DHCP-OPTIONS(5)           OpenBSD Programmer's Manual          DHCP-OPTIONS(5)

NAME
     dhcp-options - Dynamic Host Configuration Protocol options

DESCRIPTION
     The Dynamic Host Configuration protocol allows the client to receive
     options from the DHCP server describing the network configuration and
     various services that are available on the network.  When configuring
     dhcpd(8) or dhclient(8), options must often be declared.  The syntax for
     declaring options, and the names and formats of the options that can be
     declared, are documented here.

REFERENCE: OPTION STATEMENTS
     DHCP option statements always start with the option keyword, followed by
     an option name, followed by option data.  The option names and data 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 do-
     main 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 standard options are:

   RFC 1497 Vendor Extensions

     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 as-
             signed.  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 time-offset int32;
             The time-offset option specifies the offset of the client's sub-
             net in seconds from Coordinated Universal Time (UTC).

     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 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 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 or-
             der of preference.

     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 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 or-
             der of preference.

     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 or-
             der 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 or-
             der 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 or-
             der of preference.

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

     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 domain-name string;
             This option specifies the domain name that the client should use
             when resolving hostnames via the Domain Name System.

     option swap-server ip-address;
             This specifies the IP address of the client's swap server.

     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.

   IP Layer Parameters per Host

     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 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 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 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 default-ip-ttl uint8;
             This option specifies the default time-to-live that the client
             should use on outgoing datagrams.

     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.

   IP Layer Parameters per Interface

     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 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 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 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 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 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 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 or-
             der of priority.

             The routes consist of a list of IP address pairs.  The first ad-
             dress 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 stat-
             ic route.  To specify the default route, use the routers option.

   Link Layer Parameters per Interface

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

     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.

   TCP Parameters

     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 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 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.

   Application and Service Parameters

     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 or-
             der of preference.

     option ntp-servers ip-address [, ip-address ...];
             This option specifies a list of IP addresses indicating NTP (RFC
             1305) servers available to the client.  Servers should be 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.  Net-
             BIOS Name Service is currently more commonly referred to as WINS.
             WINS servers can be specified using the netbios-name-servers op-
             tion.

     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-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 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 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.

     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 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 or-
             der of preference.

     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 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 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 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 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 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 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 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 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 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 or-
             der of preference.

     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 voip-configuration-server ip-address [, ip-address ...];
             The voip-configuration-server option specifies a list of (normal-
             ly TFTP) servers that VoIP clients may download their configura-
             tion information and software images from.  Servers should be
             listed in order of preference.

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

     RFC 2131, RFC 2132.

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>.

OpenBSD 4.7                    January 13, 2009                              7
