Table of Contents

DHCP Options

This appendix contains DHCP options and BOOTP vendor extensions from the IETF Network Working Group, Request For Comments 2132 publication. It was written by S. Alexander, Silicon Graphics, Inc. and R. Droms, Bucknell University, March 1997.

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

The Dynamic Host Configuration Protocol (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.

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 fully qualified Domain Names (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

The Pad option can be used to cause the subsequent fields to align on word boundaries. The code for the Pad option is 0, and its length is 1 octet.

End Option

The End option marks the end of valid information in the vendor field. Subsequent octets should be filled with Pad options. The code for the End option is 255, and its length is
1 octet.

Subnet Mask Option

The Subnet Mask option specifies the 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. The code for the Subnet Mask option is 1, and its length is 4 octets.

Time Offset Option

The Time Offset field specifies the 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. The code for the Time Offset option is 2, and its length is 4 octets.

Router Option

The Router option specifies a list of IP addresses for routers on the client's subnet. Routers should be listed in order of preference. The code for the Router option is 3. The minimum length for the Router option is 4 octets, and the length must always be a multiple of 4.

Time Server Option

The Time Server option specifies a list of RFC 868 compliant time servers available to the client. Servers should be listed in order of preference. The code for the Time Server option is 4. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.

Name Server Option

The Name Server option specifies a list of IEN 116 name servers available to the client. Servers should be listed in order of preference. The code for the Name Server option is 5. The minimum length for this option is 4 octets, and the length must always be a
multiple of 4.

Domain Name Server Option

The Domain Name Server 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. The code for the Domain Name Server option is 6. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.

Log Server Option

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. The code for the Log Server option is 7. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.

Cookie Server Option

The Cookie Server option specifies a list of RFC 865-compliant cookie servers available to the client. Servers should be listed in order of preference. The code for the Cookie Server option is 8. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.

LPR Server Option

The LPR Server option specifies a list of RFC 1179-compliant line printer servers available to the client. Servers should be listed in order of preference. The code for the LPR Server option is 9. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.

Impress Server Option

The Impress Server option specifies a list of Imagen Impress servers available to the client. Servers should be listed in order of preference. The code for the Impress Server option is 10. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.

Resource Location Server Option

The Resource Location Server option specifies a list of RFC 887-compliant resource location servers available to the client. Servers should be listed in order of preference. The code for this option is 11. The minimum length for this option is 4 octets, and the length must be a multiple of 4.

Host Name Option

The Host Name option specifies the name of the client. The name may or may not be qualified with the local domain name. See RFS 1035 for the character set restrictions. The code for this option is 12, and its minimum length is 1.

Boot File Size Option

The Boot File Size option specifies the length in 512-octet blocks of the default boot image for the client. The file length is specified as an unsigned 16-bit integer. The code for this option is 13, and its length is 2.

Merit Dump File

The Merit Dump File option specifies the 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. The code for this option is 14. Its minimum length is 1.

Domain Name Option

The Domain Name option specifies the domain name that the client should use when resolving host names through the Domain Name System. The code for this option is 15. Its minimum length is 1.

Swap Server Option

The Swap Server option specifies the IP address of the client's swap server. The code for this option is 16, and its length is 4.

Root Path Option

The Root Path option specifies the 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. The code for this option is 17. Its minimum length is 1.

Extensions Path Option

The Extensions Path option 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. All references to Tag 18 (that is, instances of the BOOTP Extensions Path field) within the file are ignored. The code for this option is 18. Its minimum length is 1.

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

The IP Forwarding Enable/Disable 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. The code for this option is 19, and its length is 1.

Non-Local Source Routing Enable/Disable Option

The Non-Local Source Routing Enable/Disable 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. The code for this option is 20, and its length is 1.

Policy Filter Option

The Policy Filter option specifies policy filters for nonlocal 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. The code for this option is 21. The minimum length of this option is 8, and the length must be a multiple of 8.

Maximum Datagram Reassembly Size Option

The Maximum Datagram Reassembly Size option specifies the maximum size datagram that the client should be prepared to reassemble. The size is specified as a 16-bit unsigned integer. The minimum legal value is 576. The code for this option is 22, and its length is 2.

Default IP Time-to-Live Option

The Default IP Time-To-Live option specifies the default time-to-live that the client should use on outgoing datagrams. The TTL is specified as an octet with a value between 1 and 255. The code for this option is 23, and its length is 1.

Path MTU Aging Timeout Option

The Path MTU Aging Timeout option specifies the timeout (in seconds) to use when aging Path MTU values discovered by the mechanism defined in RFC 1191. The timeout is specified as a 32-bit unsigned integer. The code for this option is 24, and its length is 4.

