DHCP Tools

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Network Configuration Technologies > DHCP Technical Reference

DHCP Tools and Settings Updated: March 28, 2003

DHCP Tools and Settings

In this section

Top of page

DHCP Tools

The following tools are associated with DHCP.

DHCP Snap-in

The DHCP snap-in allows you to perform a variety of administrative tasks for your DHCP servers:

Category

The DHCP snap-in Microsoft Management Console (MMC) appears as an administrative tool after you install

DHCP from Add/Remove Windows Components in Control Panel. The DHCP snap-in can also be added to

Windows Server 2003 or Windows XP by installing the Windows Server 2003 Administrative Tools Pack. This

allows remote administration of DHCP servers running Windows 2000 Server or Windows Server 2003 from a

Windows XP-based workstation.

Version Compatibility

The Windows Server 2003 DHCP snap-in is compatible with DHCP servers running Windows Server 2003 and

Windows 2000.

Netsh

• DHCP Tools

• DHCP Options

• Related information

• Create and manage scopes, including superscopes and multicast scopes.

• Create and manage properties for scopes, such as options, reservations, and exclusion ranges.

• Review active leases for each scope.

Page 1 of 44DHCP Tools and Settings

8/25/2006http://technet2.microsoft.com/WindowsServer/en/library/a70f1bb7-d2d4-49f0-96d6-4b741...

Netsh is a command-line scripting tool that allows you to display or modify the network configuration of a

computer. Netsh also provides a scripting feature that allows you to run a group of commands in batch mode

against a specified computer. Netsh can also save a configuration script in a text file for archival purposes or for

reuse in configuring other servers.

Commands in the netsh dhcp context provide a command-line method to help with the administration of DHCP

servers and provides an equivalent alternative to console-based management. All commands in netshdhcp

context can also be executed against a specified remote server.

Category

Netsh is a command-line tool.

Version compatibility

You can use netsh commands with Windows 2000 Server and Windows Server 2003.

Network Monitor

You can use the Network Monitor tool or a commercial packet analyzer (also known as a network sniffer), to

capture and view packets such as DHCP messages.

In Windows 2000 Server and Windows Server 2003, Network Monitor is installed as an optional management

and monitoring component by using Add or Remove Programs in Control Panel. After it is installed, you can run

Network Monitor from the Administrative Tools folder.

Category

Network Monitor is available in Microsoft Systems Management Server or with Windows 2000 Server and

Windows Server 2003.

Version compatibility

You can use Network Monitor to capture and view packets in Windows XP, Windows 2000, or Windows

Server 2003.

Top of page

DHCP Options

This section lists the predefined options available for use with the Microsoft Windows Server 2003 DHCP service.

These options are defined according to the updated standards reference for DHCP options in RFC 2132, “DHCP

Options and BOOTP Vendor Extensions.”

Use the DHCP Microsoft Management Console (MMC) snap-in to specifically configure each option value, and

enable the option for assignment and distribution to DHCP clients based on server, scope, class, or client-

specific levels of preference.

In this section:

• Basic options. These options were originally defined in RFC 1497 and relisted in RFC 2123.

• IP host options. These options affect the operation of the IP layer, on a per-host basis.



Note

Basic Options

The following sections list the basic DHCP options originally defined in RFC 1497 and updated in RFC 2132, for

use with DHCP and the Boot Protocol (BOOTP) service. The BOOTP service refers to options as vendor

extensions.

The DHCP service supports configuration and distribution of any options assigned using the DHCP Manager

snap-in. By default, Microsoft DHCP-enabled clients require and provide storage and interpretation for options 1

(Subnet Mask), 3 (Router), 6 (DNS Servers), and 15 (DNS Domain Name).

Pad Option

A single octet of zero (“00”) used for padding. This option differs from most DHCP options in that it does not use

a length or value field. This option used to ensure that subsequent DHCP options are aligned on word

boundaries the same as they appear in the DHCP packet. This option does not require configuration.

Code

0

Length

Not used.

Value

Not used.

Structure of Pad Option

• IP interface options. These options affect operation of the IP layer on a per-interface basis.

• Link layer options. These options affect operation of the data-link layer on a per-interface basis.

• TCP options. These options affect operation of the TCP layer on a per-interface basis.

• Application layer options. These options affect the application layer operations on a per-interface basis.

• NetBIOS over TCP/IP options. These options are used to support NetBIOS over TCP/IP.

• Vendor-specific options. These options are specified for vendor class use..

• User class options. These options are specified for user class use.

• DHCP extensions. These options are used to implement default protocol interaction and system

behavior between servers and clients.

• Administrator-defined options. These options are not preconfigured in Windows Server 2003, but can

be defined by an administrator.

• Microsoft options. These options are only available for use with supported Microsoft DHCP clients, such

as computers running Windows Server 2003 or Windows XP.

• For all DHCP options that use a list of IP addresses as the value data, the IP addresses are always used in

order of preference by the DHCP client. For example, the first address in the list is used first.

Code



End Option

A single octet of decimal 255 (“FF”) used to indicate the end of a DHCP options area in DHCP messages. This

option differs from most DHCP options because it does not use a length or value field. Typically, it is used at the

end of the options field to indicate that there is no more option data in a DHCP message. It can also be used

within the message, in connection with vendor-specific information (option 43), to indicate the end of an

encapsulated vendor-specific options subfield. This option does not require configuration.

Code

255

Length

Not used.

Value

Not used.

Structure of End Option

Subnet Mask

This option specifies the subnet mask of the client subnet, as described in RFC 950. The value for this option is

taken from the Subnet Mask field, as defined in the DHCP ScopeProperties dialog box in DHCP Manager. A

DHCP client running Windows XP or Windows Server 2003 requests this option.

Code

1

Length

Fixed, 4 octets.

Value

Unsigned 32-bit (4 octet) integer representing the subnet mask for an IP address provided in a DHCP message.

