Exercises 6-4 and 6-5 show you how to add global and scope DHCP options using the DHCP Manager.

1. Log on as an Administrator.
2. Click the Start button and choose Programs | Administrative Tools | DHCP Manager.
3. The first DHCP option you are going to set is the DNS server address for all DHCP scopes. Select the DHCP Options menu and choose Global….
4. Highlight DNS Servers in the Unused Options: box and click the Add -> button.
5. Click the Value > > > button.
6. Click the Edit Array… button.
7. Type 172.16.12.5 in the New IP Address: dialog box.
8. Type Global DNS Server in the Server Name: dialog box.
9. Click the Add -> button and then the OK button.
10. Click the OK button. A globe icon is displayed to the left of DNS Servers within the Option Configuration window. Leave the DHCP Manager open for the next exercise.

1. Highlight the 172.16.12.0 scope that was created in Exercise 2.
2. Select the DHCP Options menu and choose Scope….
3. Highlight Router in the Unused Options: box and click the Add -> button.
4. Click the Value >>> button.
5. Click the Edit Array… button.
6. Type 172.16.12.1 in the New IP Address: dialog box.
7. Type Router for the .12 subnet in the Server Name: dialog box.
8. Click the Add -> button and then the OK button.
9. Click the OK button. You should see a series of computers on an icon to the left of Router within the Option Configuration window. If the Local Machine had more than one scope, you would see that the routeris displayed only for the .12 subnet.

Now that you know how to create scopes in DHCP Manager, here is a quick reference for possible scenario questions, and the appropriate answer:

Manipulating Client Leases

It is not difficult to configure a DHCP server to lease IP addresses to DHCP clients. But a situation may occur in which you need to see what IP addresses are leased, especially if you need to track down a client that has a certain IP address.

Viewing Active Leases

You can view the active leases for a scope by choosing Active Leases… from the Scope menu in DHCP Manager. The Active Leases dialog box not only shows you information about active leases for the scope you currently have selected but also several other statistics. The other statistics shown include the total number of addresses in the scope, the number and percentage of addresses that are unavailable because they are active or excluded, and the number and percentage of addresses that are currently available, as shown in Figure 6-8.

The count of active leases and excluded addresses shown in Active/Excluded is an aggregate so it does not just give you statistics about the active leases. However, when you deactivate a scope, the Active/Excluded count reflects only excluded addresses. If you need to know the number of active leases, then you can compare the Active/Excluded count when the scope is deactivated and activated.

Even though a lease is set for a certain period of time, it does not disappear automatically when the lease expires. The DHCP server database retains the lease for approximately one day after it expires. The expired lease is maintained in the DHCP server database to provide a grace period for the client. Your probably wondering why the client needs a grace period. A few of the reasons are to protect a client’s lease in case the client and server are located in different time zones; it also compensates for a time difference in case the DHCP client’s and DHCP server’s clocks are not synchronized; or in case the client computer was not on the network when the lease expired. Expired leases are identified by a dimmed icon and are included in the list of active clients in the Active Leases dialog box as well as the aggregate Active/Excluded count.

To view when a lease will expire, highlight the client and click the Properties… button. The lease expiration is shown at the bottom of the window as shown in Figure 6-9.

By highlighting an IP address in the Client window and clicking the Delete button, you can delete the lease for a DHCP client listed in the scope. A lease is normally deleted because it conflicts with an IP address exclusion or a client reservation that you want to add. Deleting a lease causes the same result as if the client’s lease expired. The client has to start the process from the beginning of the initialization state and obtain a new IP address and configuration information from a DHCP server. Be careful when you delete a lease. Make sure that you delete only entries for clients that are no longer using the lease. Deleting a client that is actively using a lease could result in duplicate IP addresses on the network because deleted addresses can be assigned to new clients immediately.

Viewing Client Reservations

Viewing client reservations is very similar to the method used for viewing active leases. Using DHCP Manager, choose Active Leases… from the Scope menu. Client Reservations are shown in the Client window along with all the active leases for the scope. You can scroll down the list or if you want to isolate only client reservations, you may choose the checkbox located to the left of Show Reservations Only. Figure 6-10 shows the client reservations for the 172.16.12.0 subnet by filtering out the active leases, using Show Reservations Only. The only reservation is the one you created in Exercise 6-3.

