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.\" This software has been written for the Internet Software Consortium
.\" by Ted Lemon <mellon@fugue.com> in cooperation with Vixie
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.Dd $Mdocdate: January 3 2013 $
.Dt DHCP-OPTIONS 5
.Os
.Sh NAME
.Nm dhcp-options
.Nd Dynamic Host Configuration Protocol options
.Sh DESCRIPTION
The Dynamic Host Configuration protocol allows the client to receive
.Ic options
from the DHCP server describing the network configuration and various
services that are available on the network.
When configuring
.Xr dhcpd 8
or
.Xr 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.
.Sh REFERENCE: OPTION STATEMENTS
DHCP
.Ic option
statements always start with the
.Ic 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.
.Pp
Option data comes in a variety of formats, as defined below:
.Pp
The
.Ar 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).
A domain name must resolve to a single IP address.
.Pp
The
.Ar int32
data type specifies a signed 32-bit integer.
The
.Ar uint32
data type specifies an unsigned 32-bit integer.
The
.Ar int16
and
.Ar uint16
data types specify signed and unsigned 16-bit integers.
The
.Ar int8
and
.Ar uint8
data types specify signed and unsigned 8-bit integers.
Unsigned 8-bit integers are also sometimes referred to as octets.
.Pp
The
.Ar string
data type specifies an
.Tn NVT
.Pq Network Virtual Terminal
.Tn ASCII
string, which must be enclosed in double quotes \- for example,
to specify a domain-name option, the syntax would be
.Pp
.Dl option domain-name \&"isc.org\&";
.Pp
The
.Ar 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).
.Pp
The
.Ar data-string
data type specifies either an
.Tn NVT ASCII
string enclosed in double quotes, or a series of octets specified in
hexadecimal, separated by colons.
For example:
.Pp
.Dl option dhcp-client-identifier \&"CLIENT-FOO\&";
or
.Dl option dhcp-client-identifier 43:4c:49:45:4e:54:2d:46:4f:4f;
.Pp
The documentation for the various options mentioned below is taken
from the IETF draft document on DHCP options, RFC 2132.
Options which are not listed by name may be defined by the name
.Pf option\- Ns Ar nnn ,
where
.Ar nnn
is the decimal number of the option code.
These options may be followed either by a string, enclosed in quotes, or by
a series of octets, expressed as two-digit hexadecimal numbers separated
by colons.
For example:
.Bd -literal -offset indent
option option-133 "my-option-133-text";
option option-129 1:54:c9:2b:47;
.Ed
.Pp
Because
.Xr dhcpd 8
does not know the format of these undefined option codes,
no checking is done to ensure the correctness of the entered data.
.Pp
The defined options are:
.Bl -tag -width Ds
.It Ic option all-subnets-local Ar flag ;
This option specifies whether or not the client may assume that all subnets
of the IP network to which the client is connected use the same MTU as the
subnet of that network to which the client is directly connected.
A value of 1 indicates that all subnets share the same MTU.
A value of 0 means that the client should assume that some subnets of the
directly connected network may have smaller MTUs.
.It Ic option arp-cache-timeout Ar uint32 ;
This option specifies the timeout in seconds for ARP cache entries.
.It Ic option autoproxy-script Ar string ;
Site-specific as of RFC 3942.
Widely used for Web Proxy Autodiscovery Protocol (WPAD).
.It Ic option boot-size Ar uint16 ;
This option specifies the length in 512-octet blocks of the default
boot image for the client.
.It Ic option bootfile-name Ar string ;
This option is used to identify a bootstrap file.
If supported by the client, it should have the same effect as the
.Ic filename
declaration.
BOOTP clients are unlikely to support this option.
Some DHCP clients will support it, and others actually require it.
.It Ic option broadcast-address Ar 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
RFC 1122.
.It Ic option cookie-servers Ar ip-address Oo , Ar ip-address ... Oc ;
The
.Ic cookie-servers
option specifies a list of RFC 865 cookie servers available to the client.
Servers should be listed in order of preference.
.It Ic option default-ip-ttl Ar uint8 ;
This option specifies the default time-to-live that the client should
use on outgoing datagrams.
.It Ic option default-tcp-ttl Ar uint8 ;
This option specifies the default TTL that the client should use when
sending TCP segments.
