NAME
dhcp-options - Dynamic Host Configuration Protocol options
DESCRIPTION
The Dynamic Host Configuration protocol allows the client to receive
options from the DHCP server describing the network configuration and
various services that are available on the network. When configuring
dhcpd(8) or
dhclient(8) , options must often be declared. The
syntax for declaring options, and the names and formats of the options that
can be declared, are documented here.
REFERENCE: OPTION STATEMENTS
DHCP
option statements always start with the
option keyword,
followed by an option name, followed by option data. The option names and data
formats are described below. It is not necessary to exhaustively specify all
DHCP options - only those options which are needed by clients must be
specified.
Option data comes in a variety of formats, as defined below:
The
ip-address data type can be entered either as an explicit IP address
(e.g., 239.254.197.10) or as a domain name (e.g., 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
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.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 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 definitions will
be included in the code based on the definition of an argument for the RFC
that defines the option in includes/site.h. This provides you with a method
for over-riding the ISC definitions if necessary - 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 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 example 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 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 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 segments. 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
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 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 option is specified on the server, the server
returns the specified options. This can be used to force a client to take
options that it hasn't requested, and it can also be used to tailor the
response of the DHCP server for clients that may need a more limited set of
options than those the server would normally return.
option 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 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 configured 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-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 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 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 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 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 forwarding. 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 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 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 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 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
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 NetWare/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 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 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 preference.
option slp-directory-agent boolean ip-address [,
ip-address... ]
;
This option specifies two things: the IP addresses of one or more Service
Location Protocol Directory Agents, and whether the use of these addresses is
mandatory. If the initial boolean value is true, the SLP agent should just use
the IP addresses given. If the value is false, the SLP agent may additionally
do active or passive multicast discovery of SLP agents (see RFC2165 for
details).
Please note that in this option and the slp-service-scope option, the term
"SLP Agent" is being used to refer to a Service Location Protocol
agent running on a machine that is being configured using the DHCP protocol.
Also, please be aware that some companies may refer to SLP as NDS. If you have
an NDS directory agent whose address you need to 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 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 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 Microsoft DHCP
client.
option streettalk-directory-assistance-server ip-address
[ , ip-address...];
The StreetTalk Directory Assistance (STDA) server option specifies a list of
STDA servers available to the client. Servers should be listed in order of
preference.
option streettalk-server ip-address [,
ip-address... ] ;
The StreetTalk server option specifies a list of StreetTalk servers available to
the client. Servers should be listed in order of 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 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 declaration. 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 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 attempt 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 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
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 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 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 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 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 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-specified, 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 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-option-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 configured
by the user - instead it should be used as part of an automatic DNS update
system.
option fqdn.no-client-update flag;
When the client sends this, if it is true, it means the client will not attempt
to update its A record. When sent by the server to the client, it means that
the client
should not update its own A record.
option fqdn.server-update flag;
When the client sends this to the server, it is requesting that the server
update its A record. When sent by the server, it means that the server has
updated (or is about to update) the client's A record.
option fqdn.encoded flag;
If true, this indicates that the domain name included in the option is encoded
in DNS wire format, rather than as plain ASCII text. The client normally sets
this to false if it doesn't support DNS wire format in the FQDN option. The
server should always send back the same value that the client sent. When this
value is set on the configuration side, it controls the format in which the
fqdn.fqdn suboption is encoded.
option fqdn.rcode1 flag;
option fqdn.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 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 options 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
configure 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 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 multicast 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 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
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
"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 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
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 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 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 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 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 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
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 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 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. 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 different 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
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.