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.\"     dhcp-options.5
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.\" This software has been written for the Internet Software Consortium
.\" by Ted Lemon in cooperation with Vixie Enterprises and Nominum, Inc.
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.TH dhcpd-options 5
.SH NAME
dhcp-options - Dynamic Host Configuration Protocol options
.SH DESCRIPTION
The Dynamic Host Configuration protocol allows the client to receive
.B options
from the DHCP server describing the network configuration and various
services that are available on the network.   When configuring
.B dhcpd(8)
or
.B 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.
.SH REFERENCE: OPTION STATEMENTS
.PP
DHCP \fIoption\fR statements always start with the \fIoption\fR
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.
.PP
Option data comes in a variety of formats, as defined below:
.PP
The
.B ip-address
data type can be entered either as an explicit IP
address (e.g., 239.254.197.10) or as a domain name (e.g.,
haagen.isc.org).  When entering a domain name, be sure that that
domain name resolves to a single IP address.
.PP
The
.B int32
data type specifies a signed 32-bit integer.   The 
.B uint32
data type specifies an unsigned 32-bit integer.   The 
.B int16
and
.B uint16
data types specify signed and unsigned 16-bit integers.   The 
.B int8
and
.B uint8
data types specify signed and unsigned 8-bit integers.
Unsigned 8-bit integers are also sometimes referred to as octets.
.PP
The
.B 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
.nf
.sp 1
option root-path "10.0.1.4:/var/tmp/rootfs";
.fi
.PP
The
.B domain-name
data type specifies a domain name, which must not
enclosed in double quotes.   This data type is not used for any
existing DHCP options.   The domain name is stored just as if it were
a text option.
.PP
The
.B 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).
.PP
The
.B string
data type specifies either an NVT ASCII string
enclosed in double quotes, or a series of octets specified in
hexadecimal, seperated by colons.   For example:
.nf
.sp 1
  option dhcp-client-identifier "CLIENT-FOO";
or
  option dhcp-client-identifier 43:4c:49:45:54:2d:46:4f:4f;
.fi
.SH 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 
.B 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:
.nf
.sp 1
  \fBoption \fImy-option \fB= \fIexpression \fB;\fR
.fi
.PP
For example:
.nf
.sp 1
  option hostname = binary-to-ascii (16, 8, "-",
                                     substring (hardware, 1, 6));
.fi
.SH STANDARD DHCP OPTIONS
The documentation for the various options mentioned below is taken
from the latest IETF draft document on DHCP options.  Options not
listed below may not yet be implemented, but it is possible to use
such options by defining them in the configuration file.  Please see
the DEFINING NEW OPTIONS heading later in this document for more
information.
.PP
The standard options are:
.PP
.B option \fBall-subnets-local\fR \fIflag\fR\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBarp-cache-timeout\fR \fIuint32\fR\fB;\fR
.RS 0.25i
.PP
This option specifies the timeout in seconds for ARP cache entries.
.RE
.PP
.B option \fBbootfile-name\fR \fItext\fR\fB;\fR
.RS 0.25i
.PP
This option is used to identify a bootstrap file.  If supported by the
client, it should have the same effect as the \fBfilename\fR
declaration.  BOOTP clients are unlikely to support this option.  Some
DHCP clients will support it, and others actually require it.
.RE
.PP
.B option \fBboot-size\fR \fIuint16\fR\fB;\fR
.RS 0.25i
.PP
This option specifies the length in 512-octet blocks of the default
boot image for the client.
.RE
.PP
.B option \fBbroadcast-address\fR \fIip-address\fR\fB;\fR
.RS 0.25i
.PP
This option specifies the broadcast address in use on the client's
subnet.  Legal values for broadcast addresses are specified in
section 3.2.1.3 of STD 3 (RFC1122).
.RE
.PP
.B option \fBcookie-servers\fR \fIip-address\fR [\fB,\fR \fIip-address\fR...
]\fB;\fR
.RS 0.25i
.PP
The cookie server option specifies a list of RFC 865 cookie
servers available to the client.  Servers should be listed in order
of preference.
.RE
.PP
.B option \fBdefault-ip-ttl\fR \fIuint8;\fR
.RS 0.25i
.PP
This option specifies the default time-to-live that the client should
use on outgoing datagrams.
.RE
.PP
.B option \fBdefault-tcp-ttl\fR \fIuint8\fR\fB;\fR
.RS 0.25i
.PP
This option specifies the default TTL that the client should use when
sending TCP segments.  The minimum value is 1.
.RE
.PP
.B option \fBdhcp-client-identifier\fR \fIstring\fR\fB;\fR
.RS 0.25i
.PP
This option can be used to specify the a DHCP client identifier in a
host declaration, so that dhcpd can find the host record by matching
against the client identifier.
.PP
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:
.nf