The minimum value is 1.
.It Ic option dhcp-class-identifier Ar string ;
This option is used by DHCP clients to optionally identify the vendor type
and configuration of a DHCP client.
The information is a string of n octets, interpreted by servers.
Vendors may choose to define specific vendor class identifiers to convey
particular configuration or other identification information about a client.
For example, the identifier may encode the client's hardware configuration.
Servers not equipped to interpret the class-specific information sent by a
client must ignore it (although it may be reported).
Servers that respond should only use option 43
.Pq Ic vendor-encapsulated-options
to return the vendor-specific information to the client.
.It Ic option dhcp-client-identifier Ar data-string ;
This option can be used to specify a DHCP client identifier in a
host declaration, so that
.Xr dhcpd 8
can find the host record by matching against the client identifier.
.It Ic option dhcp-lease-time Ar 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.
.It Ic option dhcp-max-message-size Ar uint16 ;
This option specifies the maximum length DHCP message that it is willing
to accept.
The length is specified as an unsigned 16-bit integer.
A client may use the maximum DHCP message size option in DHCPDISCOVER or
DHCPREQUEST messages, but should not use the option in DHCPDECLINE messages.
.It Ic option dhcp-message-type Ar uint8 ;
This option is used to convey the type of the DHCP message.
Values: 1=DHCPDISCOVER, 2=DHCPOFFER, 3=DHCPREQUEST, 4=DHCPDECLINE, 5=DHCPACK,
6=DHCPNAK, 7=DHCPRELEASE, 8=DHCPINFORM.
.It Ic option dhcp-option-overload Ar uint8 Oo , Ar uint8 ... Oc ;
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.
A value of 1 means the 'file' field is used to hold options.
A value of 2 means the 'sname' field is used to hold options.
A value of 3 means both fields are used to hold options.
.It Ic option dhcp-parameter-request-list Ar uint8 Oo , Ar uint8 ... Oc ;
This option is used by a DHCP client to request values for specified
configuration parameters.
The list of requested parameters is specified as n octets, where each
octet is a valid DHCP option code as defined in this document.
The client MAY list the options in order of preference.
The DHCP server is not required to return the options in the requested
order, but MUST try to insert the requested options in the order requested
by the client.
.It Ic option dhcp-rebinding-time Ar uint32 ;
This option specifies the time interval from address assignment until the
client transitions to the REBINDING state.
The value is in seconds.
.It Ic option dhcp-renewal-time Ar uint32 ;
This option specifies the time interval from address assignment until the
client transitions to the RENEWING state.
The value is in seconds.
.It Ic option dhcp-requested-address Ar ip-address ;
This option is used in a client request (DHCPDISCOVER) to allow the client
to request that a particular IP address be assigned.
.It Ic option dhcp-server-identifier Ar 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 identifier is the IP address of the selected server.
.It Ic option domain-name Ar string ;
This option specifies the domain name that the client should use when
resolving hostnames via the Domain Name System.
.It Ic option domain-name-servers Ar ip-address Oo , Ar ip-address ... Oc ;
The
.Ic domain-name-servers
option specifies a list of Domain Name System name servers
available to the client.
Servers should be listed in order of preference.
.It Ic option extensions-path Ar string ;
A string to specify a file, retrievable via TFTP, which contains information
which can be interpreted in the same way as the 64-octet vendor-extension
field within the BOOTP response, with exceptions; see RFC 2132, Section 3.20
for details.
.It Ic option finger-server Ar ip-address Oo , Ar ip-address ... Oc ;
The
.Ic finger-server
option specifies a list of
.Xr finger 1
servers available to the client.
Servers should be listed in order of preference.
.It Ic option font-servers Ar ip-address Oo , Ar ip-address ... Oc ;
This option specifies a list of X Window System Font servers available
to the client.
Servers should be listed in order of preference.
.It Ic option host-name Ar 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
.Ic domain-name
option to specify the domain name).
See RFC 1035 for character set restrictions.
.It Ic option ieee802-3-encapsulation Ar flag ;
This option specifies whether or not the client should use Ethernet
Version 2 (RFC 894) or IEEE 802.3 (RFC 1042) encapsulation if the
interface is an Ethernet.