Structure of Subnet Mask

Time Offset

0

Code

255

Code Length Subnet Mask

1 4 subnet mask in binary format



This option specifies an offset value (in seconds) from the Coordinated Universal Time (UTC) that applies to the

client subnet. This value is configurable as a signed 32-bit integer. Positive offset values indicate a subnet

location east of the zero meridian. Negative offset values indicate a subnet location west of the zero meridian.

Code

2

Length

Fixed, 4 octets.

Value

Signed 32-bit integer used for offset of UTC.

Structure of Time Offset

Router

This option specifies a list of IP addresses for routers on the client subnet. When more than one router is

assigned, the client interprets and uses the addresses in the specified order. This option is normally used to

assign a default gateway to DHCP clients on a subnet. A DHCP client running Windows XP or Windows

Server 2003 requests this option.

Code

3

Length

Variable. Minimum length of 4 octets; octet length increases in multiples of 4 for each router address listed.

Value

Unsigned 32-bit integer representing the IP address of each assigned router.

Structure of Router

Time Server

This option specifies a list of IP addresses for time servers, as defined in RFC 868, that are available to the

client. When more than one time server is assigned, the client interprets and uses the addresses in the specified

order.

Code Length Time Offset

2 4 time

Code Length Address 1 Address N

3 n IP address in binary format IP address in binary format



Code

4

Length

Variable. Minimum length of 4 octets; octet length increases in multiples of 4 for each time server address

listed.

Value

Unsigned 32-bit integer representing the IP address of each assigned time server.

Structure of Time Server

IEN Name Server

This option specifies a list of IP addresses for Internet Engineering Note (IEN) name servers available to the

client. When more than one server is assigned, the client interprets and uses the addresses in the specified

order.

Code

5

Length

Variable. Minimum length of 4 octets; octet length increases in multiples of 4 for each IEN name server address

listed.

Value

Unsigned 32-bit integer representing the IP address of each assigned IEN name server.

Structure of IEN Name Server

DNS Servers

This option specifies a list of IP addresses for DNS name servers available to the client. When more than one

server is assigned, the client interprets and uses the addresses in the specified order. A DHCP client running

Windows XP and Windows Server 2003 requests this option.

Code

6







Length

Variable. Minimum length of 4 octets; octet length increases in multiples of 4 for each Domain Name System

(DNS) server address listed.

Value

Unsigned 32-bit integer representing the IP address of each assigned DNS server.

Structure of DNS Servers

Log Server

This option specifies a list of IP addresses for Massachusetts Institute of Technology Lab for Computer Science

(MIT-LCS) User Datagram Protocol (UDP) log servers available to the client. When more than one server is

assigned, the client interprets and uses the addresses in the specified order.

Code

7

Length

Variable. Minimum length of 4 octets; octet length increases in multiples of 4 for each log server address listed.

Value

Unsigned 32-bit integer representing the IP address of each assigned log server.

Structure of Log Server

Cookie Server

This option specifies a list of IP addresses for cookie servers, as defined in RFC 865, available to the client.

When more than one server is assigned, the client interprets and uses the addresses in the specified order.

Code

8

Length

Variable. Minimum length of 4 octets; octet length increases in multiples of 4 for each cookie server address

listed.







Value

Unsigned 32-bit integer representing the IP address of each assigned cookie server.

Structure of Cookie Server

LPR Server

This option specifies a list of IP addresses for line printer (LPR) servers, as defined in RFC 1179, available to the


order.

Code

9

Length

Variable. Minimum length of 4 octets; octet length increases in multiples of 4 for each LPR server address listed.

Value

Unsigned 32-bit integer representing the IP address of each assigned LPR server.

Structure of LPR Server

Impress Server

This option specifies a list of IP addresses for Imagen Impress servers available to the client. When more than

one server is assigned, the client interprets and uses the addresses in the specified order.

Code

10

Length

Variable. Minimum length of 4 octets; octet length increases in multiples of 4 for each Impress server address

listed.

Value

Unsigned 32-bit integer representing the IP address of each assigned Impress server.

Structure of Impress Server







Resource Location Server

This option specifies a list of IP addresses for resource location servers, as defined in RFC 887, available to the


order.

Code

11

Length

Variable. Minimum length of 4 octets; octet length increases in multiples of 4 for each resource location server

address listed.

Value

Unsigned 32-bit integer representing the IP address of each assigned resource location server.

Structure of Resource Location Server

Host Name

This option specifies a host name for the client. In some cases, this name can also be fully qualified by

appending the name value provided here with the DNS domain name, as specified in DHCP option 15. For

Windows clients, this option is not supported for use when configuring the client’s host name, which is set for

computers running Windows XP or Windows Server 2003 on the Computer Name tab in the System

Properties dialog box on the client computer.

Code

12

Length

Length varies depending on data in value. Minimum length is 1 octet. Maximum length is limited to 63

characters, or one octet for each character used in the host name configured for use with this option.

Value

ASCII character text.

Structure of Host Name





Code Length Host Name



Boot File Size

This option specifies the size of the default boot image file for the client.

Code

13

Length

Fixed, 2 octets.

Value

Unsigned 16-bit integer to indicate the number of 512-octet blocks needed to make up the boot file.

Structure of Boot File Size

Merit Dump File

This option specifies the path name of a file to which the client’s core memory image should be dumped in the

event the client terminates abnormally. Data used for a value is in ASCII character text format. The length of

the value field depends on the number of characters used in the path specified. For example, if the path entered

has 20 characters, the value field for this option should also be 20 octets in length.

Code

14

Length

Length varies depending on data in value. Minimum length is 1 octet.

