Managing Client Leases and Reservations

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from Chapter 14, Windows NT Administrator's Pocket Consultant by William R. Stanek.

When you create a scope, you specify that client leases should be assigned either with a fixed expiration date or with an unlimited expiration date. A fixed expiration date is usually the best option. You can then supplement this with address reservations for clients that need semipermanent IP addresses.

Managing Client Leases

You manage active leases using the Active Leases dialog box shown in Figure 14-6. To open this dialog box, select the scope in the DHCP Manager and then choose Active Leases from the Scope menu. The primary fields of this dialog box are used as follows:

• Total Addresses in Scope Shows the total number of IP addresses assigned to the scope.

• Active/Excluded Shows the total number of active or excluded addresses as a numerical value and a percentage of the total available addresses.

  Tip Think of the Active/Excluded field as the number of addresses used up. If the percentage total reaches 85 percent or more, you may want to consider assigning additional addresses or freeing up addresses for use.

  

  Figure 14-6: The Active Leases dialog box allows you to view or modify lease settings.

• Available Shows the total number of addresses available for use as a numerical value and a percentage of the total available addresses.

• Client Shows the active leases or reservations for the currently selected scope. Clients are listed by IP address and by client name. If the IP address is reserved, the listing is followed by the keyword Reservation. Select the Show Reservations Only check box to only display reservations in the Client list box.

• Properties Displays the properties of the lease or reservation selected in the Client list box.

• Delete Deletes the currently selected lease or reservation.

• Reconcile Reconciles the client leases and reservations against the DHCP database on the server. This is useful if you want to ensure that the list of leases shown is actually in use.

• Refresh Refreshes the listings in the Client list box.

Reserving DHCP Addresses

DHCP provides several ways to provide permanent addresses to clients. One way is to use the Unlimited setting in the Scope dialog box to assign permanent addresses to all clients that use the scope. Another way is to reserve DHCP addresses on a per-client basis. When you reserve a DHCP address, the client is always assigned the same IP address by the DHCP server, and you can do so without sacrificing the centralized management features that make DHCP so attractive.

Figure 14-7: Use the Add Reserved Clients dialog box to reserve an IP address for a client.

To reserve a DHCP address for a client, follow these steps:

1. In the DHCP Manager, select the scope you want to work with and then choose Add Reservation from the Scope menu. This opens the dialog box shown in Figure 14-7.

2. In the IP Address field, enter the IP address you want to reserve for the client. Note that this IP address must be within the valid range of addresses for the currently selected scope.

3. The Unique Identifier field specifies the MAC (media access control) address for the client computer's network adapter card. You can obtain the MAC address by typing the command net config wksta at the Command prompt on the client computer. This value must be typed exactly for the address reservation to work.

4. In the Client Name field, enter the computer name for the client. This field is used for identification purposes only and doesn't affect the client's actual computer name.

5. Enter an optional comment in the Client Comment field if you like.

6. Select Add to create the address reservation.

Releasing Addresses and Leases

When you work with reserved addresses, there are a couple of caveats you should know about:

• Reserved addresses aren't automatically reassigned. So if the address is already in use, you'll need to release the address to ensure that the appropriate client can obtain it. You can force a client to release an address by terminating the client's lease or by logging on to the client and typing the command ipconfig/release at the Command prompt.

• Clients don't automatically switch to the reserved address. So if the client is already using a different IP address, you'll need to force the client to release the current lease and request a new one. You can do this by terminating the client's lease or by logging on to the client and typing the command ipconfig/renew at the Command prompt.

  

  Figure 14-8: The Client Properties dialog box allows you to modify lease and reservation properties. For details on the fields in this dialog box, see the section of this chapter titled "Reserving DHCP Addresses."

Modifying Lease and Reservation Properties

You can modify the properties of leases and reservations by doing the following:

1. Select the scope that contains the lease or reservation you want to modify and then choose Active Leases from the Scope menu.

2. In the Active Leases dialog box, select the lease or reservation you want to modify in the Client list box and then select Properties. This opens the dialog box shown in Figure 14-8.

3. You can now modify lease and reservation properties. Fields that are shaded can't be modified. Other fields can be modified. These fields are the same fields described in the previous section.

4. To configure lease-specific or reservation-specific options, click on the Options button. You can now configure DHCP options that only apply to this lease or reservation.

Deleting Leases and Reservations

You can delete active leases and reservations by completing the following steps:

1. Select the scope that contains the lease or reservation you want to modify and then choose Active Leases from the Scope menu.

2. In the Active Leases dialog box, select the lease or reservation you want to delete and then click on the Delete button.

3. The lease or reservation is now removed from DHCP. However, the client isn't forced to release the IP address. To force the client to release the IP address, log on to the client that holds the lease or reservation and type the command ipconfig/release at the Command prompt.

Backing Up and Restoring the DHCP Database

DHCP servers store DHCP lease and reservation information in database files. By default, these files are stored in the %SystemRoot%\System32\dhcp directory. The key files in this directory are used as follows:

• DHCP.MDB The primary database file for the DHCP server.

• DHCP.TMP A temporary working file for the DHCP server.

• *.LOG Transaction log files used to recover incomplete transactions in case of a server malfunction.

• *.CHK Check files for the DHCP server.

The Backup Directory

The backup directory in the %SystemRoot%\System32\dhcp folder contains backup information for the DHCP configuration and the DHCP database. By default, the DHCP database is backed up every 60 minutes. Registry keys that control the location and timing of DHCP backups as well as other DHCP settings are located in the folder

     HKEY_LOCAL_MACHINE
     \SYSTEM
     \CurrentControlSet
     \Services
     \DHCPServer
     \Parameters

Restoring the Database from Backup

If you want to force DHCP to restore the database from backup, follow these steps:

1. Stop the DHCP Server service by using the Services utility in the Control Panel.

2. Restore a good copy of the %SystemRoot%\System32\dhcp\backup directory from a tape or other archive source.

3. Use the Registry Editor to edit the key HKEY_LOCAL_MACHINE \SYSTEM \CurrentControlSet \Services \DHCPServer \Parameters. Set the key's RestoreFlag value to 1.

4. Start the DHCP Server service.

from Windows NT Administrator's Pocket Consultant by William R. Stanek. Copyright © 1999 Microsoft Corporation.

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