A value of 0 indicates that the client should use RFC 894 encapsulation.
A value of 1 means that the client should use RFC 1042 encapsulation.
.It Ic option ien116-name-servers Ar ip-address Oo , Ar ip-address ... Oc ;
The
.Ic ien116-name-servers
option specifies a list of IEN 116 name servers available to the client.
Servers should be listed in order of preference.
.It Ic option impress-servers Ar ip-address Oo , Ar ip-address ... Oc ;
The
.Ic impress-servers
option specifies a list of Imagen Impress servers available to the client.
Servers should be listed in order of preference.
.It Ic option interface-mtu Ar uint16 ;
This option specifies the MTU to use on this interface.
The minimum legal value for the MTU is 68.
.It Ic option ip-forwarding Ar flag ;
This option specifies whether the client should configure its IP layer
for packet forwarding.
A value of 0 means disable IP forwarding, and a value of 1 means enable
IP forwarding.
.It Ic option irc-server Ar ip-address Oo , Ar ip-address ... Oc ;
The
.Ic irc-server
option specifies a list of IRC servers available to the client.
Servers should be listed in order of preference.
.It Ic option log-servers Ar ip-address Oo , Ar ip-address ... Oc ;
The
.Ic log-servers
option specifies a list of MIT-LCS UDP log servers available to the client.
Servers should be listed in order of preference.
.It Ic option lpr-servers Ar ip-address Oo , Ar ip-address ... Oc ;
The
.Ic lpr-servers
option specifies a list of RFC 1179 line printer servers available to the
client.
Servers should be listed in order of preference.
.It Ic option mask-supplier Ar flag ;
This option specifies whether or not the client should respond to subnet mask
requests using ICMP.
A value of 0 indicates that the client should not respond.
A value of 1 means that the client should respond.
.It Ic option max-dgram-reassembly Ar uint16 ;
This option specifies the maximum size datagram that the client should be
prepared to reassemble.
The minimum legal value is 576.
.It Ic option merit-dump Ar string ;
This option specifies the pathname 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
.Tn NVT ASCII
character set.
.It Ic option mobile-ip-home-agent Ar ip-address Oo , Ar ip-address ... Oc ;
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.
.It Ic option nds-context Ar string ;
This option specifies the initial NDS context the client should use.
NDS contexts are 16-bit Unicode strings.
For transmission in the NDS Context Option, an NDS context is
transformed into octets using UTF-8.
The string should NOT be zero terminated.
A single DHCP option can only contain 255 octets.
Since an NDS context name can be longer than that, this option can
appear more than once in the DHCP packet.
The contents of all NDS Context options in the packet should be
concatenated as suggested in the DHCP specification to get the
complete NDS context.
A single encoded character could be split between two NDS Context Options.
See RFC 2241 for details.
.It Ic option nds-servers Ar ip-address Oo , Ar ip-address ... Oc ;
This option specifies one or more NDS servers for the client to contact
for access to the NDS database.
Servers should be listed in order of preference.
See RFC 2241 for details.
.It Ic option nds-tree-name Ar string ;
This option specifies the name of the NDS tree the client will be contacting.
NDS tree names are 16-bit Unicode strings.
For transmission in the NDS Tree Name Option, an NDS tree name is
transformed into octets using UTF-8.
The string should not be zero terminated.
See RFC 2241 for details.
.It Ic option netbios-dd-server Ar ip-address Oo , Ar ip-address ... Oc ;
The NetBIOS datagram distribution server (NBDD) option specifies a
list of RFC 1001/1002 NBDD servers listed in order of preference.
.It Ic option netbios-name-servers Ar ip-address Oo , Ar ip-address ... Oc ;
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
.Ic netbios-name-servers
option.
.It Ic option netbios-node-type Ar 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.
.Pp
Possible node types are:
.Bl -tag -width Ds
.It 1
B-node: Broadcast - no WINS
.It 2
P-node: Peer - WINS only
.It 4
M-node: Mixed - broadcast, then WINS
.It 8
H-node: Hybrid - WINS, then broadcast
.El
.It Ic option netbios-scope Ar string ;
The NetBIOS scope option specifies the NetBIOS over TCP/IP scope
parameter for the client as specified in RFC 1001/1002.
