isc-dhcp/common/dhcp-options.5

.\"	dhcp-options.5
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.\" This file is part of the ISC DHCP distribution.   The documentation
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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 string
data type specifies an NVT ASCII string, which must be
enclosed in double quotes - for example, to specify a domain-name
option, the syntax would be
.nf
.sp 1
	option domain-name "isc.org";
.fi
.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 data-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 client-identifier "CLIENT-FOO";
or
	option client-identifier 43:4c:49:45:54:2d:46:4f:4f;
.fi
.PP
The documentation for the various options mentioned below is taken
from the latest IETF draft document on DHCP options.   Options which
are not listed by name may be defined by the name option-\fInnn\fR,
where \fInnn\fI is the decimal number of the option code.   These
options may be followed either by a string, enclosed in quotes, or by
a series of octets, expressed as two-digit hexadecimal numbers seperated
by colons.   For example:
.PP
.nf
	option option-133 "my-option-133-text";
	option option-129 1:54:c9:2b:47;
.fi
.PP
Because dhcpd does not know the format of these undefined option codes,
no checking is done to ensure the correctness of the entered data.
.PP
The standard options are:
.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 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 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 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 \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 \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 \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 \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 \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 \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 \fBresource-location-servers\fR \fIip-address\fR [\fB,\fR \fIip-address\fR...
]\fB;\fR
.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 \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 \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 \fBmerit-dump\fR \fIstring\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 \fBdomain-name\fR \fIstring\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 \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 \fBroot-path\fR \fIstring\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 \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 0 means disable IP
forwarding, and a value of 1 means enable IP forwarding.
.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 1
means allow forwarding.
.RE
.PP
.B option \fBpolicy-filter\fR \fIip-address ip-address\fR [\fB,\fR \fIip-address ip-address\fR...
]\fB;\fR
.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 \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 \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 \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 \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 \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 1 indicates that all subnets share
the same MTU.  A value of 0 means that the client should assume that
some subnets of the directly connected network may have smaller MTUs.
.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 \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 0 indicates that the client
should not perform mask discovery.  A value of 1 means that the
client should perform mask discovery.
.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 0 indicates that the
client should not respond.  A value of 1 means that the client should
respond.
.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 0 indicates that the client should not perform
router discovery.  A value of 1 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 \fBstatic-routes\fR \fIip-address ip-address\fR [\fB,\fR \fIip-address ip-address\fR...
]\fB;\fR
.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.
.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 1 means that the client should attempt to use trailers.
.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 \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 0 indicates that the client
should use RFC 894 encapsulation.  A value of 1 means that the client
should use RFC 1042 encapsulation.
.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 \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 \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 0 indicates that a garbage octet
should not be sent. A value of 1 indicates that a garbage octet
should be sent.
.RE
.PP
.B option \fBnis-domain\fR \fIstring\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 \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 \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-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-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 \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 \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 \fBdhcp-client-identifier\fR \fIdata-string\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.
.RE
.SH RELAY AGENT INFORMATION OPTION
An IETF draft, draft-ietf-dhc-agent-options-03.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 isn't
useful to specify these options to be sent, nor is it useful to
reference them at all in the client.
.B option \fBagent.circuit-id\fR \fIdata-string\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.circuit-id\fR \fIdata-string\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.
.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 <mellon@isc.org> 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/isc.