dhcp-options(category14-security-amp-firewalls.html) - phpMan

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 spec-
       ify  all  DHCP options - only those options which are needed by clients
       must be specified.
       Option data comes in a variety of formats, as defined below:
       The ip-address data type can  be  entered  either  as  an  explicit  IP
       address  (e.g.,  239.254.197.10)  or  as  a  domain  name  (e.g.,  haa-
       gen.isc.org).  When entering a domain name, be sure  that  that  domain
       name resolves to a single IP address.
       The   ip6-address   data   specifies  an  IPv6  address,  like  ::1  or
       3ffe:bbbb:aaaa:aaaa::1.
       The int32 data type specifies a signed 32-bit integer.  The uint32 data
       type  specifies  an unsigned 32-bit integer.  The int16 and uint16 data
       types specify signed and unsigned 16-bit integers.  The int8 and  uint8
       data  types specify signed and unsigned 8-bit integers.  Unsigned 8-bit
       integers are also sometimes referred to as octets.
       The text data type  specifies  an  NVT  ASCII  string,  which  must  be
       enclosed in double quotes - for example, to specify a root-path option,
       the syntax would be
       option root-path "10.0.1.4:/var/tmp/rootfs";
       The domain-name data type specifies a domain name, which  must  not  be
       enclosed in double quotes.  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, enclosed in
       double  quotes  and  separated  by  commas  ("example.com",  "foo.exam-
       ple.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, sepa-
       rated by colons.  For example:
         option dhcp-client-identifier "CLIENT-FOO";
       or
         option dhcp-client-identifier 43:4c:49:45:54:2d:46:4f:4f;
       The destination-descriptor describe the IP  subnet  number  and  subnet
       mask  of a particular destination using a compact encoding. This encod-
       ing consists of one octet describing the width of the subnet mask, fol-
       lowed  by all the significant octets of the subnet number.  The follow-
       ing table contains some examples of how various subnet number/mask com-
       binations can be encoded:
       Subnet number   Subnet mask      Destination descriptor
       0               0                0
       10.0.0.0        255.0.0.0        8.10
       10.0.0.0        255.255.255.0    24.10.0.0
       10.17.0.0       255.255.0.0      16.10.17
       10.27.129.0     255.255.255.0    24.10.27.129
       10.229.0.128    255.255.255.128  25.10.229.0.128
       10.198.122.47   255.255.255.255  32.10.198.122.47
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 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 informa-
       tion.
       Some of the options documented here are automatically generated by  the
       DHCP  server  or by clients, and cannot be configured by the user.  The
       value of such an option can be used in the configuration  file  of  the
       receiving DHCP protocol agent (server or client), for example in condi-
       tional expressions. However, the value of the option cannot be used  in
       the  configuration  file  of  the  sending  agent, because the value is
       determined only after the configuration file has been processed. In the
       following  documentation,  such options will be shown as "not user con-
       figurable"
       The standard options are:
       option all-subnets-local flag;
         This option specifies whether or not the client may assume  that  all
         subnets  of  the  IP network to which the client is connected use the
         same MTU as the subnet  of  that  network  to  which  the  client  is
         directly connected.  A value of true indicates that all subnets share
         the same MTU.  A value of false means that the client  should  assume
         that  some subnets of the directly connected network may have smaller
         MTUs.
       option arp-cache-timeout uint32;
         This option specifies the timeout in seconds for ARP cache entries.
       option bcms-controller-address ip-address [, ip-address... ];
         This option configures a list of IPv4 addresses for use as  Broadcast
         and Multicast Controller Servers ("BCMS").
       option bcms-controller-names domain-list;
         This  option  contains the domain names of local Broadcast and Multi-
         cast 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  declara-
         tion.   BOOTP clients are unlikely to support this option.  Some DHCP
         clients will support it, and others actually require it.
       option boot-size uint16;
         This option specifies the length in 512-octet blocks of  the  default
         boot image for the client.
       option broadcast-address ip-address;
         This  option  specifies  the broadcast address in use on the client's
         subnet.  Legal values for broadcast addresses are specified  in  sec-
         tion 3.2.1.3 of STD 3 (RFC1122).
       option cookie-servers ip-address [, ip-address...  ];
         The  cookie  server option specifies a list of RFC 865 cookie servers
         available to the client.  Servers should be listed in order of  pref-
         erence.
       option default-ip-ttl uint8;
         This option specifies the default time-to-live that the client should
         use on outgoing datagrams.
       option default-tcp-ttl uint8;
         This option specifies the default TTL that the client should use when
         sending TCP segments.  The minimum value is 1.
       option default-url string;
         The  format  and meaning of this option is not described in any stan-
         dards document, but is claimed to be in use by Apple Computer.  It is
         not  known  what  clients  may  reasonably  do  if supplied with this
         option.  Use at your own risk.
       option dhcp-client-identifier string;
         This option can be used to specify a DHCP client identifier in a host
         declaration,  so  that  dhcpd  can  find  the host record by matching
         against the client identifier.
         Please be aware that some DHCP clients, when configured  with  client
         identifiers  that  are  ASCII  text, will prepend a zero to the ASCII
         text.  So you may need to write:
              option dhcp-client-identifier "\0foo";
         rather than:
              option dhcp-client-identifier "foo";
       option dhcp-lease-time uint32;
         This option is used in a client request (DHCPDISCOVER or DHCPREQUEST)
         to allow the client to request a lease time for the IP address.  In a
         server reply (DHCPOFFER), a DHCP server uses this option  to  specify
         the lease time it is willing to offer.
         This option is not directly user configurable in the server; refer to
         the  max-lease-time  and   default-lease-time   server   options   in
         dhcpd.conf(5).
       option dhcp-max-message-size uint16;
         This  option,  when sent by the client, specifies the maximum size of
         any response that the server sends to the client.  When specified  on
         the  server,  if  the  client  did  not  send a dhcp-max-message-size
         option, the size specified on the server is  used.   This  works  for
         BOOTP as well as DHCP responses.
       option dhcp-message text;
         This option is used by a DHCP server to provide an error message to a
         DHCP client in a DHCPNAK message in the event of a failure. A  client
         may  use  this  option  in  a DHCPDECLINE message to indicate why the
         client declined the offered parameters.
         This option is not user configurable.
       option dhcp-message-type uint8;
         This option, sent by both client and server, specifies  the  type  of
         DHCP  message  contained  in  the DHCP packet. Possible values (taken
         directly from RFC2132) are:
                      1     DHCPDISCOVER
                      2     DHCPOFFER
                      3     DHCPREQUEST
                      4     DHCPDECLINE
                      5     DHCPACK
                      6     DHCPNAK
                      7     DHCPRELEASE
                      8     DHCPINFORM
         This option is not user configurable.
       option dhcp-option-overload uint8;
         This option is used to indicate  that  the  DHCP  'sname'  or  'file'
         fields  are  being  overloaded by using them to carry DHCP options. A
         DHCP server inserts this  option  if  the  returned  parameters  will
         exceed the usual space allotted for options.