Value


Structure of Merit Dump File

DNS Domain Name

This option specifies the domain name that the DHCP client should use when resolving unqualified domain

names with the Domain Name System (DNS). For DHCP clients running Windows 2000, Windows XP, and

12 n name

Code Length File Size

13 02 16-bit integer

Code Length Dump File Path

14 n path name



Windows Server 2003, the DNS Domain Name option becomes the connection-specific DNS name assigned to

the DHCP-configured interface. The connection-specific DNS name is used to construct fully qualified domain

names (FQDNs) that are registered using DNS dynamic update. The length of the value field depends on the

number of characters used in the DNS domain name specified. For example, if the domain name has 20

characters, the value field for this option is 20 octets in length. A DHCP client running Windows XP or Windows

Server 2003 requests this option.

Code

15

Length


Value


Structure of DNS Domain Name

Swap Server

This option specifies the IP address of the client’s swap server.

Code

16

Length

Length is fixed at 4 octets.

Value

A single IP address for the client’s swap server (unsigned 32-bit integer).

Structure of Swap Server

Root Path

This option specifies the path of the client’s root disk. The path value is formatted as ASCII text. The length of

the value field depends on the number of characters used in the root path specified. For example, if the root

path entered has 20 characters, the value field for this option is 20 octets in length.

Code Length Domain Name

15 n domain name

Code Length Swap Server Address

16 4 IP address in binary format



Code

17

Length


Value


Structure of Root Path

Extensions Path

This option specifies an extension path file that can be retrieved using Trivial File Transfer Protocol (TFTP). The

file contains information to be interpreted as the 64-octet vendor-extension field within a BOOTP response. To

allow more than 64 octets of BOOTP vendor extension information, this option can be enabled. When enabled,

the length of the specified extension path file is not constrained in size and all references in the extensions file

to tag 18 (such as instances of the BOOTP Extensions Path field) are ignored.

Code

18

Length


Value


Structure of Extension Path

IP Host Options

The following sections describe the DHCP options that affect the operation of the IP layer, on a per-host basis.

IP Forwarding Enable/Disable

This option specifies whether the DHCP client should enable or disable forwarding of datagrams at the IP layer.

Code

Code Length Root Disk Path

17 n path name

Code Length Extensions Path

18 n file name



19

Length

Length is fixed at 1 octet.

Value

Accepted values for this option are:

Structure of IP Forwarding Enable/Disable

Non-Local Source Routing Enable/Disable

This option specifies whether the DHCP client enables or disables the forwarding at the IP layer of datagrams

that contain source routing information and were sent by a non-local host.

Code

20

Length

Length is fixed at 1 octet.

Value


Structure of Non-Local Source Routing Enable/Disable

Policy Filter

This option specifies policy filters for non-local source routing on the client. The filters consist of a list of IP

address and mask pairs specifying destination and mask pairs for which incoming datagrams should be source-

route filtered. The client discards any source routed datagram with a next-hop address that does not match one

of the filters. For further information about policy filtering as it applies to this option, see RFC 1122 in the IETF

RFC Database [http://go.microsoft.com/fwlink/?linkid=3952] .

• 1 = Enable IP forwarding

• 0 = Disable IP forwarding

Code Length Value

19 1 0 | 1

• 1 = Enable forwarding of datagrams from non-local sources

• 0 = Disable forwarding of datagrams from non-local sources

Code Length Value

20 1 0 | 1



Code

21

Length

Variable. Minimum length is 8 octets for a single destination and mask pair. Length increases in multiples of 8

octets for each additional pair used.

Value

Two consecutive, unsigned 32-bit integers indicating a paired value, consisting of an IP address followed by a

subnet mask.

Structure of Policy Filter

Maximum Datagram Reassembly Size

This option specifies the maximum size datagram that the client needs to be prepared to reassemble.

Code

22

Length

Fixed, 2 octets.

Value

Unsigned 16-bit integer specifying the maximum datagram size for reassembly. The minimum size for a

datagram is 576 octets.

Structure of Maximum Datagram Reassembly Size

Default IP Time-To-Live

This option specifies the default Time-To-Live (TTL) that the client uses for the datagrams it sends.

Code

23

Length

Code Length Address 1 Subnet Mask 1 Address N Subnet Mask N

21 n IP address in binary

format

subnet mask in

binary format

IP address in binary

format

subnet mask in

binary format

Code Length File Size

22 2 16-bit integer



Fixed, 1 octet.

Value

A number (in seconds) between 1 and 255.

Structure of Default IP Time-To-Live

Path MTU Aging Time-out

This option specifies the time-out for aging Path Maximum Transmission Unit (MTU) values. Values are found by

the Path MTU discovery process, as defined in RFC 1191.

Code

24

Length

Fixed, 4 octets.

Value

A number (in seconds) that specifies a time-out value.

Structure of Path MTU Aging Time-out

Path MTU Plateau Table

This option specifies a table of MTU sizes to use when performing Path MTU discovery, as defined in RFC 1191.

Code

25

Length

Variable. Minimum length is 2 octets and increases in multiples of 2.

Value

A table formatted as a list of 16-bit unsigned integers, ordered from smallest to largest. The minimum tabled

MTU value cannot be smaller than 68.

Structure of Path MTU Plateau Table

Code Length TTL

23 1 TTL value in seconds

Code Length Time-out

24 4 time-out value in seconds



IP Interface Options

The following sections describe the DHCP options that affect operation of the IP layer on a per-interface basis.

Interface MTU

This option specifies the MTU size that can be used on a specified host adapter interface.

Code

26

Length

Fixed, 2 octets.

Value

A 16-bit unsigned integer specifying the interface MTU. The minimum value for the MTU is 68.

Structure of Interface MTU

All Subnets Are Local

This option specifies whether the client assumes that all subnets within the client’s internetwork use the same

MTU size as the local subnet on which the client is connected.

Code

27

Length

Fixed, 1 octet.

Value


Structure of All Subnets Are Local

Code Length MTU Size 1 MTU Size N

25 n MTU size in binary format MTU size in binary format

Code Length MTU

26 2 interface MTU

• 1 = Clients assume all subnets are local and share the same MTU size

• 0 = Clients assume some subnets are not local and that smaller MTU sizes might be in use on remote

subnets



Broadcast Address

This option specifies the broadcast address used on the client’s subnet.

