Basic Options (RFC 1497)

The following tables list basic DHCP option types originally defined in RFC 1497, "BOOTP Vendor Information Extensions," for use with DHCP and the BOOTP service. In the BOOTP service, these option types are referred to as vendor extensions.

The DHCP service supports configuration and distribution of any of these options that are assigned using DHCP Manager. By default, Microsoft DHCP-enabled clients require and provide storage and interpretation only for options 1, 3, 6, and 15 of the option types listed in this section for default client configuration. Other options are predefined for assignment and distribution by the DHCP service, but are recognized only where clients use third-party DHCP software supporting these other option types.

 

Pad Option

Code

0

Length

Not used.

Value

Not used.

Description

This option type is a single octet of zero ("00") used for padding. This option differs from most DHCP option types in that it does not use a length or value field. When used, this option causes subsequent DHCP option types as they appear in the DHCP packet to align on word boundaries. This option does not require configuration.

Structure

Code

0

 

************************

End Option

Code

255

Length

Not used.

Value

Not used.

Description

This option is a single octet of decimal 255 ("FF") used to indicate the end of a DHCP options area in DHCP message packets. This option differs from most DHCP option types 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 packet. It can also be used within the message, in connection with the vendor-specific information (option 43), to indicate the end of an encapsulated vendor-specific options subfield. This option does not require configuration.

Structure

Code

255

 

************************

Subnet Mask

Code

1

Length

Fixed, 4 octets.

Value

Signed 32-bit integer representing the subnet mask for an IP address provided in a DHCP message.

Description

Specifies the subnet mask of the client subnet, as described in RFC 950, "Internet Standard Subnetting Procedure." The value for this option type is taken from the Subnet Mask field, as defined in the DHCP scope Properties dialog box in DHCP Manager.

Structure

Code

Length

Subnet Mask

1

4

m1, m2, m3, m4

 

************************

Time Offset

Code

2

Length

Fixed, 4 octets

Value

Signed 32-bit integer used for offset of Universal Coordinated Time (UCT).

Description

Specifies an offset value (in seconds) from the UCT that applies to the client's 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.

Structure

Code

Length

Time Offset

2

4

time

 

************************

Router

Code

3

Length

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

Value

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

Description

Specifies a list of IP addresses for routers on the client's subnet. When more than one router is assigned, the client interprets and uses the addresses in the specified order.

Structure

Code

Length

Address 1

Address 2

3

n

a1, a2, a3, a4

a1, a2, ...

 

************************

Time Server

Code

4

Length

Variable; minimum length of 4 octets; octet length increases in multiples of 4 for each RFC 868 time server address listed.

Value

Signed 32-bit integer representing the IP address of each assigned RFC 868 time server.

Description

Specifies a list of IP addresses for RFC 868 time servers 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.

Structure

Code

Length

Address 1

Address 2

4

n

a1, a2, a3, a4

a1, a2, ...

 

************************

IEN Name Server

Code

5

Length

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

Value

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

Description

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.

Structure

Code

Length

Address 1

Address 2

5

n

a1, a2, a3, a4

a1, a2, ...

 

************************

DNS Server

Code

6

Length

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

Value

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

Description

Specifies a list of IP addresses for Domain Name System (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. DHCP client computers that are multihomed and can obtain multiple DHCP leases can have only one DNS server list per host computer, not per adapter interface, except for Windows 2000 clients.

Structure

Code

Length

Address 1

Address 2

6

n

a1, a2, a3, a4

a1, a2, ...

 

************************

Log Server

Code

7

Length

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

Value

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

Description

Specifies a list of IP addresses for MIT-LCS 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.

Structure

Code

Length

Address 1

Address 2

7

n

a1, a2, a3, a4

a1, a2, ...

 

************************

Cookie Server

Code

8

Length

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

Value

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

Description

Specifies a list of IP addresses for RFC 865 cookie servers available to the client. When more than one server is assigned, the client interprets and uses the addresses in the specified order.

Structure

Code

Length

Address 1

Address 2

8

n

a1, a2, a3, a4

a1, a2, ...

