 36 = string;
option docsis.time-servers code 37 = array of ip6-address;
option docsis.time-offset code 38 = signed integer 32;
option vsio.docsis code 4491 = encapsulate docsis;
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 dif-
ferent classes of clients. Using the option space definition shown in
the previous example, you can send different option values to different
clients based on the vendor-class-identifier option that the clients
send, as follows:
class "vendor-classes" {
match option vendor-class-identifier;
}
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";
}
option SUNW.server-address 172.17.65.1;
option SUNW.server-name "sundhcp-server17-1";
option vivso-sample.sample "Hello world!";
option docsis.tftp-servers ::1;
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 DHCPv4 vendor-
encapsulated-options option. This is a limitation of that option - the
DHCPv4 VIVSO and the DHCPv6 VSIO options can have multiple vendor defi-
nitions all at once (even transmitted to the same client), so it is not
necessary to configure this.
SEE ALSO
dhcpd.conf(5), dhcpd.leases(5), dhclient.conf(5), dhcp-eval(5),
dhcpd(8), dhclient(8), RFC2132, RFC2131, RFC3046, RFC3315.
AUTHOR
Information about Internet Systems Consortium can be found at
https://www.isc.org.
dhcp-options(5)
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