The Dynamic Host Configuration protocol allows the client to receive
options
from the DHCP server describing the network configuration and various
services that are available on the network.
When configuring
dhcpd(8)
or
dhclient(8),
options must often be declared.
The syntax for declaring options, and the names and formats of the options
that can be declared, are documented here.
REFERENCE: OPTION STATEMENTS
DHCP
option
statements always start with the
option
keyword, followed by an option name, followed by option data.
The option names and data formats are described below.
It is not necessary to exhaustively specify all DHCP options -
only those options which are needed by clients must be specified.
Option data comes in a variety of formats, as defined below:
The
ip-address
data type can be entered either as an explicit IP address
(e.g.,
239.254.197.10
or as a domain name (e.g.,
haagen.isc.org )
A domain name must resolve 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
string
data type specifies an
NVT
(Network Virtual Terminal)
ASCII
string, which must be enclosed in double quotes - for example,
to specify a domain-name option, the syntax would be
option domain-name "isc.org";
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
data-string
data type specifies either an
NVT ASCII
string enclosed in double quotes, or a series of octets specified in
hexadecimal, separated by colons.
For example:
The documentation for the various options mentioned below is taken
from the IETF draft document on DHCP options, RFC 2132.
Options which are not listed by name may be defined by the name
option- nnn
where
nnn
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 separated
by colons.
For example:
Because
dhcpd(8)
does not know the format of these undefined option codes,
no checking is done to ensure the correctness of the entered data.
The standard options are:
RFC 1497 Vendor Extensions
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(8)
will use the subnet mask from the subnet declaration for the network on
which an address is being assigned.
However,
any
subnet-mask option declaration that is in scope for the address being
assigned will override the subnet mask specified in the subnet declaration.
option time-offset int32
The
time-offset
option specifies the offset of the client's subnet in seconds from
Coordinated Universal Time (UTC).
option routers ip-address
[, ip-address ... ]
The
routers
option specifies a list of IP addresses for routers on the client's subnet.
Routers should be listed in order of preference.
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 log-servers ip-address
[, ip-address ... ]
The
log-servers
option specifies a list of MIT-LCS UDP log servers available to the client.
Servers should be listed in order of preference.
This option specifies a list of RFC 887 Resource Location servers available
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.
option boot-size uint16
This option specifies the length in 512-octet blocks of the default
boot image for the client.
option merit-dump string
This option specifies the pathname 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 domain-name string
This option specifies the domain name that the client should use when
resolving hostnames via the Domain Name System.
option swap-server ip-address
This specifies the IP address of the client's swap server.
option root-path string
This option specifies the pathname that contains the client's root disk.
The path is formatted as a character string consisting of characters from
the
NVT ASCII
character set.
IP Layer Parameters per Host
option ip-forwarding flag
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.
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 0 means disallow forwarding of such datagrams, and a value of 1
means allow forwarding.
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 (RFC 1122) for further information.
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 default-ip-ttl uint8
This option specifies the default time-to-live that the client should
use on outgoing datagrams.
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.
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.
IP Layer Parameters per Interface
option interface-mtu uint16
This option specifies the MTU to use on this interface.
The minimum legal value for the MTU is 68.
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 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.
option broadcast-address ip-address
This option specifies the broadcast address in use on the client's subnet.
Legal values for broadcast addresses are specified in section 3.2.1.3 of
STD 3 (RFC 1122).
option perform-mask-discovery flag
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.
option mask-supplier flag
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.
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 0 indicates that the client should not perform router discovery.
A value of 1 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.
This option specifies a list of static routes that the client should
install in its routing cache.
If multiple routes to the same destination are specified, they are listed
in descending order of priority.
The routes consist of a list of IP address pairs.
The first address is the destination address,
and the second address is the router for the destination.
The default route (0.0.0.0) is an illegal destination for a static route.
To specify the default route, use the
routers
option.
Link Layer Parameters per Interface
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 0 indicates that the client should not attempt to use trailers.
A value of 1 means that the client should attempt to use trailers.
option arp-cache-timeout uint32
This option specifies the timeout in seconds for ARP cache entries.
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 0 indicates that the client should use RFC 894 encapsulation.
A value of 1 means that the client should use RFC 1042 encapsulation.
TCP Parameters
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 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 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 0 indicates that a garbage octet should not be sent.
A value of 1 indicates that a garbage octet should be sent.
Application and Service Parameters
option nis-domain string
This option specifies the name of the client's NIS (Sun Network Information
Services) domain.
The domain is formatted as a character string consisting of characters
from the
NVT ASCII
character set.
option nis-servers ip-address
[, ip-address ... ]
This option specifies a list of IP addresses indicating NIS servers
available to the client.
Servers should be listed in order of preference.
option ntp-servers ip-address
[, ip-address ... ]
This option specifies a list of IP addresses indicating NTP (RFC 1305)
servers available to the client.
Servers should be listed in order of preference.
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.
The NetBIOS datagram distribution server (NBDD) option specifies a
list of RFC 1001/1002 NBDD servers listed in order of preference.
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 RFC 1001, RFC 1002, and RFC 1035 for character-set restrictions.
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.
option dhcp-client-identifier data-string
This option can be used to specify a DHCP client identifier in a
host declaration, so that
dhcpd(8)
can find the host record by matching against the client identifier.
option nisplus-domain string
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.
This option specifies a list of IP addresses indicating NIS+ servers
available to the client.
Servers should be listed in order of preference.
option tftp-server-name string
This option is used to identify a TFTP server and, if supported by the
client, should have the same effect as the
server-name
declaration.
BOOTP clients are unlikely to support this option.
Some DHCP clients will support it, and others actually require it.
option bootfile-name string
This option is used to identify a bootstrap file.
If supported by the client, it should have the same effect as the
filename
declaration.
BOOTP clients are unlikely to support this option.
Some DHCP clients will support it, and others actually require it.
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 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 pop-server ip-address
[, ip-address ... ]
The
pop-server
option specifies a list of POP3 servers available to the client.
Servers should be listed in order of preference.
option nntp-server ip-address
[, ip-address ... ]
The
nntp-server
option specifies a list of NNTP servers available to the client.
Servers should be listed in order of preference.
option 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.
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.
