Avoid Conflicts
Regardless of which procedure you use to restore your DHCP database and DHCP services, a good practice at this point is to enable DHCP's IP address conflictdetection feature. This feature, which is present in Win2K and NT 4.0 Service Pack 2 (SP2) and later, uses a ping test to verify that an IP address isn't already in use before the DHCP server assigns the address to a DHCP client. After a restore operation, a DHCP server might not be up-to-date and can give out addresses that are already in use, so verify that this feature is enabled before you put the server back in service.
To verify that the conflict-detection feature is enabled on Win2K DHCP servers, right-click the server name in the left pane of the DHCP snap-in, select Properties, then click the Advanced tab. On this tab, set the Conflict Detections Attempt value to any integer greater than zero. The value you set tells the server to make that many ping attempts to the IP address before allocating the address to a client.
On NT 4.0 DHCP servers, select Properties from the DHCP Manager utility's Server menu. You'll find the Conflict Detection Attempts setting on the resulting dialog box's General tab.
Preparing for DHCP Disaster
In addition to knowing the techniques for recovering failed DHCP services, you can take a few preparatory steps that will make recovery easier. First, periodically run Jetpack on DHCP databases to compact them and verify their integrity. Second, regularly use the DhcpExim utility to export and back up the configuration of each of your DHCP servers, then store this data in a separate location (i.e., not on the server you're backing up). Third, if appropriate, edit the registry to increase the frequency of automatic DHCP database backups. Fourth, consider using Win2K and NT's built-in replication features or a third-party utility to replicate the DHCP database to other locations on the network. Finally, if you're running a clustered environment, consider implementing DHCP as a clustered service.
These steps, in addition to familiarity with DHCP recovery techniques, will ease the recovery process in the event of a failure. In a later article, I'll help you round out your recovery knowledge with a discussion about disaster prevention and recovery features for WINS and AD.
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Sean Daily's article lists several steps to prepare for DHCP disaster. I'd like to replicate the DHCP database to another location. The article mentions using Windows 2000 or Windows NT's replication feature to perform this task. How do you do that, and which file do you replicate (e.g., dhcp.mdb in \%systemroot%\system32\dhcp\backup\jet\newfolder)? I was thinking of using a batch job to pull all the DHCP databases in our enterprise to one location for our administrators to access. Would you recommend that we pull the backup copy of dhcp.mdb that Win2K creates?<br><br>
Rick De Bucce<br>
I'd handle this task exactly as you suggest: Create a job on each server to stop the service, back up the current database to a centralized location (perhaps with a subfolder of the server's name or some similar method of keeping different DHCP server databases labeled and separated), and restart the DHCP service. The location you mentioned would be the backup of the database*you'll probably want to back up the current version of dhcp.mdb in the \%systemroot%\system32\dhcp folder.<br><br>
Sean Daily<br>
Rick De Bucce February 05, 2002