         If  this option is present, the client interprets the specified addi-
         tional fields after  it  concludes  interpretation  of  the  standard
         option fields.
         Legal values for this option are:
                      1     the 'file' field is used to hold options
                      2     the 'sname' field is used to hold options
                      3     both fields are used to hold options
         This option is not user configurable.
       option dhcp-parameter-request-list uint16 [, uint16... ];
         This  option,  when  sent  by the client, specifies which options the
         client wishes the server  to  return.   Normally,  in  the  ISC  DHCP
         client,  this is done using the request statement.  If this option is
         not specified by the client, the DHCP  server  will  normally  return
         every  option  that  is  valid in scope and that fits into the reply.
         When this option is specified on the server, the server  returns  the
         specified  options.   This  can  be  used  to  force a client to take
         options that it hasn't requested, and it can also be used  to  tailor
         the response of the DHCP server for clients that may need a more lim-
         ited set of options than those the server would normally return.
       option dhcp-rebinding-time uint32;
         This option specifies the number of seconds from the  time  a  client
         gets an address until the client transitions to the REBINDING state.
         This option is 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  con-
         figured 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 con-
         figured in this option is also used as a DHCPv6 "T2" (rebind) time.
       option dhcp-requested-address ip-address;
         This option is used by the client in a DHCPDISCOVER to request that a
         particular IP address be assigned.
         This option is not user configurable.
       option dhcp-server-identifier ip-address;
         This  option  is  used in DHCPOFFER and DHCPREQUEST messages, and may
         optionally be included in the DHCPACK  and  DHCPNAK  messages.   DHCP
         servers  include  this  option in the DHCPOFFER in order to allow the
         client to distinguish between lease offers.   DHCP  clients  use  the
         contents  of the 'server identifier' field as the destination address
         for any DHCP messages unicast to the DHCP server.  DHCP clients  also
         indicate which of several lease offers is being accepted by including
         this option in a DHCPREQUEST message.
         The value of this option is the IP address of the server.
         This option is not directly user configurable. See the server-identi-
         fier server option in dhcpd.conf(5).
       option domain-name text;
         This  option  specifies  the  domain name that client should use when
         resolving hostnames via the Domain Name System.
       option domain-name-servers ip-address [, ip-address...  ];
         The domain-name-servers option specifies a list of Domain Name System
         (STD  13,  RFC  1035)  name servers available to the client.  Servers
         should be listed in order of preference.
       option domain-search domain-list;
         The domain-search option specifies a 'search list' of Domain Names to
         be  used  by  the  client to locate not-fully-qualified domain names.
         The difference between this option and historic use  of  the  domain-
         name  option  for  the  same  ends  is that this option is encoded in
         RFC1035 compressed labels on the wire.  For example:
           option domain-search "example.com", "sales.example.com",
                                "eng.example.com";
       option extensions-path text;
         This option specifies  the  name  of  a  file  containing  additional
         options  to  be  interpreted  according  to the DHCP option format as
         specified in RFC2132.
       option finger-server ip-address [, ip-address... ];
         The Finger server option specifies a list of Finger servers available
         to the client.  Servers should be listed in order of preference.
       option font-servers ip-address [, ip-address...  ];
         This  option  specifies a list of X Window System Font servers avail-
         able to the client. Servers should be listed in order of preference.
       option host-name string;
         This option specifies the name of the client.  The name  may  or  may
         not  be qualified with the local domain name (it is preferable to use
         the domain-name option to specify the domain name).  See RFC 1035 for
         character set restrictions.  This option is only honored by dhclient-
         script(8) if the hostname for the client machine is not set.
       option ieee802-3-encapsulation flag;
         This option specifies whether or not the client should  use  Ethernet
         Version  2  (RFC  894)  or IEEE 802.3 (RFC 1042) encapsulation if the
         interface is an Ethernet.  A value of false indicates that the client
         should  use  RFC  894  encapsulation.  A value of true means that the
         client should use RFC 1042 encapsulation.
       option ien116-name-servers ip-address [, ip-address...  ];
         The ien116-name-servers option specifies  a  list  of  IEN  116  name
         servers  available  to the client.  Servers should be listed in order
         of preference.
       option impress-servers ip-address [, ip-address...  ];
         The impress-server option specifies a list of Imagen Impress  servers
         available  to the client.  Servers should be listed in order of pref-
         erence.
       option interface-mtu uint16;
         This option specifies the MTU to use on this interface.  The  minimum
         legal value for the MTU is 68.
       option ip-forwarding flag;
         This  option  specifies  whether  the  client should configure its IP
         layer for packet forwarding.  A value of false means disable IP  for-
         warding, and a value of true means enable IP forwarding.
       option irc-server ip-address [, ip-address... ];
         The  IRC  server  option specifies a list of IRC servers available to
         the client.  Servers should be listed in order of preference.
       option log-servers ip-address [, ip-address...  ];
         The log-server option specifies a list of  MIT-LCS  UDP  log  servers
         available  to the client.  Servers should be listed in order of pref-
         erence.
       option lpr-servers ip-address  [, ip-address...  ];
         The LPR server option specifies a  list  of  RFC  1179  line  printer
         servers  available  to the client.  Servers should be listed in order
         of preference.
       option mask-supplier flag;
         This option specifies whether or not the  client  should  respond  to
         subnet mask requests using ICMP.  A value of false indicates that the
         client should not respond.  A value of true  means  that  the  client
         should respond.
       option max-dgram-reassembly uint16;
         This  option  specifies  the  maximum  size  datagram that the client
         should be prepared to reassemble.  The minimum legal value is 576.
       option merit-dump text;
         This option specifies the path-name of a file to which  the  client's
         core  image  should  be  dumped in the event the client crashes.  The
         path is formatted as a character string consisting of characters from
         the NVT ASCII character set.
       option mobile-ip-home-agent ip-address [, ip-address... ];
         This  option  specifies  a  list of IP addresses indicating mobile IP
         home agents available to the client.   Agents  should  be  listed  in
         order  of  preference,  although normally there will be only one such
         agent.
       option nds-context string;
         The nds-context option specifies the  name  of  the  initial  Netware
         Directory Service for an NDS client.
       option nds-servers ip-address [, ip-address... ];
         The  nds-servers  option  specifies  a  list  of  IP addresses of NDS
         servers.
       option nds-tree-name string;
         The nds-tree-name option specifies NDS tree name that the NDS  client
         should use.
       option netbios-dd-server ip-address [, ip-address...  ];
         The  NetBIOS  datagram  distribution server (NBDD) option specifies a
         list of RFC 1001/1002 NBDD servers listed in order of preference.
       option netbios-name-servers ip-address [, ip-address...];
         The NetBIOS name  server  (NBNS)  option  specifies  a  list  of  RFC
         1001/1002  NBNS  name servers listed in order of preference.  NetBIOS
         Name Service is currently more commonly referred to  as  WINS.   WINS
         servers can be specified using the netbios-name-servers option.
       option netbios-node-type uint8;
         The NetBIOS node type option allows NetBIOS over TCP/IP clients which
         are configurable to be configured as described in RFC 1001/1002.  The
         value  is  specified  as  a  single octet which identifies the client
         type.