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

rather than:

        option dhcp-client-identifier "foo";
.fi
.RE
.PP
.B option \fBdhcp-max-message-size\fR \fIuint16\fR\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBdhcp-parameter-request-list\fR \fIuint16\fR\fB;\fR
.RS 0.25i
.PP
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 \fIrequest\fR statement.   If this
option is not specified by the client, the DHCP server will normally
return every option that is valid in scope and that fits into the
reply.   When this option is specified on the server, the server
returns the specified options.   This can be used to force a client to
take options that it hasn't requested, and it can also be used to
tailor the response of the DHCP server for clients that may need a
more limited set of options than those the server would normally
return.
.RE
.PP
.B option \fBdomain-name\fR \fItext\fR\fB;\fR
.RS 0.25i
.PP
This option specifies the domain name that client should use when
resolving hostnames via the Domain Name System.
.RE
.PP
.B option \fBdomain-name-servers\fR \fIip-address\fR [\fB,\fR \fIip-address\fR...
]\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBextensions-path-name\fR \fItext\fR\fB;\fR
.RS 0.25i
.PP
This option specifies the name of a file containing additional options
to be interpreted according to the DHCP option format as specified in
RFC2132.
.RE
.PP
.B option \fBfinger-server\fR \fIip-address\fR [\fB,\fR
\fIip-address\fR... ]\fB;\fR
.RS 0.25i
.PP
The Finger server option specifies a list of Finger available to the
client.  Servers should be listed in order of preference.
.RE
.PP
.B option \fBfont-servers\fR \fIip-address\fR [\fB,\fR \fIip-address\fR...
]\fB;\fR
.RS 0.25i
.PP
This option specifies a list of X Window System Font servers available
to the client. Servers should be listed in order of preference.
.RE
.PP
.B option \fBhost-name\fR \fIstring\fR\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBieee802-3-encapsulation\fR \fIflag\fR\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBien116-name-servers\fR \fIip-address\fR [\fB,\fR \fIip-address\fR...
];
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBimpress-servers\fR \fIip-address\fR [\fB,\fR \fIip-address\fR...
]\fB;\fR
.RS 0.25i
.PP
The impress-server option specifies a list of Imagen Impress servers
available to the client.  Servers should be listed in order of
preference.
.RE
.PP
.B option \fBinterface-mtu\fR \fIuint16\fR\fB;\fR
.RS 0.25i
.PP
This option specifies the MTU to use on this interface.   The minimum
legal value for the MTU is 68.
.RE
.PP
.B option \fBip-forwarding\fR \fIflag\fR\fB;\fR
.RS 0.25i
.PP
This option specifies whether the client should configure its IP
layer for packet forwarding.  A value of false means disable IP
forwarding, and a value of true means enable IP forwarding.
.RE
.PP
.B option \fBirc-server\fR \fIip-address\fR [\fB,\fR
\fIip-address\fR... ]\fB;\fR
.RS 0.25i
.PP
The IRC server option specifies a list of IRC available to the
client.  Servers should be listed in order of preference.
.RE
.PP
.B option \fBlog-servers\fR \fIip-address\fR [\fB,\fR \fIip-address\fR...
]\fB;\fR
.RS 0.25i
.PP
The log-server option specifies a list of MIT-LCS UDP log servers
available to the client.  Servers should be listed in order of
preference.
.RE
.PP
.B option \fBlpr-servers\fR \fIip-address \fR [\fB,\fR \fIip-address\fR...
]\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBmask-supplier\fR \fIflag\fR\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBmax-dgram-reassembly\fR \fIuint16\fR\fB;\fR
.RS 0.25i
.PP
This option specifies the maximum size datagram that the client
should be prepared to reassemble.  The minimum value legal value is
576.
.RE
.PP
.B option \fBmerit-dump\fR \fItext\fR\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBmobile-ip-home-agent\fR \fIip-address\fR [\fB,\fR \fIip-address\fR... ]\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBnds-context\fR \fIstring\fR\fB;\fR
.RS 0.25i
.PP
The nds-context option specifies the name of the initial Netware
Directory Service for an NDS client.
.RE
.PP
.B option \fBnds-servers\fR \fIip-address\fR [\fB,\fR \fIip-address\fR... ]\fB;\fR
.RS 0.25i
.PP
The nds-servers option specifies a list of IP addresses of NDS servers.
