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dhcp-options(5)               File Formats Manual              dhcp-options(5)



NAME
       dhcp-options - Dynamic Host Configuration Protocol options

DESCRIPTION
       The Dynamic Host Configuration protocol allows the client to receive
       options from the DHCP server describing the network configuration and
       various services that are available on the network.  When configuring
       dhcpd(8) or dhclient(8) , options must often be declared.  The syntax
       for declaring options, and the names and formats of the options that
       can be 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 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 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 uint16 [, uint16... ];

         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 servesr 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 infomration 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 whcih the RDNSS has sepcial 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 string;

         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.hostname 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 Renew packets from their clients by exchanging
         UDP unicasts with them.  Normally, DHCPv6 clients will multicast
         their Renew messages.  This may be configured on the server, and
         should be configured as an address the server is ready to reply to.

       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 extentions, 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;

         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.



                                                               dhcp-options(5)