         Possible node types are:
         1    B-node: Broadcast - no WINS
         2    P-node: Peer - WINS only
         4    M-node: Mixed - broadcast, then WINS
         8    H-node: Hybrid - WINS, then broadcast
       option netbios-scope string;
         The NetBIOS scope option specifies  the  NetBIOS  over  TCP/IP  scope
         parameter  for the client as specified in RFC 1001/1002. See RFC1001,
         RFC1002, and RFC1035 for character-set restrictions.
       option netinfo-server-address ip-address [, ip-address... ];
         The netinfo-server-address option has not been described in any  RFC,
         but has been allocated (and is claimed to be in use) by Apple Comput-
         ers.  It's hard to say if the above is the correct  format,  or  what
         clients  might  be  expected to do if values were configured.  Use at
         your own risk.
       option netinfo-server-tag text;
         The netinfo-server-tag option has not been described in any RFC,  but
         has  been allocated (and is claimed to be in use) by Apple Computers.
         It's hard to say if the above is the correct format, or what  clients
         might  be  expected to do if values were configured.  Use at your own
         risk.
       option nis-domain text;
         This option specifies the name  of  the  client's  NIS  (Sun  Network
         Information Services) domain.  The domain is formatted as a character
         string consisting of characters from the NVT ASCII character set.
       option nis-servers ip-address [, ip-address...  ];
         This option specifies a list of IP addresses indicating  NIS  servers
         available  to the client.  Servers should be listed in order of pref-
         erence.
       option nisplus-domain text;
         This option specifies the name of  the  client's  NIS+  domain.   The
         domain  is  formatted  as a character string consisting of characters
         from the NVT ASCII character set.
       option nisplus-servers ip-address [, ip-address...  ];
         This option specifies a list of IP addresses indicating NIS+  servers
         available  to the client.  Servers should be listed in order of pref-
         erence.
       option nntp-server ip-address [, ip-address... ];
         The NNTP server option specifies a list of NNTP servesr available  to
         the client.  Servers should be listed in order of preference.
       option non-local-source-routing flag;
         This  option  specifies  whether  the  client should configure its IP
         layer to allow forwarding of datagrams with non-local  source  routes
         (see  Section  3.3.5 of [4] for a discussion of this topic).  A value
         of false means disallow forwarding of such datagrams, and a value  of
         true means allow forwarding.
       option ntp-servers ip-address [, ip-address...  ];
         This  option  specifies  a  list  of IP addresses indicating NTP (RFC
         1035) servers available to the client.  Servers should be  listed  in
         order of preference.
       option nwip-domain string;
         The  name  of  the  NetWare/IP domain that a NetWare/IP client should
         use.
       option nwip-suboptions string;
         A sequence of suboptions for NetWare/IP clients  -  see  RFC2242  for
         details.   Normally  this  option  is set by specifying specific Net-
         Ware/IP suboptions - see the NETWARE/IP SUBOPTIONS section  for  more
         information.
       option path-mtu-aging-timeout uint32;
         This option specifies the timeout (in seconds) to use when aging Path
         MTU values discovered by the mechanism defined in RFC 1191.
       option path-mtu-plateau-table uint16 [, uint16...  ];
         This option specifies a table of MTU sizes  to  use  when  performing
         Path MTU Discovery as defined in RFC 1191.  The table is formatted as
         a list of 16-bit unsigned integers, ordered from smallest to largest.
         The minimum MTU value cannot be smaller than 68.
       option perform-mask-discovery flag;
         This option specifies whether or not the client should perform subnet
         mask discovery using ICMP.  A  value  of  false  indicates  that  the
         client should not perform mask discovery.  A value of true means that
         the client should perform mask discovery.
       option policy-filter ip-address ip-address
                         [, ip-address ip-address...];
         This option specifies policy filters for  non-local  source  routing.
         The filters consist of a list of IP addresses and masks which specify
         destination/mask pairs with which to filter incoming source routes.
         Any source routed datagram whose next-hop address does not match  one
         of the filters should be discarded by the client.
         See STD 3 (RFC1122) for further information.
       option pop-server ip-address [, ip-address... ];
         The  POP3 server option specifies a list of POP3 servers available to
         the client.  Servers should be listed in order of preference.
       option resource-location-servers ip-address
                                     [, ip-address...];
         This option specifies a list of RFC  887  Resource  Location  servers
         available  to the client.  Servers should be listed in order of pref-
         erence.
       option root-path text;
         This option specifies the path-name that contains the  client's  root
         disk.   The  path  is  formatted  as a character string consisting of
         characters from the NVT ASCII character set.
       option router-discovery flag;
         This option specifies  whether  or  not  the  client  should  solicit
         routers  using the Router Discovery mechanism defined in RFC 1256.  A
         value of false indicates that the client should  not  perform  router
         discovery.   A  value  of  true  means that the client should perform
         router discovery.
       option router-solicitation-address ip-address;
         This option specifies the address to which the client should transmit
         router solicitation requests.
       option routers ip-address [, ip-address...  ];
         The  routers  option  specifies a list of IP addresses for routers on
         the client's subnet.  Routers should be listed in  order  of  prefer-
         ence.
       option slp-directory-agent boolean ip-address [, ip-address... ];
         This  option  specifies  two  things: the IP addresses of one or more
         Service Location Protocol Directory Agents, and whether  the  use  of
         these  addresses is mandatory.  If the initial boolean value is true,
         the SLP agent should just use the IP addresses given.  If  the  value
         is  false, the SLP agent may additionally do active or passive multi-
         cast discovery of SLP agents (see RFC2165 for details).
         Please note that in this option and the slp-service-scope option, the
         term  "SLP Agent" is being used to refer to a Service Location Proto-
         col agent running on a machine that is  being  configured  using  the
         DHCP protocol.
         Also,  please  be  aware that some companies may refer to SLP as NDS.