.RE
.PP
.B option \fBnds-tree-name\fR \fIstring\fR\fB;\fR
.RS 0.25i
.PP
The nds-context option specifies NDS tree name that the NDS client
should use.
.RE
.PP
.B option \fBnetbios-dd-server\fR \fIip-address\fR [\fB,\fR \fIip-address\fR...
]\fB;\fR
.RS 0.25i
.PP
The NetBIOS datagram distribution server (NBDD) option specifies a
list of RFC 1001/1002 NBDD servers listed in order of preference.
.RE
.PP
.B option \fBnetbios-name-servers\fR \fIip-address\fR [\fB,\fR \fIip-address\fR...]\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBnetbios-node-type\fR \fIuint8\fR\fB;\fR
.RS 0.25i
.PP
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.
.PP
Possible node types are:
.PP
.TP 5
.I 1
B-node: Broadcast - no WINS
.TP
.I 2
P-node: Peer - WINS only.
.TP
.I 4
M-node: Mixed - broadcast, then WINS
.TP
.I 8
H-node: Hybrid - WINS, then broadcast
.RE
.PP
.B option \fBnetbios-scope\fR \fIstring\fR\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBnwip-domain\fR \fIstring\fR\fB;\fR
.RS 0.25i
.PP
The name of the NetWare/IP domain that a NetWare/IP client should
use.
.RE
.PP
.B option \fBnwip-suboptions\fR \fIstring\fR\fB;\fR
.RS 0.25i
.PP
A sequence of suboptions for NetWare/IP clients - see RFC2242 for
details.   Normally this option is set by specifying specific
NetWare/IP suboptions - see the NETWARE/IP SUBOPTIONS section for more
information.
.RE
.PP
.B option \fBnis-domain\fR \fItext\fR\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBnis-servers\fR \fIip-address\fR [\fB,\fR \fIip-address\fR...
]\fB;\fR
.RS 0.25i
.PP
This option specifies a list of IP addresses indicating NIS servers
available to the client.  Servers should be listed in order of
preference.
.RE
.PP
.B option \fBnisplus-domain\fR \fItext\fR\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBnisplus-servers\fR \fIip-address\fR [\fB,\fR \fIip-address\fR...
]\fB;\fR
.RS 0.25i
.PP
This option specifies a list of IP addresses indicating NIS+ servers
available to the client.  Servers should be listed in order of
preference.
.RE
.PP
.B option \fBnntp-server\fR \fIip-address\fR [\fB,\fR
\fIip-address\fR... ]\fB;\fR
.RS 0.25i
.PP
The NNTP server option specifies a list of NNTP available to the
client.  Servers should be listed in order of preference.
.RE
.PP
.B option \fBnon-local-source-routing\fR \fIflag\fR\fB;\fR
.RS 0.25i
.PP
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 0 means disallow forwarding of such datagrams, and a value of true
means allow forwarding.
.RE
.PP
.B option \fBntp-servers\fR \fIip-address\fR [\fB,\fR \fIip-address\fR...
]\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBpath-mtu-aging-timeout\fR \fIuint32\fR\fB;\fR
.RS 0.25i
.PP
This option specifies the timeout (in seconds) to use when aging Path
MTU values discovered by the mechanism defined in RFC 1191.
.RE
.PP
.B option \fBpath-mtu-plateau-table\fR \fIuint16\fR [\fB,\fR \fIuint16\fR...
]\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBperform-mask-discovery\fR \fIflag\fR\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.nf
.B option \fBpolicy-filter\fR \fIip-address ip-address\fR
                  [\fB,\fR \fIip-address ip-address\fR...]\fB;\fR
.RE
.fi
.RS 0.25i
.PP
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.
.PP
Any source routed datagram whose next-hop address does not match one
of the filters should be discarded by the client.
.PP
See STD 3 (RFC1122) for further information.
.RE
.PP
.B option \fBpop-server\fR \fIip-address\fR [\fB,\fR \fIip-address\fR... ]\fB;\fR
.RS 0.25i
.PP
The POP3 server option specifies a list of POP3 available to the
client.  Servers should be listed in order of preference.
.RE
.PP
.nf
.B option \fBresource-location-servers\fR \fIip-address\fR
                              [\fB, \fR\fIip-address\fR...]\fB;\fR
.fi
.RS 0.25i
.PP
This option specifies a list of RFC 887 Resource Location
servers available to the client.  Servers should be listed in order
of preference.
.RE
.PP
.B option \fBroot-path\fR \fItext\fB;\fR\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBrouter-discovery\fR \fIflag\fR\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBrouter-solicitation-address\fR \fIip-address\fR\fB;\fR
.RS 0.25i
.PP
This option specifies the address to which the client should transmit
