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

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), 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:

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.
uint32;
This option specifies the timeout in seconds for ARP cache entries.
string;
Site-specific as of RFC 3942. Widely used for Web Proxy Autodiscovery Protocol (WPAD).
uint16;
This option specifies the length in 512-octet blocks of the default boot image for the client.
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;
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.
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.
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.
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.
uint8;
This option specifies the default time-to-live that the client should use on outgoing datagrams.
uint8;
This option specifies the default TTL that the client should use when sending TCP segments. The minimum value is 1.
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.
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.
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.
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.
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.
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.
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.
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.
uint32;
This option specifies the time interval from address assignment until the client transitions to the REBINDING state. The value is in seconds.
uint32;
This option specifies the time interval from address assignment until the client transitions to the RENEWING state. The value is in seconds.
ip-address;
This option is used in a client request (DHCPDISCOVER) to allow the client to request that a particular IP address be assigned.
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.
string;
This option specifies the domain name that the client should use when resolving hostnames via the Domain Name System.
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.
string [, string ...];
The domain-search option specifies a list of the domain names that should be used during DNS name resolution.
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.
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 ...];
This option specifies a list of X Window System Font 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.
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.
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 impress-servers option specifies a list of Imagen Impress servers available to the client. Servers should be listed in order of preference.
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 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.
uint32;
This option specifies that NAT64 is available and the pool is IPv6-mostly capable. This option is specified in RFC 8925.
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 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 lpr-servers option specifies a list of RFC 1179 line printer servers available to the client. Servers should be listed in order of preference.
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.
uint16;
This option specifies the maximum size datagram that the client should be prepared to reassemble. The minimum legal value is 576.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.

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.
data-string;
This is a "container" option for specific agent-supplied sub-options. See RFC 3046 for details.
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 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 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 ...];
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 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 [, 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.

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.
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;
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.
ip-address;
This specifies the IP address of the client's swap server.
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.
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.
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.
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.
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 time-server option specifies a list of RFC 868 time servers available to the client. Servers should be listed in order of preference.
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.
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.
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.
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.
ip-address [, ip-address ...];
The www-server option specifies a list of WWW servers available to the client.
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.

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

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.

dhcpd(8) 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>.

June 27, 2024 OpenBSD-7.6