Configuring DHCP with NetWare Cluster Services
Novell Cool Solutions: Feature
By Todd Grant
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Posted: 24 Aug 2000
It has happened again: the power to part of your office has failed and some of the NetWare 5.1 servers are down. Of course you have UPS backup, but the UPS supplying backup power to your DHCP Server has also failed. Some of your users can't connect to the network because they can't get an IP address.
Well, at least this is better than it used to be--when you were an NT shop. Back then, it didn't take a power failure to cause your users to lose network access....but you're trying to forget those days. It's better now, and with the advent of NetWare Cluster Services, it will be even better.
With NetWare Cluster Services, if your UPS fails you're still in business. In such a situation your DHCP Server can be automatically migrated to another NetWare server on your network. In addition, with NetWare Cluster Services you can upgrade or service hardware during business hours, and your users won't lose access to the network or critical network resources.
When configured with NetWare Cluster Services, that DHCP Server you have running on your NetWare network can be accessible virtually all the time.
In this article, I'll tell you how to configure DHCP with NetWare Cluster Services, but first, let's review some of the benefits you gain by doing this.
Benefits of Configuring DHCP for High Availability
A NetWare 5 DHCP server automatically assigns IP addresses and other configuration information to clients upon request or when the clients are restarted. If for some reason the NetWare 5 DHCP server is not accessible, clients lose their ability to connect to the network, because they cannot obtain an IP address. Configuring DHCP with NetWare Cluster Services ensures the IP address range required by users to connect to the network are highly available. This is possible because DHCP configuration information and IP address ranges are stored in NDS, and the DHCP server is automatically started, stopped and restarted on different servers in the cluster by NetWare Cluster Services.
DHCP Installation and Configuration
NetWare Cluster Services 1.01 must be installed before configuring DHCP. NetWare Cluster Services (NCS) 1.01 provides a DHCP resource template, which facilitates configuring DHCP in a cluster environment. The DHCP resource template is not available with NCS 1.0 and is not installed during the upgrade from NCS 1.0 to NCS 1.01. The DHCP resource template is included only during a new installation of NCS 1.01.
Before running DHCP with NetWare" Cluster ServicesTM, DHCP must be configured properly on one server in the cluster. See DHCP documentation for instructions on installing and configuring DHCP with NetWare 5.1.
DHCP server software is included and installed automatically when you install NetWare 5.1, so you only need to configure DHCP once your NetWare 5.1 server is installed. As stated earlier, configure DHCP on only one server in the cluster.
Since all DHCP configuration information and IP address ranges are stored in NDS, no shared storage is required to run DHCP with NetWare Cluster Services.
Any server in the cluster that will run DHCP must have at least a Read/Write NDS replica. This allows NetWare Cluster Services to modify DHCP specific NDS objects regardless of which server in the cluster holds the objects.
NetWare Cluster Services Configuration and Setup
Once DHCP is properly configured on one server in your cluster, you must create and configure a DHCP resource in NetWare Cluster Services. This includes configuring DHCP load and unload scripts; setting DHCP Failover and Failback modes; and assigning the DHCP resource to specific servers in your cluster.
Creating a DHCP Cluster Resource
NetWare Cluster Services includes a DHCP resource template, which greatly simplifies the process for creating a DHCP cluster resource. Much of the DHCP cluster resource configuration is perfomed automatically by the DHCP resource template.
To create a cluster resource:
- In ConsoleOne, browse and select the Cluster object.
- Click File > New > Cluster > Cluster Resource.
- Enter a name for the DHCP cluster resource.
- Enter the DHCP template name in the Inherit From Template field, or browse and select it from the list.
- Click Create.
The DHCP resource template configures the DHCP resource by automatically creating DHCP load and unload scripts, setting failover and failback modes and assigning DHCP as a resource to all nodes in the cluster.
After creating a DHCP resource, view or edit the DHCP resource to ensure it is configured correctly.
Edit DHCP Load and Unload Scripts
The DHCP resource template automatically creates a load and unload script to start and stop the DHCP service on servers in your cluster. The load script contains a command that you must customize for your specific DHCP configuration.
To edit the DHCP load script:
- In ConsoleOne, right click the DHCP Resource object and select Properties.
- Click the Load Script tab on the property page.
The load script will appear similar to the following example:
CLUSTER_START DHCP CN=SERVER.O=ORG.T=TREE
The CLUSTER_START command is necessary to pass DHCP specific configuration information to another server in the cluster should the server where DHCP is running fail. The DHCPSRVR command is used to start the DHCP server (DHCPSRVR.NLM) on a NetWare server.
- Edit the CLUSTER_START command to add the context of the server object for the server in your cluster that has DHCP configured.