router solicitation requests.
.RE
.PP
.B option routers \fIip-address\fR [\fB,\fR \fIip-address\fR...
]\fB;\fR
.RS 0.25i
.PP
The routers option specifies a list of IP addresses for routers on the
client's subnet.  Routers should be listed in order of preference.
.RE
.PP
.B option slp-directory-agent \fIboolean ip-address
[\fB,\fR \fIip-address\fR... ]\fB;\fR
.RS 0.25i
.PP
This option specifies two things: the IP addresses of one or more
Service Location Protocol Directory Agents, and whether the use of
these addresses is mandatory.   If the initial boolean value is true,
the SLP agent should just use the IP addresses given.   If the value
is false, the SLP agent may additionally do active or passive
multicast discovery of SLP agents (see RFC2165 for details).
.PP
Please note that in this option and the slp-service-scope option, the
term "SLP Agent" is being used to refer to a Service Location Protocol
agent running on a machine that is being configured using the DHCP
protocol.
.PP
Also, please be aware that some companies may refer to SLP as NDS.
If you have an NDS directory agent whose address you need to
configure, the slp-directory-agent option should work.
.RE
.PP
.B option slp-service-scope \fIboolean text\fR\fB;\fR
.RS 0.25i
.PP
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.
.PP
The text string should be a comma-seperated 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.
.RE
.PP
.B option \fBsmtp-server\fR \fIip-address\fR [\fB,\fR
\fIip-address\fR... ]\fB;\fR
.RS 0.25i
.PP
The SMTP server option specifies a list of SMTP servers available to
the client.  Servers should be listed in order of preference.
.RE
.PP
.nf
.B option \fBstatic-routes\fR \fIip-address ip-address\fR
                  [\fB,\fR \fIip-address ip-address\fR...]\fB;\fR
.fi
.RS 0.25i
.PP
This option specifies a list of static routes that the client should
install in its routing cache.  If multiple routes to the same
destination are specified, they are listed in descending order of
priority.
.PP
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.
.PP
The default route (0.0.0.0) is an illegal destination for a static
route.  To specify the default route, use the
.B routers
option.   Also, please note that this option is not intended for
classless IP routing - it does not include a subnet mask.   Since
classless IP routing is now the most widely deployed routing standard,
this option is virtually useless, and is not implemented by any of the
popular DHCP clients, for example the Microsoft DHCP client.
.RE
.PP
.nf
.B option \fBstreettalk-directory-assistance-server\fR \fIip-address\fR
                                           [\fB,\fR \fIip-address\fR...]\fB;\fR
.fi
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBstreettalk-server\fR \fIip-address\fR [\fB,\fR \fIip-address\fR... ]\fB;\fR
.RS 0.25i
.PP
The StreetTalk server option specifies a list of StreetTalk servers
available to the client.  Servers should be listed in order of
preference.
.RE
.PP
.B option subnet-mask \fIip-address\fR\fB;\fR
.RS 0.25i
.PP
The subnet mask option specifies the client's subnet mask as per RFC
950.  If no subnet mask option is provided anywhere in scope, as a
last resort dhcpd will use the subnet mask from the subnet declaration
for the network on which an address is being assigned.  However,
.I any
subnet-mask option declaration that is in scope for the address being
assigned will override the subnet mask specified in the subnet
declaration.
.RE
.PP
.B option \fBswap-server\fR \fIip-address\fR\fB;\fR
.RS 0.25i
.PP
This specifies the IP address of the client's swap server.
.RE
.PP
.B option \fBtcp-keepalive-garbage\fR \fIflag\fR\fB;\fR
.RS 0.25i
.PP
This option specifies the whether or not the client should send TCP
keepalive messages with a 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.
.RE
.PP
.B option \fBtcp-keepalive-interval\fR \fIuint32\fR\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBtftp-server-name\fR \fItext\fR\fB;\fR
.RS 0.25i
.PP
This option is used to identify a TFTP server and, if supported by the
client, should have the same effect as the \fBserver-name\fR
declaration.   BOOTP clients are unlikely to support this option.
Some DHCP clients will support it, and others actually require it.