 

************************

LPR Server

Code

9

Length

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

Value

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

Description

Specifies a list of IP addresses for RFC 1179 line printer servers available to the client. When more than one server is assigned, the client interprets and uses the addresses in the specified order.

Structure

Code

Length

Address 1

Address 2

9

n

a1, a2, a3, a4

a1, a2, ...

 

************************

Impress Server

Code

10

Length

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

Value

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

Description

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.

Structure

Code

Length

Address 1

Address 2

10

n

a1, a2, a3, a4

a1, a2, ...

 

************************

Resource Location Server

Code

11

Length

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

Value

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

Description

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

Structure

Code

Length

Address 1

Address 2

11

n

a1, a2, a3, a4

a1, a2, ...

 

************************

Host Name

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.

Description

Specifies a host name for the client of up to 63 characters in length. (See RFC 1035, Domain NamesImplementation and Specification, for possible character set restrictions.) 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 option 15. For Windows clients, this option is not supported for use when configuring the client's host name, which is set in Computer Name in the Network Identification Properties dialog box on the client computer.

Structure

Code

Length

Host Name

12

n

name

 

************************

Boot File Size

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.

Description

Specifies the size of the default boot image file for the client.

Structure

Code

Length

File Size

13

02

16-bit integer

 

************************

Merit Dump File

Code

14

Length

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

Value

ASCII character text

Description

Specifies a path name of a file to which the client's core memory image should be dumped in the event the client crashes. For this option type, data used for a value is ASCII character text. The length of the value field depends on the number of characters used in the path specified here. For example, if the path entered has 20 characters, the value field for this option should also be 20 octets in length.

Structure

Code

Length

Dump File Path

14

n

pathname

 

************************

DNS Domain Name

Code

15

Length

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

Value

ASCII character text

Description

Specifies the domain name that the DHCP client should use when resolving host names using the DNS. For this option type, ASCII character text is used for the data value. The length of the value field depends on the number of characters used in the DNS domain name specified here. For example, if the domain name has 20 characters, the value field for this option should also be 20 octets in length.

Structure

Code

Length

Domain Name

15

n

domain name

 

************************

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).

Description

Specifies the IP address of the client's swap server.

Structure

Code

Length

Swap Server Address

16

n

a1, a2, a3, a4

 

************************

Root Path

Code

17

Length

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

Value

ASCII character text.

Description

Specifies the path that contains the client's root disk. The path is formatted as ASCII text. For this option type, data used for a value is ASCII character text. The length of the value field depends on the number of characters used in the root path specified here. For example, if the root path entered has 20 characters, the value field for this option should also be 20 octets in length.

Structure

Code

Length

Root Disk Path

17

n

pathname

 

************************

Extensions Path

Code

18

Length

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

Value

ASCII character text.

Description

Specifies a file that can be retrieved using Trivial File Transfer Protocol (TFTP) containing information to be interpreted in the same way 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 used. When this option is used, 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.

Structure

Code

Length

Extensions Path

18

n

filename



Note

For all option types provided in "Extensions Path" that use a list of IP addresses as the valued options data, IP addresses are always used in order of preference by the DHCP client, so that the first address in the list is used first.

 

***********************************************************

IP Host Options

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

 

IP Forwarding Enable/Disable

Code

19

Length

Length is fixed at 1 octet.

Value

1 = Enable IP forwarding.
0 = Disable IP forwarding.

Description

Used to determine whether the DHCP client enables or disables forwarding of datagrams at the IP layer.

Structure

Code

Length

Value

19

1

0 | 1

 

************************

Nonlocal Source Routing Enable/Disable

Code

20

Length

Length is fixed at 1 octet.

Value

1 = Enable forwarding of datagrams from nonlocal sources.
0 = Disable forwarding of datagrams from nonlocal sources.

Description

Used to determines whether the DHCP client enables or disables forwarding of datagrams at the IP layer, based on whether a received datagram is from a local or nonlocal source.

Structure

Code Length Value
20 1 0 | 1

 

************************

Policy Filter

Code

21

Length

Variable. Minimum length is 8 octets for a single destination-and-mask pairing. Length increases in multiples of 8 octets for each additional pairing used.