As mentioned earlier, it is possible to set DHCP options for a client that has a reservation. Exercise 6-6 shows you how to add a client option using the DHCP Manager.

1. Log on as an Administrator.
2. Click the Start button and choose Programs | Administrative Tools | DHCP Manager.
3. Highlight the 172.16.12.0 scope and choose Active Leases… from the Scope menu.
4. Highlight 172.16.12.50 and choose Properties….
5. Click the Options… button in the Client Properties window.
6. Scroll down the Unused Options: list and highlight Domain Name.
7. Click the Add - > button.
8. Click the Value > > > button.
9. Type exercise6.com in the String dialog box and click the OK button.
10. Click the OK button.
11. Click the OK button.

The DHCP client that is assigned the IP address of 172.16.12.50 will use exercise6.com as its domain name for client resolutions.

For purposes of the exercise, there was a reserved client already visible when you opened the Active Leases window. On a scope that is used in a production environment you may want to click the check box located to the left of Show Reservations Only so that only the clients with a reservation are displayed.

Manipulating an IP Address with IPCONFIG

IPCONFIG is a command-line utility that displays current TCP/IP configuration information for each NIC in a Windows NT system. IPCONFIG is very useful when you need to know the IP address assigned to a Windows NT DHCP client, especially since the client could receive a different IP address every time it boots.

Typing IPCONFIG without any switches will display the IP address, subnet mask, and default gateway for all NICs that are bound to TCP/IP as shown in Figure 6-11.

Using IPCONFIG without any switches may tell you all the information that you want to know. However, there are many other items of information that you can glean from IPCONFIG if you use the /all switch. Using IPCONFIG /all displays all of the TCP/IP configuration information for each NIC, including DHCP information. Figure 6-12 shows the output of using IPCONFIG /all for the same system shown in Figure 6-11.

There are two other switches that are very useful when using IPCONFIG in a DHCP environment.

Renew Address

The first switch is IPCONFIG /renew. IPCONFIG /renew actually serves two purposes. It can renew an existing lease for an IP address and it can also obtain a new IP address if you have released the address using another switch. If your system has multiple NICs, then you need to specify which NIC to renew the address for by typing IPCONFIG /renew [adapter]. Replace [adapter] with the name of your NIC. Figure 6-13 shows a lease being renewed for a Windows NT DHCP client. Does the output on the screen look familiar? It should, because it is the same as what you see using an IPCONFIG with no switches, however your lease is renewed.

Release Address

The second switch is IPCONFIG /release. The /release switch can be used to release the IP address obtained from a DHCP server, so that the NIC no longer has an IP address. If your system has multiple NICs, then you need to specify which NIC to release the address for by typing IPCONFIG /release [adapter]. Replace [adapter] with the name of your NIC. Figure 6-14 shows the IP address being released from a system.

Creating a DHCP Relay Agent

As discussed earlier in this chapter, any computer running Windows NT workstation or server can be a DHCP Relay Agent. Successful configuration of a DHCP Relay Agent is a two-step process. The first step is the installation of the DHCP Relay Agent service. After installation of the DHCP Relay Agent service, you can move to the second step, which is configuring the parameters for the service. Use the Network applet in the Control Panel to install the service and configure the agent.

Installing and Configuring the DHCP Relay Agent Service

The procedure for installing the DHCP Relay Agent is the same as for any other service. Figure 6-15 shows the window for installing network services. Exercise 7 shows you how to install the DHCP Relay Agent service for use on your system.

1. Log on as an Administrator.
2. Click the Start button and select Settings | Control Panel.
3. Double-click the Network applet.
4. Select the Services tab.
5. Click the Add… button.
6. Highlight DHCP Relay Agent from the Select Network Service window and click the OK button.
7. Click the Close button.
8. After the bindings are reviewed, the system notifies you to shut down and restart so the new settings can take effect. Click the Yes button to restart your system.

