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

NAME
       dhcp-options - Dynamic Host Configuration Protocol options

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

REFERENCE: OPTION STATEMENTS
       DHCP  option  statements always start with the option keyword, followed by an option name,
       followed by option data.  The option names and data formats are described  below.   It  is
       not  necessary  to  exhaustively  specify  all DHCP options - only those options which are
       needed by clients must be specified.

       Option data comes in a variety of formats, as defined below:

       The ip-address data  type  can  be  entered  either  as  an  explicit  IP  address  (e.g.,
       239.254.197.10)  or as a domain name (e.g., haagen.isc.org).  When entering a domain name,
       be sure that that domain name resolves to a single IP address.

       The ip6-address data specifies an IPv6 address, like ::1 or 3ffe:bbbb:aaaa:aaaa::1.

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

       The  text data type specifies an NVT ASCII string, which must be enclosed in double quotes
       - for example, to specify a root-path option, the syntax would be

       option root-path "10.0.1.4:/var/tmp/rootfs";

       The domain-name data type specifies a domain name, which must not be  enclosed  in  double
       quotes.  The domain name is stored just as if it were a text option.

       The  domain-list data type specifies a list of domain names, enclosed in double quotes and
       separated by commas ("example.com", "foo.example.com").

       The flag data type specifies a boolean value.  Booleans can be either true or false (or on
       or off, if that makes more sense to you).

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

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

SETTING OPTION VALUES USING EXPRESSIONS
       Sometimes it's helpful to be able to set the value of a DHCP option based  on  some  value
       that  the  client  has  sent.   To  do this, you can use expression evaluation.  The dhcp-
       eval(5) manual page describes how to write expressions.  To assign the result of an evalu‐
       ation to an option, define the option as follows:

         option my-option = expression ;

       For example:

         option hostname = binary-to-ascii (16, 8, "-",
                                            substring (hardware, 1, 6));

INCLUDING OPTION DEFINITIONS
       Starting  with  4.3.0  when  ISC  adds  new  option  definitions those definitions will be
       included in the code based on the definition of an argument for the RFC that  defines  the
       option  in includes/site.h.  This provides you with a method for over-riding the ISC defi‐
       nitions if necessary - for example if you have previously defined the option with  a  dif‐
       ferent format using the mechanism from DEFINING NEW OPTIONS below.

       By  default  all of the options are enabled.  In order to disable an option you would edit
       the includes/site.h file and comment out the definition for the proper RFC.

STANDARD DHCPV4 OPTIONS
       The documentation for the various options mentioned below is taken from  the  latest  IETF
       draft  document on DHCP options.  Options not listed below may not yet be implemented, but
       it is possible to use such options by defining them in the configuration file.  Please see
       the DEFINING NEW OPTIONS heading later in this document for more information.

       Some  of  the options documented here are automatically generated by the DHCP server or by
       clients, and cannot be configured by the user.  The value of such an option can be used in
       the  configuration file of the receiving DHCP protocol agent (server or client), for exam‐
       ple 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 sub‐
         nets 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 associated-ip ip-address [, ip-address... ];

         This  option is part of lease query.  It is used to return all of the IP addresses asso‐
         ciated with a given DHCP client.

         This option is not user configurable.

       option bcms-controller-address ip-address [, ip-address... ];

         This option configures a list of IPv4 addresses for use as Broadcast and Multicast  Con‐
         troller Servers ("BCMS").

       option bcms-controller-names domain-list;

         This  option  contains  the  domain  names  of  local Broadcast and Multicast Controller
         Servers ("BCMS") controllers which the client may use.

       option bootfile-name text;

         This option is used to identify a bootstrap file.  If supported by the client, it should
         have the same effect as the filename declaration.  BOOTP clients are unlikely to support
         this option.  Some DHCP clients will support it, and others actually require it.

       option boot-size uint16;

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

       option broadcast-address ip-address;

         This option specifies the broadcast address in use on the client's subnet.  Legal values
         for broadcast addresses are specified in section 3.2.1.3 of STD 3 (RFC1122).

       option capwap-ac-v4 ip-address [, ip-address ... ] ;

         A list of IPv4 addresses of CAPWAP ACs that the WTP may use.  The addresses  are  listed
         in preference order.

         This option is included based on RFC 5417.

       option client-last-transaction-time uint32;

         This option is part of lease query.  It allows the receiver to determine the time of the
         most recent access by the client.  The value is a duration  in  seconds  from  when  the
         client last communicated with the DHCP server.

         This option is not user configurable.

       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 seg‐
         ments.  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  con‐
         tained 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 over‐
         loaded 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 uint8 [, uint8... ];

         This option, when sent by the client, specifies which  options  the  client  wishes  the
         server  to  return.   Normally,  in  the ISC DHCP client, this is done using the request
         statement.  If this option is not specified by the client, the DHCP server will normally
         return  every  option  that  is  valid in scope and that fits into the reply.  When this
         option is specified on the server, the server returns the specified options.   This  can
         be  used  to force a client to take options that it hasn't requested, and it can also be
         used to tailor the response of the DHCP server for clients that may need a more  limited
         set of options than those the server would normally return.

       option dhcp-rebinding-time uint32;

         This option specifies the number of seconds from the time a client gets an address until
         the client transitions to the REBINDING state.

         This option is user configurable, but it will be ignored if the value is greater than or
         equal to the lease time.

         To  make  DHCPv4+DHCPv6  migration  easier  in  the future, any value configured in this
         option is also used as a DHCPv6 "T1" (renew) time.

       option dhcp-renewal-time uint32;

         This option specifies the number of seconds from the time a client gets an address until
         the client transitions to the RENEWING state.

         This option is user configurable, but it will be ignored if the value is greater than or
         equal to the rebinding time, or lease time.

         To make DHCPv4+DHCPv6 migration easier in the  future,  any  value  configured  in  this
         option is also used as a DHCPv6 "T2" (rebind) time.

       option dhcp-requested-address ip-address;

         This  option  is  used  by  the client in a DHCPDISCOVER to request that a particular IP
         address be assigned.

         This option is not user configurable.

       option dhcp-server-identifier ip-address;

         This option is used in  DHCPOFFER  and  DHCPREQUEST  messages,  and  may  optionally  be
         included  in  the DHCPACK and DHCPNAK messages.  DHCP servers include this option in the
         DHCPOFFER in order to allow the  client  to  distinguish  between  lease  offers.   DHCP
         clients use the contents of the ´server identifier´ field as the destination address for
         any DHCP messages unicast to the DHCP server.  DHCP clients also indicate which of  sev‐
         eral lease offers is being accepted by including this option in a DHCPREQUEST message.