Value

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

Description

Specifies policy filters for nonlocal source routing on the client. The filters consist of a list of IP address and mask pairs specifying destination-and-mask pairs for which to filter source routes of any incoming datagrams. 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 type, see RFC 1122, "Requirements for Internet Hosts—Communication Layers."

Structure

Code Length Address 1 Mask 1
21 n a1, a2, a3, a4 m1, m2, m3, m4
    Address 2 Mask 2
    a1, a2, a3, a4 m1, m2, m3, m4, ...

 

************************

Maximum Datagram Reassembly Size

Code

22

Length

Fixed, 2 octets.

Value

A 16-bit integer specifying the maximum datagram size for reassembly. The minimum size for a datagram is 576.

Description

Specifies the maximum size datagram that the client can reassemble.

Structure

Code Length Size
22 2 16 bit integer

 

************************

Default IP Time-To-Live

Code

23

Length

Fixed, 1 octet.

Value

A number (in seconds) between 1 and 255.

Description

Specifies the default Time-To-Live (TTL) that the client uses on outgoing datagrams.

Structure

Code Length TTL
23 1 TTL value in seconds

 

************************

Path MTU Aging Time-out

Code

24

Length

Fixed, 4 octets.

Value

A 32-bit unsigned integer that specifies a time-out value (in seconds).

Description

Specifies the time-out for aging Path Maximum Transmission Unit (MTU) values. (Values are discovered by the mechanism defined in RFC 1191, "Path MTU Discovery.")

Structure

Code Length Time-out
24 4 timeout value in seconds

 

************************

Path MTU Plateau Table

Code

25

Length

Variable; minimum length is 2 octets. When length is more than 2, it 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.

Description

Specifies a table of MTU sizes to use when performing Path MTU Discovered, as defined in RFC 1191, "Path MTU Discovery."

Structure

Code Length Size Size 2
25 n s1, s2 table of MTU sizes

 

***********************************************************

IP Interface Options

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

 

Interface MTU

Code

26

Length

Fixed, 2 octets.

Value

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

Description

Specifies the MTU size that can be used on a specified host adapter interface.

Structure

Code Length MTU
26 2 interface MTU

 

************************

All Subnets Are Local

Code

27

Length

Fixed, 1 octet.

Value

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.

Description

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.

Structure

Code Length Value
27 1 0 | 1

 

************************

Broadcast Address

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, Requirements for Internet HostsCommunication Layers.

Description

Specifies the broadcast address used on the client's subnet.

Structure

Code Length Broadcast Address
28 4 b1, b2, b3, b4

 

************************

Perform Mask Discovery

Code

29

Length

Fixed, 1 octet.

Value

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

Description

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

Structure

Code Length Value
29 1 0 | 1

 

************************

Mask Supplier

Code

30

Length

Fixed, 1 octet.

Value

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

Description

Specifies whether the client responds to subnet mask requests using ICMP.

Structure

Code Length Value
30 1 0 | 1

 

************************
Perform Router Discovery

Code

31

Length

Fixed, 1 octet.

Value

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

Description

Specifies whether the client solicits routers using the router discovery method in RFC 1256, ICMP Router Discovery Messages.

Structure

Code Length Value
31 1 0 | 1

 

************************
Router Solicitation Address

Code

32

Length

Fixed, 4 octets.

Value

An IP address (unsigned 32-bit integer).

Description

Specifies the IP address to which the client submits router solicitation requests.

Structure

Code Length Address
32 4 a1, a2, a3, a4

 

************************
Static Route

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 8 octets provides two consecutive IP addresses pairing the destination and router addresses used for each route. The first 4 octets specifies the destination address, and the second 4 octets specifies the router for the destination address.

Description

Specifies a list of static routes the client installs in its routing cache. Any 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.

Structure

Code Length Destination 1 Router 1
33 n d1, d2, d3, d4 r1, r2, r3, r4
    Destination 2 Router 2
    d1, d2, d3, d4 r1, r2, r3, r4, ...

