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DHCP-OPTIONS(5)             BSD File Formats Manual            DHCP-OPTIONS(5)

NAME
     dhcp-options -- DHCP 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.

     DHCP option statements always start with the option keyword, followed by
     an option name, followed by option data.  The option names and data for-
     mats 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).  A
     domain name must resolve to a single IP address.

     The int32 data type specifies a signed 32-bit integer.  The uint32 data
     type specifies an unsigned 32-bit integer.  The int16 and uint16 data
     types specify signed and unsigned 16-bit integers.  The int8 and uint8
     data types specify signed and unsigned 8-bit integers.  Unsigned 8-bit
     integers are also sometimes referred to as octets.

     The string data type specifies an NVT (Network Virtual Terminal) ASCII
     string, which must be enclosed in double quotes - for example, to specify
     a domain-name option, the syntax would be

           option domain-name "isc.org";

     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 data-string data type specifies either an NVT ASCII string enclosed
     in double quotes, or a series of octets specified in hexadecimal, sepa-
     rated by colons.  For example:

           option dhcp-client-identifier "CLIENT-FOO";
     or
           option dhcp-client-identifier 43:4c:49:45:4e:54:2d:46:4f:4f;

     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 option-nnn, where 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:

           option option-133 "my-option-133-text";
           option option-129 1:54:c9:2b:47;

     Because dhcpd(8) does not know the format of these undefined option
     codes, no checking is done to ensure the correctness of the entered data.

     The defined 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 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.

     option arp-cache-timeout uint32;
             This option specifies the timeout in seconds for ARP cache
             entries.

     option autoproxy-script string;
             Site-specific as of RFC 3942.  Widely used for Web Proxy Autodis-
             covery Protocol (WPAD).

     option boot-size uint16;
             This option specifies the length in 512-octet blocks of the
             default boot image for the client.

     option bootfile-name string;
             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 broadcast-address ip-address;
             This option specifies the broadcast address in use on the
             client's subnet.  Legal values for broadcast addresses are speci-
             fied in section 3.2.1.3 of RFC 1122.

     option classless-static-routes ip/prefix ip [, ip/prefix ip ...];
             This option specifies a list of static routes in CDIR notation,
             which should be sent to the client.

     option classless-ms-static-routes ip/prefix ip [, ip/prefix ip ...];
             This option does the same as classless-static-routes, but uses
             option code 249 instead of 121, since Windows XP and Windows
             Server 2003 ignore option 121.

     option cookie-servers ip-address [, ip-address ...];
             The cookie-servers 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 dhcp-class-identifier 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 par-
             ticular 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 (vendor-encapsulated-options) to return the vendor-
             specific information to the client.

     option dhcp-client-identifier data-string;
             This option can be used to specify a DHCP client identifier in a
             host declaration, so that dhcpd(8) can find the host record by
             matching against the client identifier.

     option dhcp-lease-time uint32;
             This option is used in a client request (DHCPDISCOVER or DHCPRE-
             QUEST) 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.

     option dhcp-max-message-size 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.

     option dhcp-message-type uint8;
             This option is used to convey the type of the DHCP message.  Val-
             ues: 1=DHCPDISCOVER, 2=DHCPOFFER, 3=DHCPREQUEST, 4=DHCPDECLINE,
             5=DHCPACK, 6=DHCPNAK, 7=DHCPRELEASE, 8=DHCPINFORM.

     option dhcp-option-overload uint8 [, 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.
             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.

     option dhcp-parameter-request-list uint8 [, uint8 ...];
             This option is used by a DHCP client to request values for speci-
             fied 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.

     option dhcp-rebinding-time uint32;
             This option specifies the time interval from address assignment
             until the client transitions to the REBINDING state.  The value
             is in seconds.

     option dhcp-renewal-time uint32;
             This option specifies the time interval from address assignment
             until the client transitions to the RENEWING state.  The value is
             in seconds.

     option dhcp-requested-address ip-address;
             This option is used in a client request (DHCPDISCOVER) to allow
             the client to request that a particular IP address be assigned.

     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 mes-
             sage.  The identifier is the IP address of the selected server.

     option domain-name string;
             This option specifies the domain name that the 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 name servers available to the client.  Servers should be
             listed in order of preference.

     option extensions-path 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.

     option finger-server ip-address [, ip-address ...];
             The finger-server option specifies a list of finger(1) 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 host-name string;
             This option specifies the name of the client.  The name may or
             may not be qualified with the local domain name (it is preferable
             to use the domain-name option to specify the domain name).  See
             RFC 1035 for character set restrictions.

     option ieee802-3-encapsulation flag;
             This option specifies whether or not the client should use Ether-
             net 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.