For example, if the server context is CN=PROD1.O=SALES.T=ACME_TREE, the load script would appear as follows:
CLUSTER_START DHCP CN=PROD1.O=SALES.T=ACME_TREE
The server context specified in the load script does not change even though during failover the load script will run on another server in the cluster.
- Click the Unload Script tab on the property page to view the unload script. The UNLOAD DHCPSRVR command is by default the only command in the unload script.
Setting DHCP Start, Failover, and Failback Modes
The DHCP resource template sets the DHCP resource Start Mode and Failover Mode to AUTO, and the Failback Mode to DISABLE.
If the DHCP resource Start Mode is set to AUTO, the DHCP server automatically loads on a designated server when the cluster is first brought up. If the DHCP Start Mode is set to MANUAL, you can manually start DHCP on a specific server when you want, instead of having it automatically start when servers in the cluster are brought up.
If the DHCP Failover Mode is set to AUTO, the DHCP server automatically moves to the next server in the Assigned Nodes list in the event of a hardware or software failure. If the DHCP Failover Mode is set to Manual, you can intervene after a failure occurs and before the DHCP server is started on another node.
If the DHCP Failback Mode is set to DISABLE, the DHCP server will continue running on the node it has failed to. If the DHCP Failback Mode is set to AUTO, the DHCP server automatically moves back to its preferred node when the preferred node is brought back online. Set the DHCP Failback Mode to MANUAL to prevent the DHCP server from moving back to its preferred node when that node is brought back online, until you are ready to allow it to happen.
To view or change DHCP Start, Failover, and Failback modes:
- In ConsoleOne, right click the DHCP Resource object and select Properties.
- Click the Policies tab on the property page.
- View or change the Start, Failover or Failback modes for DHCP.
View or Edit DHCP Resource Server Assignments
The DHCP resource template automatically assigns the DHCP resource to all nodes in the cluster. The order of assignment is the order the nodes appear in the resource list. To view or edit DHCP resource node assignments or change the server failover order:
- In ConsoleOne, right click the DHCP Resource object and select Properties
- Click the Nodes tab on the property page.
- View the current DHCP resource server assignments, and if necessary, click the right or left arrow button to assign or unassign servers to the DHCP resource.
You can click the up or down arrow button to change the failover order of the servers assigned to DHCP.
Start the DHCP Server
After you have completed the configuration described above and saved it, start the DHCP Server on your cluster. Bringing down the servers in your cluster and restarting them will automatically start the DHCP Server, but you can also do this without bringing down the servers in your cluster.
To start the DHCP Server without restarting the servers in your cluster:
- Ensure the right half of ConsoleOne displays the Cluster View State by selecting View > Cluster State from the menu at the top of the screen.
- Browse and select the Cluster object that contains the DHCP server resource.
- Click the DHCP Server resource.
- Click the Online button to start the DHCP Server.
Test DHCP Failover
After the DHCP Server is started and running on your cluster, you might want to test it to ensure DHCP still functions properly after moving it to another server in the cluster. To do this migrate the DHCP Server resource to another server in the cluster and verify your clients can still connect and login to the network.
To migrate the DHCP Server resource:
- In ConsoleOne, browse and select the DHCP Server resource.
The DHCP Server resource must be in a running state to be migrated.
- In the Cluster Resource List, select the DHCP Server resource.
The Cluster Resource Manager screen appears, displaying the server the DHCP server is currently running on, and a list of possible servers to which you can migrate it.
- Select a server from the list and click Migrate to move the DHCP Server to the selected server.
Additional NetWare Cluster Services Configuration Information
If you have DHCP clients that are located on a separate LAN segment or IP subnet from the servers in your cluster, some additional configuration is necessary. This additional configuration consists of adding and deleting a secondary IP address on the cluster server where DHCP is running, and setting up your router to forward DHCP requests.
To add a secondary IP address:
- Edit the DHCP cluster resource load script and add the following line:
ADD SECONDARY IP ADDRESS A.B.C.D
Replace A.B.C.D with a unique IP address. Your router will forward DHCP requests from other LAN segments to this address.
- Edit the DHCP cluster resource unload script and add the following line:
DEL SECONDARY IP ADDRESS A.B.C.D
Replace A.B.C.D with the same IP address you added in the load script.
To configure a router to forward DHCP requests, see your router documentation. If you are using a NetWare server as a router, ensure the following line is added to the AUTOEXEC.NCF file of the server.
Replace A.B.C.D with the secondary IP address you specified in the load and unload scripts above.
For additional information on setting up and configuring NetWare Cluster Services, go to the Online NDK.
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