 

***********************************************************

Link Layer Options

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

 
Trailer Encapsulation

Code

34

Length

Fixed, 1 octet.

Value

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

Description

Specifies whether the client negotiates the use of trailers, as described in RFC 893, ISO Transport Services on Top of the TCP, when using the Address Resolution Protocol (ARP).

Structure

Code Length Value
34 1 0 | 1

 

************************
ARP Cache Time-Out

Code

35

Length

Fixed, 4 octets.

Value

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

Description

Specifies the time-out for ARP cache entries.

Structure

Code Length Time
35 4 timeout value in seconds

 

************************
Ethernet Encapsulation

Code

36

Length

Fixed, 1 octet.

Value

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

Description

Specifies whether the client uses Ethernet v.2 (RFC 894) or IEEE 802.3 (RFC 1042) encapsulation if the interface is Ethernet.

Structure

Code Length Value
36 1 0 | 1

 

***********************************************************

TCP Options

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

 
 
TCP Default TTL

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.

Description

Specifies the default TTL the client uses when sending TCP segments.

Structure

Code Length TTL
37 1 TTL value in seconds

 

************************
TCP Keep-Alive Interval

Code

38

Length

Fixed, 4 octets.

Value

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

Description

Specifies the interval the client waits before sending a keep-alive message on a TCP connection. A value of 0 indicates that the client does not send keep-alive messages on connections unless specifically requested by an application.

Structure

Code Length Time
38 4 keepalive interval in seconds

 

************************
TCP Keep-Alive Garbage

Code

39

Length

Fixed, 1 octet.

Value

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

Description

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

Structure

Code Length Value
39 1 0 | 1

 

***********************************************************

Application Layer Options

The following tables describe the DHCP options that affect operation of the application layer on a per-interface basis. These are miscellaneous options you can use to configure programs and services.

For these options, you can dynamically configure DHCP clients that have more than one interface on a per-interface basis using additional DHCP leases (one per interface) for any of these option types.

 
 
NIS Domain Name

Code

40

Length

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

Value

ASCII character text.

Description

Specifies the Network Information Service (NIS) domain name as an ASCII string.

Structure

Code Length NIS Domain Name
40 n NIS domain name

 

************************
NIS Servers

Code

41

Length

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

Value

Signed 32-bit integer representing the IP address of an NIS server.

Description

Lists the IP addresses in the order of preference for Network Information Service (NIS) servers available to the client.

Structure

Code Length Address 1 Address 2
41 n a1, a2, a3, a4 a1, a2, ...

 

************************
NTP Servers

Code

42

Length

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

Value

Signed 32-bit integer representing the IP address of an NTP server.

Description

Lists the IP addresses in the order of preference for Network Time Protocol (NTP) servers available to the client.

Structure

Code Length Address 1 Address 2
42 n a1, a2, a3, a4 a1, a2, ...

 

************************
X Window System Font Servers

Code

48

Length

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

Value

Signed 32-bit integer representing each server IP address.

Description

Lists the IP addresses in the order of preference for X Window System font servers available to the client.

Structure

Code Length Address 1 Address 2
48 n a1, a2, a3, a4 a1, a2, ...

 

************************
X Window System Display Manager Servers

Code

49

Length

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

Value

Signed 32-bit integer representing each server IP address.

Description

Lists the IP addresses in the order of preference for X Window System display manager servers available to the client.

Structure

Code Length Address 1 Address 2
49 n a1, a2, a3, a4 a1, a2, ...

 

************************
NIS+ Domain Name

Code

64

Length

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

Value

ASCII character text.

Description

Specifies the name of the client's Network Information Service Plus (NIS+) domain name as an ASCII string.

Structure

Code Length NIS+ Domain Name
64 n NIS domain name

 

************************
NIS+ Servers

Code

65

Length

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

Value

Signed 32-bit integer representing IP address of NTP servers.

Description

Lists the IP addresses in the order of preference for Network Information Service Plus (NIS+) servers available to the client.

Structure

Code Length Address 1 Address 2
65 n a1, a2, a3, a4 a1, a2, ...

