Managing DHCP Options

DHCP options can be configured for specific values and enabled for assignment and distribution to DHCP clients based on either server, scope, class or client-specific levels. The most specific take precedence over the least specific. In most cases, the client values provided are taken from the DHCP Options Properties dialog box on the DHCP server. These properties can be configured and set for an entire scope or for a single reserved scope client.

Although these options are not required for use by DHCP, assign and configure these options to automate client TCP/IP configuration when you have a sizable number of Microsoft-based DHCP client computers in active operation on your network. Options can also be used for DHCP communication between the server computer and client computers.

Options can be managed using different levels assigned for each managed DHCP server, including:

• Default global options
  These options are applied globally for all scopes and classes defined at each DHCP server and any clients that it services. Active global option types always apply unless they are overridden by other scope, class, or reserved client settings for the option type.

• Scope options
  These options are applied to any clients that obtain a lease within that particular scope. Active scope option types always apply to all computers obtaining a lease in a given scope unless they are overridden by class or reserved client settings for the option type.

• Class options
  These options are applied to any clients that specify that particular DHCP Class ID value when obtaining a scope lease. Active class option types always apply to all computers configured as members in a specified DHCP option class unless they are overridden by a reserved client setting for the option type.

• Reserved client options
  These options apply to any appropriate, reserved, client computer—any computer that has a reservation in the scope for its IP address. Where reserved client option types are active, settings for these option types override all other possible defaults (server, scope, or class assigned option settings for the option type).

In general, options are applied at each DHCP server at the server or scope level. To precisely manage or customize option settings, specify either a user or vendor class assignment that overrides the broader server or scope option defaults. For special requirements, such as clients with special functions, narrow the spectrum even further by assigning options for reserved clients.

Options can also be used to separate and distribute appropriate options for clients with similar or special configuration needs. For example, DHCP-enabled clients on the same floor can be assigned membership in the same option class (that is, configured with the same DHCP Class ID value). This class can then be used to distribute additional or varied option data during the lease process, overriding any scope or globally provided default options.

Many of these option types are predefined in Windows 2000 DHCP. Other standard DHCP option types can be added as needed to support any other DHCP client software that recognizes or requires the use of these additional option types. All DHCP options supported by the Windows 2000 DHCP service are defined in RFC 2132, although most DHCP clients use or support only a small subset of the available RFC-specified option types. This feature enables custom applications for enterprise networks to be introduced quickly. Equipment from multiple vendors on a network can also use different option numbers for different functions. The vendor class and vendor options are described in RFC 2132.

The Microsoft-based DHCP server usually allocates 312 bytes for DHCP options. That is more than enough for most option configurations. Some other DHCP servers and clients support option overlay, in which unused space in other standard DHCP message header fields within the DHCP packet can be overlaid to store and carry additional options. Microsoft DHCP service does not support this feature. If you attempt to use more than 312 bytes, some option settings will be lost. In that case, you should delete any unused or low-priority options. Table 4.5 contains a list of default DHCP options used by Microsoft Windows 2000 DHCP clients.

Table 4.5 Default DHCP Options

Code	Option name	Meaning
1	Subnet mask	Specifies the subnet mask of the client subnet. This option is defined in the DHCP Manager Create Scope or Scope Properties dialog box. It cannot be set directly in the DHCP Options dialog box.
3	Router	Specifies a list of IP addresses for routers on the client's subnet. Multihomed computers can have only one list per computer, not one per network adapter.
6	DNS servers	Specifies a list of IP addresses for DNS name servers available to the client.
15	Domain name	Specifies the DNS domain name that the client should use for DNS computer name resolution.
44	WINS/NBNS servers	Specifies a list of IP addresses for NetBIOS name servers (NBNS).
46	WINS/NBT node type	Allows configurable NetBIOS over TCP/IP (NetBT) clients to be configured as described in RFC 1001/1002, where 1 = b-node, 2 = p-node, 4 = m-node, and 8 = h-node. On multihomed computers, the node type is assigned to the entire computer, not to individual network adapters.
47	NetBIOS scope ID 1	Specifies a text string that is the NetBIOS over TCP/IP scope ID for the client, as specified in RFC 1001/1002.
51	Lease time	Specifies the time, in seconds, from address assignment until the client's lease on the address expires. Lease time is specified in the DHCP Manager Create Scope or Scope Properties dialog box, and can be set directly in the DHCP Options dialog box.
58	Renewal (T1) time value	Specifies the time in seconds from address assignment until the client enters the Renewing state. Renewal time is a function of the lease time option, which is specified in the DHCP Manager Create Scope or Scope Properties dialog box and can be set directly in the DHCP Options dialog box.
59	Rebinding (T2) time value	Specifies the time, in seconds, from address assignment until the client enters the Rebinding state. Rebinding time is a function of the lease time option, which is specified in the DHCP Manager Create Scope or Scope Properties dialog box andcan be set directly in the DHCP Options dialog box.
1 Option 47 (NetBIOS scope ID) is provided for backward compatibility. Don't use this option unless you already employ NetBIOS scope IDs in your environment.

Note

If you are using Microsoft DHCP service to configure computers that should use the services of a WINS server for name resolution, be sure to use option 44, WINS Servers, and option 46, Node Type. These DHCP options automatically configure the DHCP client as an h-node computer that directly contacts WINS servers for NetBIOS name registration and name query instead of using only broadcasts.