         The value of this option is the IP address of the server.

         This  option  is not directly user configurable. See the server-identifier server option
         in dhcpd.conf(5).

       option domain-name text;

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

       option domain-name-servers ip-address [, ip-address...  ];

         The domain-name-servers option specifies a list of Domain Name System (STD 13, RFC 1035)
         name servers available to the client.  Servers should be listed in order of preference.

       option domain-search domain-list;

         The domain-search option specifies a ´search list´ of Domain Names to  be  used  by  the
         client  to  locate not-fully-qualified domain names.  The difference between this option
         and historic use of the domain-name option for the same ends  is  that  this  option  is
         encoded in RFC1035 compressed labels on the wire.  For example:

           option domain-search "example.com", "sales.example.com",
                                "eng.example.com";

       option extensions-path text;

         This option specifies the name of a file containing additional options to be interpreted
         according to the DHCP option format as specified in RFC2132.

       option finger-server ip-address [, ip-address... ];

         The Finger server option specifies a list of Finger servers  available  to  the  client.
         Servers should be listed in order of preference.

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

         This  option  specifies  a list of X Window System Font servers available to the client.
         Servers should be listed in order of preference.

       option geoconf-civic string;

         A string to hold the geoconf civic structure.

         This option is included based on RFC 4776.

       option host-name string;

         This option specifies the name of the client.  The name may or may not be qualified with
         the  local  domain  name  (it is preferable to use the domain-name option to specify the
         domain name).  See RFC 1035 for character set restrictions.  This option is only honored
         by dhclient-script(8) if the hostname for the client machine is not set.

       option ieee802-3-encapsulation flag;

         This  option specifies whether or not the client should use Ethernet Version 2 (RFC 894)
         or IEEE 802.3 (RFC 1042) encapsulation if the interface is  an  Ethernet.   A  value  of
         false indicates that the client should use RFC 894 encapsulation.  A value of true means
         that the client should use RFC 1042 encapsulation.

       option ien116-name-servers ip-address [, ip-address...  ];

         The ien116-name-servers option specifies a list of IEN 116 name servers available to the
         client.  Servers should be listed in order of preference.

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

         The  impress-server  option  specifies a list of Imagen Impress servers available to the
         client.  Servers should be listed in order of preference.

       option interface-mtu uint16;

         This option specifies the MTU to use on this interface.  The minimum legal value for the
         MTU is 68.

       option ip-forwarding flag;

         This  option  specifies whether the client should configure its IP layer for packet for‐
         warding.  A value of false means disable IP forwarding, and a value of true means enable
         IP forwarding.

       option irc-server ip-address [, ip-address... ];

         The  IRC server option specifies a list of IRC servers available to the client.  Servers
         should be listed in order of preference.

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

         The log-server option specifies a list of MIT-LCS  UDP  log  servers  available  to  the
         client.  Servers should be listed in order of preference.

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

         The LPR server option specifies a list of RFC 1179 line printer servers available to the
         client.  Servers should be listed in order of preference.

       option mask-supplier flag;

         This option specifies whether or not the client should respond to subnet  mask  requests
         using  ICMP.  A value of false indicates that the client should not respond.  A value of
         true means that the client should respond.

       option max-dgram-reassembly uint16;

         This option specifies the maximum size datagram that the client should  be  prepared  to
         reassemble.  The minimum legal value is 576.

       option merit-dump text;

         This option specifies the path-name of a file to which the client's core image should be
         dumped in the event the client crashes.  The path is formatted  as  a  character  string
         consisting of characters from the NVT ASCII character set.

       option mobile-ip-home-agent ip-address [, ip-address... ];

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

       option name-service-search uint16 [, uint6... ];

         This  option specifies a list of name services in the order the client should attempt to
         use them.

         This option is included based on RFC 2937.

       option nds-context string;

         The nds-context option specifies the name of the initial Netware Directory  Service  for
         an NDS client.

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

         The nds-servers option specifies a list of IP addresses of NDS servers.

       option nds-tree-name string;

         The nds-tree-name option specifies NDS tree name that the NDS client should use.

       option netbios-dd-server ip-address [, ip-address...  ];

         The NetBIOS datagram distribution server (NBDD) option specifies a list of RFC 1001/1002
         NBDD servers listed in order of preference.

       option netbios-name-servers ip-address [, ip-address...];

         The NetBIOS name server (NBNS) option specifies  a  list  of  RFC  1001/1002  NBNS  name
         servers  listed in order of preference.  NetBIOS Name Service is currently more commonly
         referred to as WINS.  WINS servers  can  be  specified  using  the  netbios-name-servers
         option.

       option netbios-node-type uint8;

         The  NetBIOS  node type option allows NetBIOS over TCP/IP clients which are configurable
         to be configured as described in RFC 1001/1002.  The value  is  specified  as  a  single
         octet which identifies the client type.

         Possible node types are:

         1    B-node: Broadcast - no WINS

         2    P-node: Peer - WINS only

         4    M-node: Mixed - broadcast, then WINS

         8    H-node: Hybrid - WINS, then broadcast

       option netbios-scope string;

         The  NetBIOS  scope  option  specifies  the  NetBIOS over TCP/IP scope parameter for the
         client as specified in RFC 1001/1002. See RFC1001, RFC1002, and RFC1035  for  character-
         set restrictions.

       option netinfo-server-address ip-address [, ip-address... ];

         The  netinfo-server-address option has not been described in any RFC, but has been allo‐
         cated (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 config‐
         ured.  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  servers  available  to  the  client.
         Servers should be listed in order of preference.

       option non-local-source-routing flag;

         This option specifies whether the client should configure its IP layer to allow forward‐
         ing of datagrams with non-local source routes (see Section 3.3.5 of [4] for a discussion
         of  this  topic).   A  value of false means disallow forwarding of such datagrams, and a
         value of true means allow forwarding.

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

         This option specifies a list of IP addresses indicating NTP (RFC 5905) servers available
         to the client.  Servers should be listed in order of preference.

       option nwip-domain string;

         The name of the NetWare/IP domain that a NetWare/IP client should use.

       option nwip-suboptions string;

         A  sequence  of  suboptions  for NetWare/IP clients - see RFC2242 for details.  Normally
         this option is set by specifying specific NetWare/IP suboptions  -  see  the  NETWARE/IP
         SUBOPTIONS section for more information.

       option option-6rd uint8 uint8 ip6-address ip-address [, ip-address ...];

         This  option  contains  information  about the rapid deployment option.  It is 8 bits of
         ipv4 mask length, 8 bits of 6rd prefix length, an ipv6 prefix as an ipv6 address  and  a
         list of one or more ipv4 addresses.