         If you have an NDS directory agent whose address you need to  config-
         ure, the slp-directory-agent option should work.
       option slp-service-scope boolean text;
         The  Service  Location  Protocol  Service  Scope Option specifies two
         things: a list of service scopes for SLP, and whether the use of this
         list  is  mandatory.   If  the initial boolean value is true, the SLP
         agent should only use the list of scopes  provided  in  this  option;
         otherwise,  it  may use its own static configuration in preference to
         the list provided in this option.
         The text string should be a comma-separated list of scopes  that  the
         SLP agent should use.  It may be omitted, in which case the SLP Agent
         will use the aggregated list of scopes of all directory agents  known
         to the SLP agent.
       option smtp-server ip-address [, ip-address... ];
         The  SMTP server option specifies a list of SMTP servers available to
         the client.  Servers should be listed in order of preference.
       option static-routes ip-address ip-address
                         [, ip-address ip-address...];
         This option specifies a list of static routes that the client  should
         install  in its routing cache.  If multiple routes to the same desti-
         nation are specified, they are listed in descending order  of  prior-
         ity.
         The  routes consist of a list of IP address pairs.  The first address
         is the destination address, and the second address is the router  for
         the destination.
         The  default  route  (0.0.0.0) is an illegal destination for a static
         route.  To specify the default route, use the routers option.   Also,
         please note that this option is not intended for classless IP routing
         - it does not include a subnet mask.  Since classless IP  routing  is
         now  the most widely deployed routing standard, this option is virtu-
         ally useless, and is not implemented  by  any  of  the  popular  DHCP
         clients, for example the Microsoft DHCP client.
         NOTE to Fedora dhclient users:
         dhclient-script  interprets  trailing 0 octets of the target as indi-
         cating the subnet class of the route, so for  the  following  static-
         routes value:
                 option static-routes 172.0.0.0 172.16.2.254,
                                      192.168.0.0 192.168.2.254;
         dhclient-script will create routes:
                 172/8 via 172.16.2.254 dev $interface
                 192.168/16 via 192.168.2.254 dev $interface
       option classless-static-routes destination-descriptor ip-address
                                   [, destination-descriptor ip-address...];
         This option (see RFC3442) specifies a list of classless static routes
         that the client should install in its routing cache.
         This option can contain one or more static routes, each of which con-
         sists  of  a  destination descriptor and the IP address of the router
         that should be used to reach that destination.
         Many clients may not implement the Classless  Static  Routes  option.
         DHCP  server  administrators  should  therefore  configure their DHCP
         servers to send both a Router option and a  Classless  Static  Routes
         option,  and  should specify the default router(s) both in the Router
         option and in the Classless Static Routes option.
         If the DHCP server returns both a Classless Static Routes option  and
         a Router option, the DHCP client ignores the Router option.
       option streettalk-directory-assistance-server ip-address
                                                  [, ip-address...];
         The  StreetTalk Directory Assistance (STDA) server option specifies a
         list of STDA servers available to  the  client.   Servers  should  be
         listed in order of preference.
       option streettalk-server ip-address [, ip-address... ];
         The  StreetTalk  server option specifies a list of StreetTalk servers
         available to the client.  Servers should be listed in order of  pref-
         erence.
       option subnet-mask ip-address;
         The  subnet mask option specifies the client's subnet mask as per RFC
         950.  If no subnet mask option is provided anywhere in  scope,  as  a
         last  resort  dhcpd will use the subnet mask from the subnet declara-
         tion for the network on which an address is being assigned.  However,
         any  subnet-mask  option declaration that is in scope for the address
         being assigned will override the subnet mask specified in the  subnet
         declaration.
       option subnet-selection string;
         Sent  by  the client if an address is required in a subnet other than
         the one that would  normally  be  selected  (based  on  the  relaying
         address  of  the  connected subnet the request is obtained from). See
         RFC3011. Note that the option number used by this server is 118; this
         has  not  always  been the defined number, and some clients may use a
         different value. Use of this option should be  regarded  as  slightly
         experimental!
       This option is not user configurable in the server.
       option swap-server ip-address;
         This specifies the IP address of the client's swap server.
       option tcp-keepalive-garbage flag;
         This  option  specifies  whether  or  not  the client should send TCP
         keepalive messages with an octet of garbage  for  compatibility  with
         older  implementations.   A  value  of false indicates that a garbage
         octet should not be sent. A value of true indicates  that  a  garbage
         octet should be sent.
       option tcp-keepalive-interval uint32;
         This  option  specifies the interval (in seconds) that the client TCP
         should wait before sending a keepalive message on a  TCP  connection.
         The  time is specified as a 32-bit unsigned integer.  A value of zero
         indicates that the client should not generate keepalive  messages  on
         connections unless specifically requested by an application.
       option tftp-server-name text;
         This  option  is  used to identify a TFTP server and, if supported by
         the client, should have the same effect as the  server-name  declara-
         tion.   BOOTP clients are unlikely to support this option.  Some DHCP
         clients will support it, and others actually require it.
       option time-offset int32;
         The time-offset option specifies the offset of the client's subnet in
         seconds from Coordinated Universal Time (UTC).
       option time-servers ip-address [, ip-address...  ];
         The  time-server  option  specifies  a  list  of RFC 868 time servers
         available to the client.  Servers should be listed in order of  pref-
         erence.
       option trailer-encapsulation flag;
         This  option specifies whether or not the client should negotiate the
         use of trailers (RFC 893 [14]) when using the ARP protocol.  A  value
         of  false  indicates that the client should not attempt to use trail-
         ers.  A value of true means that the client  should  attempt  to  use
         trailers.
       option uap-servers text;
         This option specifies a list of URLs, each pointing to a user authen-
         tication  service  that  is  capable  of  processing   authentication
         requests encapsulated in the User Authentication Protocol (UAP).  UAP
         servers can accept either HTTP 1.1 or SSLv3 connections.  If the list
         includes  a  URL  that  does not contain a port component, the normal
         default port is assumed (i.e., port 80 for  http  and  port  443  for
         https).  If the list includes a URL that does not contain a path com-
         ponent, the path /uap is assumed.  If more than one URL is  specified
         in this list, the URLs are separated by spaces.
       option user-class string;
         This  option is used by some DHCP clients as a way for users to spec-
         ify identifying information to the client.  This can  be  used  in  a
         similar  way  to the vendor-class-identifier option, but the value of
         the option is specified by the user, not  the  vendor.   Most  recent
         DHCP  clients  have  a way in the user interface to specify the value
         for this identifier, usually as a text string.
       option vendor-class-identifier string;
         This option is used by some DHCP clients to identify the vendor  type
         and  possibly the configuration of a DHCP client.  The information is
         a string of bytes whose contents are specific to the vendor  and  are
         not  specified  in  a  standard.  To see what vendor class identifier
         clients are sending, you can write the following in your DHCP  server
         configuration file:
         set vendor-string = option vendor-class-identifier;
         This  will  result  in  all entries in the DHCP server lease database
         file for clients that sent vendor-class-identifier options  having  a
         set statement that looks something like this:
         set vendor-string = "SUNW.Ultra-5_10";
         The  vendor-class-identifier  option  is  normally  used  by the DHCP
         server to determine the options that  are  returned  in  the  vendor-
         encapsulated-options  option.   Please  see  the  VENDOR ENCAPSULATED
         OPTIONS section later in this manual page for further information.
       option vendor-encapsulated-options string;
         The vendor-encapsulated-options option can contain  either  a  single
         vendor-specific  value  or  one  or  more vendor-specific suboptions.
