dhclient-options man page on CentOS

Man page or keyword search:  
man Server   8420 pages
apropos Keyword Search (all sections)
Output format
CentOS logo
[printable version]

dhcpd-options(5)					      dhcpd-options(5)

NAME
       dhcp-options - Dynamic Host Configuration Protocol options

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

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

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

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

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

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

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

       The  domain-name	 data  type  specifies	a  domain name, which must not
       enclosed in double quotes.   This data type is not used for any	exist‐
       ing DHCP options.   The domain name is stored just as if it were a text
       option.

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

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

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

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

	 option my-option = expression ;

       For example:

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

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

       Some of the options documented here are automatically generated by  the
       DHCP  server  or by clients, and cannot be configured by the user.  The
       value of such an option can be used in the configuration	 file  of  the
       receiving DHCP protocol agent (server or client), for example in condi‐
       tional expressions. However, the value of the option cannot be used  in
       the  configuration  file	 of  the  sending  agent, because the value is
       determined only after the configuration file has been processed. In the
       following  documentation,  such options will be shown as "not user con‐
       figurable"

       The standard options are:

       option all-subnets-local flag;

	 This option specifies whether or not the client may assume  that  all
	 subnets  of  the  IP network to which the client is connected use the
	 same MTU as the subnet	 of  that  network  to	which  the  client  is
	 directly connected.  A value of true indicates that all subnets share
	 the same MTU.	A value of false means that the client	should	assume
	 that  some subnets of the directly connected network may have smaller
	 MTUs.

       option arp-cache-timeout uint32;

	 This option specifies the timeout in seconds for ARP cache entries.

       option 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 dhcp-client-identifier string;

	 This option can be used to specify a DHCP client identifier in a host
	 declaration,  so  that	 dhcpd	can  find  the host record by matching
	 against the client identifier.

	 Please be aware that some DHCP clients, when configured  with	client
	 identifiers  that  are	 ASCII	text, will prepend a zero to the ASCII
	 text.	 So you may need to write:

	      option dhcp-client-identifier "\0foo";

	 rather than:

	      option dhcp-client-identifier "foo";

       option dhcp-lease-time uint32;

	 This option is used in a client request (DHCPDISCOVER or DHCPREQUEST)
	 to allow the client to request a lease time for the IP address.  In a
	 server reply (DHCPOFFER), a DHCP server uses this option  to  specify
	 the lease time it is willing to offer.

	 This option is not directly user configurable in the server; refer to
	 the  max-lease-time  and   default-lease-time	 server	  options   in
	 dhcpd.conf(5).

       option dhcp-max-message-size uint16;

	 This  option,	when sent by the client, specifies the maximum size of
	 any response that the server sends to the client.   When specified on
	 the  server,  if  the	client	did  not  send a dhcp-max-message-size
	 option, the size specified on the server is used.    This  works  for
	 BOOTP as well as DHCP responses.

       option dhcp-message text;

	 This option is used by a DHCP server to provide an error message to a
	 DHCP client in a DHCPNAK message in the event of a failure. A	client
	 may  use  this	 option	 in  a DHCPDECLINE message to indicate why the
	 client declined the offered parameters.

	 This option is not user configurable.

       option dhcp-message-type uint8;

	 This option, sent by both client and server, specifies	 the  type  of
	 DHCP  message	contained  in  the DHCP packet. Possible values (taken
	 directly from RFC2132) are:

		      1	    DHCPDISCOVER
		      2	    DHCPOFFER
		      3	    DHCPREQUEST
		      4	    DHCPDECLINE
		      5	    DHCPACK
		      6	    DHCPNAK
		      7	    DHCPRELEASE
		      8	    DHCPINFORM

	 This option is not user configurable.

       option dhcp-option-overload uint8;

	 This option is used to indicate  that	the  DHCP  'sname'  or	'file'
	 fields	 are  being  overloaded by using them to carry DHCP options. A
	 DHCP server inserts this  option  if  the  returned  parameters  will
	 exceed the usual space allotted for options.

	 If  this option is present, the client interprets the specified addi‐
	 tional fields after  it  concludes  interpretation  of	 the  standard
	 option fields.

	 Legal values for this option are:

		      1	    the 'file' field is used to hold options
		      2	    the 'sname' field is used to hold options
		      3	    both fields are used to hold options

	 This option is not user configurable.

       option dhcp-parameter-request-list uint16;

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

       option dhcp-rebinding-time uint32;

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

	 This option is not user configurable.

       option dhcp-renewal-time uint32;

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

	 This option is not user configurable.

       option dhcp-requested-address ip-address;

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

	 This option is not user configurable.

       option dhcp-server-identifier ip-address;

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

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

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

       option domain-name text;

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

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

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

       option 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 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 Red Hat dhclient users:
	 The  RedHat dhclient-script interprets trailing 0 octets of the  tar‐
	 get as	 indicating  the  subnet  class	 of  the route - so  for  this
	 static-routes value:
		 option static-routes 172.0.0.0 172.16.2.254,
				      192.168.0.0 192.168.2.254;
	 the Red Hat dhclient-script will create routes:
		 172/8 via 172.16.2.254 dev $interface
		 192.168/16 via 192.168.2.254 dev $interface
	 which slightly increases the usefulness of the static-routes 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 www-server ip-address [, ip-address... ];

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

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

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

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

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

       option agent.circuit-id string;

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

       option agent.remote-id string;

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

       option agent.DOCSIS-device-class uint32;

	 The DOCSIS-device-class suboption is intended to  convey  information
	 about the host endpoint, hardware, and software, that either the host
	 operating system or the DHCP server may not  otherwise	 be  aware  of
	 (but  the  relay  is  able to distinguish).  This is implemented as a
	 32-bit field (4 octets), each bit representing a flag describing  the
	 host  in  one	of these ways.	So far, only bit zero (being the least
	 significant bit) is defined in RFC3256.  If this bit is set  to  one,
	 the  host  is	considered  a  CPE Controlled Cable Modem (CCCM).  All
	 other bits are reserved.