         This option is included based on RFC 5969.

       option pana-agent ip-address [, ip-address ... ] ;

         A  set  of  IPv4  addresses of a PAA for the client to use.  The addresses are listed in
         preferred order.

         This option is included based on RFC 5192.

       option path-mtu-aging-timeout uint32;

         This option specifies the timeout (in seconds) to use when aging Path MTU values discov‐
         ered by the mechanism defined in RFC 1191.

       option path-mtu-plateau-table uint16 [, uint16...  ];

         This  option specifies a table of MTU sizes to use when performing Path MTU Discovery as
         defined in RFC 1191.  The table is formatted as a  list  of  16-bit  unsigned  integers,
         ordered from smallest to largest.  The minimum MTU value cannot be smaller than 68.

       option pcode text;

         This option specifies a string suitable for the TZ variable.

         This option is included based on RFC 4833.

       option perform-mask-discovery flag;

         This  option  specifies  whether  or not the client should perform subnet mask discovery
         using ICMP.  A value of false indicates that the client should not perform mask  discov‐
         ery.  A value of true means that the client should perform mask discovery.

       option policy-filter ip-address ip-address
                         [, ip-address ip-address...];

         This  option specifies policy filters for non-local source routing.  The filters consist
         of a list of IP addresses and masks which specify destination/mask pairs with  which  to
         filter incoming source routes.

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

         See STD 3 (RFC1122) for further information.

       option pop-server ip-address [, ip-address... ];

         The POP3 server option specifies a  list  of  POP3  servers  available  to  the  client.
         Servers should be listed in order of preference.

       option rdnss-selection uint8 ip-address ip-address domain-name;

         The  rdnss-selection  option  specifies an 8 bit flags field, a primary and secondary ip
         address for the name server and a domainlist of domains for which the RDNSS has  special
         knowledge.

         This option is included based on RFC 6731.

       option resource-location-servers ip-address
                                     [, ip-address...];

         This  option  specifies  a  list  of  RFC 887 Resource Location servers available to the
         client.  Servers should be listed in order of preference.

       option root-path text;

         This option specifies the path-name that contains the client's root disk.  The  path  is
         formatted  as  a  character string consisting of characters from the NVT ASCII character
         set.

       option router-discovery flag;

         This option specifies whether or not the client should solicit routers using the  Router
         Discovery  mechanism  defined  in  RFC 1256.  A value of false indicates that the client
         should not perform router discovery.  A value of true means that the client should  per‐
         form router discovery.

       option router-solicitation-address ip-address;

         This  option  specifies the address to which the client should transmit router solicita‐
         tion 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 Pro‐
         tocol 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  dis‐
         covery 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 ser‐
         vice 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 Mi‐
         crosoft 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 ip-address;

         Sent by the client if an address is required in a subnet other than the one  that  would
         normally  be selected (based on the relaying address of the connected subnet the request
         is obtained from). See RFC3011. Note that the option number used by this server is  118;
         this has not always been the defined number, and some clients may use a different value.
         Use of this option should be regarded as slightly experimental!

       This option is not user configurable in the server.

       option swap-server ip-address;

         This specifies the IP address of the client's swap server.

       option tcp-keepalive-garbage flag;

         This option specifies whether or not the client should send TCP keepalive messages  with
         an  octet  of  garbage  for  compatibility with older implementations.  A value of false
         indicates that a garbage octet should not be sent. A value  of  true  indicates  that  a
         garbage octet should be sent.

       option tcp-keepalive-interval uint32;

         This  option  specifies the interval (in seconds) that the client TCP should wait before
         sending a keepalive message on a TCP connection.  The time  is  specified  as  a  32-bit
         unsigned  integer.   A  value  of  zero  indicates  that  the client should not generate
         keepalive messages on connections unless specifically requested by an application.

       option tcode text;

         This option specifies a name of a zone entry in the TZ database.

         This option is included based on RFC 4833.

       option tftp-server-name text;

         This option is used to identify a TFTP server and, if supported by  the  client,  should
         have the same effect as the server-name declaration.  BOOTP clients are unlikely to sup‐
         port 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 Coor‐
         dinated 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 Authenti‐
         cation 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-iden‐
         tifier  option,  but  the  value of the option is specified by the user, not the vendor.
         Most recent DHCP clients have a way in the user interface to specify the value for  this
         identifier, usually as a text string.

       option v4-access-domain domain-name;

         The domain name associated with the access network for use with LIS Discovery.

         This option is included based on RFC 5986.

       option v4-lost domain-name;

         The domain name of the LoST server for the client to use.

         This option is included based on RFC 5223.

       option vendor-class-identifier string;

         This  option  is  used by some DHCP clients to identify the vendor type and possibly the
         configuration of a DHCP client.  The information is a string of bytes whose contents are
         specific  to  the  vendor and are not specified in a standard.  To see what vendor class
         identifier clients are sending, you can write the following in your DHCP server configu‐
         ration 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  con‐
         figured manually - see the VENDOR ENCAPSULATED OPTIONS section later in this manual page
         for details.

       option vivso string;

         The vivso option can contain multiple separate options, one for each  32-bit  Enterprise
         ID.  Each Enterprise-ID discriminated option then contains additional options whose for‐
         mat is defined by the vendor who holds that ID.  This option is usually  not  configured
         manually,  but rather is configured via intervening option definitions.  Please also see
         the VENDOR ENCAPSULATED OPTIONS section later in this manual page for details.

       option www-server ip-address [, ip-address... ];

         The WWW server option specifies a list of WWW servers available to the client.   Servers
         should be listed in order of preference.

       option x-display-manager ip-address [, ip-address...  ];

         This  option  specifies  a  list of systems that are running the X Window System Display
         Manager and are available to the client.  Addresses should be listed in order of prefer‐
         ence.

RELAY AGENT INFORMATION OPTION
       An  IETF  draft,  draft-ietf-dhc-agent-options-11.txt,  defines  a  series of encapsulated
       options that a relay agent can add to a DHCP packet when relaying it to the  DHCP  server.
       The  server  can  then  make  address allocation decisions (or whatever other decisions it
       wants) based on these options.  The server also returns these options in  any  replies  it
       sends  through  the  relay agent, so that the relay agent can use the information in these
       options for delivery or accounting purposes.