Now that the DHCP Relay Agent is installed on your system, it is time to configure it for use on your network. Exercise 6-8 shows you how to configure the DHCP Relay Agent so that it can act as a router for the DHCP broadcast message sent from the clients on the subnet.

1. Log on as an Administrator.
2. Click the Start button and select Settings | Control Panel.
3. Double-click the Network applet.
4. Select the Protocols tab.
5. Highlight TCP/IP Protocol and click the Properties… button.
6. Select the DHCP Relay tab.
7. Ensure that the Seconds threshold: and Maximum hops: boxes are set to 4.
8. Click the Add… button.
9. Type 172.16.12.8 in the DHCP Server: dialog box. The address appears in the DHCP Servers window.
10. Click the OK button.
11. Click the Close button.
12. The system notifies you to shut down and restart so the new settings can take effect. Click the Yes button to restart your system.

Figure 6-16 shows the DHCP Relay tab with a server identified where all DHCP requests from clients are forwarded.

Maintaining a DHCP Database

DHCP data, such as active IP address leases and excluded addresses, are stored by the DHCP server service in a database. The following files are located in the %systemroot%\system32\dhcp folder:

Don’t try to open the DHCP database file using Microsoft Access. Even though the file ends with an .mdb extension, Access does not recognize the format.

Database Maintenance

The DHCP database is automatically backed up to the %systemroot%\system32\dhcp\backup\jet folder every 60 minutes. The default backup interval can be changed by modifying the following registry key:

JETPACK Utility

Microsoft recommends that you compact the DHCP database when it reaches 30Mb. The DHCP database is automatically compacted by Windows NT Server 4.0 so you should not have to run the following procedure very often.

JETPACK is the command-line utility that compacts the DHCP database. The syntax for JETPACK is: JETPACK <DHCP database name> <temp database name>. For example:

Before you can use JETPACK, the DHCP server service must be stopped.

Certification Summary

There are two methods to assign an IP address to a system on your network. The first method is to assign the address by manually entering the correct information for each machine. The second method is to configure a DHCP server and let it automatically issue the IP address to each client connected to the network. Allowing the DHCP server to issue the IP addresses eliminates the possibility of errors that could occurs from manual entry.

A DHCP client goes through a number of different states during the process of receiving an IP address from the DHCP server. During each state, messages are exchanged between the DHCP client and the DHCP server until the DHCP client is fully bound with an IP address.

If you decide to use a single DHCP server across subnets, then the routers must support RFC 1542. This is required so that the broadcasts from the DHCP clients can reach the single DHCP server. Using multiple DHCP servers on different subnets increases redundancy. The multiple DHCP servers can be managed from a central location using the DHCP Manager.

The system that you are using for a DHCP server must have a static IP address. The static IP address is required so that DHCP clients are able to find the DHCP server when renewing their IP address.

A scope is required for each subnet that has DHCP clients. It contains a pool of IP addresses to be issued, as well as other configuration parameters such as a subnet mask and default gateway. Scopes are created using the DHCP Manager and each scope can have different options. Besides scope options, there are also global options, default options, and client options. Client options can be applied only to a DHCP client that has a reserved IP address.

Client leases for a scope can be viewed from the Active Leases window. You can also delete a lease by highlighting the IP address and clicking the delete button while in the Active Leases window.

The IPCONFIG command-line utility can be used to determine the IP address for a DHCP client, as well as other TCP/IP configuration information. You can also renew or release an IP address by using the correct switch with IPCONFIG.

If your routers do not comply with RFC 1542, then it is possible for you to set up a Windows NT workstation or server as a DHCP Relay Agent. Once configured, the DHCP Relay Agent will forward requests for IP addresses from DHCP clients to the correct DHCP server.

The DHCP database is named dhcp.mdb and is located in the %systemroot%\system32\dhcp folder. It is automatically backed up into the %systemroot%\system32\dhcp\backup folder every 15 minutes. If your DHCP database is larger than 30 Mb then you may want to run the JETPACK command-line utility to compact it. Prior to using JETPACK, the DHCP server service must be stopped.

Two-Minute Drill