Code

28

Length

Fixed, 4 octets.

Value

Typically, the limited broadcast IP address (255.255.255.255), but can be modified using legal values for

broadcast addresses, as specified in section 3.2.1.3 of RFC 1122.

Structure of Broadcast Address

Perform Mask Discovery

This option specifies whether the client uses Internet Control Message Protocol (ICMP) for subnet mask

discovery.

Code

29

Length

Fixed, 1 octet.

Value


Structure of Perform Mask Discovery

Code Length Value

27 1 0 | 1

Code Length Broadcast Address

28 4 broadcast address in binary format

• 1 = Client performs subnet mask discovery

• 0 = Client does not perform subnet mask discovery

Code Length Value

29 1 0 | 1



Mask Supplier

This option specifies whether the client responds to subnet mask requests using ICMP.

Code

30

Length

Fixed, 1 octet.

Value


Structure of Mask Supplier

Perform Router Discovery

This option specifies whether the client solicits routers using the router discovery method in RFC 1256. A DHCP

client running Windows XP or Windows Server 2003 requests this option.

Code

31

Length

Fixed, 1 octet.

Value


Structure of Perform Router Discovery

Router Solicitation Address

• 1 = Client responds to subnet mask requests

• 0 = Client does not respond to subnet mask requests

Code Length Value

30 1 0 | 1

• 1 = Client performs router discovery

• 0 = Client does not perform router discovery

Code Length Value

31 1 0 | 1



This option specifies the IP address to which the client submits router solicitation requests.

Code

32

Length

Fixed, 4 octets.

Value

Unsigned 32-bit integer representing an IP address.

Structure of Router Solicitation Address

Static Routes

This option specifies a list of classful static routes that the DHCP client automatically adds to its IP routing table.

Multiple routes to the same destination are listed in descending order of priority. The default route of 0.0.0.0 is

an illegal destination for a static route. To configure the default route, use the Router DHCP option to assign a

default gateway. A DHCP client running Windows XP or Windows Server 2003 requests this option.

Code

33

Length

Variable. Minimum length of 8 octets; octet length increases in multiples of 8 for each additional static route

provided with this option.

Value

A list of IP address pairs. Each set of 8 octets provides two consecutive IP addresses pairing the destination (as

a classful network ID) and a router address (the IP address of the router interface on the subnet to which this

scope-specific option is configured) used for each route. The first 4 octets specify the destination classful

network ID, and the second 4 octets specify the router IP address.

Structure of Static Routes

Classless Static Route

This option specifies a list of classful static routes that the client automatically adds to its IP routing table. A

DHCP client running Windows XP or Windows Server 2003 requests this option. The Classless Static Route can

Code Length Address


Code Length Destination 1 Router 1 Destination N Router N

33 n IP address in binary

format


format


format


format



be used to configure split tunneling for remote access virtual private network (VPN) clients running Windows XP

and Windows Server 2003. For more information, see “VPN Technical

Reference [http://technet2.microsoft.com/WindowsServer/en/library/7fd37ece-f0df-4c84-b284-

c9cf9e1401981033.mspx] .”

Code

249

Note

Length

Variable. Minimum length of 5 octets; maximum octet length depends on the nature and number of the

classless static routes. Each route entry includes a destination descriptor and a router and can vary from a

minimum of 5 octets in length to maximum 9 octets in length. For more information about how classless static

routes are constructed using a destination descriptor, see RFC 3442 in the IETF RFC

Database [http://go.microsoft.com/fwlink/?linkid=3952] .

Value

The destination descriptor and the router IP address. The value is encoded. For the encoding scheme, see RFC

3442.

Structure of Classless Static Route

Link Layer Options

The following sections describe the DHCP options that affect operation of the data link layer on a per-interface

basis.

Trailer Encapsulation

This option specifies whether the client negotiates the use of trailers, as described in RFC 893, when using the

Address Resolution Protocol (ARP).

Code

34

Length

Fixed, 1 octet.

Value


• This is the same as option 121, as defined in RFC 3442.

Code Length Destination 1 Router 1 Destination N Router N

249 n Destination descriptor, as

defined in RFC 3442

IP address in

binary format

Destination descriptor, as

defined in RFC 3442

IP address in

binary format



Structure of Trailer Encapsulation

ARP Cache Time-Out

This option specifies the time-out for ARP cache entries.

Code

35

Length

Fixed, 4 octets.

Value

An unsigned 32-bit integer specifying a time-out value, in seconds.

Structure of ARP Cache Time-Out

Ethernet Encapsulation

This option specifies whether the client uses Ethernet II (RFC 894) or IEEE 802.3 (RFC 1042) encapsulation (if

the interface is Ethernet).

Code

36

Length

Fixed, 1 octet.

Value


Structure of Ethernet Encapsulation

• 1 = Client attempts to use trailer

• 0 = Client does not attempt to use trailers

Code Length Value

34 1 0 | 1

Code Length Time

35 4 time-out value in seconds

• 1 = Client uses RFC 1042 encapsulation

• 0 = Client uses RFC 894 encapsulation



TCP Options

The following sections describe the DHCP options that affect operation of the TCP layer on a per-interface basis.

TCP Default TTL

This option specifies the default TTL that the client uses when sending TCP segments.

Code

37

Length

Fixed, 1 octet.

Value

An unsigned 8-bit integer specifying a Time-To-Live (TTL) value in seconds. The minimum TTL value is 1.

Structure of TCP Default TTL

TCP Keep-Alive Interval

This option specifies the interval the client waits before sending a keep-alive message on a TCP connection. A

value of 0 indicates that the client should not send keep-alive messages on connections unless specifically

requested by an application.

Code

38

Length

Fixed, 4 octets.

Value

An unsigned 32-bit integer that specifies a keep-alive interval, in seconds.