       The current draft defines two options.  To reference these options  in  the  dhcp  server,
       specify the option space name, "agent", followed by a period, followed by the option name.
       It is not normally useful to define values for these options in the server, although it is
       permissible.  These options are not supported in the client.

       option agent.circuit-id string;

         The  circuit-id  suboption encodes an agent-local identifier of the circuit from which a
         DHCP client-to-server packet was received.  It is intended for use by agents in relaying
         DHCP  responses  back  to  the  proper  circuit.  The format of this option is currently
         defined to be vendor-dependent, and will probably remain that way, although the  current
         draft allows for the possibility of standardizing the format in the future.

       option agent.remote-id string;

         The  remote-id  suboption  encodes  information  about the remote host end of a circuit.
         Examples of what it might contain include caller ID information,  username  information,
         remote  ATM  address,  cable modem ID, and similar things.  In principal, the meaning is
         not well-specified, and it should generally be assumed to be an opaque  object  that  is
         administratively guaranteed to be unique to a particular remote end of a circuit.

       option agent.DOCSIS-device-class uint32;

         The  DOCSIS-device-class suboption is intended to convey information about the host end‐
         point, 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 imple‐
         mented 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 config‐
       ured by the user - instead it should be used as part of an automatic DNS update system.

       option fqdn.no-client-update flag;

         When  the  client  sends  this,  if  it is true, it means the client will not attempt to
         update its A record.  When sent by the server to the client, it means  that  the  client
         should not update its own A record.

       option fqdn.server-update flag;

         When  the client sends this to the server, it is requesting that the server update its A
         record.  When sent by the server, it means that the server has updated (or is  about  to
         update) the client's A record.

       option fqdn.encoded flag;

         If  true,  this  indicates that the domain name included in the option is encoded in DNS
         wire format, rather than as plain ASCII text.  The client normally sets this to false if
         it  doesn't  support  DNS wire format in the FQDN option.  The server should always send
         back the same value that the client sent.  When this value is set on  the  configuration
         side, it controls the format in which the fqdn.fqdn suboption is encoded.

       option fqdn.rcode1 flag;

       option fqdn.rcode2 flag;

         These  options specify the result of the updates of the A and PTR records, respectively,
         and are only sent by the DHCP server to the DHCP client.  The values of these fields are
         those defined in the DNS protocol specification.

       option fqdn.fqdn text;

         Specifies  the domain name that the client wishes to use.  This can be a fully-qualified
         domain name, or a single label.  If there is no trailing ´.´ character in the  name,  it
         is  not fully-qualified, and the server will generally update that name in some locally-
         defined domain.

       option fqdn.hostname --never set--;

         This option should never be set, but it can be read back using the  option  and  config-
         option  operators  in  an  expression,  in  which case it returns the first label in the
         fqdn.fqdn suboption - for example, if the value of fqdn.fqdn is "foo.example.com.", then
         fqdn.hostname will be "foo".

       option fqdn.domainname --never set--;

         This  option  should  never be set, but it can be read back using the option and config-
         option operators in an expression, in which case it returns all labels after  the  first
         label  in the fqdn.fqdn suboption - for example, if the value of fqdn.fqdn is "foo.exam‐
         ple.com.", then fqdn.domainname will be "example.com.".  If this suboption value is  not
         set,  it  means  that  an  unqualified name was sent in the fqdn option, or that no fqdn
         option was sent at all.

       If you wish to use any of these suboptions, we strongly recommend that you  refer  to  the
       Client  FQDN  option  draft  (or standard, when it becomes a standard) - the documentation
       here is sketchy and incomplete in comparison, and is just intended for reference by people
       who already understand the Client FQDN option specification.

THE NETWARE/IP SUBOPTIONS
       RFC2242 defines a set of encapsulated options for Novell NetWare/IP clients.  To use these
       options in the dhcp server, specify the option space name, "nwip", followed by  a  period,
       followed by the option name.  The following options can be specified:

       option nwip.nsq-broadcast flag;

         If  true,  the client should use the NetWare Nearest Server Query to locate a NetWare/IP
         server.  The behaviour of the Novell client if  this  suboption  is  false,  or  is  not
         present, is not specified.

       option nwip.preferred-dss ip-address [, ip-address... ];

         This  suboption specifies a list of up to five IP addresses, each of which should be the
         IP address of a NetWare Domain SAP/RIP server (DSS).

       option nwip.nearest-nwip-server ip-address
                                    [, ip-address...];

         This suboption specifies a list of up to five IP addresses, each of which should be  the
         IP address of a Nearest NetWare IP server.

       option nwip.autoretries uint8;

         Specifies  the  number  of  times that a NetWare/IP client should attempt to communicate
         with a given DSS server at startup.

       option nwip.autoretry-secs uint8;

         Specifies the number of seconds that a Netware/IP client  should  wait  between  retries
         when attempting to establish communications with a DSS server at startup.

       option nwip.nwip-1-1 uint8;

         If  true,  the  NetWare/IP  client  should support NetWare/IP version 1.1 compatibility.
         This is only needed if the client will be contacting Netware/IP version 1.1 servers.

       option nwip.primary-dss ip-address;

         Specifies the IP address of the Primary Domain SAP/RIP Service  server  (DSS)  for  this
         NetWare/IP domain.  The NetWare/IP administration utility uses this value as Primary DSS
         server when configuring a secondary DSS server.