         This option is not normally specified in the DHCP  server  configura-
         tion  file - instead, a vendor class is defined for each vendor, ven-
         dor class suboptions are defined, values  for  those  suboptions  are
         defined, and the DHCP server makes up a response on that basis.
         Some  default  behaviours  for  well-known  DHCP client vendors (cur-
         rently, the Microsoft Windows 2000 DHCP client) are configured  auto-
         matically,  but  otherwise this must be configured manually - see the
         VENDOR ENCAPSULATED OPTIONS section later in  this  manual  page  for
         details.
       option vivso string;
         The  vivso option can contain multiple separate options, one for each
         32-bit Enterprise ID.  Each Enterprise-ID discriminated  option  then
         contains additional options whose format is defined by the vendor who
         holds that ID.  This option is usually not configured  manually,  but
         rather is configured via intervening option definitions.  Please also
         see the VENDOR ENCAPSULATED OPTIONS section later in this manual page
         for details.
       option www-server ip-address [, ip-address... ];
         The  WWW  server  option specifies a list of WWW servers available to
         the client.  Servers should be listed in order of preference.
       option x-display-manager ip-address [, ip-address...  ];
         This option specifies a list of systems that are running the X Window
         System  Display  Manager  and are available to the client.  Addresses
         should be listed in order of preference.
RELAY AGENT INFORMATION OPTION
       An IETF draft, draft-ietf-dhc-agent-options-11.txt, defines a series of
       encapsulated  options  that a relay agent can add to a DHCP packet when
       relaying it to the DHCP server.  The server can then make address allo-
       cation  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 informa-
       tion in these options for delivery or accounting purposes.
       The current draft defines two options.  To reference these  options  in
       the  dhcp server, specify the option space name, "agent", followed by a
       period, followed by the option name.  It  is  not  normally  useful  to
       define values for these options in the server, although it is permissi-
       ble.  These options are not supported in the client.
       option agent.circuit-id string;
         The circuit-id suboption encodes an  agent-local  identifier  of  the
         circuit  from  which a DHCP client-to-server packet was received.  It
         is intended for use by agents in relaying DHCP responses back to  the
         proper circuit.  The format of this option is currently defined to be
         vendor-dependent, and will probably remain  that  way,  although  the
         current  draft  allows  for  for the possibility of standardizing the
         format in the future.
       option agent.remote-id string;
         The remote-id suboption encodes information about the remote host end
         of  a  circuit.   Examples of what it might contain include caller ID
         information, username information, remote ATM  address,  cable  modem
         ID, and similar things.  In principal, the meaning is not well-speci-
         fied, and it should generally be assumed to be an opaque object  that
         is  administratively  guaranteed  to be unique to a particular remote
         end of a circuit.
       option agent.DOCSIS-device-class uint32;
         The DOCSIS-device-class suboption is intended to  convey  information
         about the host endpoint, hardware, and software, that either the host
         operating system or the DHCP server may not  otherwise  be  aware  of
         (but  the  relay  is  able to distinguish).  This is implemented as a
         32-bit field (4 octets), each bit representing a flag describing  the
         host  in  one  of these ways.  So far, only bit zero (being the least
         significant bit) is defined in RFC3256.  If this bit is set  to  one,
         the  host  is  considered  a  CPE Controlled Cable Modem (CCCM).  All
         other bits are reserved.
       option agent.link-selection ip-address;
         The link-selection suboption is provided by relay  agents  to  inform
         servers what subnet the client is actually attached to.  This is use-
         ful in those cases where the giaddr (where responses must be sent  to
         the  relay agent) is not on the same subnet as the client.  When this
         option is present in a packet from a relay  agent,  the  DHCP  server
         will use its contents to find a subnet declared in configuration, and
         from here take one step further backwards to any  shared-network  the
         subnet  may  be  defined within...the client may be given any address
         within that shared network, as normally appropriate.
THE CLIENT FQDN SUBOPTIONS
       The Client FQDN option, currently defined in the Internet Draft  draft-
       ietf-dhc-fqdn-option-00.txt  is  not  a  standard yet, but is in suffi-
       ciently wide use already that we have implemented it.  Due to the  com-
       plexity  of  the  option  format, we have implemented it as a suboption
       space rather than a single option.  In general this option  should  not
       be  configured  by  the  user - instead it should be used as part of an
       automatic DNS update system.
       option fqdn.no-client-update flag;
         When the client sends this, if it is true, it means the  client  will
         not  attempt  to update its A record.  When sent by the server to the
         client, it means that the client should not update its own A record.
       option fqdn.server-update flag;
         When the client sends this to the server, it is requesting  that  the
         server  update  its A record.  When sent by the server, it means that
         the server has updated (or is about to update) the client's A record.
       option fqdn.encoded flag;
         If true, this indicates that the domain name included in  the  option
         is  encoded in DNS wire format, rather than as plain ASCII text.  The
         client normally sets this to false if it  doesn't  support  DNS  wire
         format  in  the  FQDN option.  The server should always send back the
         same value that the client sent.  When this value is set on the  con-
         figuration side, it controls the format in which the fqdn.fqdn subop-
         tion is encoded.
       option fqdn.rcode1 flag;
       option fqdn.rcode2 flag;
         These options specify the result of the updates  of  the  A  and  PTR
         records,  respectively,  and  are only sent by the DHCP server to the
         DHCP client.  The values of these fields are those defined in the DNS
         protocol specification.
       option fqdn.fqdn text;
         Specifies the domain name that the client wishes to use.  This can be
         a fully-qualified domain name, or a single label.   If  there  is  no
         trailing  '.'  character  in the name, it is not fully-qualified, and
         the server will generally update that name  in  some  locally-defined
         domain.
       option fqdn.hostname --never set--;
         This  option  should  never be set, but it can be read back using the
         option and config-option operators in an expression, in which case it
         returns  the first label in the fqdn.fqdn suboption - for example, if
         the value of fqdn.fqdn is "foo.example.com.", then fqdn.hostname will
         be "foo".
       option fqdn.domainname --never set--;
         This  option  should  never be set, but it can be read back using the
         option and config-option operators in an expression, in which case it
         returns all labels after the first label in the fqdn.fqdn suboption -
         for example, if the value of fqdn.fqdn  is  "foo.example.com.",  then
         fqdn.hostname will be "example.com.".  If this suboption value is not
         set, it means that an unqualified name was sent in the  fqdn  option,
         or that no fqdn option was sent at all.
