DHCP Tools and Options

Article
07/02/2012

Applies To: Windows Server 2008

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:

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.

Version compatibility

The Windows Server 2008 DHCP snap-in is compatible with DHCP servers running Windows Server 2003 and Windows 2000 Server.

Netsh

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. Netsh provides an equivalent alternative to console-based management. All commands in netshdhcp context can also be executed against a specified remote server. For more information, see Netsh Commands for Dynamic Host Configuration Protocol server (https://go.microsoft.com/fwlink/?LinkID=125702).

Version compatibility

You can use netsh commands with Windows 2000 Server, Windows Server 2003, and Windows Server 2008.

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, Windows Server 2003, and Windows Server 2008, Network Monitor is installed as an optional management and monitoring component by using Control Panel. After it is installed, you can run Network Monitor from the Administrative Tools folder.

Version compatibility

You can use Network Monitor to capture and view packets in Windows XP, Windows Vista, Windows 2000 Server, Windows Server 2003, and Windows Server 2008.

DHCP options

This section lists the predefined options available for use with the Windows Server 2008 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 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.

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.
IP interface options. These options affect the 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 already configured in Windows Server 2008, but can be defined by an administrator.
Microsoft options. These options are available for use with supported Microsoft DHCP clients only, such as computers running Windows Server 2003 or Windows XP.

Note

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.

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

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

Length

Not used.

Value

Not used.

Structure of Pad option

Code
0

End

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

Code
255

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 Scope Properties dialog box in DHCP Manager. A DHCP requests this option.

Code

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 option

Code	Length	Subnet Mask
1	4	subnet mask in binary format

Time Offset

This option specifies an offset value (in seconds) from the Coordinated Universal Time (UTC) that applies to the client subnet. This value can be configured 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

Length

Fixed, 4 octets.

Value

Signed 32-bit integer used for offset of UTC.

Structure of Time Offset option

Code	Length	Time Offset
2	4	time

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 requests this option.

Code

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 option

Code	Length	Address 1	Address N
3	n	IP address in binary format	IP address in binary format

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

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 option

Code	Length	Address 1	Address N
4	n	IP address in binary format	IP address in binary format

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

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 option

Code	Length	Address 1	Address N
5	n	IP address in binary format	IP address in binary format

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 requests this option.

Code

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 option

Code	Length	Address 1	Address N
6	n	IP address in binary format	IP address in binary format

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

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 option

Code	Length	Address 1	Address N
7	n	IP address in binary format	IP address in binary format

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

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 option

Code	Length	Address 1	Address N
8	n	IP address in binary format	IP address in binary format

LPR Server

This option specifies a list of IP addresses for line printer (LPR) servers, as defined in RFC 1179, available to the client. When more than one server is assigned, the client interprets and uses the addresses in the specified order.

Code

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 option

Code	Length	Address 1	Address N
9	n	IP address in binary format	IP address in binary format

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

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 option

Code	Length	Address 1	Address N
10	n	IP address in binary format	IP address in binary format

Resource Location Server

This option specifies a list of IP addresses for resource location servers, as defined in RFC 887, available to the client. When more than one server is assigned, the client interprets and uses the addresses in the specified order.

Code

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 option

Code	Length	Address 1	Address N
11	n	IP address in binary format	IP address in binary format

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 Vista and Windows Server 2008 on the Computer Name tab in the System Properties dialog box on the client computer.

Code

Length

Variable. Minimum length of 1 octet. Maximum length 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 option

Code	Length	Host Name
12	n	name

Boot File Size

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

Code

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 option

Code	Length	File Size
13	02	16-bit integer

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

Length

Variable. Minimum length of 1 octet.

Value

ASCII character text.

Structure of Merit Dump File option

Code	Length	Dump File Path
14	n	path name

DNS Domain Name

This option specifies the domain name that the DHCP client should use when resolving unqualified domain names with DNS. For DHCP clients running Windows 2000, Windows XP, and Windows Server 2003, this 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 requests this option.

Code

Length

Variable. Minimum length of 1 octet.

Value

ASCII character text.

Structure of DNS Domain Name option

Code	Length	Domain Name
15	n	domain name

Swap Server

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

Code

Length

Fixed, 4 octets.

Value

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

Structure of Swap Server option

Code	Length	Swap Server Address
16	4	IP address in binary format

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

Variable. Minimum length of 1 octet.

Value

ASCII character text.

Structure of Root Path option

Code	Length	Root Disk Path
17	n	path name

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

Length

Variable. Minimum length of 1 octet.

Value

ASCII character text.

Structure of Extension Path option

Code	Length	Extensions Path
18	n	file name

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

Length

Fixed, 1 octet.