STANDARD DHCPV6 OPTIONS
       DHCPv6 options differ from DHCPv4 options partially due to using 16-bit  code  and  length
       tags,  but  semantically zero-length options are legal in DHCPv6, and multiple options are
       treated differently.  Whereas in DHCPv4 multiple options would be concatenated to form one
       option, in DHCPv6 they are expected to be individual instantiations.  Understandably, many
       options are not "allowed" to have multiple instances in a  packet  -  normally  these  are
       options  which  are  digested  by the DHCP protocol software, and not by users or applica‐
       tions.

       option dhcp6.client-id string;

         This option specifies the client's DUID identifier.  DUIDs  are  similar  but  different
         from DHCPv4 client identifiers - there are documented duid types:

         duid-llt

         duid-en

         duid-ll

         This value should not be configured, but rather is provided by clients and treated as an
         opaque identifier key blob by servers.

       option dhcp6.server-id string;

         This option specifies the server's DUID identifier.  One may use this option to  config‐
         ure an opaque binary blob for your server's identifier.

       option dhcp6.ia-na string;

         The  Identity Association for Non-temporary Addresses (ia-na) carries assigned addresses
         that are not temporary addresses for use by the DHCPv6 client.  This option is  produced
         by the DHCPv6 server software, and should not be configured.

       option dhcp6.ia-ta string;

         The  Identity  Association  for Temporary Addresses (ia-ta) carries temporary addresses,
         which may change upon every renewal.  There is no support for this in the current DHCPv6
         software.

       option dhcp6.ia-addr string;

         The Identity Association Address option is encapsulated inside ia-na or ia-ta options in
         order to represent addresses associated with those IA's.  These options are manufactured
         by the software, so should not be configured.

       option dhcp6.oro uint16 [ , uint16, ... ];

         The  Option  Request  Option  ("ORO") is the DHCPv6 equivalent of the parameter-request-
         list.  Clients supply this option to ask servers to reply with options relevant to their
         needs  and  use.   This  option  must  not be directly configured, the request syntax in
         dhclient.conf (5) should be used instead.

       option dhcp6.preference uint8;

         The preference option informs a DHCPv6 client which server is ´preferred´ for use  on  a
         given  subnet.   This preference is only applied during the initial stages of configura‐
         tion - once a client is bound to an IA, it will remain bound to that IA until it  is  no
         longer  valid  or  has  expired.   This  value  may  be configured on the server, and is
         digested by the client software.

       option dhcp6.elapsed-time uint16;

         The elapsed-time option is constructed by the DHCPv6 client software, and is potentially
         consumed by intermediaries.  This option should not be configured.

       option dhcp6.relay-msg string;

         The  relay-msg  option  is constructed by intervening DHCPv6 relay agent software.  This
         option is entirely used by protocol software, and is not meant for user configuration.

       option dhcp6.unicast ip6-address;

         The unicast option is provided by DHCPv6  servers  which  are  willing  (or  prefer)  to
         receive  Request,  Renew,  Decline,  and Release packets from their clients via unicast.
         Normally, DHCPv6 clients will multicast these messages.  Per RFC 3315, the  server  will
         reject a unicast message received from a client unless it previously sent (or would have
         sent) the unicast option to that client.  This option may be configured on the server at
         the  global  and  shared  network level.  When a unicast message is received, the server
         will check for an applicable definition of the unicast option.  If  such  an  option  is
         found the message will be accepted, if not it will be rejected.

       option dhcp6.status-code status-code [ string ] ;

         The  status-code  option is provided by DHCPv6 servers to inform clients of error condi‐
         tions during protocol communication.  This option is manufactured and digested by proto‐
         col software, and should not be configured.

       option dhcp6.rapid-commit ;

         The  rapid-commit  option  is  a  zero-length  option that clients use to indicate their
         desire to enter into rapid-commit with the server.

       option dhcp6.vendor-opts string;

         The vendor-opts option is actually an encapsulated sub-option space, in which each  Ven‐
         dor-specific  Information  Option (VSIO) is identified by a 32-bit Enterprise-ID number.
         The encapsulated option spaces within these options are defined by the vendors.

         To make use of this option, the best way is to examine the section titled VENDOR  ENCAP‐
         SULATED OPTIONS below, in particular the bits about the "vsio" option space.

       option dhcp6.interface-id string;

         The  interface-id  option is manufactured by relay agents, and may be used to guide con‐
         figuration differentiating clients by the interface they are remotely attached  to.   It
         does  not  make  sense  to  configure  a value for this option, but it may make sense to
         inspect its contents.

       option dhcp6.reconf-msg dhcpv6-message;

         The reconf-msg option is manufactured by servers, and sent  to  clients  in  Reconfigure
         messages  to  inform them of what message the client should Reconfigure using.  There is
         no support for DHCPv6 Reconfigure extensions, and this option is documented information‐
         ally only.

       option dhcp6.reconf-accept ;

         The  reconf-accept  option  is  included  by DHCPv6 clients that support the Reconfigure
         extensions, advertising that they will respond if the server were to ask them to  Recon‐
         figure.  There is no support for DHCPv6 Reconfigure extensions, and this option is docu‐
         mented informationally only.

       option dhcp6.sip-servers-names domain-list;

         The sip-servers-names option allows SIP clients to locate a local SIP server that is  to
         be  used for all outbound SIP requests, a so-called"outbound proxy server."  If you wish
         to use manually entered IPv6 addresses instead,  please  see  the  sip-servers-addresses
         option below.

       option dhcp6.sip-servers-addresses ip6-address [, ip6-address ... ] ;

         The sip-servers-addresses option allows SIP clients to locate a local SIP server that is
         to be used for all outbound SIP requests, a so-called "outbound proxy servers."  If  you
         wish  to  use  domain names rather than IPv6 addresses, please see the sip-servers-names
         option above.

       option dhcp6.name-servers ip6-address [, ip6-address ... ] ;

         The name-servers option instructs clients about locally available recursive DNS servers.
         It is easiest to describe this as the "nameserver" line in /etc/resolv.conf.

       option dhcp6.domain-search domain-list;

         The  domain-search  option  specifies  the  client's domain search path to be applied to
         recursive DNS queries.  It  is  easiest  to  describe  this  as  the  "search"  line  in
         /etc/resolv.conf.

       option dhcp6.ia-pd string;

         The  ia-pd  option  is manufactured by clients and servers to create a Prefix Delegation
         binding - to delegate an IPv6 prefix to the  client.   It  is  not  directly  edited  in
         dhcpd.conf(5)  or dhclient.conf(5), but rather is manufactured and consumed by the soft‐
         ware.

       option dhcp6.ia-prefix string;

         The ia-prefix option is placed inside ia-pd options in order to identify the  prefix(es)
         allocated   to   the   client.    It   is   not  directly  edited  in  dhcpd.conf(5)  or
         dhclient.conf(5), but rather is manufactured and consumed by the software.

       option dhcp6.nis-servers ip6-address [, ip6-address ... ] ;

         The nis-servers option identifies, in order, NIS servers available to the client.

       option dhcp6.nisp-servers ip6-address [, ip6-address ... ] ;

         The nisp-servers option identifies, in order, NIS+ servers available to the client.

       option nis-domain-name domain-list;

         The nis-domain-name option specifies the NIS domain name the client is expected to  use,
         and is related to the nis-servers option.