THE CLIENT FQDN SUBOPTIONS
       The Client FQDN option, currently defined in the Internet Draft	draft-
       ietf-dhc-fqdn-option-00.txt  is	not  a	standard yet, but is in suffi‐
       ciently wide use already that we have implemented it.   Due to the com‐
       plexity	of  the	 option	 format, we have implemented it as a suboption
       space rather than a single option.   In general this option should  not
       be  configured  by  the	user - instead it should be used as part of an
       automatic DNS update system.

       option fqdn.no-client-update flag;

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

       option fqdn.server-update flag;

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

       option fqdn.encoded flag;

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

       option fqdn.rcode1 flag;

       option fqdn.rcode2 flag;

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

       option fqdn.fqdn text;

	 Specifies  the	 domain name that the client wishes to use.   This can
	 be a fully-qualified domain name, or a single label.	If there is no
	 trailing generally update that name in some locally-defined domain.

       option fqdn.hostname --never set--;

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

       option fqdn.domainname --never set--;

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

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

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

       option nwip.nsq-broadcast flag;

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

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

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

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

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

       option nwip.autoretries uint8;

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

       option nwip.autoretry-secs uint8;

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

       option nwip.nwip-1-1 uint8;

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

       option nwip.primary-dss ip-address;

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

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

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

       Once you have chosen a name, you must choose a  code.   For  site-local
       options,	 all  codes between 128 and 254 are reserved for DHCP options,
       so you can pick any one of  these.   In	practice,  some	 vendors  have
       interpreted  the protocol rather loosely and have used option code val‐
       ues greater than 128 themselves.	  There's no real way  to  avoid  this
       problem, but it's not likely to cause too much trouble in practice.

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

       New options are declared as follows:

       option new-name code new-code = definition ;

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

       The following simple option type definitions are supported:

       BOOLEAN

       option new-name code new-code = boolean ;

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

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

       INTEGER

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

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

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

       IP-ADDRESS

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

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

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

       TEXT

       option new-name code new-code = text ;

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

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

       DATA STRING

       option new-name code new-code = string ;

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

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

       ENCAPSULATION

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

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

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

       ARRAYS

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

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

       RECORDS

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

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

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

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

VENDOR ENCAPSULATED OPTIONS
       The  DHCP  protocol  defines  the   vendor-encapsulated-options option,
       which allows vendors to define their own	 options  that	will  be  sent
       encapsulated  in	 a  standard  DHCP option.   The format of the vendor-
       encapsulated-options option is either a series of bytes whose format is
       not  specified,	or  a sequence of options, each of which consists of a
       single-byte vendor-specific option  code,  followed  by	a  single-byte
       length,	followed  by  as  many	bytes  of data as are specified in the
       length (the length does not include itself or the option code).

       The value of this option can be set in one of two ways.	 The first way
       is to simply specify the data directly, using a text string or a colon-
       separated list of hexadecimal values.   For example:

       option vendor-encapsulated-options
	   2:4:AC:11:41:1:
	   3:12:73:75:6e:64:68:63:70:2d:73:65:72:76:65:72:31:37:2d:31:
	   4:12:2f:65:78:70:6f:72:74:2f:72:6f:6f:74:2f:69:38:36:70:63;

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

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

       option space name ;

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

       option space SUNW;
       option SUNW.server-address code 2 = ip-address;
       option SUNW.server-name code 3 = text;
       option SUNW.root-path code 4 = text;

       Once  you  have defined an option space and the format of some options,
       you can set up scopes that define values for those options, and you can
       say  when  to  use  them.   For example, suppose you want to handle two
       different classes of clients.   Using the option space definition shown
       in  the	previous example, you can send different option values to dif‐
       ferent clients based on the  vendor-class-identifier  option  that  the
       clients send, as follows:

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

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

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

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

       As  you	can see in the preceding example, regular scoping rules apply,
       so you can define values that are global in the global scope, and  only
       define  values  that  are  specific  to a particular class in the local
       scope.	The vendor-option-space declaration tells the DHCP  server  to
       use  options  in the SUNW option space to construct the vendor-encapsu‐
       lated-options option.

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

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

							      dhcpd-options(5)
[top]

List of man pages available for CentOS

Copyright (c) for man pages and the logo by the respective OS vendor.

For those who want to learn more, the polarhome community provides shell access and support.

[legal] [privacy] [GNU] [policy] [cookies] [netiquette] [sponsors] [FAQ]
Tweet
Polarhome, production since 1999.
Member of Polarhome portal.
Based on Fawad Halim's script.
....................................................................
Vote for polarhome
Free Shell Accounts :: the biggest list on the net