dhcp-options
—
DHCP options
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.
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 cidr data type specifies a network in
CIDR notion. e.g. 1.2.3/24.
The int32 data type specifies a signed
32-bit integer. The uint32 data type specifies an
unsigned 32-bit integer. The uint16 data type
specifies unsigned 16-bit integers. The uint8 data
type specifies unsigned 8-bit integers, 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:
option dhcp-client-identifier
"CLIENT-FOO";
or
option dhcp-client-identifier
43:4c:49:45:4e:54:2d:46:4f:4f;
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:
option option-133 "my-option-133-text";
option option-129 1:54:c9:2b:47;
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 defined 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 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
arp-cache-timeout
uint32;
- This option specifies the timeout in seconds for ARP cache entries.
option
autoproxy-script
string;
- Site-specific as of RFC 3942. Widely used for Web Proxy Autodiscovery
Protocol (WPAD).
option
boot-size
uint16;
- This option specifies the length in 512-octet blocks of the default boot
image for the client.
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.
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
RFC 1122.
option
classless-static-routes
cidr ip-address [,
cidr ip-address ...];
- This option specifies a list of destination networks and the associated
gateways. This option is defined in RFC 3442. The RFC says that clients
supporting this option must ignore the
Routers
option when both are present. Thus default routes, if any, must be
included in the list. The cidr of a default route is
0/0.
option
classless-ms-static-routes
cidr ip-address
[, cidr ip-address ...];
- This option does the same as
classless-static-routes
, but uses option code 249
instead of 121, since Windows XP and Windows Server 2003 ignore option
121.
option
cookie-servers
ip-address [,
ip-address ...];
- The
cookie-servers
option specifies a list of RFC
865 cookie servers available to the client. Servers should be listed in
order of preference.
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-class-identifier
string;
- This option is used by DHCP clients to optionally identify the vendor type
and configuration of a DHCP client. The information is a string of n
octets, interpreted by servers. Vendors may choose to define specific
vendor class identifiers to convey particular configuration or other
identification information about a client. For example, the identifier may
encode the client's hardware configuration. Servers not equipped to
interpret the class-specific information sent by a client must ignore it
(although it may be reported). Servers that respond should only use option
43 (
vendor-encapsulated-options
) to return the
vendor-specific information to the client.
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
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.
option
dhcp-max-message-size
uint16;
- This option specifies the maximum length
option
dhcp-message
that the DHCP client is willing to accept. The length
is specified as an unsigned 16-bit integer. A client may use the maximum
DHCP message size option in DHCPDISCOVER or DHCPREQUEST messages, but
should not use the option in DHCPDECLINE messages.
option
dhcp-message
string;
- This option is used to send an error message to a DHCP client in the event
of a failure. The maximum acceptable length of the message can be set with
option dhcp-max-message-size
.
option
dhcp-message-type
uint8;
- This option is used to convey the type of the DHCP message. Values:
1=DHCPDISCOVER, 2=DHCPOFFER, 3=DHCPREQUEST, 4=DHCPDECLINE, 5=DHCPACK,
6=DHCPNAK, 7=DHCPRELEASE, 8=DHCPINFORM.
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 additional fields after it concludes interpretation of the
standard option fields. A value of 1 means the 'file' field is used to
hold options. A value of 2 means the 'sname' field is used to hold
options. A value of 3 means both fields are used to hold options.
option
dhcp-parameter-request-list
uint8 [,
uint8 ...];
- This option is used by a DHCP client to request values for specified
configuration parameters. The list of requested parameters is specified as
n octets, where each octet is a valid DHCP option code as defined in this
document. The client MAY list the options in order of preference. The DHCP
server is not required to return the options in the requested order, but
MUST try to insert the requested options in the order requested by the
client.
option
dhcp-rebinding-time
uint32;
- This option specifies the time interval from address assignment until the
client transitions to the REBINDING state. The value is in seconds.
option
dhcp-renewal-time
uint32;
- This option specifies the time interval from address assignment until the
client transitions to the RENEWING state. The value is in seconds.
option
dhcp-requested-address
ip-address;
- This option is used in a client request (DHCPDISCOVER) to allow the client
to request that a particular IP address be assigned.
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 identifier is the IP address of the selected server.
option
domain-name
string;
- This option specifies the domain name that the 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 name servers available to the client. Servers should be
listed in order of preference.
option
domain-search
string [,
string ...];
- The
domain-search
option specifies a list of the
domain names that should be used during DNS name resolution.
When dhclient(8)
constructs resolv.conf(5) it
will use this list of domains in preference to any information provided
by the domain-name
option.
option
extensions-path
string;
- A string to specify a file, retrievable via TFTP, which contains
information which can be interpreted in the same way as the 64-octet
vendor-extension field within the BOOTP response, with exceptions; see RFC
2132, Section 3.20 for details.
option
finger-server
ip-address [,
ip-address ...];
- The
finger-server
option specifies a list of
finger(1) 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 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
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.
