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DHCP-OPTIONS(5) File Formats Manual DHCP-OPTIONS(5)

dhcp-options — Dynamic Host Configuration Protocol 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 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:

option dhcp-client-identifier "CLIENT-FOO";
or
option dhcp-client-identifier 43:4c:49:45:54:2d: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 standard options are:

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, subnet-mask option declaration that is in scope for the address being assigned will override the subnet mask specified in the subnet declaration.
int32;
The time-offset option specifies the offset of the client's subnet in seconds from Coordinated Universal Time (UTC).
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.
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.
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.
ip-address [, ip-address ...];
The domain-name-servers option specifies a list of Domain Name System (STD 13, RFC 1035) name servers available to the client. Servers should be listed in order of preference.
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.
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.
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.
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.
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.
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.
uint16;
This option specifies the length in 512-octet blocks of the default boot image for the client.
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.
string;
This option specifies the domain name that the client should use when resolving hostnames via the Domain Name System.
string;
This option specifies a list of domain names that the client should use when resolving hostnames via the Domain Name System. This option is defined in RFC 3397.
ip-address;
This specifies the IP address of the client's swap server.
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.

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.
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.
ip-address ip-address [, ip-address ip-address ...];
This option specifies policy filters for non-local source routing. The filters consist of a list of IP addresses and masks which specify destination/mask pairs with which to filter incoming source routes.

Any source-routed datagram whose next-hop address does not match one of the filters should be discarded by the client.

See STD 3 (RFC 1122) for further information.

uint16;
This option specifies the maximum size datagram that the client should be prepared to reassemble. The minimum legal value is 576.
uint8;
This option specifies the default time-to-live that the client should use on outgoing datagrams.
uint32;
This option specifies the timeout (in seconds) to use when aging Path MTU values discovered by the mechanism defined in RFC 1191.
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.

uint16;
This option specifies the MTU to use on this interface. The minimum legal value for the MTU is 68.
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.
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).
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.
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.
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.
ip-address;
This option specifies the address to which the client should transmit router solicitation requests.
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.

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.
uint32;
This option specifies the timeout in seconds for ARP cache entries.
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.

uint8;
This option specifies the default TTL that the client should use when sending TCP segments. The minimum value is 1.
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.
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.

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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.

dhclient.conf(5), dhcpd.conf(5), dhcpd.leases(5), dhclient(8), dhcpd(8)

RFC 2131, RFC 2132.

The dhcpd(8) utility was written by Ted Lemon <mellon@vix.com> under a contract with Vixie Labs.

The current implementation was reworked by Henning Brauer <henning@openbsd.org>.

January 1, 1995 FreeBSD-12.0