 

************************
Mobile IP Home Agents

Code

68

Length

Variable; minimum length is 4 octets; octet length increases in multiples of 4 for each mobile IP home agent address listed.

Value

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

Description

Lists the IP addresses in the order of preference for mobile IP home agents available to the client.

Structure

Code Length Address 1 Address 2
68 n a1, a2, a3, a4 a1, a2, ...

 

***********************************************************

NetBIOS over TCP/IP Options

The following option types are used for NetBIOS over TCP/IP support. All Microsoft-based DHCP clients and DHCP servers can recognize and support the use of these option types.

 
NetBIOS Name Server

Code

44

Length

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

Value

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

Description

Lists the IP addresses for Windows Internet Name Service (WINS) servers or NetBIOS name servers (NBNS).

Structure

Code Length Address 1 Address 2
44 n a1, a2, a3, a4 b1, b2, b3, ,, ...

 

************************
NetBIOS Datagram Distribution (NBDD) Server

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 a listed NBDD server IP address, specified as an unsigned 32-bit integer.

Description

Lists the IP addresses for NetBIOS datagram distribution (NBDD) servers.

Structure

Code Length Address 1 Address 2
45 n a1, a2, a3, a4 b1, b2, b3, b4, ...

 

************************
NetBIOS Node Type

Code

46

Length

Fixed, 1 octet.

Value

1 = b-node, 2 = p-node, 4 = m-node, and 8 = h-node.

Description

Configures the client node type for NetBIOS over TCP/IP (NetBT) clients, as described in RFCs 1001and 1002, Protocol Standard for a NetBIOS Service on a TCP/UDP Transport: Concepts and Methods and Protocol Standard for a NetBIOS Service on a TCP/UDP Transport: Detailed Specifications, respectively. On multihomed computers, the node type is assigned for the computer, not to individual network adapters.

Structure

Code Length Node Type
46 1 See above.

 

************************
NetBIOS Scope ID

Code

47

Length

Variable; octet length is equal to the number of characters used in NetBIOS scope ID.

Value

Specifies the NetBIOS over TCP/IP scope identifier used by the client. The format used for these scope IDs is further described in RFCs 1001 and 1002, "Protocol Standard for a NetBIOS Service on a TCP/UDP Transport: Concepts and Methods" and "... Detailed Specifications," respectively. See the RFCs for character-set restrictions.

Description

Specifies a string that is 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.

Structure

Code Length NetBIOS Scope
47 n scope identifier string

 

***********************************************************

Vendor-Specific Options

This section describes reserved DHCP option types specified for vendor class use. The vendor-specific option types are specified in RFC 2132. Vendor classes can be used by the DHCP service and DHCP-enabled client computers running under Windows 2000. 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.

In addition, DHCP Manager provides a single default vendor class, the DHCP Standard options class, that can be used to group and classify clients that do not identify a vendor class to the DHCP service.

 
Vendor-Specific Information

Code

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 type is vendor specific, and values provided here are presumed to be interpreted by vendor-specific code on DHCP clients and the DHCP server.

Description

This option is used by clients and servers to exchange vendor-specific information. Servers that are not equipped to interpret the information ignore it. Clients that expect but don't receive the information attempt to operate without it.

In some cases, a vendor uses this option type to send more than one information item; therefore, this option can serve as an encapsulated options subfield for encapsulating vendor-specific options. When encapsulating options, DHCP servers maintain the same syntax (that is, the same code/length/value fields sequence) for each encapsulated option type as it would normally appear in the full standard options field, with the following exceptions for the encapsulated vendor-specific subfield:

"Magic cookies" cannot be used.

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

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.

Structure

Code Length Vendor-Specific Information
43 ... n i1, i2, ...

 

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

Code Length Data Item Code Length Data Item Code
T1 n d1, d2, ... T2 n D1, D2, ... ...

 

************************
Vendor Class Identifier

Code

60

Length

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

Value

A value of n octets interpreted by DHCP servers that can support vendor classification of clients.

Description

Can be used by DHCP clients to identify their vendor type and configuration. When using this option, vendors can define their own specific identifier values, such as to convey a particular hardware or operating system configuration or other identifying information.