.RE
.PP
.B option time-offset \fIint32\fR\fB;\fR
.RS 0.25i
.PP
The time-offset option specifies the offset of the client's subnet in
seconds from Coordinated Universal Time (UTC).
.RE
.PP
.B option time-servers \fIip-address\fR [, \fIip-address\fR...
]\fB;\fR
.RS 0.25i
.PP
The time-server option specifies a list of RFC 868 time servers
available to the client.  Servers should be listed in order of
preference.
.RE
.PP
.B option \fBtrailer-encapsulation\fR \fIflag\fR\fB;\fR
.RS 0.25i
.PP
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 0 indicates that the client should not attempt to use trailers.  A
value of true means that the client should attempt to use trailers.
.RE
.PP
.B option \fBuap-servers\fR \fItext\fR\fB;\fR
.RS 0.25i
.PP
This option specifies a list of URLs, each pointing to a user
authentication service that is capable of processing authentication
requests encapsulated in the User Authentication Protocol (UAP).  UAP
servers can accept either HTTP 1.1 or SSLv3 connections.  If the list
includes a URL that does not contain a port component, the normal
default port is assumed (i.e., port 80 for http and port 443 for
https).  If the list includes a URL that does not contain a path
component, the path /uap is assumed.   If more than one URL is
specified in this list, the URLs are seperated by spaces.
.RE
.PP
.B option \fBuser-class\fR \fIstring\fR\fB;\fR
.RS 0.25i
.PP
This option is used by some DHCP clients as a way for users to
specify identifying information to the client.   This can be used in a
similar way to the vendor-class-identifier option, but the value of
the option is specified by the user, not the vendor.   Most recent
DHCP clients have a way in the user interface to specify the value for
this identifier, usually as a text string.
.PP
.B option \fBvendor-class-identifier\fR \fIstring\fR\fB;\fR
.RS 0.25i
.PP
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 a
clients are sending, you can write the following in your DHCP server
configuration file:
.nf
.PP
set vendor-class option vendor-class-identifier;
.fi
.PP
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:
.nf
.PP
set vendor-class "SUNW.Ultra-5_10";
.fi
.PP
The vendor-class-identifier option is normally used by the DHCP server
to determine the options that are returned in the
.B vendor-encapsulated-options
option.   Please see the VENDOR ENCAPSULATED OPTIONS section of the
dhcpd.conf manual page for further information.
.RE
.PP
.B option \fBvendor-encapsulated-options\fR \fIstring\fR\fB;\fR
.RS 0.25i
.PP
The \fBvendor-encapsulated-options\fR option can contain either a
single vendor-specific value or one or more vendor-specific
suboptions.   This option is not normally specified in the DHCP server
configuration file - instead, a vendor class is defined for each
vendor, vendor class suboptions are defined, values for those
suboptions are defined, and the DHCP server makes up a response on
that basis.
.PP
Some default behaviours for well-known DHCP client vendors (currently,
the Microsoft Windows 2000 DHCP client) are configured automatically,
but otherwise this must be configured manually - see the VENDOR
ENCAPSULATED OPTIONS section of the \fIdhcpd.conf\fI manual page for
details.
.RE
.PP
.B option \fBx-display-manager\fR \fIip-address\fR [\fB,\fR \fIip-address\fR...
]\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBwww-server\fR \fIip-address\fR [\fB,\fR
\fIip-address\fR... ]\fB;\fR
.RS 0.25i
.PP
The WWW server option specifies a list of WWW available to the
client.  Servers should be listed in order of preference.
.RE
.SH 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.
.PP
The current draft defines two options.   To reference
these options in the dhcp server, specify the option space name,
"agent", followed by a period, followed by the option name.   It is
not normally useful to define values for these options in the server,
although it is permissible.   These options are not supported in the
client.
.PP
.B option \fBagent.circuit-id\fR \fIstring\fR\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBagent.remote-id\fR \fIstring\fR\fB;\fR
.RS 0.25i
.PP
The remote-id suboption encodes information about the remote host end
