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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 de-
clared, are documented here.
REFERENCE: OPTION STATEMENTS
DHCP option statements always start with the option keyword, followed by an option name,
followed by option data. The option names and data formats are described below. It is
not necessary to exhaustively specify all DHCP options - only those options which are
needed by clients must be specified.
Option data comes in a variety of formats, as defined below:
The ip-address data type can be entered either as an explicit IP address (e.g.,
239.254.197.10) or as a domain name (e.g., haagen.isc.org). 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 inte-
gers. 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.example.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, separated by colons. For example:
option dhcp-client-identifier "CLIENT-FOO";
or
option dhcp-client-identifier 43:4c:49:45:54:2d:46:4f:4f;
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 evalu-
ation 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 definitions will be in-
cluded in the code based on the definition of an argument for the RFC that defines the op-
tion in includes/site.h. This provides you with a method for over-riding the ISC defini-
tions if necessary - for example if you have previously defined the option with a differ-
ent 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 definition 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 information.
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 exam-
ple in conditional 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 configurable"
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 sub-
nets 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 asso-
ciated 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 Con-
troller Servers ("BCMS").
option bcms-controller-names domain-list;
This option contains the domain names of local Broadcast and Multicast 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 declaration. 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 section 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 determine 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 preference.
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 seg-
ments. The minimum value is 1.
option default-url string;
The format and meaning of this option is not described in any standards 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 con-
tained 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 over-
loaded by using them to carry DHCP options. A DHCP server inserts this option if the re-
turned parameters will exceed the usual space allotted for options.
If this option is present, the client interprets the specified additional 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 op-
tion 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 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 configured in this op-
tion 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 configured in this op-
tion 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 ad-
dress 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 in-
cluded 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 sev-
eral 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-identifier 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 en-
coded 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 available 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 do-
main 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 preference.
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 for-
warding. A value of false means disable IP forwarding, 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 loader-configfile text
This option is used to specify a boot loading configuration file a PXE client should
use.
This option is included based on RFC 5071.
option loader-pathprefix text
This option is used to specify a path prefix a PXE client should use in conjunction with
the boot load configuration file.
This option is included based on RFC 5071.
option loader-reboottime uint32
This option is used to dictate the maximum amount of time a PXE client should allow it-
self to achieve configured network resources before rebooting.
This option is included based on RFC 5071.
option log-servers ip-address [, ip-address... ];
The log-server option specifies a list of MIT-LCS UDP log servers available to the
client. Servers should be listed in order of preference.
option 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 op-
tion.
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 allo-
cated (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 config-
ured. 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 preference.
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 preference.
option nntp-server ip-address [, ip-address... ];
The NNTP server option specifies a list of NNTP servers available to the client.
Servers should be listed in order of preference.
option non-local-source-routing flag;
This option specifies whether the client should configure its IP layer to allow forward-
ing 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 NetWare/IP suboptions - see the NETWARE/IP
SUBOPTIONS section for more information.
option pxe-system-type uint16 [, uint16 ... ];
A list of one ore more 16-bit integers which allows a client to specify its pre-boot ar-
chitecture type(s).
This option is included based on RFC 4578.
option pxe-interface-id uint8 uint8 uint8
A three octet value which allows a client to specify its network interface type.
This option is included based on RFC 4578.
option pxe-client-id uint8 string
A single octet indicating type, followed by a string that allows a client to specify its
PXE client identity.
This option is included based on RFC 4578.
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 discov-
ered 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, or-
dered 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 us-
ing 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 preference.
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 per-
form router discovery.
option router-solicitation-address ip-address;
This option specifies the address to which the client should transmit router solicita-
tion 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 preference.
option slp-directory-agent boolean ip-address [, ip-address... ];
This option specifies two things: the IP addresses of one or more Service Location Pro-
tocol 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 dis-
covery 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 configure, 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 ser-
vice 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 destination are specified, they are
listed in descending order of priority.
The routes consist of a list of IP address pairs. The first address is the destination
address, and the second address is the router for the destination.
The default route (0.0.0.0) is an illegal destination for a static route. To specify
the default route, use the routers option. 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 Mi-
crosoft 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 preference.