For Windows 2000, all computers that function either as DHCP servers or DHCP clients can use and support this option type. When vendor classes are used, the DHCP server responds to identifying clients using option code 43 (described earlier), the reserved option type for returning vendor-specific information to the client.

DHCP servers that do not automatically interpret this option are expected to ignore it when it is specified by clients. For earlier Windows-based clients and other clients that do not support this option type, the DHCP service classifies these clients as part of the default vendor class, the DHCP Standard options class, which is predefined for Microsoft-based DHCP servers.

Structure

Code Length Vendor Class Identifier
60 n i1, i2, ...

 

***********************************************************

User Class Options

This section describes reserved DHCP option types specified for user class use. The user class option type is an additional DHCP draft specification currently under proposal as an Internet standard. User classes can be used by the DHCP service and DHCP-enabled client computers running under Windows 2000. 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

Code

77

Length

Variable; minimum is 2 octets.

Value

ASCII character text.

Description

A DHCP client can use this option to identify the type or category of user or applications it represents. The information contained in this option is an NVT ASCII text object that represents the user class of which the client is a member.

You can use DHCP Manager 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, default user classes can be 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.

For Microsoft DHCP clients, only computers running Windows 2000 support sending or using this option type. Other legacy DHCP clients do not send a class ID or have the ability to recognize the concept of a user class. Therefore, these clients are assigned as members to the Default User Class, which is a user class predefined for immediate use in DHCP Manager. Other user classes must be manually created.

Structure

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

 

***********************************************************

DHCP Extensions

The following option types 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 DHCP Manager.

 
Requested IP Address

Code

50

Length

Fixed, 4 octets.

Value

Single, signed 32-bit integer indicating a requested IP address.

Description

Can be used by clients when sending a DHCPDiscover message to request that a specific IP address be assigned by the server.

Structure

Code Length Requested IP Address
50 n a1, a2, a3, a4

 

************************
IP Address Lease Time

Code

51

Length

Fixed, 4 octets.

Value

Single, signed 32-bit integer representing a client's lease time, in seconds.

Description

This option type 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 the server can offer to the client.

Structure

Code Length Lease Time
51 4 lease time in seconds

 

************************
Option Overload

Code

52

Length

Fixed, 1 octet.

Value

Predefined, accepted values for this option type include:

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

Description

Used in messages sent by the 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) are to be overloaded (used to hold options).

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.

Structure

Code Length Value
52 1 1 | 2 | 3

 

************************
TFTP Server Name

Code

66

Length

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

Value

ASCII character text.

Description

Specifies the host name of the Trivial File Transfer Protocol (TFTP) server when the server_host_name (sname) field in a DHCP message packet is overloaded and used for carrying additional DHCP options.

Structure

Code Length TFTP Server
66 n TFTP hostname

 

************************
Boot File Name

Code

67

Length

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

Value

ASCII character text.

Description

Specifies the name of a boot image file on the TFTP server when the boot_file_name (file) field in a DHCP message packet is overloaded and used for carrying additional DHCP options.

Structure

Code Length Bootfile Name
67 n boot image filename

 

************************
DHCP Message Type

Code

53

Length

Fixed, 1 octet.

Value

Predefined, accepted values for this option type 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).

Description

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

Structure

Code Length Type
53 1 1–8

 

************************

Server Identifier

Code

54

Length

Fixed, 4 octets.

Value

A single, signed 32-bit IP address that identifies the DHCP server.

Description

This option is used in DHCPOffer and DHCPRequest messages and sometimes appears in DHCP Acknowledgment messages (DHCPAck, DHCPNak). The server identifier is the IP address of a selected DHCP server. This option type is used in two possible ways. First, servers include this option type in DHCPOffer messages so that clients can distinguish between multiple lease offers.

Second, clients include this option type in DHCPRequest messages to select a lease and indicate which offer is accepted from multiple lease offers. Also, clients can use the contents of this field for unicast sending of request messages to specific DHCP servers to renew a current lease.