See RFC 1001, RFC 1002, and RFC 1035 for character-set restrictions.
.It Ic option nis-domain Ar string ;
This option specifies the name of the client's NIS (Sun Network Information
Services) domain.
The domain is formatted as a character string consisting of characters
from the
.Tn NVT ASCII
character set.
.It Ic option nis-servers Ar ip-address Oo , Ar ip-address ... Oc ;
This option specifies a list of IP addresses indicating NIS servers
available to the client.
Servers should be listed in order of preference.
.It Ic option nisplus-domain Ar string ;
This option specifies the name of the client's NIS+ domain.
The domain is formatted as a character string consisting of characters
from the
.Tn NVT ASCII
character set.
.It Ic option nisplus-servers Ar ip-address Oo , Ar ip-address ... Oc ;
This option specifies a list of IP addresses indicating NIS+ servers
available to the client.
Servers should be listed in order of preference.
.It Ic option nntp-server Ar ip-address Oo , Ar ip-address ... Oc ;
The
.Ic nntp-server
option specifies a list of NNTP servers available to the client.
Servers should be listed in order of preference.
.It Ic option non-local-source-routing Ar flag ;
This option specifies whether the client should configure its IP
layer to allow forwarding of datagrams with non-local source routes.
A value of 0 means disallow forwarding of such datagrams, and a value of 1
means allow forwarding.
.It Ic option ntp-servers Ar ip-address Oo , Ar ip-address ... Oc ;
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.
.It Ic option path-mtu-aging-timeout Ar uint32 ;
This option specifies the timeout (in seconds) to use when aging Path
MTU values discovered by the mechanism defined in RFC 1191.
.It Ic option path-mtu-plateau-table Ar uint16 Oo , Ar uint16 ... Oc ;
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.
.It Ic option perform-mask-discovery Ar flag ;
This option specifies whether or not the client should perform subnet mask
discovery using ICMP.
A value of 0 indicates that the client should not perform mask discovery.
A value of 1 means that the client should perform mask discovery.
.It Ic option policy-filter Ar ip-address ip-address Oo , Ar ip-address ip-address ... Oc ;
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.
.Pp
Any source-routed datagram whose next-hop address does not match one
of the filters should be discarded by the client.
.Pp
See RFC 1122 for further information.
.It Ic option pop-server Ar ip-address Oo , Ar ip-address ... Oc ;
The
.Ic pop-server
option specifies a list of POP3 servers available to the client.
Servers should be listed in order of preference.
.It Ic option relay-agent-information Ar string ;
This is a "container" option for specific agent-supplied sub-options.
See RFC 3046 for details.
.It Ic option resource-location-servers Ar ip-address Oo , Ar ip-address ... Oc ;
This option specifies a list of RFC 887 Resource Location servers available
to the client.
Servers should be listed in order of preference.
.It Ic option root-path Ar string ;
This option specifies the pathname that contains the client's root disk.
The path is formatted as a character string consisting of characters from
the
.Tn NVT ASCII
character set.
.It Ic option router-discovery Ar flag ;
This option specifies whether or not the client should solicit routers using
the Router Discovery mechanism defined in RFC 1256.
A value of 0 indicates that the client should not perform router discovery.
A value of 1 means that the client should perform router discovery.
.It Ic option router-solicitation-address Ar ip-address ;
This option specifies the address to which the client should transmit
router solicitation requests.
.It Ic option routers Ar ip-address Oo , Ar ip-address ... Oc ;
The
.Ic routers
option specifies a list of IP addresses for routers on the client's subnet.
Routers should be listed in order of preference.
.It Ic option smtp-server Ar ip-address Oo , Ar ip-address ... Oc ;
The
.Ic smtp-server
option specifies a list of SMTP servers available to the client.
Servers should be listed in order of preference.
.It Ic option static-routes Ar ip-address ip-address Oo , Ar ip-address ip-address ... Oc ;
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.
.Pp
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.
.Pp
The default route (0.0.0.0) is an illegal destination for a static route.
To specify the default route, use the
.Ic routers
option.
.It Ic option streettalk-directory-assistance-server Ar ip-address Oo , Ar ip-address ... Oc ;
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.
.It Ic option streettalk-server Ar ip-address Oo , Ar ip-address ... Oc ;
The
.Ic streettalk-server
option specifies a list of StreetTalk servers available to the client.