       option dhcp6.nis-domain-name domain-name;

         The  dhcp6.nis-domain-name  option  specifies  NIS domain name the client is expected to
         use, and is related to dhcp6.nis-servers option.

       option nisp-domain-name domain-list;

         The nisp-domain-name option specifies the NIS+ domain name the  client  is  expected  to
         use, and is related to the nisp-servers option.

       option dhcp6.nisp-domain-name domain-name;

         The  dhcp6.nis-domain-name  option  specifies NIS+ domain name the client is expected to
         use, and is related to dhcp6.nisp-servers option.

       option dhcp6.sntp-servers ip6-address [, ip6-address ... ] ;

         The sntp-servers option specifies a list of local SNTP servers available for the  client
         to synchronize their clocks.

       option dhcp6.info-refresh-time uint32;

         The  info-refresh-time  option gives DHCPv6 clients using Information-request messages a
         hint as to how long they should between refreshing  the  information  they  were  given.
         Note  that this option will only be delivered to the client, and be likely to affect the
         client's behaviour, if the client requested the option.

       option dhcp6.bcms-server-d domain-list;

         The bcms-server-d option contains the domain names of local BCMS (Broadcast  and  Multi‐
         cast Control Services) controllers which the client may use.

       option dhcp6.bcms-server-a ip6-address [, ip6-address ... ] ;

         The bcms-server-a option contains the IPv6 addresses of local BCMS (Broadcast and Multi‐
         cast Control Services) controllers which the client may use.

       option dhcp6.geoconf-civic string;

         A string to hold the geoconf civic structure.

         This option is included based on RFC 4776.

       option dhcp6.remote-id string;

         The remote-id option is constructed by relay agents, to inform  the  server  of  details
         pertaining  to  what  the relay knows about the client (such as what port it is attached
         to, and so forth).  The contents of this  option  have  some  vendor-specific  structure
         (similar to VSIO), but we have chosen to treat this option as an opaque field.

       option dhcp6.subscriber-id string;

         The  subscriber-id option is an opaque field provided by the relay agent, which provides
         additional information about the subscriber in question.  The  exact  contents  of  this
         option  depend upon the vendor and/or the operator's configuration of the remote device,
         and as such is an opaque field.

       option dhcp6.fqdn string;

         The fqdn option is normally constructed by the client  or  server,  and  negotiates  the
         client's  Fully Qualified Domain Name, as well as which party is responsible for Dynamic
         DNS Updates.  See the section on the Client FQDN SubOptions for full details (the DHCPv4
         and  DHCPv6  FQDN  options  use  the same "fqdn." encapsulated space, so are in all ways
         identical).

       option dhcp6.pana-agent ip6-address [, ip6-address ... ] ;

         A set of IPv6 addresses of a PAA for the client to use.  The  addresses  are  listed  in
         preferred order.

         This option is included based on RFC 5192.

       option dhcp6.new-posix-timezone text;

         This option specifies a string suitable for the TZ variable.

         This option is included based on RFC 4833.

       option dhcp6.new-tzdb-timezone text;

         This option specifies a name of a zone entry in the TZ database.

         This option is included based on RFC 4833.

       option dhcp6.ero uint16 [, uint16 ... ] ;

         A list of the options requested by the relay agent.

         This option is included based on RFC 4994.

       option dhcp6.lq-query string;

         The lq-query option is used internally for lease query.

       option dhcp6.client-data string;

         The client-data option is used internally for lease query.

       option dhcp6.clt-time uint32;

         The clt-time option is used internally for lease query.

       option dhcp6.lq-relay-data ip6-address string;

         The lq-relay-data option is used internally for lease query.

       option dhcp6.lq-client-link ip6-address [, ip6-address ... ] ;

         The lq-client-link option is used internally for lease query.

       option dhcp6.v6-lost domain-name;

         The domain name of the LoST server for the client to use.

         This option is included based on RFC 5223.

       option dhcp6.capwap-ac-v6 ip6-address [, ip6-address ... ] ;

         A  list  of IPv6 addresses of CAPWAP ACs that the WTP may use.  The addresses are listed
         in preference order.

         This option is included based on RFC 5417.

       option dhcp6.relay-id string;

         The DUID for the relay agent.

         This option is included based on RFC 5460.

       option dhcp6.v6-access-domain domain-name;

         The domain name associated with the access network for use with LIS Discovery.

         This option is included based on RFC5986.

       option dhcp6.sip-ua-cs-list domain-list;

         The list of domain names in the SIP User Agent Configuration Service Domains.

         This option is included based on RFC 6011.

       option dhcp6.bootfile-url text;

         The URL for a boot file.

         This option is included based on RFC 5970.

       option dhcp6.bootfile-param string;

         A string for the parameters to the bootfile.  See RFC 5970 for more description  of  the
         layout of the parameters within the string.

         This option is included based on RFC 5970.

       option dhcp6.client-arch-type uint16 [, uint16 ... ] ;

         A list of one or more architecture types described as 16 bit values.

         This option is included based on RFC 5970.

       option dhcp6.nii uint8 uint8 uint8;

         The  client  network  interface  identitier option supplies information about a client's
         level of UNDI support.  The values are, in order, the type,  the  major  value  and  the
         minor value.

         This option is included based on RFC5970.

       option dhcp6.aftr-name domain-name;

         A domain name of the AFTR tunnel endpoint.

         This option is included based on RFC 6334.

       option dhcp6.erp-local-domain-name domain-name;

         A domain name for the ERP domain.

         This option is included based on RFC 6440.

       option dhcp6.rdnss-selection ip6-address uint8 domain-name;

         RDNSS  information  consists  of  an  IPv6  address of RDNSS, an 8 bit flags field and a
         domain-list of domains for which the RDNSS has special knowledge.

         This option is included based on RFC 6731.

       option dhcp6.client-linklayer-addr string;

         A client link-layer address.  The first two bytes must be the  type  of  the  link-layer
         followed by the address itself.

         This option is included based on RFC 6939.

       option dhcp6.link-address ip6-address;

         An  IPv6  address  used by a relay agent to indicate to the server the link on which the
         client is located.

         This option is included based on RFC 6977.

       option dhcp6.solmax-rt uint32;

         A value to override the default for SOL_MAX_RT.  This is a 32 bit value.

         This option is included based on RFC 7083.

       option dhcp6.inf-max-rt uint32;

         A value to override the default for INF_MAX_RT.  This is a 32 bit value.

         This option is included based on RFC 7083.