Structure

Code Length Address
54 4 a1, a2, a3, a4

 

************************
Parameter Request List

Code

55

Length

Minimum of 1 octet. Length increases by 1 octet for each additional option code included in the request list.

Value

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

Description

Used by a DHCP client to request specific option type values from the DHCP server. Each option type 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 type, the list can be ordered by preference, although the DHCP server is not required to return options in the order they are requested. However, the DHCP server attempts to insert the requested options in the order requested by the client.

Structure

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

 

************************
Optional Message

Code

56

Length

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

Value

ASCII character text.

Description

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

A server can use this option type to provide and embed an error message in a DHCP Negative Acknowledgment message (DHCPNak) in the event of a failure.

A client can use this option type 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.

Structure

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

 

************************
Maximum Message Size

Code

57

Length

Fixed, 2 octets.

Value

A 16-bit integer indicating the maximum size, in bytes/octets, for a DHCP message packet. The maximum legal value for this option type is 576.

Description

Used by client to specify the maximum length for a DHCP message packet that it can accept. A client can include this option type in DHCPDiscover or DHCPRequest messages; however, it does not include this option type in DHCPDecline messages.

Structure

Code Length Length
57 2 maxsize

 

************************
Renewal Time Value (T1)

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.

Description

This time is a function that is typically 50 percent of the full configured duration (or lease time) for a client's lease. To adjust this time 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 using NetShell (for more information, see the online Help).

Structure

Code Length T1 Interval
58 4 begin renewal interval

 

************************
Rebinding Time Value (T2)

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.

Description

This time is a function (typically, 87.5 percent) of the full configured duration (or lease time) for a client's lease. To adjust this time 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 using NetShell (For more information, see the online Help).

Structure

Code Length T2 Interval
59 4 begin rebinding interval

 

************************
Client Unique Identifier

Code

61

Length

Variable length; minimum length is 2 octets.

Value

A series of 2 or more octets that is treated as a variable object by the DHCP server. Servers can interpret and use this value to uniquely identify clients.

Description

Used by clients to specify their unique identifier to the server. This option type is most useful for reserved clients. When a reserved client contacts the server, the DHCP service can check and match the client's 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. For this reason, each client's identifier must be unique among all other client identifiers used on the effective DHCP network to which the client is attached (that is, the 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 common approach to ensure uniqueness is to configure client reservations at the DHCP server based on the client's media access control 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 as way as to ensure that they are unique for each device.

Structure

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

 

***********************************************************

Options Not Defined

This section describes DHCP option types that are reserved and specified for use in RFC 2132, DHCP Options and BOOTP Vender Extensions, but are not predefined for use in DHCP Manager. These option types can be added to support third-party DHCP clients that recognize these options.

 
Simple Mail Transport Protocol (SMTP) Server

Code

69

Length

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

Value

Signed 32-bit integers representing IP addresses of servers.

Description

Lists the IP addresses in order of preference for SMTP servers available to the client.

Structure

Code Length Address 1 Address 2
69 n a1, a2, a3, a4 a1, a2, ...

 

************************
Post Office Protocol (POP3) Server

Code

70

Length

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

Value

Signed 32-bit integers representing IP addresses of servers.

Description

Lists the IP addresses in order of preference for POP3 servers available to the client.

Structure

Code Length Address 1 Address 2
70 n a1, a2, a3, a4 a1, a2, ...

 

************************
Network News Transport Protocol (NNTP) Server

Code

71

Length

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

Value

Signed 32-bit integers representing IP addresses of servers.

Description

Lists the IP addresses in order of preference for NNTP servers available to the client.

Structure

Code Length Address 1 Address 2
71 n a1, a2, a3, a4 a1, a2, ...

 

************************
Default World Wide Web Server

Code

72

Length

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

Value

Signed 32-bit integers representing IP addresses of servers.

Description

Lists the IP addresses in order of preference for default Web servers available to the client.

Structure

Code Length Address 1 Address 2
72 n a1, a2, a3, a4 a1, a2, ...

 

************************
Default Finger Server

Code

73

Length

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

Value

Signed 32-bit integers representing IP addresses of servers.

Description

Lists the IP addresses in order of preference for default Finger servers available to the client.