Servers should be listed in order of preference.
.It Ic option subnet-mask Ar ip-address ;
The
.Ic 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
.Xr dhcpd 8
will use the subnet mask from the subnet declaration for the network on
which an address is being assigned.
However,
.Em any
subnet-mask option declaration that is in scope for the address being
assigned will override the subnet mask specified in the subnet declaration.
.It Ic option swap-server Ar ip-address ;
This specifies the IP address of the client's swap server.
.It Ic option tcp-keepalive-garbage Ar 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 0 indicates that a garbage octet should not be sent.
A value of 1 indicates that a garbage octet should be sent.
.It Ic option tcp-keepalive-interval Ar 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.
.It Ic option tftp-config-file Ar string ;
Option 144.
Per RFC 2132 options 128 \- 254 are site-specific.
RFC 3942 reclassifies options 128 to 223 as publicly defined options and
puts them in "Unavailable" state by IANA.
See RFC 3679 for "Unused DHCP Option Codes to be Reassigned to Future
DHCP Options".
See RFC 2939 for procedures for definitions of new DHCP options.
.It Ic option tftp-server-name Ar string ;
This option is used to identify a TFTP server and, if supported by the
client, should have the same effect as the
.Ic server-name
declaration.
BOOTP clients are unlikely to support this option.
Some DHCP clients will support it, and others actually require it.
.It Ic option time-offset Ar int32 ;
The
.Ic time-offset
option specifies the offset of the client's subnet in seconds from
Coordinated Universal Time (UTC).
.It Ic option time-servers Ar ip-address Oo , Ar ip-address ... Oc ;
The
.Ic time-server
option specifies a list of RFC 868 time servers available to the client.
Servers should be listed in order of preference.
.It Ic option trailer-encapsulation Ar flag ;
This option specifies whether or not the client should negotiate the
use of trailers (RFC 893) when using the ARP protocol.
A value of 0 indicates that the client should not attempt to use trailers.
A value of 1 means that the client should attempt to use trailers.
.It Ic option user-class Ar string ;
This option is used by a DHCP client to optionally identify the type or
category of user or applications it represents.
A DHCP server uses the User Class option to choose the address pool it
allocates an address from and/or to select any other configuration option.
This option may carry multiple User Classes.
Servers may interpret the meanings of multiple class specifications in an
implementation dependent or configuration dependent manner, and so the use
of multiple classes by a DHCP client should be based on the specific server
implementation and configuration which will be used to process that User
class option.
See RFC 3004 for details.
.It Ic option vendor-encapsulated-options Ar data-string ;
This option is used by clients and servers to exchange vendor-specific
information.
The information is an opaque object of n octets, presumably interpreted by
vendor-specific code on the clients and servers.
See RFC 2132, Section 8.4 for details.
.It Ic option voip-configuration-server Ar ip-address Oo , Ar ip-address ... Oc ;
The
.Ic voip-configuration-server
option specifies a list of (normally TFTP) servers that VoIP clients
may download their configuration information and software images
from.
Servers should be listed in order of preference.
.It Ic option www-server Ar ip-address Oo , Ar ip-address ... Oc ;
The
.Ic www-server
option specifies a list of WWW servers available to the client.
.It Ic option x-display-manager Ar ip-address Oo , Ar ip-address ... Oc ;
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.
.El
.Sh SEE ALSO
.Xr dhclient.conf 5 ,
.Xr dhcpd.conf 5 ,
.Xr dhcpd.leases 5 ,
.Xr dhclient 8 ,
.Xr dhcpd 8
.Sh STANDARDS
.Rs
.%A R. Droms
.%D March 1997
.%R RFC 2131
.%T Dynamic Host Configuration Protocol
.Re
.Pp
.Rs
.%A S. Alexander
.%A R. Droms
.%D March 1997
.%R RFC 2132
.%T DHCP Options and BOOTP Vendor Extensions
.Re
.Sh AUTHORS
.An -nosplit
.Xr dhcpd 8
was written by
.An Ted Lemon Aq mellon@vix.com
under a contract with Vixie Labs.
.Pp
The current implementation was reworked by
.An Henning Brauer Aq henning@openbsd.org .