ACCESSING DHCPV6 RELAY OPTIONS
       v6relay (relay-number option This option allows access to an option that has been added to
       a  packet by a relay agent.  Relay-number value selects the relay to examine and option is
       the option to find.  In DHCPv6 each relay encapsulates the entire previous message into an
       option,  adds  its own options (if any) and sends the result onwards.  The RFC specifies a
       limit of 32 hops.  A relay-number of 0 is a no-op and means don't look at the  relays.   1
       is  the relay that is closest to the client, 2 would be the next in from the client and so
       on.  Any value greater than the max number of hops is which is closest to the server inde‐
       pendent  of number.  To use this option in a class statement you would have something like
       this:

       match if v6relay(1, option dhcp6.subscriber-id) = "client_1";

DEFINING NEW OPTIONS
       The Internet Systems Consortium DHCP client and server provide the  capability  to  define
       new  options.   Each DHCP option has a name, a code, and a structure.  The name is used by
       you to refer to the option.  The code is a number, used by the DHCP server and  client  to
       refer to an option.  The structure describes what the contents of an option looks like.

       To define a new option, you need to choose a name for it that is not in use for some other
       option - for example, you can't use "host-name" because the DHCP protocol already  defines
       a  host-name  option,  which is documented earlier in this manual page.  If an option name
       doesn't appear in this manual page, you can use it, but it's probably a good idea  to  put
       some  kind  of unique string at the beginning so you can be sure that future options don't
       take your name.  For example, you might define an option, "local-host-name", feeling  some
       confidence that no official DHCP option name will ever start with "local".

       Once  you  have  chosen a name, you must choose a code.  All codes between 224 and 254 are
       reserved as ´site-local´ DHCP options, so you can pick any one of these for your site (not
       for  your  product/application).   In RFC3942, site-local space was moved from starting at
       128 to starting at 224.  In practice, some vendors have interpreted  the  protocol  rather
       loosely and have used option code values greater than 128 themselves.  There's no real way
       to avoid this problem, and it was thought to be unlikely to  cause  too  much  trouble  in
       practice.   If  you  come  across a vendor-documented option code in either the new or old
       site-local spaces, please contact your vendor and inform them about rfc3942.

       The structure of an option is simply the format in which the option data appears.  The ISC
       DHCP server currently supports a few simple types, like integers, booleans, strings and IP
       addresses, and it also supports the ability to define arrays of single types or arrays  of
       fixed sequences of types.

       New options are declared as follows:

       option new-name code new-code = definition ;

       The  values of new-name and new-code should be the name you have chosen for the new option
       and the code you have chosen.  The definition should be the definition of the structure of
       the option.

       The following simple option type definitions are supported:

       BOOLEAN

       option new-name code new-code = boolean ;

       An  option  of type boolean is a flag with a value of either on or off (or true or false).
       So an example use of the boolean type would be:

       option use-zephyr code 180 = boolean;
       option use-zephyr on;

       INTEGER

       option new-name code new-code = sign integer width ;

       The sign token should either be blank, unsigned or signed.  The width can be either 8,  16
       or 32, and refers to the number of bits in the integer.  So for example, the following two
       lines show a definition of the sql-connection-max option and its use:

       option sql-connection-max code 192 = unsigned integer 16;
       option sql-connection-max 1536;

       IP-ADDRESS

       option new-name code new-code = ip-address ;

       An option whose structure is an IP address can be expressed either as a domain name or  as
       a dotted quad.  So the following is an example use of the ip-address type:

       option sql-server-address code 193 = ip-address;
       option sql-server-address sql.example.com;

       IP6-ADDRESS

       option new-name code new-code = ip6-address ;

       An  option  whose  structure is an IPv6 address must be expressed as a valid IPv6 address.
       The following is an example use of the ip6-address type:

       option dhcp6.some-server code 1234 = array of ip6-address;
       option dhcp6.some-server 3ffe:bbbb:aaaa:aaaa::1, 3ffe:bbbb:aaaa:aaaa::2;

       TEXT

       option new-name code new-code = text ;

       An option whose type is text will encode an ASCII text string.  For example:

       option sql-default-connection-name code 194 = text;
       option sql-default-connection-name "PRODZA";

       DATA STRING

       option new-name code new-code = string ;

       An option whose type is a data string is essentially just a collection of bytes,  and  can
       be  specified  either as quoted text, like the text type, or as a list of hexadecimal con‐
       tents separated by colons whose values must be between 0 and FF.  For example:

       option sql-identification-token code 195 = string;
       option sql-identification-token 17:23:19:a6:42:ea:99:7c:22;

       DOMAIN-LIST

       option new-name code new-code = domain-list [compressed] ;

       An option whose type is domain-list is an RFC1035 formatted (on the  wire,  "DNS  Format")
       list of domain names, separated by root labels.  The optional compressed keyword indicates
       if the option should be compressed relative to the start of the option contents  (not  the
       packet contents).

       When  in doubt, omit the compressed keyword.  When the software receives an option that is
       compressed and the compressed keyword is omitted, it  will  still  decompress  the  option
       (relative  to the option contents field).  The keyword only controls whether or not trans‐
       mitted packets are compressed.

       Note that when domain-list formatted  options  are  output  as  environment  variables  to
       dhclient-script(8), the standard DNS -escape mechanism is used: they are decimal.  This is
       appropriate for direct use in eg /etc/resolv.conf.

       ENCAPSULATION

       option new-name code new-code = encapsulate identifier ;

       An option whose type is encapsulate will encapsulate the  contents  of  the  option  space
       specified in identifier.  Examples of encapsulated options in the DHCP protocol as it cur‐
       rently exists  include  the  vendor-encapsulated-options  option,  the  netware-suboptions
       option and the relay-agent-information option.

       option space local;
       option local.demo code 1 = text;
       option local-encapsulation code 197 = encapsulate local;
       option local.demo "demo";

       ARRAYS

       Options  can  contain arrays of any of the above types except for the text and data string
       types, which aren't currently supported in arrays.  An example of an array  definition  is
       as follows:

       option kerberos-servers code 200 = array of ip-address;
       option kerberos-servers 10.20.10.1, 10.20.11.1;

       RECORDS

       Options  can also contain data structures consisting of a sequence of data types, which is
       sometimes called a record type.  For example:

       option contrived-001 code 201 = { boolean, integer 32, text };
       option contrived-001 on 1772 "contrivance";