Structure

Code Length Address 1 Address 2
73 n a1, a2, a3, a4 a1, a2, ...

 

************************
Default Internet Relay Chat Server

Code

74

Length

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

Value

Signed 32-bit integers representing IP addresses of servers.

Description

Lists the IP addresses in order of preference for default IRC servers available to the client.

Structure

Code Length Address 1 Address 2
74 n a1, a2, a3, a4 a1, a2, ...

 

************************
StreetTalk Server

Code

75

Length

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

Value

Signed 32-bit integers representing IP addresses of servers.

Description

Lists the IP addresses in order of preference for StreetTalk servers available to the client.

Structure

Code Length Address 1 Address 2
75 n a1, a2, a3, a4 a1, a2, ...

 

************************
StreetTalk Directory Assistance Server

Code

76

Length

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

Value

Signed 32-bit integers representing IP addresses of servers.

Description

Lists the IP addresses in order of preference for STDA servers available to the client.

Structure

Code Length Address 1 Address 2
76 n a1, a2, a3, a4 a1, a2, ...

 

***********************************************************

Microsoft Options

This section describes reserved DHCP option types defined by Microsoft. These options are only available for use with supporting Microsoft DHCP clients, such as computers running Windows 2000.

These options are provided as encapsulated vendor-specific data fields within the vendor-specific information option.

Currently, these options are only assignable in the DHCP console through the following vendor classes: Microsoft options and Microsoft Windows 2000 options.

 

 
Disable NetBIOS over TCP/IP (NetBT)

Code

1

Length

4

Value

1= NetBT remains enabled.

2=Disable NetBIOS over TCP/IP (NetBT) for Windows 2000 DHCP clients.
 

Description

This option can be used to selectively enable or disable NetBT for DHCP-enabled computers running Windows 2000 only. By installation default, if this option is not present Windows 2000 enables the use of NetBT for network connections that are configured to use TCP/IP. Earlier Windows clients require NetBT and do not support this option.

Structure

Code Length NetBT
001 4 On/Off

 

************************
Release DHCP Lease on Shutdown

Code

2

Length

4B

Value

0=Windows 2000 DHCP clients do not send a DHCPRelease message on proper shutdown.

1=Windows 2000 DHCP clients send a DHCPRelease message on proper shutdown.

Description

This option can be used to control whether DHCP-enabled computers running Windows 2000 send a release for their current DHCP lease to the DHCP server when shutdown occurs. It is actually implemented and interpreted as a bitmasked value by the DHCP client service. In most cases, the default (that is, the functional equivalent to this option value not being used or present in DHCP messages) is that Windows 2000 clients do not send DHCPRelease messages on proper shutdown.

Structure

Code Length Release
002 4 On/Off

 

************************
Default Router Metric Base

Code

3

Length

4B

Value

This value is a specified router metric base to be used for all default gateway routes used at Windows 2000 DHCP-enabled client computers.

This value can be assigned as an integer 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.

Description

This option can be used to set the default base metric for Windows 2000 DHCP clients. When this option is set, the DHCP client service uses the value configured here as the base metric for its default gateways.

Structure

Code Length Route Metric
003 4 router metric base

 

************************
Proxy Autodiscovery for Internet Explorer 5 Only

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. The file this URL points to can be a .pac, .jvs, .js, or .ins configuration file created by your system or Web administrator when deploying Internet Explorer 5 on your intranet. It might include settings for other Internet Explorer 5 configurable options, such as which home page to use, or settings for locating and using a proxy server.

Description

This option is communicated between Internet Explorer 5 client computers and the DHCP server using the DHCPInform message, which is currently only supported for Windows 2000 DHCP server and clients.

The use of additional DHCP configuration is only supported by Internet Explorer 5, not earlier versions that use different methods for automatic detection and configuration of proxy server settings.

You can also add and configure an alias (CNAME) resource record at the DNS server to support Internet Explorer 5 proxy server autodiscovery and configuration features.

For further details, see the Microsoft® Internet Explorer 5 Resource Kit.

Structure

Code Length URL
252 n url name