Table of Contents

DHCP Options

DHCP provides a framework for passing configuration information to hosts on a TCP/IP network. Configuration parameters and other control information are carried in tagged data items that are stored in the options field of the DHCP message. The data items themselves are also called options.

This appendix contains DHCP options and BOOTP vendor extensions from RFC 2132, and includes the validation type for each option, as indicated in Table B-5.

This appendix also contains the standard Microsoft client options and several tables displaying the options sorted by different categories.

Option Descriptions

The following sections describe the DHCP options in detail:

• BOOTP Extensions/DHCP Option Field Format
  
  
• RFC 1497 Vendor Extensions
  
  
• IP Layer Parameters Per Host
  
  
• Link Layer Parameters for Each Interface
  
  
• TCP Parameters
  
  
• Application and Service Parameters
  
  
• DHCP Extensions
  
  
• Microsoft Client Options
  
  

BOOTP Extensions/DHCP Option Field Format

DHCP options have the same format as the BOOTP vendor extensions defined in RFC 1497. Options may be fixed length or variable length. All options begin with a tag octet, which uniquely identifies the option. Fixed-length options without data consist of only a tag octet. Only options 0 and 255 are fixed length. All other options are variable-length with a length octet following the tag octet. The value of the length octet does not include the two octets specifying the tag and length. The length octet is followed by length octets of data. Options containing NVT ASCII data should not include a trailing NULL; however, the receiver of such options must be prepared to delete trailing nulls if they exist. The receiver must not require that a trailing null be included in the data. In the case of some variable-length options, the length field is a constant but must still be specified.

Any options defined subsequent to this document must contain a length octet even if the length is fixed or zero. All multi-octet quantities are in network byte-order.

Except for the options in section 9, all options may be used with either DHCP or BOOTP. Many of these options have their default values specified in other documents. In particular, RFC 1122 [4] specifies default values for most IP and TCP configuration parameters.

Many options supply one or more 32-bit IP address. Use of IP addresses instead of FQDNs may make future renumbering of IP hosts more difficult. Use of these addresses is discouraged at sites that may require renumbering.

When used with BOOTP, the first four octets of the vendor information field have been assigned to the magic cookie (as suggested in RFC 951). This field identifies the mode in which the succeeding data is to be interpreted. The value of the magic cookie is the 4 octet dotted decimal 99.130.83.99 (or hexadecimal number 63.82.53.63) in network byte order.

All of the vendor extensions defined in RFC 1497 are also DHCP options.

Option codes 128 to 254 (decimal) are reserved for site-specific options.

RFC 1497 Vendor Extensions

This section lists the vendor extensions as defined in RFC 1497.

Pad Option

Option code: 0
Length: 1 octet

Causes the subsequent fields to align on word boundaries.

End Option

Option code: 255
Length: 1 octet

End of valid information in the vendor field. Subsequent octets should be filled with Pad options.

Subnet Mask Option

Option code: 1
Length: 4 octets

Client's subnet mask, as per RFC 950. If both the Subnet Mask and the Router option are specified in a DHCP reply, the Subnet Mask option must be first.

Time Offset Option

Option code: 2
Length: 4 octets

Offset of the client's subnet in seconds from Coordinated Universal Time (UTC). The offset is expressed as a two's complement 32-bit integer. A positive offset indicates a location east of the zero meridian and a negative offset indicates a location west of the zero meridian.

Router Option

Option code: 3
Length: 4 octet minimum; multiples of 4

List of IP addresses for routers on the client's subnet. Routers should be listed in order of preference.

Time Server Option

Option code: 4
Length: 4 octet minimum; multiples of 4

List of RFC 868 compliant time servers available to the client. Servers should be listed in order of preference.

Name Server Option

Option code: 5
Length: 4 octet minimum; multiples of 4

List of IEN 116 name servers available to the client. Servers should be listed in order of preference.

Domain Name Server Option

Option code: 6
Length: 4 octet minimum; multiples of 4

List of Domain Name System (STD 13, RFC 1035) name servers available to the client. Servers should be listed in order of preference.

Log Server Option

Option code: 7
Length: 4 octet minimum; multiples of 4

The Log Server option specifies a list of MIT-LCS UDP log servers available to the client. Servers should be listed in order of preference.

Cookie Server Option

Option code: 8
Length: 4 octet minimum; multiples of 4

List of RFC 865-compliant cookie servers available to the client. Servers should be listed in order of preference.

LPR Server Option

Option code: 9
Length: 4 octet minimum; multiples of 4

List of RFC 1179-compliant line printer servers available to the client. Servers should be listed in order of preference.

Impress Server Option

Option code: 10
Length: 4 octet minimum; multiples of 4

List of Imagen Impress servers available to the client. Servers should be listed in order of preference.