Structure of TCP Keep-Alive Interval

Code Length Value

36 1 0 | 1

Code Length TTL

37 1 TTL value in seconds

Code Length Time

38 4 keep-alive interval in seconds



TCP Keep-Alive Garbage

This option specifies whether or not the client sends TCP keep-alive messages with an octet of garbage data for

compatibility with older implementations.

Code

39

Length

Fixed, 1 octet.

Value


Structure of TCP Keep-Alive Garbage

Application Layer Options

The following sections describe the DHCP options that affect the application layer operations on a per-interface

basis. These are miscellaneous options used to configure programs and services.

NIS Domain Name

This option specifies the Network Information Service (NIS) domain name as an ASCII string.

Code

40

Length


Value


Structure of NIS Domain Name

NIS Servers

• 1 = Client sends keep-alive garbage octet

• 0 = Client does not send keep-alive garbage octet

Code Length Value

39 1 0 | 1

Code Length NIS Domain Name

40 n NIS domain name



This option lists the IP addresses in order of preference for Network Information Service (NIS) servers available

to the client.

Code

41

Length

Variable. Minimum length is 4 octets; octet length increases in multiples of 4 for each server address listed.

Value

Unsigned 32-bit integer representing the IP address of each NIS server.

Structure of NIS Servers

NTP Servers

This option lists the IP addresses in the order of preference for Network Time Protocol (NTP) servers available to

the client.

Code

42

Length


Value

Unsigned 32-bit integer representing the IP address of each NTP server.

Structure of NTP Servers

X Window System Font Servers

This option lists the IP addresses in the order of preference for X Window System font servers available to the

client.

Code

48







Length


Value

Unsigned 32-bit integer representing each server IP address.

Structure of X Window System Font Servers

X Window System Display Manager Servers

This option lists the IP addresses in the order of preference for X Window System display manager servers

available to the client.

Code

49

Length


Value

Unsigned 32-bit integer representing each server IP address.

Structure of X Window System Display Manager Servers

NIS+ Domain Name

This option specifies the name of the client’s Network Information Service Plus (NIS+) domain name as an

ASCII string.

Code

64

Length

Length varies depending on the data in its value. Minimum length is 1 octet.

Value








Structure of NIS+ Domain Name

NIS+ Servers

This option lists the IP addresses in the order of preference for Network Information Service Plus (NIS+) servers


Code

65

Length


Value

Unsigned 32-bit integer representing IP address of NIS+ servers.

Structure of NIS+ Servers

Mobile IP Home Agents

This option lists the IP addresses in the order of preference for mobile IP home agents available to the client.

Code

68

Length

Variable. Minimum length is 0 octets. A length of 0 octets signifies that no mobile IP home agents are available.

Octet length increases in multiples of 4 for each mobile IP home agent address listed.

Value

Unsigned 32-bit integer representing IP address of a mobile IP home agent.

Structure of Mobile IP Home Agents

Simple Mail Transfer Protocol (SMTP) Server

Code Length NIS+ Domain Name

64 n NIS+ domain name







This option lists the IP addresses in order of preference for SMTP servers available to the client.

Code

69

Length

Variable. Minimum length is 4 octets; octet length increases in multiples of 4 for each server IP address listed.

Value

Unsigned 32-bit integers representing IP addresses of servers.

Structure of Simple Mail Transfer Protocol (SMTP) Server

Post Office Protocol 3 (POP3) Server

This option lists the IP addresses in order of preference for POP3 servers available to the client.

Code

70

Length


Value


Structure of Post Office Protocol 3 (POP3) Server

Network News Transport Protocol Server

This option lists the IP addresses in order of preference for NNTP servers available to the client.

Code

71

Length








Value


Structure of Network News Transport Protocol Server

Default World Wide Web Server

This option lists the IP addresses in order of preference for default Web servers available to the client.

Code

72

Length


Value


Structure of Default World Wide Web Server

Top of page

Default FFinger Server

This option lists the IP addresses in order of preference for default Finger servers available to the client.

Code

73

Length


Value








Structure of Default Finger Server

Default Internet Relay Chat Server

This option lists the IP addresses in order of preference for default IRC servers available to the client.

Code

74

Length


Value


Structure of Default Internet Relay Chat Server

StreetTalk Server

This option lists the IP addresses in order of preference for StreetTalk servers available to the client.

Code

75

Length


Value


Structure of StreetTalk Server

StreetTalk Directory Assistance Server









This option lists the IP addresses in order of preference for StreetTalk Directory Assistance (STDA) servers


Code

76

Length


Value


Structure of StreetTalk Directory Assistance Server

NetBIOS over TCP/IP Options

The following options are used to support NetBIOS over TCP/IP. All Microsoft-based DHCP clients and DHCP

servers can recognize and support the use of these options.

WINS/NBNS Servers

This option lists the IP addresses for NetBIOS name servers (NBNSes) on the network. In Windows, a Windows

Internet Name Service (WINS) server is an NBNS. A DHCP client running Windows XP or Windows Server 2003

requests this option.

Code

44

Length

Variable. Minimum length is 4 octets; length can be increased by multiples of 4 for each address listed.

Value

Each 4 octets in this field contains an NBNS server IP address, specified as an unsigned 32-bit integer.

Structure of WINS/NBNS Servers

NetBIOS over TCP/IP Datagram Distribution Server

This option lists the IP addresses for NetBIOS datagram distribution (NBDD) servers.







Code

45

Length

Variable. Minimum length is 4 octets; length can be increased only by multiples of 4.

Value

Each 4 octets in this field contains an NBDD server IP address, specified as an unsigned 32-bit integer.

Structure of NetBIOS over TCP/IP Datagram Distribution Server

NetBIOS over TCP/IP Node Type

Configures the client node type for NetBIOS over TCP/IP (NetBT) clients, as described in RFCs 1001and 1002.

