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dhcpd-options(5)              File Formats Manual             dhcpd-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.,  haa-
       gen.isc.org).  When entering a domain name, be sure  that  that  domain
       name resolves 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  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
       enclosed in double quotes.   This data type is not used for any  exist-
       ing DHCP options.   The domain name is stored just as if it were a text
       option.

       The domain-list data type specifies a list of  domain  names,  a  space
       between  each  name  and  the  entire string enclosed in double quotes.
       These types of data are used for the domain-search option for  example,
       and encodes an RFC1035 compressed DNS label list on the wire.

       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, sepa-
       rated 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));

STANDARD DHCP 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  informa-
       tion.

       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 condi-
       tional  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  con-
       figurable"

       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 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 declara-
         tion.  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 sec-
         tion 3.2.1.3 of STD 3 (RFC1122).

       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 pref-
         erence.

       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  stan-
         dards 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  addi-
         tional  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;

         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 lim-
         ited 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 not user configurable.

       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 not user configurable.

       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-identi-
         fier 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  avail-
         able 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.  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  pref-
         erence.

       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 for-
         warding, 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  pref-
         erence.

       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 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 Comput-
         ers.   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  pref-
         erence.

       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  pref-
         erence.

       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
         1035)  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  Net-
         Ware/IP  suboptions  - see the NETWARE/IP SUBOPTIONS section for more
         information.

       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 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 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  pref-
         erence.

       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 prefer-
         ence.

       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  multi-
         cast 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  Proto-
         col  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 config-
         ure, 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  desti-
         nation  are  specified, they are listed in descending order of prior-
         ity.

         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  virtu-
         ally  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 pref-
         erence.

       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  declara-
         tion 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 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 declara-
         tion.   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 pref-
         erence.

       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  trail-
         ers.   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 authen-
         tication   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 com-
         ponent, 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  spec-
         ify  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 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 configura-
         tion file - instead, a vendor class is defined for each vendor,  ven-
         dor  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  (cur-
         rently,  the Microsoft Windows 2000 DHCP client) are configured auto-
         matically, but otherwise this must be configured manually -  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 permissi-
       ble.   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 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-spec-
         ified, 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 use-
         ful  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  suffi-
       ciently wide use already that we have implemented it.   Due to the com-
       plexity 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 con-
         figuration side, it controls the format in which the fqdn.fqdn subop-
         tion 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 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  com-
       parison,  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 sec-
         ondary DSS server.

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 spa-
       ces, 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;


       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 ;

       An option whose type is domain-list is an  RFC1035  formatted  (on  the
       wire, "DNS Format") list of domain names, separated by root labels.

       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 e.g.
       /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.   The  format  of  the  vendor-
       encapsulated-options option is either a series of bytes whose format is
       not specified, or a sequence of options, each of which  consists  of  a
       single-byte  vendor-specific  option  code,  followed  by a single-byte
       length, followed by as many bytes of  data  as  are  specified  in  the
       length (the length does not include itself or the option code).

       The value of this option 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 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;

       The  second way of setting the value of this option 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  vendor-encapsulated-options
       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.

       The code and length widths are used in DHCP protocol - you must config-
       ure these numbers to match the applicable option space you are  config-
       uring.   They each default to 1.  Valid values for code widths are 1, 2
       or 4.  Valid values for length widths are 1 or 2.

       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.   Due to limitations in 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;

       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 dif-
       ferent clients based on the  vendor-class-identifier  option  that  the
       clients send, as follows:

       class "vendor-classes" {
         match option vendor-class-identifier;
       }

       option SUNW.server-address 172.17.65.1;
       option SUNW.server-name "sundhcp-server17-1";

       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";
       }

       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 vendor-encapsu-
       lated-options option.

SEE ALSO
       dhcpd.conf(5),   dhcpd.leases(5),    dhclient.conf(5),    dhcp-eval(5),
       dhcpd(8),    dhclient(8),   RFC2132,   RFC2131,   draft-ietf-dhc-agent-
       options-??.txt.

AUTHOR
       The Internet Systems Consortium DHCP Distribution was  written  by  Ted
       Lemon  under  a contract with Vixie Labs.  Funding for this project was
       provided through Internet Systems Consortium.  Information about Inter-
       net Systems Consortium can be found at http://www.isc.org.



                                                              dhcpd-options(5)