Resource Location Server Option

Option code: 11
Length: 4 octet minimum; multiples of 4

List of RFC 887-compliant resource location servers available to the client. Servers should be listed in order of preference.

Host Name Option

Option code: 12
Length: 1 octet minimum; multiples of 4

Name of the client. The name may or may not be qualified with the local domain name. See RFC 1035 for the character set restrictions.

Boot File Size Option

Option code: 13
Length: 2 octets

Length in 512-octet blocks of the default boot image for the client.

Merit Dump File Option

Option code: 14
Length: 1 octet minimum

Path name of a file to which the client's core image should be placed in the event the client crashes. The path is formatted as a character string consisting of characters from the NVT ASCII character set.

Domain Name Option

Option code: 15
Length: 1 octet minimum

Domain name that the client should use when resolving host names through the Domain Name System.

Swap Server Option

Option code: 16
Length: 4 octets

IP address of the client's swap server.

Root Path Option

Option code: 17
Length: 1 octet minimum

Path name that contains the client's root disk. The path is formatted as a character string consisting of characters from the NVT ASCII character set.

Extensions Path Option

Option code: 18
Length: 1 octet minimum

Uses a string to specify a file, retrievable through TFTP. The file contains information that can be interpreted in the same way as the 64-octet vendor-extension field within the BOOTP response, with the following exceptions: the length of the file is unconstrained, and all references to instances of this option in the file are ignored.

IP Layer Parameters Per Host

This section describes the options that affect the operation of the IP layer on a per-host basis.

IP Forwarding Enable/Disable Option

Option code: 19
Length: 1 octet
Values: 0=disable; 1=enable

Specifies whether the client should configure its IP layer for packet forwarding.

Non-Local Source Routing Enable/Disable Option

Option code: 20
Length: 1 octet
Values: 0=disable; 1=enable

Specifies whether the client should configure its IP layer to allow forwarding of datagrams with non-local source routes.

Policy Filter Option

Option code: 21
Length: 8 octet minimum; multiples of 8

Policy filters for non-local source routing. The filters consist of a list of IP addresses and masks that 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.

Maximum Datagram Reassembly Size Option

Option code: 22
Length: 2 octets
Value: 576 minimum

Maximum size datagram that the client should be prepared to reassemble.

Default IP Time-to-Live Option

Option code: 23
Length: 1octet
Values: 1 to 255

Default TTL that the client should use on outgoing datagrams.

Path MTU Aging Timeout Option

Option code: 24
Length: 4 octets

Timeout (in seconds) to use when aging Path MTU values discovered by the mechanism defined in RFC 1191.

Path MTU Plateau Table Option

Option code: 25
Length: 2 octets minimum; multiples of 2
Value: 68 minimum

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.

IP Layer Parameters per Interface

This section describes the options that affect the operation of the IP layer on a per-interface basis. It is expected that a client can issue multiple requests, one per interface, in order to configure interfaces with their specific parameters.

Interface MTU Option

Option code: 26
Length: 2 octets
Value: 68 minimum

Maximum time to live to use on this interface.

All Subnets Are Local Option

Option code: 27
Length: 1 octet
Values: 1=all subnets share same MTU; 0=some directly-connected subnets may have smaller MTUs

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.

Broadcast Address Option

Option code: 28
Length: 4 octets

Broadcast address in use on the client's subnet.

Perform Mask Discovery Option

Option code: 29
Length: 1 octet
Values: 0=disable; 1=enable

Specifies whether or not the client should perform subnet mask discovery using ICMP.

Mask Supplier Option

Option code: 30
Length: 1 octet
Values: 0=do not respond; 1=respond

Specifies whether or not the client should respond to subnet mask requests using ICMP.

Perform Router Discovery Option

Option code: 31
Length: 1 octet
Values: 0=disable; 1=enable

Specifies whether or not the client should solicit routers using the Router Discovery mechanism defined in RFC 1256.

Router Solicitation Address Option

Option code: 32
Length: 4 octets

Address to which the client should transmit router solicitation requests.

Static Route Option

Option code: 33
Length: 8 octet minimum; multiples of 8

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.

Link Layer Parameters for Each Interface

This section describes the options that affect the operation of the data link layer on a per-interface basis.

Trailer Encapsulation Option

Option code: 34
Length: 1 octet
Values: 0=do not use; 1=use

Specifies whether or not the client should negotiate the use of trailers (RFC 893) when using the ARP protocol.

ARP Cache Timeout Option

Option code: 35
Length: 4 octets

Timeout in seconds for ARP cache entries.

Ethernet Encapsulation Option

Option code: 36
Length: 1 octet
Value: 0=use RFC 894 encapsulation; 1=use RFC 1042 encapsulation

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.

TCP Parameters

This section describes the options that affect the operation of the TCP layer on a per-interface basis.