       If  you wish to use any of these suboptions, we strongly recommend that
       you refer to the Client FQDN option draft (or standard, when it becomes
       a  standard) - the documentation here is sketchy and incomplete in com-
       parison, and is just intended  for  reference  by  people  who  already
       understand the Client FQDN option specification.
THE NETWARE/IP SUBOPTIONS
       RFC2242  defines  a  set  of encapsulated options for Novell NetWare/IP
       clients.  To use these options in the dhcp server, specify  the  option
       space  name, "nwip", followed by a period, followed by the option name.
       The following options can be specified:
       option nwip.nsq-broadcast flag;
         If true, the client should use the NetWare Nearest  Server  Query  to
         locate  a  NetWare/IP  server.  The behaviour of the Novell client if
         this suboption is false, or is not present, is not specified.
       option nwip.preferred-dss ip-address [, ip-address... ];
         This suboption specifies a list of up to five IP addresses,  each  of
         which  should  be  the  IP address of a NetWare Domain SAP/RIP server
         (DSS).
       option nwip.nearest-nwip-server ip-address
                                    [, ip-address...];
         This suboption specifies a list of up to five IP addresses,  each  of
         which should be the IP address of a Nearest NetWare IP server.
       option nwip.autoretries uint8;
         Specifies the number of times that a NetWare/IP client should attempt
         to communicate with a given DSS server at startup.
       option nwip.autoretry-secs uint8;
         Specifies the number of seconds that a Netware/IP client should  wait
         between  retries  when  attempting to establish communications with a
         DSS server at startup.
       option nwip.nwip-1-1 uint8;
         If true, the NetWare/IP client should support NetWare/IP version  1.1
         compatibility.   This is only needed if the client will be contacting
         Netware/IP version 1.1 servers.
       option nwip.primary-dss ip-address;
         Specifies the IP address of the Primary Domain SAP/RIP Service server
         (DSS)  for  this  NetWare/IP  domain.   The NetWare/IP administration
         utility uses this value as Primary DSS server when configuring a sec-
         ondary DSS server.
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.  Understand-
       ably, many options are not "allowed" to have multiple  instances  in  a
       packet - normally these are options which are digested by the DHCP pro-
       tocol software, and not by users or applications.
       option dhcp6.client-id string;
         This option specifies the client's DUID identifier.  DUIDs are  simi-
         lar  but  different  from DHCPv4 client identifiers - there are docu-
         mented duid types:
         duid-llt
         duid-en
         duid-ll
         This value should not  be  configured,  but  rather  is  provided  by
         clients and treated as an opaque identifier key blob by servers.
       option dhcp6.server-id string;
         This option specifies the server's DUID identifier.  One may use this
         option to configure an opaque binary blob for your  server's  identi-
         fier.
       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 soft-
         ware, and should not be configured.
       option dhcp6.ia-ta string;
         The Identity Association for Temporary Addresses (ia-ta) carries tem-
         porary  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 'pre-
         ferred' for use on a given subnet.  This preference is  only  applied
         during  the  initial stages of configuration - once a client is bound
         to an IA, it will remain bound to that IA until it is no longer valid
         or  has  expired.  This value may be configured on the server, and is
         digested by the client software.
       option dhcp6.elapsed-time uint16;
         The elapsed-time option is constructed by the DHCPv6 client software,
         and  is  potentially  consumed by intermediaries.  This option should
         not be configured.
       option dhcp6.relay-msg string;
         The relay-msg option is constructed by intervening DHCPv6 relay agent
         software.   This option is entirely used by protocol software, and is
         not meant for user configuration.
       option dhcp6.unicast ip6-address;
         The unicast option is provided by DHCPv6 servers  which  are  willing
         (or prefer) to receive Renew packets from their clients by exchanging
         UDP unicasts with them.   Normally,  DHCPv6  clients  will  multicast
         their  Renew  messages.   This  may  be configured on the server, and
         should be configured as an address the server is ready to reply to.
       option dhcp6.status-code status-code [ string ] ;
         The status-code option  is  provided  by  DHCPv6  servers  to  inform
         clients  of  error  conditions  during  protocol communication.  This
         option is manufactured and digested by protocol software, and  should
         not be configured.
       option dhcp6.rapid-commit ;
         The  rapid-commit  option is a zero-length option that clients use to
         indicate their desire to enter into rapid-commit with the server.
       option dhcp6.vendor-opts string;
         The vendor-opts option is actually an encapsulated sub-option  space,
         in which each Vendor-specific Information Option (VSIO) is identified
         by a 32-bit Enterprise-ID number.   The  encapsulated  option  spaces
         within these options are defined by the vendors.
         To  make  use  of this option, the best way is to examine the section
         titled VENDOR ENCAPSULATED OPTIONS  below,  in  particular  the  bits
         about the "vsio" option space.
       option dhcp6.interface-id string;
         The  interface-id  option is manufactured by relay agents, and may be
         used to guide configuration differentiating clients by the  interface
         they are remotely attached to.  It does not make sense to configure a
         value for this option, but it may make sense to inspect its contents.
       option dhcp6.reconf-msg dhcpv6-message;
         The reconf-msg option is manufactured by servers, and sent to clients
         in  Reconfigure  messages  to  inform them of what message the client
         should Reconfigure using.  There is no support for DHCPv6 Reconfigure
         extensions, and this option is documented informationally only.
       option dhcp6.reconf-accept ;
         The  reconf-accept  option is included by DHCPv6 clients that support
         the Reconfigure extentions, advertising that they will respond if the
         server  were  to  ask  them  to Reconfigure.  There is no support for
         DHCPv6 Reconfigure extensions, and this option is documented informa-
         tionally 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  "name-
         server" line in /etc/resolv.conf.
       option dhcp6.domain-search domain-list;
         The domain-search option specifies the client's domain search path to
         be applied to recursive DNS queries.  It is easiest to describe  this
         as the "search" line in /etc/resolv.conf.
       option dhcp6.ia-pd string;
         The  ia-pd  option is manufactured by clients and servers to create a
         Prefix Delegation binding - to delegate an IPv6 prefix to the client.