On multihomed computers, the node type is assigned for the computer, not to individual network adapters. A

DHCP client running Windows XP or Windows Server 2003 requests this option.

Code

46

Length

Fixed, 1 octet.

Value


Structure of NetBIOS over TCP/IP Node Type

NetBIOS over TCP/IP Scope ID

This option specifies a string representing the NetBIOS over TCP/IP scope ID for the client, as specified in

RFCs 1001 and 1002. On multihomed computers, the node type is assigned for the computer, not to individual

network adapters. A DHCP client running Windows XP or Windows Server 2003 requests this option. The use of



• 1 = b-node

• 2 = p-node

• 4 = m-node

• 8 = h-node

Code Length Value

46 1 1 | 2 | 4 | 8



NetBIOS scope IDs is not recommended.

Code

47

Length

Variable. Minimum length is 1 octet. Octet length is equal to the number of characters used in NetBIOS scope

ID.

Value

This option specifies the NetBIOS over TCP/IP scope identifier used by the client. The format used for these

scope IDs is described in RFCs 1001 and 1002. For character-set restrictions, see the RFCs.

Structure of NetBIOS over TCP/IP Scope ID

Vendor-Specific Options

This section describes reserved DHCP options specified for vendor class use. The vendor-specific options are

specified in RFC 2132. Vendor classes can be used by the DHCP service and DHCP clients running

Windows 2000, Windows XP, and Windows Server 2003. For any other DHCP clients, default classes provided by

the DHCP service can be used to group and classify non-identifying clients at the DHCP server.

The Windows Server 2003 DHCP snap-in provides a default vendor class called DHCP Standard. This class can

be used to group and classify clients that do not identify a vendor class to the DHCP service.

Vendor-Specific Information

This option is used by clients and servers to exchange vendor-specific information. Servers not equipped to

interpret the information ignore it. Clients that expect but do not receive the information attempt to operate

without it.

In some cases, a vendor uses this option to send more than one information item; therefore, this option can

serve as a subfield for encapsulating vendor-specific options. When encapsulating options, DHCP servers

maintain the same syntax (the same sequence of code, length, and value fields) for each encapsulated option

as it would normally appear in the full standard options field. The following exceptions are for the encapsulated,

vendor-specific subfield:

Code

Code Length NetBIOS Scope

47 n scope identifier

• A “Magic cookie” field cannot be used.

• All standard option codes — other than the padding option (0) or the end option (255) — can be

redefined, but should conform to the code, length, value syntax sequence described in RFC 2131.

• If present, the end option (255) signifies the end of the encapsulated vendor options, but not the end of

the encapsulated vendor-specific subfield. If no end option is present, the end for the encapsulated

vendor-specific subfield is taken from its stated length. For more information, see RFC 2132 in the IETF

RFC Database [http://go.microsoft.com/fwlink/?linkid=3952] .



43

Length

Variable. Minimum length is 1 octet.

Value

An object of n octets (where n is equal to the length specified with this option). The definition of values stored

for this option is vendor specific, and values provided here are presumed to be interpreted by vendor-specific

code on DHCP clients and the DHCP server.

Structure of Vendor-Specific Information

When this option uses an encapsulated vendor-specific subfield, the information bytes 1–n have the following

format.

Vendor Class Identifier

Can be used by DHCP clients to identify their vendor type and configuration when communicating with DHCP

servers. Vendors can define their own specific identifier values, like conveying a particular hardware or

operating system configuration.

For Windows Server 2003, all computers that function either as DHCP servers or DHCP clients can use and

support this option. When vendor classes are used, the DHCP server responds to identifying clients by using

option 43, the reserved option for returning vendor-specific information to the client.

DHCP servers that do not automatically interpret this option are expected to ignore it. For earlier, Windows-

based clients and other clients that do not support this option, the DHCP service classifies these clients as part

of the default vendor class — the DHCP Standard option class — predefined for Windows-based DHCP servers.

Code

60

Length

Variable. Minimum is 1 octet; Length varies according to n (the number of octets used as an identifier).

Value

A value of n octets, which can be interpreted by DHCP servers that support vendor-specific classification of

clients.

Structure of Vendor Class Identifier

Code Length Value

43 n vendor-specific information (which can include subfield bytes 1-n)

Code Length Data Item

T1 n d1, d2, ...dn

T2 n d1, d2, dn



User Class Options

This section describes reserved DHCP options specified for user classes. User classes can be used by the DHCP

service and DHCP-enabled client computers running Windows Server 2003. For other DHCP clients, default

classes provided by the DHCP service can be used to group and classify non-identifying clients at the DHCP

server.

User Class Information

A DHCP client can use this option to identify the user class of which it is a member when communicating with

the DHCP server. The information contained in this option is a Network Virtual Terminal (NVT) ASCII text object

that represents the user class ID.

You can use the DHCP snap-in to define specific user classes. When user classes are created, each class sets an

identifying string of information to be used by the DHCP service to classify identifying clients. Also, a default

user class is created for classifying clients that are unable to support a user class ID.

User classes can be helpful for separating client computers that have a shared or common need for similar

software configuration or user preferences. For example, an identifier can specify that a particular DHCP client

be a member of the class “accounting auditors,” who have special service needs, such as a particular database

server.

Computers running Windows Server 2003 support sending or using this option. Legacy DHCP clients do not send

a class ID and cannot recognize DHCP user class IDs. Such client’s are assigned to the Default User Class, a

user class predefined for immediate use in the DHCP snap-in. Other user classes must be manually created.

Code

77

Length

Variable. Minimum is 2 octets.

Value


Structure of User Class Information

DHCP Extensions

The following options are specific to DHCP and are used to implement default protocol interaction and system

behavior between servers and clients. Some of these options are implicitly set when you configure server and

scope properties using the DHCP snap-in.

Code Length Value

60 n vendor class identifier

Code Length User Class Information

77 n c1, c2, c3, c4 ...cn



Requested IP Address

This option can be used by clients when sending a DHCPDiscover message to request a specific IP address from

the DHCP server.