Path MTU Plateau Table Option

The Path MTU Plateau Table 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. The code for this option is 25. Its minimum length is 2, and the length must be a multiple of 2.

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

The Interface MTU option specifies the MTU to use on this interface. The MTU is specified as a 16-bit unsigned integer. The minimum legal value for the MTU is 68. The code for this option is 26, and its length is 2.

All Subnets are Local Option

The All Subnets are Local 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. The code for this option is 27, and its length is 1.

Broadcast Address Option

The Broadcast Address option specifies the broadcast address in use on the client's subnet. The code for this option is 28, and its length is 4.

Perform Mask Discovery Option

The Perform Mask Discovery 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. The code for this option is 29, and its length is 1.

Mask Supplier Option

The Mask Supplier 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. The code for this option is 30, and its length is 1.

Perform Router Discovery Option

The Perform Router Discovery 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. The code for this option is 31, and its length is 1.

Router Solicitation Address Option

The Router Solicitation Address option specifies the address to which the client should transmit router solicitation requests. The code for this option is 32, and its length is 4.

Static Route Option

The Static Route 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. The code for this option is 33. The minimum length of this option is 8, and the length must be a multiple of 8.

Link Layer Parameters Per Interface

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

Trailer Encapsulation Option

The Trailer Encapsulation 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. The code for this option is 34, and its length is 1.

ARP Cache Timeout Option

The ARP Cache Timeout option specifies the timeout in seconds for ARP cache entries. The time is specified as a 32-bit unsigned integer. The code for this option is 35, and its length is 4.

Ethernet Encapsulation Option

The Ethernet Encapsulation 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. The code for this option is 36, and its length is 1.

TCP Parameters

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

TCP Default TTL Option

The TCP Default TTL option specifies the default TTL that the client should use when sending TCP segments. The value is represented as an 8-bit unsigned integer. The minimum value is 1. The code for this option is 37, and its length is 1.

TCP Keepalive Interval Option

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

TCP Keepalive Garbage Option

The TCP Keepalive Garbage option specifies the whether or not the client should send TCP keep-alive 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. The code for this option is 39, and its length is 1.

Application and Service Parameters

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

Network Information Service Domain Option

The Network Information Service Domain 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. The code for this option is 40. Its minimum length is 1.

Network Information Servers Option

The Network Information Servers option specifies a list of IP addresses indicating NIS servers available to the client. Servers should be listed in order of preference. The code for this option is 41. Its minimum length is 4, and the length must be a multiple of 4.

Network Time Protocol Servers Option

The Network Time Protocol Servers option specifies a list of IP addresses indicating NTP servers available to the client. Servers should be listed in order of preference. The code for this option is 42. Its minimum length is 4, and the length must be a multiple of 4.

Vendor-Specific Information

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:

The code for this option is 43 and its minimum length is 1.

NetBIOS over TCP/IP Name Server Option

The NetBIOS Name Server (NBNS) option specifies a list of RFC 1001/1002 NBNS name servers listed in order of preference. The code for this option is 44. The minimum length of the option is 4 octets, and the length must always be a multiple of 4.

NetBIOS over TCP/IP Datagram Distribution Server Option

The NetBIOS Datagram Distribution Server (NBDD) option specifies a list of RFC 1001/1002 NBDD servers listed in order of preference. The code for this option is 45. The minimum length of the option is 4 octets, and the length must always be a multiple of 4.

NetBIOS over TCP/IP Node Type Option

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

In Table B-1, the notation 0x indicates a number in base-16 (hexadecimal).

The code for this option is 46. The length of this option is always 1.

NetBIOS over TCP/IP Scope Option

The NetBIOS Scope option specifies the NetBIOS over TCP/IP scope parameter for the client as specified in RFC 1001/1002. The code for this option is 47. The minimum length of this option is 1.

X Window System Font Server Option

The X Window System Font Server option specifies a list of X Window System Font servers available to the client. Servers should be listed in order of preference. The code for this option is 48. The minimum length of this option is 4 octets, and the length must be a multiple of 4.

X Window System Display Manager Option

The X Window System Display Manager option specifies a 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. The code for this option is 49. The minimum length of this option is 4, and the length must be a multiple of 4.

Network Information Service+ Domain Option

The Network Information Service+ Domain 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. The code for this option is 64. Its minimum length is 1.