Value

Accepted values for this option are:

1 = Enable IP forwarding
0 = Disable IP forwarding

Structure of IP Forwarding Enable/Disable option

Code	Length	Value
19	1	0 \| 1

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

Length

Fixed, 1 octet.

Value

Accepted values for this option are:

1 = Enable forwarding of datagrams from non-local sources
0 = Disable forwarding of datagrams from non-local sources

Structure of Non-Local Source Routing Enable/Disable option

Code	Length	Value
20	1	0 \| 1

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.

Code

Length

Variable. Minimum length of 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 option

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

Maximum Datagram Reassembly Size

This option specifies the maximum size of the datagram that the client must reassemble.

Code

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 option

Code	Length	File Size
22	2	16-bit integer

Default IP Time-To-Live

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

Code

Length

Fixed, 1 octet.

Value

A number (in seconds) between 1 and 255.

Structure of Default IP Time-To-Live option

Code	Length	TTL
23	1	TTL value in seconds

Path MTU Aging Time-out

This option specifies the timeout for aging Path Maximum Transmission Unit (MTU) values. Values are found by the Path MTU discovery process, as defined in RFC 1191.

Code

Length

Fixed, 4 octets.

Value

A number (in seconds) that specifies a timeout value.

Structure of Path MTU Aging Timeout option

Code	Length	Time-out
24	4	timeout value in seconds

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

Length

Variable. Minimum length of 2 octets; 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 option

Code	Length	MTU Size 1	MTU Size N
25	n	MTU size in binary format	MTU size in binary format

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

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 option

Code	Length	MTU
26	2	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

Length

Fixed, 1 octet.

Value

Accepted values for this option are:

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

Structure of All Subnets Are Local option

Code	Length	Value
27	1	0 \| 1

Broadcast Address

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

Code

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 option

Code	Length	Broadcast Address
28	4	broadcast address in binary format

Perform Mask Discovery

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

Code

Length

Fixed, 1 octet.

Value

Accepted values for this option are:

1 = Client performs subnet mask discovery
0 = Client does not perform subnet mask discovery

Structure of Perform Mask Discovery option

Code	Length	Value
29	1	0 \| 1

Mask Supplier

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

Code

Length

Fixed, 1 octet.

Value

Accepted values for this option are:

1 = Client responds to subnet mask requests
0 = Client does not respond to subnet mask requests

Structure of Mask Supplier option

Code	Length	Value
30	1	0 \| 1

Perform Router Discovery

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

Code

Length

Fixed, 1 octet.

Value

Accepted values for this option are:

1 = Client performs router discovery
0 = Client does not perform router discovery

Structure of Perform Router Discovery option

Code	Length	Value
31	1	0 \| 1

Router Solicitation Address

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

Code

Length

Fixed, 4 octets.

Value

Unsigned 32-bit integer representing an IP address.

Structure of Router Solicitation Address option

Code	Length	Address
32	4	IP address in binary format

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 requests this option.

Code

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 option

Code	Length	Destination 1	Router 1	Destination N	Router N
33	n	IP address in binary format	IP address in binary format	IP address in binary format	IP address in binary format

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

Code

249

Note

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

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.

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 option

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

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

Length

Fixed, 1 octet.

Value

Accepted values for this option are:

1 = Client attempts to use trailer
0 = Client does not attempt to use trailers

Structure of Trailer Encapsulation option

Code	Length	Value
34	1	0 \| 1

ARP Cache Time-Out

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

Code

Length

Fixed, 4 octets.

Value

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

Structure of ARP Cache Time-Out option

Code	Length	Time
35	4	timeout value in seconds

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

Length

Fixed, 1 octet.

Value

Accepted values for this option are:

1 = Client uses RFC 1042 encapsulation
0 = Client uses RFC 894 encapsulation

Structure of Ethernet Encapsulation option

Code	Length	Value
36	1	0 \| 1

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

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 option

Code	Length	TTL
37	1	TTL value in seconds

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

Length

Fixed, 4 octets.

Value

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

Structure of TCP Keep-Alive Interval option

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

Length

Fixed, 1 octet.

Value

Accepted values for this option are:

1 = Client sends keep-alive garbage octet
0 = Client does not send keep-alive garbage octet

Structure of TCP Keep-Alive Garbage option

Code	Length	Value
39	1	0 \| 1

Application Layer options

Code	Length	Address 1	Address N
76	n	IP address in binary format	IP address in binary format

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 requests this option.

Code

Length

Variable. Minimum length of 4 octets; octet 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 option

Code	Length	Address 1	Address N
44	n	IP address in binary format	IP address in binary format

NetBIOS over TCP/IP Datagram Distribution Server

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

Code

Length

Variable. Minimum length of 4 octets; octet 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 option

Code	Length	Address 1	Address N
45	n	IP address in binary format	IP address in binary format

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 requests this option.