       It's also possible to have options that are arrays of records, for example:

       option new-static-routes code 201 = array of {
            ip-address, ip-address, ip-address, integer 8 };
       option static-routes
            10.0.0.0 255.255.255.0 net-0-rtr.example.com 1,
            10.0.1.0 255.255.255.0 net-1-rtr.example.com 1,
            10.2.0.0 255.255.224.0 net-2-0-rtr.example.com 3;

VENDOR ENCAPSULATED OPTIONS
       The DHCP protocol defines the vendor-encapsulated-options option, which allows vendors  to
       define  their  own  options  that will be sent encapsulated in a standard DHCP option.  It
       also defines the Vendor Identified Vendor Sub Options option  ("VIVSO"),  and  the  DHCPv6
       protocol  defines  the  Vendor-specific Information Option ("VSIO").  The format of all of
       these options is usually internally a string of options, similarly to  other  normal  DHCP
       options.   The  VIVSO and VSIO options differ in that they contain options that correspond
       to vendor Enterprise-ID numbers (assigned by IANA), which then contain  options  according
       to each Vendor's specifications.  You will need to refer to your vendor's documentation in
       order to form options to their specification.

       The value of these options can be set in one of two ways.  The  first  way  is  to  simply
       specify  the  data  directly, using a text string or a colon-separated list of hexadecimal
       values.  For help in forming these strings, please refer to RFC2132 for the DHCPv4  Vendor
       Specific  Information Option, RFC3925 for the DHCPv4 Vendor Identified Vendor Sub Options,
       or RFC3315 for the DHCPv6 Vendor-specific Information Option.  For example:

       option vendor-encapsulated-options
           2:4:
            AC:11:41:1:
           3:12:
            73:75:6e:64:68:63:70:2d:73:65:72:76:65:72:31:37:2d:31:
           4:12:
            2f:65:78:70:6f:72:74:2f:72:6f:6f:74:2f:69:38:36:70:63;
       option vivso
           00:00:09:bf:0E:
            01:0c:
                48:65:6c:6c:6f:20:77:6f:72:6c:64:21;
       option dhcp6.vendor-opts
           00:00:09:bf:
            00:01:00:0c:
                48:65:6c:6c:6f:20:77:6f:72:6c:64:21;

       The second way of setting the value of these options is to have the DHCP server generate a
       vendor-specific  option  buffer.   To  do  this, you must do four things: define an option
       space, define some options in that option space, provide values for them, and specify that
       that option space should be used to generate the relevant option.

       To  define  a new option space in which vendor options can be stored, use the option space
       statement:

       option space name [ [ code width number ] [ length width number ] [ hash size number ] ] ;

       Where the numbers following code width, length width, and hash size respectively  identify
       the number of bytes used to describe option codes, option lengths, and the size in buckets
       of the hash tables to hold options in this space (most DHCPv4 option  spaces  use  1  byte
       codes  and  lengths,  which  is  the default, whereas most DHCPv6 option spaces use 2 byte
       codes and lengths).

       The code and length widths are used in DHCP protocol - you must configure these numbers to
       match  the  applicable  option  space you are configuring.  They each default to 1.  Valid
       values for code widths are 1, 2 or 4.  Valid values for length widths are 0, 1 or 2.  Most
       DHCPv4  option  spaces  use  1  byte codes and lengths, which is the default, whereas most
       DHCPv6 option spaces use 2 byte codes and lengths.  A zero-byte  length  produces  options
       similar to the DHCPv6 Vendor-specific Information Option - but not their contents!

       The hash size defaults depend upon the code width selected, and may be 254 or 1009.  Valid
       values range between 1 and 65535.  Note that the higher you configure this value, the more
       memory will be used.  It is considered good practice to configure a value that is slightly
       larger than the estimated number of options you plan to configure within the space.   Pre‐
       vious versions of ISC DHCP (up to and including DHCP 3.0.*), this value was fixed at 9973.

       The  name  can  then be used in option definitions, as described earlier in this document.
       For example:

       option space SUNW code width 1 length width 1 hash size 3;
       option SUNW.server-address code 2 = ip-address;
       option SUNW.server-name code 3 = text;
       option SUNW.root-path code 4 = text;

       option space ISC code width 1 length width 1 hash size 3;
       option ISC.sample code 1 = text;
       option vendor.ISC code 2495 = encapsulate vivso-sample;
       option vendor-class.ISC code 2495 = text;

       option ISC.sample "configuration text here";
       option vendor-class.ISC "vendor class here";

       option space docsis code width 2 length width 2 hash size 17;
       option docsis.tftp-servers code 32 = array of ip6-address;
       option docsis.cablelabs-configuration-file code 33 = text;
       option docsis.cablelabs-syslog-servers code 34 = array of ip6-address;
       option docsis.device-id code 36 = string;
       option docsis.time-servers code 37 = array of ip6-address;
       option docsis.time-offset code 38 = signed integer 32;
       option vsio.docsis code 4491 = encapsulate docsis;

       Once you have defined an option space and the format of  some  options,  you  can  set  up
       scopes  that define values for those options, and you can say when to use them.  For exam‐
       ple, 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 differ‐
       ent clients based on the vendor-class-identifier option that the clients send, as follows:

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

       subclass "vendor-classes" "SUNW.Ultra-5_10" {
         vendor-option-space SUNW;
         option SUNW.root-path "/export/root/sparc";
       }

       subclass "vendor-classes" "SUNW.i86pc" {
         vendor-option-space SUNW;
         option SUNW.root-path "/export/root/i86pc";
       }

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

       option vivso-sample.sample "Hello world!";

       option docsis.tftp-servers ::1;

       As you can see in the preceding example, regular scoping rules apply, so  you  can  define
       values  that are global in the global scope, and only define values that are specific to a
       particular class in the local scope.  The vendor-option-space declaration tells  the  DHCP
       server  to  use  options  in the SUNW option space to construct the DHCPv4 vendor-encapsu‐
       lated-options option.  This is a limitation of that option -  the  DHCPv4  VIVSO  and  the
       DHCPv6  VSIO options can have multiple vendor definitions all at once (even transmitted to
       the same client), so it is not necessary to configure this.

SEE ALSO
       dhcpd.conf(5), dhcpd.leases(5),  dhclient.conf(5),  dhcp-eval(5),  dhcpd(8),  dhclient(8),
       RFC2132, RFC2131, RFC3046, RFC3315.

AUTHOR
       Information about Internet Systems Consortium can be found at https://www.isc.org.

                                                                                  dhcp-options(5)

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