Code

50

Length

Fixed, 4 octets.

Value

Single, unsigned 32-bit integer representing a requested IP address.

Structure of Requested IP Address

IP Address Lease Time

This option is used to negotiate and exchange lease-time information between DHCP clients and servers in two

possible ways. First, the option can be used in a DHCPDiscover or DHCPRequest message sent by a client to

request a lease time for its IP address. Second, the option can be used in a DHCPOffer message sent by a

server to specify a lease time for the client. This option is configured on the Scope Properties dialog box.

Code

51

Length

Fixed, 4 octets.

Value

Single, unsigned 32-bit integer representing a clients lease time (in seconds).

Structure of IP Address Lease Time

Option Overload

Used in messages sent by a DHCP server to indicate that either of the standard message fields in a DHCP packet

for server_host_name (sname) and boot_file_name (file) can be used to hold options (a condition also known

as overloaded).

When this option is used, it extends the options area in each packet by indicating that unused space for one or

Code Length Requested IP Address


Code Length Lease Time

51 4 lease time in seconds



both of these two standard fields should be allocated to the area used to carry DHCP options.

Code

52

Length

Fixed, 1 octet.

Value

Accepted values for this option include:

Structure of Option Overload

TFTP Server Name

This option specifies the host name of the Trivial File Transfer Protocol (TFTP) server when the

server_host_name (sname) field in a DHCP message is overloaded and used for carrying additional DHCP

options.

Code

66

Length

Variable depending on data in value. Minimum length is 1 octet.

Value


Structure of TFTP Server Name

Boot File Name

This option specifies the name of a boot image file on the TFTP server when the boot_file_name (file) field in a

DHCP message is overloaded and used for carrying additional DHCP options.

• 1 = File field is overloaded

• 2 = Sname field is overloaded

• 3 = Both file and sname fields are overloaded

Code Length Value

52 1 1 | 2 | 3

Code Length TFTP Server

66 n TFTP server host name



Code

67

Length

Variable depending on data in value. Minimum length is 1 octet.

Value


Structure of Boot File Name

DHCP Message Type

This option is required for use in all DHCP messages to convey the type of message being sent.

Code

53

Length

Fixed, 1 octet.

Value


Structure of DHCP Message Type

Code Length Boot File Name

67 n boot image file name

• 1 = DHCP Discover message (DHCPDiscover)

• 2 = DHCP Offer message (DHCPOffer)

• 3 = DHCP Request message (DHCPRequest)

• 4 = DHCP Decline message (DHCPDecline)

• 5 = DHCP Acknowledgment message (DHCPAck)

• 6 = DHCP Negative Acknowledgment message (DHCPNak)

• 7 = DHCP Release message (DHCPRelease)

• 8 = DHCP Informational message (DHCPInform)

Code Length Value

53 1 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8



Server Identifier

This option is used in DHCPOffer and DHCPRequest messages, and might appear in DHCP Acknowledgment

messages (DHCPAck, DHCPNak). The server identifier is the IP address of a selected DHCP server.

This option is used in three possible ways. First, servers include this option in DHCPOffer messages so that

clients can distinguish between multiple lease offers. Second, clients include this option in DHCPRequest

messages to select a lease and indicate which offer is accepted from multiple lease offers. Third, clients can use

the contents of this option for unicast transmission of DHCPRequest messages to specific DHCP servers to renew

a current lease.

Code

54

Length

Fixed, 4 octets.

Value

A single, unsigned 32-bit integer representing the IP address that identifies the DHCP server.

Structure of Server Identifier

Parameter Request List

Used by a DHCP client to request specific option values from the DHCP server. Each option is requested and

listed by a single octet value containing a valid or recognized DHCP option Code for the server.

For clients that use this option, the list can be ordered by preference. Although the DHCP server is not required

to return options in the order they are requested it does attempt to insert the requested options in the

requested order.

Code

55

Length

Variable. Minimum of 1 octet; length increases by 1 octet for each option code included in the request list.

Value

List of 8-bit values, each representing an option code between 0 and 255.

Structure of Parameter Request List

Code Length Address




Optional Message

Can be used by both the DHCP server and DHCP clients in the following ways:

The message consists of a variable-length ASCII text string, which the receiving computer can then either log or

display.

Code

56

Length

Minimum of 1 octet. Length depends on the length of the sent message.

Value


Structure of Optional Message

Maximum Message Size

Used by client to specify the maximum length for a DHCP message it can accept. A client can include this option

in DHCPDiscover or DHCPRequest messages; however, it does not include this option in DHCPDecline messages.

Code

57

Length

Fixed, 2 octets.

Value

A 16-bit integer indicating the maximum size, in octets, for a DHCP message. The minimum value for this option

is 576.

Code Length Option Codes

55 n c1, c2...cn

• A server can use this option to provide and embed an error message in a DHCP Negative

Acknowledgment (DHCPNak) message in the event of a failure.

• A client can use this option in a DHCPDecline message to indicate why it declined offered parameters.

Code Length Text

56 n c1, c2...cn



Structure of Maximum Message Size

Renewal Time Value (T1)

The time value of this option is typically 50 percent of the client’s lease time. To adjust the value, change the

length of the client lease in the client scope properties or the per-user class on the DHCP server. You can also

change the value by using the Netsh tool.

Code

58

Length

Fixed, 4 octets.

Value

A 32-bit unsigned integer indicating the number of seconds before the client begins to renew its address lease

with the DHCP server.

Structure of Renewal Time Value (T1)

Rebinding Time Value (T2)

The time value of this option is typically 87.5 percent of the full configured duration (lease time) for a client’s

lease. To adjust the value, change the length of the client lease in the properties for the client’s scope or per-

user class on the DHCP server. You can also change the value by using the Netsh tool.

Code

59

Length

Fixed, 4 octets.