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 declaration for the network on which an address is being assigned.
However, any subnet-mask option declaration that is in scope for the address being as-
signed 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 tftp-server-address ip-address [, ip-address... ];
This option configures a list of one or more IPv4 addresses of tftp servers a client may
use.
This option is included based on RFC 5859
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 in-
dicates 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 un-
signed 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 declaration. BOOTP clients are unlikely to sup-
port 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 Coor-
dinated 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 preference.
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 trailers. A value of true means that the client should at-
tempt to use trailers.
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 Authenti-
cation 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 component, 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 specify identifying in-
formation to the client. This can be used in a similar way to the vendor-class-identi-
fier 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 iden-
tifier, 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 configu-
ration 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 configuration 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 (currently, the Microsoft
Windows 2000 DHCP client) are configured automatically, but otherwise this must be con-
figured 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 for-
mat 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 prefer-
ence.
RELAY AGENT INFORMATION OPTION
An IETF draft, draft-ietf-dhc-agent-options-11.txt, defines a series of encapsulated op-
tions 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
permissible. 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 de-
fined 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. Ex-
amples of what it might contain include caller ID information, username information, re-
mote 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 admin-
istratively 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 end-
point, 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 imple-
mented 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 useful 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-op-
tion-00.txt is not a standard yet, but is in sufficiently wide use already that we have
implemented it. Due to the complexity of the option format, we have implemented it as a
suboption space rather than a single option. In general this option should not be config-
ured 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 up-
date 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.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-op-
tion 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-op-
tion operators in an expression, in which case it returns all labels after the first la-
bel in the fqdn.fqdn suboption - for example, if the value of fqdn.fqdn is "foo.exam-
ple.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 op-
tion 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 comparison, 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 secondary 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. Understandably, many
options are not "allowed" to have multiple instances in a packet - normally these are op-
tions which are digested by the DHCP protocol software, and not by users or applications.
option dhcp6.client-id string;
This option specifies the client's DUID identifier. DUIDs are similar but different
from DHCPv4 client identifiers - there are documented 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 config-
ure an opaque binary blob for your server's identifier.
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 software, and should not be configured.
option dhcp6.ia-ta string;
The Identity Association for Temporary Addresses (ia-ta) carries temporary 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 'preferred' for use on a
given subnet. This preference is only applied during the initial stages of configura-
tion - 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 di-
gested 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 re-
ceive Request, Renew, Decline, and Release packets from their clients via unicast. Nor-
mally, DHCPv6 clients will multicast these messages. Per RFC 3315, the server will re-
ject 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 condi-
tions during protocol communication. This option is manufactured and digested by proto-
col 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 de-
sire 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 Ven-
dor-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 ENCAP-
SULATED 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 con-
figuration 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 in-
spect 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 information-
ally only.
option dhcp6.reconf-accept ;
The reconf-accept option is included by DHCPv6 clients that support the Reconfigure ex-
tensions, advertising that they will respond if the server were to ask them to Reconfig-
ure. There is no support for DHCPv6 Reconfigure extensions, and this option is docu-
mented informationally 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 op-
tion 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 "nameserver" 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 re-
cursive DNS queries. It is easiest to describe this as the "search" line in /etc/re-
solv.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 soft-
ware.
option dhcp6.ia-prefix string;
The ia-prefix option is placed inside ia-pd options in order to identify the prefix(es)
allocated 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.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 available 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 behaviour, 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 Multi-
cast 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 Multi-
cast 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 op-
tion 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 mi-
nor 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 do-
main-list of domains for which the RDNSS has special 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 inde-
pendent 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 spaces, 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 con-
tents 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 compressed 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 trans-
mitted 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 cur-
rently exists include the vendor-encapsulated-options option, the netware-suboptions op-
tion 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. 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 Vendor's specifications. You will need to refer to your vendor's documentation 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 Information 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 configure these numbers to
match the applicable option space you are configuring. 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. Pre-
vious 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 exam-
ple, 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 differ-
ent 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-encapsu-
lated-options option. This is a limitation of that option - the DHCPv4 VIVSO and the
DHCPv6 VSIO options can have multiple vendor definitions 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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