of a circuit.   Examples of what it might contain include caller ID
information, username information, remote ATM address, cable modem ID,
and similar things.   In principal, the meaning is not well-specified,
and it should generally be assumed to be an opaque object that is
administratively guaranteed to be unique to a particular remote end of
a circuit.
.RE
.SH THE CLIENT FQDN SUBOPTIONS
The Client FQDN option, currently defined in the Internet Draft
draft-ietf-dhc-fqdn-option-00.txt is not a standard yet, but is in
sufficiently wide use already that we have implemented it.   Due to
the complexity of the option format, we have implemented it as a
suboption space rather than a single option.   In general this
option should not be configured by the user - instead it should be
used as part of an automatic DNS update system.
.PP
.B option fqdn.no-client-update \fIflag\fB;
.RS 0.25i
.PP
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 \fIshould not\fR update its own A record.
.RE
.PP
.B option fqdn.server-update \fIflag\fB;
.RS 0.25i
.PP
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.
.RE
.PP
.B option fqdn.encoded \fIflag\fB;
.RS 0.25i
.PP
If true, this indicates that the domain name included in the option is
encoded in DNS wire format, rather than as plain ASCII text.   The client
normally sets this to false if it doesn't support DNS wire format in the
FQDN option.   The server should always send back the same value that the
client sent.   When this value is set on the configuration side, it controls
the format in which the \fIfqdn.fqdn\fR suboption is encoded.
.RE
.PP
.B option fqdn.rcode1 \fIflag\fB;
.PP
.B option fqdn.rcode1 \fIflag\fB;
.RS 0.25i
.PP
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.
.RE
.PP
.B option fqdn.fqdn \fItext\fB;
.RS 0.25i
.PP
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.
.RE
.PP
If you wish to use any of these suboptions, we strongly recommend that you
refer to the Client FQDN option draft (or standard, when it becomes a
standard) - the documentation here is sketchy and incomplete in comparison,
and is just intended for reference by people who already understand the
Client FQDN option specification.
.SH 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:
.PP
.B option \fBnwip.nsq-broadcast\fR \fIflag\fR\fB;\fR
.RS 0.25i
.PP
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.
.PP
.RE
.B option \fBnwip.preferred-dss\fR \fIip-address\fR [\fB,\fR \fIip-address\fR... ]\fR\fB;\fR
.RS 0.25i
.PP
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).
.RE
.PP
.B option \fBnwip.nearest-nwip-server\fR \fI\fIip-address\fR
                             [\fB,\fR \fIip-address\fR...]\fR\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBnwip.autoretries\fR \fIuint8\fR\fB;\fR
.RS 0.25i
.PP
Specifies the number of times that a NetWare/IP client should attempt
to communicate with a given DSS server at startup.
.RE
.PP
.B option \fBnwip.autoretry-secs\fR \fIuint8\fR\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBnwip.nwip-1-1\fR \fIuint8\fR\fB;\fR
.RS 0.25i
.PP
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.
.RE
.PP
.B option \fBnwip.primary-dss\fR \fIip-address\fR\fB;\fR
.RS 0.25i
.PP
Specifies the IP address of the Primary Domain SAP/RIP Service server
(DSS) for this NetWare/IP domain.   The NetWare/IP administration
utility uses this value as Primary DSS server when configuring a
secondary DSS server.
.RE
.SH DEFINING NEW OPTIONS
The Internet Software 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.
.PP
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".
.PP
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
values 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.
.PP
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.
.PP
New options are declared as follows:
.PP
.B option
.I new-name
.B code
.I new-code
.B =
.I definition
.B ;
.PP
The values of
.I new-name
and
.I new-code
should be the name you have chosen for the new option and the code you
have chosen.   The
.I definition
should be the definition of the structure of the option.
.PP
The following simple option type definitions are supported:
.PP
.B BOOLEAN
.PP
.B option
.I new-name
.B code
.I new-code
.B =
.B boolean
.B ;
.PP
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:
.nf