Value

A 32-bit unsigned integer indicating the number of seconds before the client enters the rebinding state (if it has

not renewed its current address lease with the DHCP server).

Structure of Rebinding Time Value (T2)

Code Length Value

57 2 maximum size

Code Length T1 Interval

58 4 renewal interval in seconds

Code Length T2 Interval



Client Unique Identifier

This option is used by clients to specify their unique identifier to the server. This option is most useful for

reserved clients. When a reserved client contacts the server, the DHCP service checks and matches the client

identifier value to a corresponding identifier used to configure an address reservation in the server’s database.

When a matching reservation is found, the DHCP server returns the reserved address and its related parameters

to the correct client.

Each clients identifier must be unique among all other client identifiers used on the DHCP client’s local subnet

and any remote subnets reachable using DHCP relay. Vendors and system administrators are responsible for

choosing client identifiers that meet this requirement for uniqueness. One way to ensure unique values is to use

the client’s media access control (MAC) address as the client identifier value. Media access control addresses are

encoded in the client’s network adapter hardware, and are assigned to hardware manufacturers in such a way

as to ensure that they are unique for each device.

Code

61

Length

Variable length; minimum length is 2 octets.

Value

A series of 2 or more octets treated as a single value by the DHCP server. Servers can interpret and use this

value to uniquely identify clients. The client identifier can consist of type-value pairs similar to the DHCP Header

fields “htype” and “chaddr,” which are defined by the DHCP protocol.

Structure of Client Unique Identifier

Administrator-Defined Options

This section describes DHCP options that are reserved and specified for use by RFC 2132, but are not predefined

for use in the DHCP snap-in. Administrators can add these options to support DHCP clients that recognize these

options.

Proxy Autodiscovery for Internet Explorer 5

This option points to the configuration file that the client uses for automatic configuration of Internet Explorer

5.x and Internet Explorer 6. The configuration file can be a .pac, .jvs, .js, or .ins file created by a system or

Web administrator when deploying Internet Explorer 5.x on an intranet. It might include settings for other

Internet Explorer configurable options, such as which home page to use, or settings for locating and using a

proxy server.

The option is communicated between Internet Explorer client computers and the DHCP server using the

DHCPInform message. DHCPInform is currently supported for DHCP servers running Windows Server 2003 and

DHCP clients running Windows 2000, Windows XP, and Windows Server 2003.

59 4 rebinding interval in seconds

Code Length Type Client Identifier

61 n t1 i1, i2...in



The use of additional DHCP configuration is supported by Internet Explorer 5.x and Internet Explorer 6, but not

earlier versions, because those use different methods for automatic detection and configuration of proxy server

settings.

You can add and configure an alias (CNAME) resource record at the DNS server to support Internet Explorer

proxy server auto-discovery and configuration features.

Code

252

Length

Variable

Value

A URL that points to the configuration file that the client should use for automatic configuration of Internet

Explorer 5.x and Internet Explorer 6.

Structure of Proxy Autodiscovery for Internet Explorer 5

Microsoft Options

This section describes reserved DHCP options defined by Microsoft. These options are only available for use with

supported Windows-based DHCP clients, such as computers running Windows Server 2003.

The Microsoft options are provided as encapsulated vendor-specific data fields within the vendor-specific

information option.

Currently, administrators can assign these options are by using the DHCP snap-in through the following vendor

classes: Microsoft options and Microsoft Windows 2000 options.

Disable NetBIOS over TCP/IP (NetBT)

This option can be used to selectively enable or disable NetBT for DHCP-enabled computers running Windows

Server 2003, Windows XP, and Windows 2000. By default, if this option is not present, Windows Server 2003,

Windows XP, and Windows 2000 enable the use of NetBT for network connections that are configured to use

TCP/IP. Earlier Windows-based clients require NetBT and do not support this option.

Code

1

Length

4 octets

Value

Code Length Value

252 n URL name




Structure of Disable NetBIOS over TCP/IP (NetBT)

Release DHCP Lease on Shutdown

This option can be used to control whether DHCP-enabled computers running Windows 2000, Windows XP, or

Windows Server 2003 sends a DHCPRelease message to the DHCP server when a shutdown occurs. It is actually

implemented and interpreted as a bit-masked value by the DHCP Client service. By default, these clients do not

send DHCPRelease messages on proper shutdown.

Code

2

Length

4 octets

Value


Structure of Release DHCP Lease on Shutdown

Default Router Metric Base

This option can be used to set the default base metric for DHCP clients running Windows Server 2003. When

this option is set, the DHCP Client service uses the value configured here as the base metric for its default

gateways.

Code

3

Length

4 octets

• 1= NetBT remains enabled

• 2=Disable NetBT for DHCP clients

Code Length NetBT

001 4 1 | 2

• 1 = DHCP clients send a DHCPRelease message on proper shutdown

• 0 = DHCP clients do not send a DHCPRelease message on proper shutdown

Code Length Release

002 4 0 | 1



Value

This value represents a specified router metric base to be used for all default gateway routes used by DHCP-

enabled clients running Windows Server 2003. This value can be assigned as an integer representing a cost

metric ranging from 1 through 9,999. It is used in calculating the fastest, most reliable, and least expensive

routes. If a value is not specified, a default of either one (1) or the currently set interface-specific metric is

used.

Structure of Default Router Metric Base

Related Information

The following resources contain additional information that is relevant to this section.

Top of page

Code Length Route Metric

003 4 Router metric base

• The IETF RFC Database [http://go.microsoft.com/fwlink/?linkid=3952]

• What Is DHCP? [http://technet2.microsoft.com/WindowsServer/en/library/8832c551-939a-4b7e-b1b5-

c2a0ae8651ce1033.mspx]

• How DHCP Technology Works [http://technet2.microsoft.com/WindowsServer/en/library/8006f246-2029-

4bad-b9f0-4f31a56b05901033.mspx]

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