Class Options

Class Options are a new feature found in Windows 2000. Class Options can be used to identify a particular set of DHCP clients that have needs different from other clients found in the environment.

A good example of this is Windows 2000. Besides the options listed earlier in this section, Windows 2000's DHCP Client can use two additional options:

Perform Router Discovery (31)

This option specifies whether the DHCP client can use ICMP to discover Routers by sending Router Solicitation messages.

Static Route (33)

This option specifies a list of static routes to be installed into the client's routing table. Since other Microsoft clients will not support these options, a Class Option can be created for the Windows 2000 DHCP clients.

Windows 2000 supports the use of two types of class options: User Classes and Vendor Classes.

5.7.4.1 User Classes

User Classes are supported with the use of the User Class (77) Option. This option is not part of the DHCP standard and is proposed in a draft RFC called "The User Class Option for DHCP" (http://www.ietf.org/internet-drafts/draft-ietf-dhc-userclass-10.txt).

For example, users in separate departments might require the use of different DHCP options, even though they both use the same scope. Users in the accounting department may use a third-party application that requires a database server to be specified via DHCP options. By using User Class IDs, the DHCP server is able to differentiate between users in the accounting

department and other users. Only users in the accounting department receive the DHCP option for the database server.

The User Class Option is simply ASCII text used to identify a DHCP client as a member of a particular User Class. Windows 2000 has several predefined User Classes (see Table 5.3).

Table 5.3. Predefined User Classes

Class Name Class Data Description

Default User Class (none)

This class is used for clients that do not identify themselves as members of any User Class. Windows 2000 DHCP clients with user classes unknown to the DHCP server are placed in this class.

Default

Remote Access Class

RRAS.Microsoft This class is used to identify clients that are connected to the network via a _{RAS server.} Default

BOOTP class BOOTP.Microsoft This class is used to identify BOOTP clients.

When a Windows 2000 DHCP server receives a DHCPREQUEST that contains the User Class Identifier, it sends options assigned to that User Class if available.

To use User Classes, the User Class must be defined at the DHCP server and the DHCP clients.

To define a User Class at the DHCP server, follow these steps:

1. In the DHCP Console, right-click on the DHCP server where the User Class is to be defined. Select "Define User Classes . . . "

2. In the DHCP User Classes dialog box, click Add to add a new User Class.

3. In the New Class dialog box (see Figure 5.22), enter a name in the "Display name" field. The display name is used to display the User Class; it is not the actual name of the class. Enter a description in the Description field. To enter the name of the class, click in the area right below ASCII. Enter the name here. Click OK to add the User Class.

At this point you can begin assigning DHCP options to the new User Class: 1. Select the scope that will use the new User Class.

2. Right-click on Scope Options and select "Configure Options . . . " 3. Click on the Advanced tab.

4. Select the new User Class from the dropdown box and select the DHCP options that will be a part of this User Class. Click on OK.

Figure 5.22. Configuring a new User Class

To assign a DHCP client to a User Class, use the following command at the client:

ipconfig /setclassid "Local Area Connection" NewUserClassID

To view which User Class the DHCP client belongs to, use this command at the client:

ipconfig /showclassid "Local Area Connection" 5.7.4.2 Vendor Classes

Vendor Classes are supported with two options that are part of the DHCP options standard, defined in RFC2132.

Vendor-specific options are used to exchange specific configuration options between a DHCP server and its clients. Servers that are not designed to handle vendor-specific options will ignore client requests for these options. Clients that do not receive the requested options should attempt to operate without them.

The vendor is identified by the Vendor Class Identifier Option (60). Clients use this option to identify their Vendor Class. When the Windows 2000 DHCP server receives a DHCPREQUEST that contains the Vendor Class Identifier, it sends vendor-specific information pertaining to that vendor, if available.

In Windows 2000's DHCP Server, there are several predefined DHCP Vendor Classes (see

Table 5.4).

Table 5.4. Microsoft Vendor Classes

Class Name Class _Data Description

Microsoft Windows 2000

Options MSFT 5.0 This class is used to identify clients running Windows 2000. Microsoft Windows 98

Options MSFT 98 This class is used to identify clients running Windows 98. There are no options defined in this class. Microsoft Options MSFT This class is used to identify all clients running either Windows 2000 or _{Windows 98.} DHCP Standard Options This class is used to identify all clients that are not running Windows _{2000 or Windows 98.}

The Microsoft Options and Microsoft Windows 2000 Vendor Classes provide the following options:

Disable NetBIOS over TCP/IP (NetBT) Option

This option disables NetBIOS over TCP/IP. With Windows 2000, NetBT is not required, and therefore it can be disabled. Earlier Microsoft operating systems required NetBT. A value of 2 disables NetBT.

Release DHCP Lease on Shutdown Option

This option controls whether a Windows 2000 DHCP client releases its IP address lease upon shutdown. A value of tells the client not to send a DHCPRELEASE message to the server. A value of 1 tells the client to send a DHCPRELEASE message.

Default Router Metric Base Option

This option is used to set a base metric for all default gateway routes used on Windows 2000 DHCP clients. A metric is used to calculate the fastest and cheapest routes.

To define a Vendor Class at the DHCP server, follow these steps:

1. In the DHCP Console, right-click on the DHCP server where the Vendor Class is to be defined. Select "Define Vendor Classes . . . "

2. In the DHCP Vendor Classes dialog box, click Add to add a new Vendor Class.

3. In the New Class dialog box, enter a name in the "Display name" field. The display name is used to display the Vendor Class; it is not the actual name of the class. Enter a description in the description field. To enter the name of the class, click in the area right below ASCII. Enter the name here. Click OK to add the Vendor Class.

5.7.4.3 Assigning Class Options

Class Options can be assigned much like assigning options at the server and scope levels. Class Options can be found under the Advanced tab on the Server and Scope Options dialog boxes (see Figure 5.23). Class Options assigned at the server level flow down to the scope and client levels.

Figure 5.23. Assigning Class Options