Code

Length

Fixed, 1 octet.

Value

Accepted values for this option are:

1 = b-node
2 = p-node
4 = m-node
8 = h-node

Structure of NetBIOS over TCP/IP Node Type option

Code	Length	Value
46	1	1 \| 2 \| 4 \| 8

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 requests this option. The use of NetBIOS scope IDs is not recommended.

Code

Length

Variable. Minimum length of 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 option

Code	Length	NetBIOS Scope
47	n	scope identifier

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. 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 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:

A “Magic cookie” field cannot be used.
All standard option codes — other than the pad 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.

Code

Length

Variable. Minimum length of 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 option

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

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

Code	Length	Data Item
T1	n	d1, d2, ... dn
T2	n	d1, d2, dn

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 identifier values, like conveying a particular hardware or operating system configuration.

All computers running Windows Server 2008 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

Length

Variable. Minimum of 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 option

Code	Length	Value
60	n	vendor class identifier

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 2008 and 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 2008 and 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

Length

Variable. Minimum of 2 octets.

Value

ASCII character text.

Structure of User Class Information option

Code	Length	User Class Information
77	n	c1, c2, c3, c4 ... cn

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.

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

Length

Fixed, 4 octets.

Value

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

Structure of Requested IP Address option

Code	Length	Requested IP Address
50	4	IP address in binary format

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

Length

Fixed, 4 octets.

Value

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

Structure of IP Address Lease Time option

Code	Length	Lease Time
51	4	lease time in seconds

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

Code

Length

Fixed, 1 octet.

Value

Accepted values for this option include:

1 = File field is overloaded
2 = Sname field is overloaded
3 = Both file and sname fields are overloaded

Structure of Option Overload option

Code	Length	Value
52	1	1 \| 2 \| 3

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

Length

Variable, depending on data in value. Minimum length of 1 octet.

Value

ASCII character text.

Structure of TFTP Server Name option

Code	Length	TFTP Server
66	n	TFTP server host 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.

Code

Length

Variable, depending on data in value. Minimum length of 1 octet.

Value

ASCII character text.

Structure of Boot File Name option

Code	Length	Boot File Name
67	n	boot image file name

DHCP Message Type

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

Code

Length

Fixed, 1 octet.

Value

Accepted values for this option are:

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)

Structure of DHCP Message Type option

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

Length

Fixed, 4 octets.

Value

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

Structure of Server Identifier option

Code	Length	Address
54	4	IP address in binary format

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

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 option

Code	Length	Option Codes
55	n	c1, c2... cn

Optional Message

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

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.

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

Code

Length

Minimum of 1 octet. Length varies according to the length of the sent message.

Value

ASCII character text.

Structure of Optional Message option

Code	Length	Text
56	n	c1, c2... cn

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

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.

Structure of Maximum Message Size option

Code	Length	Value
57	2	maximum 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

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) option

Code	Length	T1 Interval
58	4	renewal interval in seconds

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

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) option

Code	Length	T2 Interval
59	4	rebinding interval in seconds

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 client’s 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. MAC 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

Length

Variable. Minimum of 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 option

Code	Length	Type	Client Identifier
61	n	t1	i1, i2... in

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 2008 and Windows Server 2003, and DHCP clients running Windows Vista, Windows XP and Windows 2000.

The use of additional DHCP configuration is supported by Internet Explorer 5.x and up, 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 up.

Structure of Proxy Autodiscovery for Internet Explorer 5 option

Code	Length	Value
252	n	URL name

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 2008 or 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 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 2008, Windows Server 2003, Windows Vista, Windows XP, and Windows 2000. By default, if this option is not present, Windows Server 2008, Windows Server 2003, Windows Vista, 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

Length

4 octets

Value

Accepted values for this option are:

1= NetBT remains enabled
2=Disable NetBT for DHCP clients

Structure of Disable NetBIOS over TCP/IP (NetBT) option

Code	Length	NetBT
001	4	1 \| 2

Release DHCP Lease on Shutdown

This option can be used to control whether DHCP-enabled computers 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

Length

4 octets

Value

Accepted values for this option are:

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

Structure of Release DHCP Lease on Shutdown option

Code	Length	Release
002	4	0 \| 1

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

Length

4 octets

Value

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

Code	Length	Route Metric
003	4	Router metric base

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

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DHCP Tools and Options

DHCP tools

DHCP snap-in

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Netsh

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Version compatibility

Network Monitor

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Version compatibility

DHCP options

Basic options

Pad

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End

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Subnet Mask

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Time Offset

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Router

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Time Server

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IEN Name Server

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DNS Servers

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Log Server

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Cookie Server

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LPR Server

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Impress Server

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Resource Location Server

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Host Name

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Boot File Size

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Merit Dump File

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DNS Domain Name

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