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dhcpd-options

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NAME

dhcp-options - Dynamic Host Configuration Protocol options

DESCRIPTION

The Dynamic Host Configuration protocol allows the client to receiveoptionsfrom the DHCP server describing the network configuration and variousservices that are available on the network. When configuringdhcpd(8)ordhclient(8) ,options must often be declared. The syntax for declaring options,and the names and formats of the options that can be declared, aredocumented here.

REFERENCE: OPTION STATEMENTS

DHCP option statements always start with the optionkeyword, followed by an option name, followed by option data. Theoption names and data formats are described below. It is notnecessary to exhaustively specify all DHCP options - only thoseoptions which are needed by clients must be specified.

Option data comes in a variety of formats, as defined below:

Theip-addressdata type can be entered either as an explicit IPaddress (e.g., 239.254.197.10) or as a domain name (e.g.,haagen.isc.org). When entering a domain name, be sure that thatdomain name resolves to a single IP address.

Theint32data type specifies a signed 32-bit integer. The uint32data type specifies an unsigned 32-bit integer. The int16anduint16data types specify signed and unsigned 16-bit integers. The int8anduint8data types specify signed and unsigned 8-bit integers.Unsigned 8-bit integers are also sometimes referred to as octets.

Thetextdata type specifies an NVT ASCII string, which must beenclosed in double quotes - for example, to specify a root-pathoption, the syntax would be

option root-path "10.0.1.4:/var/tmp/rootfs";

Thedomain-namedata type specifies a domain name, which must notenclosed in double quotes. This data type is not used for anyexisting DHCP options. The domain name is stored just as if it werea text option.

Theflagdata type specifies a boolean value. Booleans can be either true orfalse (or on or off, if that makes more sense to you).

Thestringdata type specifies either an NVT ASCII stringenclosed in double quotes, or a series of octets specified inhexadecimal, 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;

SETTING OPTION VALUES USING EXPRESSIONS

Sometimes it's helpful to be able to set the value of a DHCP optionbased on some value that the client has sent. To do this, you canuse expression evaluation. The dhcp-eval(5)manual page describes how to write expressions. To assign the resultof an evaluation to an option, define the option as follows:

  option my-option = expression ;

For example:

  option hostname = binary-to-ascii (16, 8, "-",                                     substring (hardware, 1, 6));

STANDARD DHCP OPTIONS

The documentation for the various options mentioned below is takenfrom the latest IETF draft document on DHCP options. Options notlisted below may not yet be implemented, but it is possible to usesuch options by defining them in the configuration file. Please seethe DEFINING NEW OPTIONS heading later in this document for moreinformation.

Some of the options documented here are automatically generated bythe DHCP server or by clients, and cannot be configured by the user.The value of such an option can be used in the configuration file ofthe receiving DHCP protocol agent (server or client), for example inconditional expressions. However, the value of the option cannot beused in the configuration file of the sending agent, because the valueis determined only after the configuration file has beenprocessed. In the following documentation, such options will be shownas "not user configurable"

The standard options are:

option all-subnets-local flag;

: This option specifies whether or not the client may assume that allsubnets of the IP network to which the client is connected use thesame MTU as the subnet of that network to which the client isdirectly connected. A value of true indicates that all subnets sharethe same MTU. A value of false means that the client should assume thatsome 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 bootfile-name text;

: This option is used to identify a bootstrap file. If supported by theclient, it should have the same effect as the filenamedeclaration. BOOTP clients are unlikely to support this option. SomeDHCP clients will support it, and others actually require it.

option boot-size uint16;

: This option specifies the length in 512-octet blocks of the defaultboot image for the client.

option broadcast-address ip-address;

: This option specifies the broadcast address in use on the client'ssubnet. Legal values for broadcast addresses are specified insection 3.2.1.3 of STD 3 (RFC1122).

option cookie-servers ip-address [, ip-address...];

: The cookie server option specifies a list of RFC 865 cookieservers available to the client. Servers should be listed in orderof preference.

option default-ip-ttl uint8;

: This option specifies the default time-to-live that the client shoulduse on outgoing datagrams.

option default-tcp-ttl uint8;

: This option specifies the default TTL that the client should use whensending TCP segments. The minimum value is 1.

option dhcp-client-identifier string;

This option can be used to specify the a DHCP client identifier in ahost declaration, so that dhcpd can find the host record by matchingagainst the client identifier.

Please be aware that some DHCP clients, when configured with clientidentifiers that are ASCII text, will prepend a zero to the ASCIItext. So you may need to write:

        option dhcp-client-identifier "\0foo";rather than:        option dhcp-client-identifier "foo";

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 aserver reply (DHCPOFFER), a DHCP server uses this option to specifythe lease time it is willing to offer.

This option is not directly user configurable in the server; refer to themax-lease-time and default-lease-time server options indhcpd.conf(5).

option dhcp-max-message-size uint16;

: This option, when sent by the client, specifies the maximum size ofany response that the server sends to the client. When specified onthe server, if the client did not send a dhcp-max-message-size option,the size specified on the server is used. This works for BOOTP aswell as DHCP responses.

option dhcp-message text;

This option is used by a DHCP server to provide an error message to aDHCP client in a DHCPNAK message in the event of a failure. A clientmay use this option in a DHCPDECLINE message to indicate why theclient declined the offered parameters.