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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  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
       existing 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,
       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));

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
       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 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 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;

          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 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-
          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 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 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
          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
          NetWare/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
          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 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 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 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 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  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.

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;

       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
       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 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
       different  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-
       encapsulated-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
       Internet Systems Consortium can be found at http://www.isc.org.

                                                              dhcpd-options(5)