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-servers
option specifies a list of
Imagen Impress servers available to the client. Servers should be listed
in order of preference.
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 0 means disable IP forwarding, and a value
of 1 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-servers
option specifies a list of MIT-LCS
UDP log servers available to the client. Servers should be listed in order
of preference.
option
lpr-servers
ip-address [,
ip-address ...];
- The
lpr-servers
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 0 indicates that the client should
not respond. A value of 1 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
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
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
data-string;
- This option specifies the initial NDS context the client should use. NDS
contexts are 16-bit Unicode strings. For transmission in the NDS Context
Option, an NDS context is transformed into octets using UTF-8. The string
should NOT be zero terminated. A single DHCP option can only contain 255
octets. Since an NDS context name can be longer than that, this option can
appear more than once in the DHCP packet. The contents of all NDS Context
options in the packet should be concatenated as suggested in the DHCP
specification to get the complete NDS context. A single encoded character
could be split between two NDS Context Options. See RFC 2241 for
details.
option
nds-servers
ip-address [,
ip-address ...];
- This option specifies one or more NDS servers for the client to contact
for access to the NDS database. Servers should be listed in order of
preference. See RFC 2241 for details.
option
nds-tree-name
data-string;
- This option specifies the name of the NDS tree the client will be
contacting. NDS tree names are 16-bit Unicode strings. For transmission in
the NDS Tree Name Option, an NDS tree name is transformed into octets
using UTF-8. The string should not be zero terminated. See RFC 2241 for
details.
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 RFC 1001, RFC 1002, and
RFC 1035 for character-set restrictions.
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
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.
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
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
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. A value of 0
means disallow forwarding of such datagrams, and a value of 1 means allow
forwarding.
option
ntp-servers
ip-address [,
ip-address ...];
- This option specifies a list of IP addresses indicating NTP (RFC 5905)
servers available to the client. Servers should be listed in order of
preference.
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 0 indicates that the client should not
perform mask discovery. A value of 1 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 RFC 1122 for further information.
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
relay-agent-information
data-string;
- This is a "container" option for specific agent-supplied
sub-options. See RFC 3046 for details.
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
preference.
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.
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.
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.
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 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. Note that this option is
obsolete and should be replaced by the
classless-static-routes
option.
dhclient(8) ignores this
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 preference.
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
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 0 indicates that a garbage octet should not be
sent. A value of 1 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-config-file
string;
- Option 144. Per RFC 2132 options 128 - 254 are site-specific. RFC 3942
reclassifies options 128 to 223 as publicly defined options and puts them
in "Unavailable" state by IANA. See RFC 3679 for "Unused
DHCP Option Codes to be Reassigned to Future DHCP Options". See RFC
2939 for procedures for definitions of new DHCP options.
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
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
preference.
option
trailer-encapsulation
flag;
- This option specifies whether or not the client should negotiate the use
of trailers (RFC 893) 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
user-class
string;
- This option is used by a DHCP client to optionally identify the type or
category of user or applications it represents. A DHCP server uses the
User Class option to choose the address pool it allocates an address from
and/or to select any other configuration option. This option may carry
multiple User Classes. Servers may interpret the meanings of multiple
class specifications in an implementation dependent or configuration
dependent manner, and so the use of multiple classes by a DHCP client
should be based on the specific server implementation and configuration
which will be used to process that User class option. See RFC 3004 for
details.
option
vendor-encapsulated-options
data-string;
- This option is used by clients and servers to exchange vendor-specific
information. The information is an opaque object of n octets, presumably
interpreted by vendor-specific code on the clients and servers. See RFC
2132, Section 8.4 for details.
option
voip-configuration-server
ip-address [,
ip-address ...];
- The
voip-configuration-server
option specifies a
list of (normally TFTP) servers that VoIP clients may download their
configuration information and software images from. 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.
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.
R. Droms,
Interoperation Between DHCP and BOOTP,
RFC 1534, October
1993.
R. Droms,
Dynamic Host Configuration Protocol,
RFC 2131, March
1997.
S. Alexander and
R. Droms, DHCP Options and BOOTP
Vendor Extensions, RFC 2132,
March 1997.
T. Lemon and
S. Cheshire, Encoding Long
Options in the Dynamic Host Configuration Protocol (DHCPv4),
RFC 3396, November
2002.
T. Lemon,
S. Cheshire, and B. Volz,
The Classless Static Route Option for Dynamic Host
Configuration Protocol (DHCP) version 4, RFC
3442, December 2002.