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

.fi
.B INTEGER
.PP
.B option
.I new-name
.B code
.I new-code
.B =
.I sign
.B integer
.I width
.B ;
.PP
The \fIsign\fR token should either be blank, \fIunsigned\fR
or \fIsigned\fR.   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:
.nf

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

.fi
.B IP-ADDRESS
.PP
.B option
.I new-name
.B code
.I new-code
.B =
.B ip-address
.B ;
.PP
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:
.nf

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

.fi
.PP
.B TEXT
.PP
.B option
.I new-name
.B code
.I new-code
.B =
.B text
.B ;
.PP
An option whose type is text will encode an ASCII text string.   For
example:
.nf

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

.fi
.PP
.B DATA STRING
.PP
.B option
.I new-name
.B code
.I new-code
.B =
.B string
.B ;
.PP
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 seperated by colons whose
values must be between 0 and FF.   For example:
.nf

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

.fi
.PP
.B ENCAPSULATION
.PP
.B option
.I new-name
.B code
.I new-code
.B =
.B encapsulate
.I identifier
.B ;
.PP
An option whose type is \fBencapsulate\fR will encapsulate the
contents of the option space specified in \fIidentifier\fR.   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.
.nf

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

.fi
.PP
.B ARRAYS
.PP
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:
.nf

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

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

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

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

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;

.fi     
.SH VENDOR ENCAPSULATED OPTIONS
The DHCP protocol defines the \fB vendor-encapsulated-options\fR
option, which allows vendors to define their own options that will be
sent encapsulated in a standard DHCP option.   The format of the
.B 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).
.PP
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-seperated list of hexadecimal values.   For example:
.PP
.nf
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;
.fi
.PP
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
.B vendor-encapsulated-options
option.
.PP
To define a new option space in which vendor options can be stored,
use the \fRoption space\fP statement:
.PP
.B option
.B space
.I name
.B ;
.PP
The name can then be used in option definitions, as described earlier in
this document.   For example:
.nf

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;

.fi
Once you have defined an option space and the format of some options,
you can set up scopes that define values for those options, and you
can say when to use them.   For example, suppose you want to handle
two different classes of clients.   Using the option space definition
shown in the previous example, you can send different option values to
different clients based on the vendor-class-identifier option that the
clients send, as follows:
.PP
.nf
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";
}
.fi
.PP
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 \fBvendor-option-space\fR declaration tells the DHCP
server to use options in the SUNW option space to construct the
.B vendor-encapsulated-options
option.
.SH 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.
.SH AUTHOR
The Internet Software Consortium DHCP Distribution was written by Ted
Lemon under a contract with Vixie Labs.  Funding for
this project was provided through the Internet Software Consortium.
Information about the Internet Software Consortium can be found at
.B http://www.isc.org.