TCP Default TTL Option

Option code: 37
Length: 1 octet
Value: minimum 1

Default TTL that the client should use when sending TCP segments.

TCP Keepalive Interval Option

Option code: 38
Length: 4 octets
Value: 32-bit unsigned; 0=do not generate keepalive messages unless specifically requested

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. The code for this option is 38, and its length is 4.

TCP Keepalive Garbage Option

Option code: 39
Length: 1 octet
Values: 0=do not send; 1=send

Specifies the whether or not the client should send TCP keep-alive messages with an octet of garbage for compatibility with older implementations.

Application and Service Parameters

This section details some miscellaneous options used to configure miscellaneous applications and services.

Network Information Service Domain Option

Option code: 40
Length: 1 octet minimum

Name of the client's NIS domain. The domain is formatted as a character string consisting of characters from the NVT ASCII character set.

Network Information Servers Option

Option code: 41
Length: 4 octet minimum; multiples of 4

List of IP addresses indicating NIS servers available to the client. Servers should be listed in order of preference.

Network Time Protocol Servers Option

Option code: 42
Length: 4 octet minimum; multiples of 4

List of IP addresses indicating NTP servers available to the client. Servers should be listed in order of preference.

Vendor-Specific Information

Option code: 43
Length: 1 octet minimum

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. The definition of this information is vendor specific. The vendor is indicated in the vendor-class-identifier option. Servers not equipped to interpret the vendor-specific information sent by a client must ignore it (although it may be reported). Clients that do not receive desired vendor-specific information should make an attempt to operate without it, although they may do so (and announce they are doing so) in a degraded mode.

If a vendor potentially encodes more than one item of information in this option, then the vendor should encode the option using encapsulated vendor-specific options as described here.

The encapsulated vendor-specific options field should be encoded as a sequence of code/length/value fields of identical syntax to the DHCP options field with the following exceptions:

• There should not be a magic cookie field in the encapsulated vendor-specific extensions field.
  
  
• Codes other than 0 or 255 may be redefined by the vendor within the encapsulated vendor-specific extensions field, but should conform to the tag-length-value syntax defined in section 2.
  
  
• Code 255 (END), if present, signifies the end of the encapsulated vendor extensions, not the end of the vendor extensions field. If no code 255 is present, then the end of the enclosing vendor-specific information field is taken as the end of the encapsulated vendor-specific extensions field.
  
  

NetBIOS over TCP/IP Name Server Option

Option code: 44
Length: 4 octet minimum; multiples of 4

List of RFC 1001/1002 NBNS name servers listed in order of preference.

NetBIOS over TCP/IP Datagram Distribution Server Option

Option code: 45
Length: 4 octet minimum; multiples of 4

List of RFC 1001/1002 NBDD servers listed in order of preference.

NetBIOS over TCP/IP Node Type Option

Option code: 46
Length: 1 octet

Allows NetBIOS over TCP/IP client, which are configured as described in RFC 1001/1002. The value is a single octet that identifies the client type, as shown in Table B-1.

NetBIOS over TCP/IP Scope Option

Option code: 47
Length: 1 octet minimum

NetBIOS over TCP/IP scope parameter for the client as specified in RFC 1001/1002.

X Window System Font Server Option

Option code: 48
Length: 4 octet minimum; multiples of 4

List of X Window System Font servers available to the client. Servers should be listed in order of preference.

X Window System Display Manager Option

Option code: 49
Length: 4 octet minimum; multiples of 4

List of IP addresses 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.

Network Information Service+ Domain Option

Option code: 64
Length: 1 octet minimum

Name of the client's NIS+ domain. The domain is formatted as a character string consisting of characters from the NVT ASCII character set.

Network Information Service+ Servers Option

Option code: 65
Length: 4 octet minimum; multiples of 4

List of IP addresses indicating NIS+ servers available to the client. Servers should be listed in order of preference.

Mobile IP Home Agent Option

Option code: 68
Length: 0 octets minimum; multiples of 4; expected, 4 octets (single home agent's address)
Value: 32-bit address; 0=no home agents available

List of IP addresses indicating mobile IP home agents available to the client. Agents should be listed in order of preference.

Simple Mail Transport Protocol (SMTP) Server Option

Option code: 69
Length: 4 octet minimum; multiples of 4

List of SMTP servers available to the client. Servers should be listed in order of preference.

Post Office Protocol (POP3) Server Option

Option code: 70
Length: 4 octet minimum; multiples of 4

List of POP3 servers available to the client. Servers should be listed in order of preference.

Network News Transport Protocol (NNTP) Server Option

Option code: 71
Length: 4 octet minimum; multiples of 4

List of NNTP servers available to the client. Servers should be listed in order of preference.

Default World Wide Web (WWW) Server Option

Option code: 72
Length: 4 octet minimum; multiples of 4

List of World Wide Web (WWW) servers available to the client. Servers should be listed in order of preference.