         It  is  not directly edited in dhcpd.conf(5) or dhclient.conf(5), but
         rather is manufactured and consumed by the software.
       option dhcp6.ia-prefix string;
         The ia-prefix option is placed inside ia-pd options in order to iden-
         tify  the  prefix(es)  allocated  to  the client.  It is not directly
         edited in dhcpd.conf(5) or dhclient.conf(5), but rather  is  manufac-
         tured 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 specfies 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 specfies 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 avail-
         able 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 behav-
         iour, if the client requested the option.
       option dhcp6.bcms-server-d domain-list;
         The bcms-server-d option contains the  domain  names  of  local  BCMS
         (Broadcast  and  Multicast  Control  Services)  controllers which the
         client may use.
       option dhcp6.bcms-server-a ip6-address [, ip6-address ... ] ;
         The bcms-server-a option contains the IPv6 addresses  of  local  BCMS
         (Broadcast  and  Multicast  Control  Services)  controllers which the
         client may use.
       option dhcp6.remote-id string;
         The remote-id option is constructed by relay agents,  to  inform  the
         server of details pertaining to what the relay knows about the client
         (such as what port it is attached to, and so forth).  The contents of
         this  option  have  some vendor-specific structure (similar to VSIO),
         but we have chosen to treat this option as an opaque field.
       option dhcp6.subscriber-id;
         The subscriber-id option is an opaque field  provided  by  the  relay
         agent,  which provides additional information about the subscriber in
         question.  The exact contents of this option depend upon  the  vendor
         and/or the operator's configuration of the remote device, and as such
         is an opaque field.
       option dhcp6.fqdn string;
         The fqdn option is normally constructed by the client or server,  and
         negotiates the client's Fully Qualified Domain Name, as well as which
         party is responsible for Dynamic DNS Updates.  See the section on the
         Client  FQDN  SubOptions for full details (the DHCPv4 and DHCPv6 FQDN
         options use the same "fqdn." encapsulated space, so are in  all  ways
         identical).
       option dhcp6.lq-query string;
         The lq-query option is used internally by for lease query.
       option dhcp6.client-data string;
         The client-data option is used internally by for lease query.
       option dhcp6.clt-time uint32;
         The clt-time option is used internally by for lease query.
       option dhcp6.lq-relay-data ip6-address string;
         The lq-relay-data option is used internally by for lease query.
       option dhcp6.lq-client-link ip6-address [, ip6-address ... ] ;
         The lq-client-link option is used internally by for lease query.
       option dhcp6.bootfile-url string ;
         The  server  sends  this option to inform the client about a URL to a
         boot file.  Used primarily for UEFI network booting, it  contains  an
         RFC3986  compliant  URI which the client may use to boot an operating
         system.  This option is defined in RFC5970
       option dhcp6.arch-type arch-id [, arch-id...]  ;
         A client will send this option to a server so  that  the  server  may
         make  decisions on what options and addresses to offer the requesting
         client.  The option consists of a list of 16 bit unsigned values that
         represent  the  architecture  of the requesting client.  These values
         corespond to the values available to the dhcpv4 option  architecture-
         type,  as defined in RFC4578, section 2.1.  This option is defined in
         RFC5970
          .RE
         option dhcp6.net-id uint8 uint8 uint8 ;
            A client will send this option to a server to inform it about  the
            clients level of UNDI support.  The option consists of 3 octets (a
            type, major and minor value).  Specific meanings of  these  values
            are  doumented  in section 2.2 of RFC4578.  This option is defined
            in RFC5970
DEFINING NEW OPTIONS
       The Internet Systems Consortium DHCP  client  and  server  provide  the
       capability to define new options.  Each DHCP option has a name, a code,
       and a structure.  The name is used by you to refer to the option.   The
       code  is  a  number,  used by the DHCP server and client to refer to an
       option.  The structure describes what the contents of an  option  looks
       like.
       To define a new option, you need to choose a name for it that is not in
       use for some other option - for  example,  you  can't  use  "host-name"
       because  the DHCP protocol already defines a host-name option, which is
       documented earlier in this manual page.   If  an  option  name  doesn't
       appear  in  this  manual page, you can use it, but it's probably a good
       idea to put some kind of unique string at the beginning so you  can  be
       sure  that future options don't take your name.  For example, you might
       define an option, "local-host-name", feeling some  confidence  that  no
       official DHCP option name will ever start with "local".
       Once you have chosen a name, you must choose a code.  All codes between
       224 and 254 are reserved as 'site-local' DHCP options, so you can  pick
       any  one of these for your site (not for your product/application).  In
       RFC3942, site-local space was moved from starting at 128 to starting at
       224.   In  practice,  some vendors have interpreted the protocol rather
       loosely and have used option code values greater than  128  themselves.
       There's  no  real  way  to avoid this problem, and it was thought to be
       unlikely to cause too much trouble in practice.  If you come  across  a
       vendor-documented  option code in either the new or old site-local spa-
       ces, please contact your vendor and inform them about rfc3942.
       The structure of an option is simply the format  in  which  the  option
       data  appears.   The  ISC  DHCP  server currently supports a few simple
       types, like integers, booleans, strings and IP addresses, and  it  also
       supports  the  ability  to  define  arrays of single types or arrays of
       fixed sequences of types.
       New options are declared as follows:
       option new-name code new-code = definition ;
       The values of new-name and new-code should be the name you have  chosen
       for the new option and the code you have chosen.  The definition should
       be the definition of the structure of the option.
       The following simple option type definitions are supported:
       BOOLEAN
       option new-name code new-code = boolean ;
       An option of type boolean is a flag with a value of either  on  or  off
       (or true or false).  So an example use of the boolean type would be:
       option use-zephyr code 180 = boolean;
       option use-zephyr on;
       INTEGER
       option new-name code new-code = sign integer width ;
       The  sign  token should either be blank, unsigned or signed.  The width
       can be either 8, 16 or 32, and refers to the  number  of  bits  in  the
       integer.   So for example, the following two lines show a definition of
       the sql-connection-max option and its use:
       option sql-connection-max code 192 = unsigned integer 16;
       option sql-connection-max 1536;
       IP-ADDRESS
       option new-name code new-code = ip-address ;
       An option whose structure is an IP address can be expressed either as a
       domain name or as a dotted quad.  So the following is an example use of
       the ip-address type:
       option sql-server-address code 193 = ip-address;
       option sql-server-address sql.example.com;
       IP6-ADDRESS
       option new-name code new-code = ip6-address ;
       An option whose structure is an IPv6 address must  be  expressed  as  a
       valid IPv6 address.  The following is an example use of the ip6-address
       type:
       option dhcp6.some-server code 1234 = array of ip6-address;
       option dhcp6.some-server 3ffe:bbbb:aaaa:aaaa::1, 3ffe:bbbb:aaaa:aaaa::2;

       TEXT
       option new-name code new-code = text ;
       An option whose type is text will encode an  ASCII  text  string.   For
       example:
       option sql-default-connection-name code 194 = text;
       option sql-default-connection-name "PRODZA";

       DATA STRING
       option new-name code new-code = string ;
       An  option whose type is a data string is essentially just a collection
       of bytes, and can be specified either as quoted  text,  like  the  text
       type,  or  as  a list of hexadecimal contents separated by colons whose
       values must be between 0 and FF.  For example:
       option sql-identification-token code 195 = string;
       option sql-identification-token 17:23:19:a6:42:ea:99:7c:22;

       DOMAIN-LIST
       option new-name code new-code = domain-list [compressed] ;
       An option whose type is domain-list is an  RFC1035  formatted  (on  the
       wire,  "DNS  Format")  list  of domain names, separated by root labels.