     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-servers 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 min-
             imum 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 0 means disable IP for-
             warding, and a value of 1 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-servers 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-servers 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 0 indicates that the
             client should not respond.  A value of 1 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 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 NVT ASCII character set.

     option mobile-ip-home-agent ip-address [, ip-address ...];
             This option specifies a list of IP addresses indicating mobile IP
             home agents available to the client.  Agents should be listed in
             order of preference, although normally there will be only one
             such agent.

     option nds-context string;
             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.

     option nds-servers ip-address [, ip-address ...];
             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.

     option nds-tree-name 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.

     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 prefer-
             ence.

     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.  Net-
             BIOS 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 iden-
             tifies 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 RFC
             1001, RFC 1002, and RFC 1035 for character-set restrictions.

     option nis-domain string;
             This option specifies the name of the client's NIS (Sun Network
             Information Services) domain.  The domain is formatted as a char-
             acter string consisting of characters from the NVT ASCII charac-
             ter 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 string;
             This option specifies the name of the client's NIS+ domain.  The
             domain is formatted as a character string consisting of charac-
             ters 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.  A value of 0 means disallow forwarding of such data-
             grams, and a value of 1 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 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 format-
             ted as a list of 16-bit unsigned integers, ordered from smallest
             to largest.  The minimum MTU value cannot be smaller than 68.

     option perform-mask-discovery flag;
             This option specifies whether or not the client should perform
             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.

     option policy-filter ip-address ip-address [, ip-address ip-address ...];
             This option specifies policy filters for non-local source rout-
             ing.  The filters consist of a list of IP addresses and masks
             which specify destination/mask pairs with which to filter incom-
             ing source routes.

             Any source-routed datagram whose next-hop address does not match
             one of the filters should be discarded by the client.

             See RFC 1122 for further information.

     option pop-server ip-address [, ip-address ...];
             The pop-server option specifies a list of POP3 servers available
             to the client.  Servers should be listed in order of preference.

     option relay-agent-information string;
             This is a "container" option for specific agent-supplied sub-
             options.  See RFC 3046 for details.

     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 string;
             This option specifies the pathname that contains the client's
             root disk.  The path is formatted as a character string consist-
             ing 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 0 indicates that the client should not perform router
             discovery.  A value of 1 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 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.

     option streettalk-directory-assistance-server ip-address [, ip-address
             ...];
             The StreetTalk Directory Assistance (STDA) server option speci-
             fies 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(8) will use the subnet mask from the sub-
             net 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 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 0 indicates that a
             garbage octet should not be sent.  A value of 1 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 con-
             nection.  The time is specified as a 32-bit unsigned integer.  A
             value of zero indicates that the client should not generate
             keepalive messages on connections unless specifically requested
             by an application.

     option tftp-config-file 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.

     option tftp-server-name string;
             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 sub-
             net 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) 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.

     option user-class 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 configura-
             tion option.  This option may carry multiple User Classes.
             Servers may interpret the meanings of multiple class specifica-
             tions 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 config-
             uration which will be used to process that User class option.
             See RFC 3004 for details.

     option vendor-encapsulated-options 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.

     option voip-configuration-server ip-address [, ip-address ...];
             The voip-configuration-server option specifies a list of (nor-
             mally TFTP) servers that VoIP clients may download their configu-
             ration information and software images from.  Servers should be
             listed in order of preference.

     option www-server ip-address [, ip-address ...];
             The www-server option specifies a list of WWW servers available
             to the client.

     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.

SEE ALSO
     dhclient.conf(5), dhcpd.conf(5), dhcpd.leases(5), dhclient(8), dhcpd(8)

STANDARDS
     R. Droms, Interoperation Between DHCP and BOOTP, RFC 1534, October 1993.

     R. Droms, Dynamic Host Configuration Protocol, RFC 2131, March 1997.

     S. Alexander and R. Droms, DHCP Options and BOOTP Vendor Extensions, RFC
     2132, March 1997.

     T. Lemon and S. Cheshire, Encoding Long Options in the Dynamic Host
     Configuration Protocol (DHCPv4), RFC 3396, November 2002.

     T. Lemon, S. Cheshire, and B. Volz, The Classless Static Route Option for
     Dynamic Host Configuration Protocol (DHCP) version 4, RFC 3442, December
     2002.

AUTHORS
     dhcpd(8) was written by Ted Lemon <mellon@vix.com> under a contract with
     Vixie Labs.

     The current implementation was reworked by Henning Brauer
     <henning@openbsd.org>.

BSD                             March 30, 2017                             BSD