Default Finger Server Option

Option code: 73
Length: 4 octet minimum; multiples of 4

List of Finger servers available to the client. Servers should be listed in order of preference.

Default Internet Relay Chat Server Option

Option code: 74
Length: 4 octet minimum; multiples of 4

List of IRC servers available to the client. Servers should be listed in order of preference.

StreetTalk Server Option

Option code: 75
Length: 4 octet minimum; multiples of 4

List of StreetTalk servers available to the client. Servers should be listed in order of preference.

StreetTalk Directory Assistance (STDA) Server Option

Option code: 76
Length: 4 octet minimum; multiples of 4

List of STDA servers available to the client. Servers should be listed in order of preference.

DHCP Extensions

This section describes the options that are specific to DHCP.

Requested IP Address Option

Option code: 50
Length: 4 octets

Used in a client request (DHCPDISCOVER) to allow the client to request that a particular IP address be assigned.

IP Address Lease Time Option

Option code: 51
Length: 4 octets
Value: seconds, as 32-bit unsigned integer

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 Overload Option

Option code: 52
Length: 1 octet
Values: (see Table B-2)

Indicates 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.

TFTP Server Name Option

Option code: 66
Length: 1 octet minimum

Identifies a TFTP server when the sname field in the DHCP header has been used for DHCP options.

Bootfile Name Option

Option code: 67
Length: 1 octet minimum

Identifies a bootfile when the file field is the DHCP header that has been used for DHCP options.

DHCP Message Type Option

Option code: 53
Length: 1 octet
Values: (see Table B-3)

Used to convey the type of DHCP message. The default is 1 (DHCPDISCOVER).

Server Identifier Option

Option code: 54
Length: 4 octets

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.

Parameter Request List Option

Option code: 55
Length: 1 octet minimum

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 can 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.

Message Option

Option code: 56
Length: 1 octet minimum

Used by a DHCP server to provide an error message to a DHCP client in a DHCPNAK message in the event of a failure. A client may use this option in a DHCPDECLINE message to indicate why the client declined the offered parameters. The message consists of n octets of NVT ASCII text, which the client may display on an available output device.

Maximum DHCP Message Size Option

Option code: 57
Length: 2 octets
Value: 576 minimum

Maximum length DHCP message that it 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.

Renewal (T1) Time Value Option

Option code: 58
Length: 4 octets
Value: seconds, as 32-bit unsigned integer

Time interval from address assignment until the client transitions to the RENEWING state.

Rebinding (T2) Time Value Option

Option code: 59
Length: 4 octets
Value: seconds, as 32-bit unsigned integer

Time interval from address assignment until the client transitions to the REBINDING state.

Vendor Class Identifier Option

Option code: 60
Length: 1 octet minimum
Value: seconds, as 32-bit unsigned integer

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 to return the vendor-specific information to the client.

Client-Identifier Option

Option code: 61
Length: 2 octet minimum
Value: seconds, as 32-bit unsigned integer

Used by DHCP clients to specify their unique identifier. DHCP servers use this value to index their database of address bindings. This value is expected to be unique for all clients in an administrative domain.

Identifiers should be treated as opaque objects by DHCP servers. The client identifier may consist of type-value pairs similar to the htype/chaddr fields. For instance, it may consist of a hardware type and hardware address. In this case, the type field should be one of the ARP hardware types defined in STD2. A hardware type of 0 (zero) should be used when the value field contains an identifier other than a hardware address (for example, a fully qualified domain name).

For correct identification of clients, each client's client-identifier must be unique among the client-identifiers used on the subnet to which the client is attached. Vendors and system administrators are responsible for choosing client-identifiers that meet this requirement for uniqueness.

Microsoft Client Options

Table B-4 lists the standard Microsoft client options.

Options Tables

The following tables display the DHCP options in various ways. They show the options sorted numerically, by Network Registrar name, and by category.

DHCP options have a prescribed format and allowed values for their option parameters. Each DCHP option and parameter type ("Validation") is listed in Table B-5. The parameter formats and allowed values come from the DHCP and Internet RFCs. All the DHCP options are listed, but some are not controlled by the user, some are only offered by the GUI, and some only by the CLI.

Options by Number

Table B-5 shows the DHCP options sorted by option number, and includes the validation type. (See Table B-8 for details on the option validation types found in the "Validation" column.)

.

Options by Network Registrar Name

Table B-6 lists the DHCP options by Network Registrar name. (For each option's validation type, cross-reference it by number to Table B-5 and check the "Validation" column.)

Options by Category

Table B-7 list each option by category. (For each option's validation type, cross-reference it by number to Table B-5 and check the "Validation" column.)

Option Validation Types

Table B-8 defines the DHCP option validation types.