       The optional compressed keyword indicates if the option should be  com-
       pressed  relative  to  the start of the option contents (not the packet
       contents).
       When in doubt, omit the compressed keyword.  When the software recieves
       an  option that is compressed and the compressed keyword is omitted, it
       will still decompress the  option  (relative  to  the  option  contents
       field).   The  keyword only controls whether or not transmitted packets
       are compressed.
       Note that when domain-list formatted options are output as  environment
       variables  to dhclient-script(8), the standard DNS -escape mechanism is
       used: they are decimal.  This is  appropriate  for  direct  use  in  eg
       /etc/resolv.conf.

       ENCAPSULATION
       option new-name code new-code = encapsulate identifier ;
       An  option  whose  type is encapsulate will encapsulate the contents of
       the option space specified in  identifier.   Examples  of  encapsulated
       options in the DHCP protocol as it currently exists include the vendor-
       encapsulated-options option,  the  netware-suboptions  option  and  the
       relay-agent-information option.
       option space local;
       option local.demo code 1 = text;
       option local-encapsulation code 197 = encapsulate local;
       option local.demo "demo";

       ARRAYS
       Options  can  contain  arrays  of any of the above types except for the
       text and data string types, which aren't currently supported in arrays.
       An example of an array definition is as follows:
       option kerberos-servers code 200 = array of ip-address;
       option kerberos-servers 10.20.10.1, 10.20.11.1;
       RECORDS
       Options  can  also  contain data structures consisting of a sequence of
       data types, which is sometimes called a record type.  For example:
       option contrived-001 code 201 = { boolean, integer 32, text };
       option contrived-001 on 1772 "contrivance";
       It's also possible to have options that  are  arrays  of  records,  for
       example:
       option new-static-routes code 201 = array of {
            ip-address, ip-address, ip-address, integer 8 };
       option static-routes
            10.0.0.0 255.255.255.0 net-0-rtr.example.com 1,
            10.0.1.0 255.255.255.0 net-1-rtr.example.com 1,
            10.2.0.0 255.255.224.0 net-2-0-rtr.example.com 3;

VENDOR ENCAPSULATED OPTIONS
       The DHCP protocol defines the vendor-encapsulated-options option, which
       allows vendors to define their own options that will be  sent  encapsu-
       lated 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
       that they contain options that correspond to vendor Enterprise-ID  num-
       bers  (assigned  by IANA), which then contain options according to each
       Vendor's specifications.  You will need to refer to your vendor's docu-
       mentation 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  Infor-
       mation  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 config-
       ure  these numbers to match the applicable option space you are config-
       uring.  They each default to 1.  Valid values for code widths are 1,  2
       or  4.   Valid  values  for  length  widths are 0, 1 or 2.  Most DHCPv4
       option spaces use 1 byte codes  and  lengths,  which  is  the  default,
       whereas  most  DHCPv6  option  spaces  use 2 byte codes and lengths.  A
       zero-byte length produces options similar to the DHCPv6 Vendor-specific
       Information Option - but not their contents!
       The  hash size defaults depend upon the code width selected, and may be
       254 or 1009.  Valid values range between 1 and 65535.   Note  that  the
       higher  you  configure this value, the more memory will be used.  It is
       considered good practice to configure a value that is  slightly  larger
       than  the  estimated number of options you plan to configure within the
       space.  Previous versions of ISC DHCP (up to and including DHCP 3.0.*),
       this value was fixed at 9973.
       The  name  can then be used in option definitions, as described earlier
       in this document.  For example:
       option space SUNW code width 1 length width 1 hash size 3;
       option SUNW.server-address code 2 = ip-address;
       option SUNW.server-name code 3 = text;
       option SUNW.root-path code 4 = text;
       option space ISC code width 1 length width 1 hash size 3;
       option ISC.sample code 1 = text;
       option vendor.ISC code 2495 = encapsulate vivso-sample;
       option vendor-class.ISC code 2495 = text;
       option ISC.sample "configuration text here";
       option vendor-class.ISC "vendor class here";
       option space docsis code width 2 length width 2 hash size 17;
       option docsis.tftp-servers code 32 = array of ip6-address;
       option docsis.cablelabs-configuration-file code 33 = text;
       option docsis.cablelabs-syslog-servers code 34 = array of ip6-address;
       option docsis.device-id code 36 = string;
       option docsis.time-servers code 37 = array of ip6-address;
       option docsis.time-offset code 38 = signed integer 32;
       option vsio.docsis code 4491 = encapsulate docsis;
       Once you have defined an option space and the format of  some  options,
       you can set up scopes that define values for those options, and you can
       say when to use them.  For example, suppose you want to handle two dif-
       ferent  classes of clients.  Using the option space definition shown in
       the previous example, you can send different option values to different
       clients  based  on  the vendor-class-identifier option that the clients
       send, as follows:
       class "vendor-classes" {
         match option vendor-class-identifier;
       }
       subclass "vendor-classes" "SUNW.Ultra-5_10" {
         vendor-option-space SUNW;
         option SUNW.root-path "/export/root/sparc";
       }
       subclass "vendor-classes" "SUNW.i86pc" {
         vendor-option-space SUNW;
         option SUNW.root-path "/export/root/i86pc";
       }
       option SUNW.server-address 172.17.65.1;
       option SUNW.server-name "sundhcp-server17-1";
       option vivso-sample.sample "Hello world!";
       option docsis.tftp-servers ::1;

       As you can see in the preceding example, regular scoping  rules  apply,
       so  you can define values that are global in the global scope, and only
       define values that are specific to a  particular  class  in  the  local
       scope.   The  vendor-option-space  declaration tells the DHCP server to
       use options in the SUNW option space to construct  the  DHCPv4  vendor-
       encapsulated-options option.  This is a limitation of that option - the
       DHCPv4 VIVSO and the DHCPv6 VSIO options can have multiple vendor defi-
       nitions 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
       The  Internet  Systems  Consortium DHCP Distribution was written by Ted
       Lemon under a contract with Vixie Labs.  Funding for this  project  was
       provided through Internet Systems Consortium.  Information about Inter-
       net Systems Consortium can be found at https://www.isc.org.

                                                               dhcp-options(5)