Windows Server 2003 : Managing DHCP in Windows Networks (part 3) - Backing Up the DHCP Server Database & Using Options Classes

Changing the Addressing of a Subnet

In the course of DHCP administration, you might find it necessary to modify or completely change a subnet’s addressing.

If you merely need to modify the range of a current scope, you can achieve this task by changing the address range or exclusion ranges of the subnet’s defined DHCP scope.

If you need to migrate to a completely new scope, however, you must first add a new scope to the DHCP server and then migrate to that new scope. To perform such a migration, first create and activate the new scope, and then deactivate the old scope. Do not delete the deactivated scope until the clients have migrated to the new scope. To migrate clients, either wait for the clients to automatically renew their leases (after 50 percent of the configured scope lease time has elapsed) or manually renew clients by executing the Ipconfig /release command followed by the Ipconfig /renew command on the client computers. Once all clients have been moved or forced to seek leases in another scope, you can safely delete the inactive scope.

Backing Up the DHCP Server Database

Maintaining a backup of the DHCP database protects you from data loss if the DHCP database is lost (for example, due to hard disk failure) or becomes corrupted.

The two backup methods supported by the DHCP server are synchronous (automatic) backups that occur automatically every 60 minutes and asynchronous (manual) backups, performed using the Backup command in the DHCP console. Only manual backups can be used to perform manual database restorations. Automatic backups are used to restore the database only when the DHCP service detects database corruption.

When a backup occurs, the entire DHCP database is saved, including the following:

• All scopes, including superscopes and multicast scopes

• Reservations

• Leases

• All options, including server options, scope options, reservation options, and class options

However, some DHCP data is not saved in any type of backup. For example, credentials specified for DNS dynamic update are not backed up with a manual or automatic backup. (You can configure these credentials through the Advanced tab of DHCP server properties.)

Performing a Manual Backup

To perform a manual backup of the DHCP database, right-click the DHCP server icon in the DHCP console, and then select Backup. To restore the DHCP database, right-click the DHCP server icon in the DHCP console, and then select Restore.

Backup Locations

The default DHCP database manual backup path is \WINDOWS\System32\Dhcp\Backup. You can change the database backup folder by selecting a different local folder during a manual backup or by changing the backup folder location in the DHCP server properties dialog box.

When planning your backup strategy, keep the following in mind:

• When you perform manual backups, you don’t need to stop the DHCP service unless you’re moving your database to a new server.

• The backup folder location must be a local path.

Migrating a DHCP Server

Migrating a DHCP server from one server to another requires you to move the DHCP database to the new server. To move the DHCP database, simply back it up and then restore it to another location, as explained in the following procedures.

To back up the DHCP database (at the source server), complete the following steps:

To restore the DHCP database (at the destination server), complete the following steps:

Manually Compacting a DHCP Server

To support the offline compaction and repair of Jet databases, such as the DHCP database and the Windows Internet Name Service (WINS) database, Windows Server 2003 includes the Jetpack.exe tool.

The DHCP Server service performs dynamic Jet compaction of the DHCP database while the server is online—which reduces the need to use Jetpack.exe for offline compaction. However, offline compaction is a more efficient means of defragmenting the DHCP database than is dynamic compaction.

You should plan to use Jetpack.exe to compact a Jet database periodically whenever the database grows beyond 30 MB. In addition, performing offline compaction is recommended if you receive error messages indicating that the DHCP database has become corrupted.

To manually compact or repair the DHCP database, complete the following steps:

The correct syntax for Jetpack.exe is as follows:

jetpack database_name temporary_database_name
						

The following are example commands to compact the DHCP database:

cd WINDOWS\system32\dhcp
net stop dhcpserver
jetpack dhcp.mdb tmp.mdb
net start dhcpserver

Using Options Classes

An options class is a way for the server to manage options provided to clients within a scope. When an options class is added to the server, clients of that class can be provided class-specific option types for their configuration. Options classes can be of two types:

• Vendor classes are used to assign vendor-specific options to clients identified as sharing a commonly defined vendor type.

• User classes are used to assign options to clients identified as sharing a common need for similar DHCP options configuration.

You can view the available options classes by selecting Define User Classes, Define Vendor Classes, or Set Predefined Options from the server node shortcut menu. By default, the options you can configure for a reservation, scope, or server are those that belong to the DHCP Standard Options vendor class and to the Default User Class user class.

Implementing User Classes

User classes allow you to apply a particular configuration of DHCP options to any subset of DHCP clients you define. To implement a user class, you first define the class at the DHCP server by assigning an ID and set of options for the class. Then you assign selected client computers to the class by using the Ipconfig /setclassid command. When these clients subsequently communicate with DHCP servers, they announce their class ID and inherit the options of that class.

A custom user class is helpful when you need to assign distinct options to distinct sets of client computers. For example, your network might require that clients with the security clearance to bypass the company firewall be assigned a unique default gateway. In this example, you could configure options to distribute the unique default gateway to the security-exempt class.

To create a custom or new user class, right-click the DHCP server icon in the DHCP console and select Define User Classes, as shown in Figure 6.

You then need to set an ID string for the class. You designate this class ID string when you first create the class in the DHCP console, as shown in Figure 7. After defining a new class and specifying an ID string for that class, you can then set desired options for that class.

Finally, for the desired computers to inherit the options of the new class, you need to set the class ID of appropriate client computers to match the ID you have defined for that class at the DHCP server. You can achieve this task by executing the Ipconfig /setclassid command at a command prompt at each client computer. The user class option permits only one class ID to be used for identifying clients. In other words, each client computer can be identified as a member of only a single user class at the DHCP server.

To create a new user or vendor class, complete the following steps:

To set DHCP class ID information at a client computer, complete the following steps:

C:\>ipconfig /setclassid "Local Area Connection" MyNewClassId

Windows IP Configuration

Successfully set the class id for adapter Local Area Connection.