Network Information Service+ Servers Option

The Network Information Service+ Servers option specifies a list of IP addresses indicating NIS+ servers available to the client. Servers should be listed in order of preference. The code for this option is 65. Its minimum length is 4, and the length must be a multiple of 4.

Mobile IP Home Agent Option

The Mobile IP Home Agent option specifies a list of IP addresses indicating mobile IP home agents available to the client. Agents should be listed in order of preference. The code for this option is 68. Its minimum length is 0 (indicating no home agents available) and the length must be a multiple of 4. It is expected that the usual length will be 4 octets, containing a single home agent's address.

Simple Mail Transport Protocol (SMTP) Server Option

The SMTP Server option specifies a list of SMTP servers available to the client. Servers should be listed in order of preference. The code for the SMTP server option is 69. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.

Post Office Protocol (POP3) Server Option

The POP3 Server option specifies a list of POP3 servers available to the client. Servers should be listed in order of preference. The code for the POP3 Server option is 70. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.

Network News Transport Protocol (NNTP) Server Option

The NNTP Server option specifies a list of NNTP servers available to the client. Servers should be listed in order of preference. The code for the NNTP Server option is 71. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.

Default World Wide Web (WWW) Server Option

The Default WWW server option specifies a list of World Wide Web (WWW) servers available to the client. Servers should be listed in order of preference. The code for the Default WWW Server option is 72. The minimum length for this option is 4 octets and the length must always be a multiple of 4.

Default Finger Server Option

The Default Finger Server option specifies a list of Finger servers available to the client. Servers should be listed in order of preference. The code for the Default Finger Server option is 73. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.

Default Internet Relay Chat Server Option

The Default IRC Server option specifies a list of IRC servers available to the client. Servers should be listed in order of preference. The code for the Default IRC server option is 74. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.

StreetTalk Server Option

The Streettalk Server option specifies a list of StreetTalk servers available to the client. Servers should be listed in order of preference. The code for the StreetTalk Server option is 75. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.

StreetTalk Directory Assistance (STDA) Server Option

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. The code for the StreetTalk Directory Assistance Server option is 76. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.

DHCP Extensions

This section describes the options that are specific to DHCP.

Requested IP Address Option

The Requested IP Address option is used in a client request (DHCPDISCOVER) to allow the client to request that a particular IP address be assigned. The code for this option is 50, and its length is 4.

IP Address Lease Time Option

The IP Address Lease Time 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. The time is in units of seconds, and is specified as a 32-bit unsigned integer. The code for this option is 51 and its length is 4.

Option Overload Option

The Option Overload 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. The code for this option is 52, and its length is 1. Legal values for this option are shown in Table B-2:

TFTP Server Name Option

The TFTP Server Name option is used to identify a TFTP server when the sname field in the DHCP header has been used for DHCP options. The code for this option is 66, and its minimum length is 1.

Bootfile Name Option

The Bootfile Name option is used to identify a bootfile when the file field is the DHCP header that has been used for DHCP options. The code for this option is 67, and its minimum length is 1.

DHCP Message Type Option

The DHCP Message Type option is used to convey the type of DHCP message. The code for this option is 53 and its length is 1. Legal values for this option are shown in Table B-3.

Server Identifier Option

The Server Identifier 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.

The code for this option is 54, and its length is 4.

Parameter Request List Option

The Parameter Request List 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.

The code for this option is 55. Its minimum length is 1.

Message Option

The Message option is 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.

The code for this option is 56 and its minimum length is 1.

Maximum DHCP Message Size Option

The Maximum DHCP Message Size option specifies the 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.

The code for this option is 57, and its length is 2. The minimum legal value is 576 octets.

Renewal (T1) Time Value Option

The Renewal Time Value option specifies the time interval from address assignment until the client transitions to the RENEWING state. The value is in units of seconds, and is specified as a 32-bit unsigned integer.

The code for this option is 58, and its length is 4.

Rebinding (T2) Time Value Option

The Rebinding Time Value option specifies the time interval from address assignment until the client transitions to the REBINDING state. The value is in units of seconds, and is specified as a 32-bit unsigned integer.

The code for this option is 59, and its length is 4.

Vendor Class Identifier Option

The Vendor Class Identifier 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 to return the vendor-specific information to the client. The code for this option is 60 and its minimum length is 1.

Client-Identifier Option

The Client-Identifier option is 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. The code for this option is 61, and its minimum length is 2.

Microsoft Client Options

Table B-4 lists the standard Microsoft client options.

Option Tables

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

Table B-5 shows the DHCP options sorted by option number.

.

Table B-6 displays the DHCP options by Network Registrar name.

Table B-7 displays the DHCP options by category.