DHCP servers store DHCP lease and reservation information in database files. By default, these files are stored in the %SystemRoot%System32DHCP directory. The key files in this directory are used as follows:
- Dhcp.mdb The primary database file for the DHCP server
- J50.log A transaction log file used to recover incomplete transactions in case of a server malfunction
- J50.chk A checkpoint file used in truncating the transaction log for the DHCP server
- Res1.log A reserved log file for the DHCP server
- Res2.log A reserved log file for the DHCP server
- Tmp.edb A temporary working file for the DHCP server
Backing Up the DHCP Database
The Backup directory in the %SystemRoot%System32DHCP folder contains backup information for the DHCP configuration and the DHCP database. By default, the DHCP database is backed up every 60 minutes automatically. To manually back up the DHCP database at any time, follow these steps:
1. In the DHCP console, right-click the server you want to back up, and then click Backup.
2. In the Browse For Folder dialog box, select the folder that will contain the backup DHCP database, and then click OK.
Registry keys that control the location and timing of DHCP backups, as well as other DHCP settings, are located under HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesDHCPServerParameters.
The following keys control the DHCP database and backup configuration:
- BackupDatabasePath Sets the location of the DHCP database. You should set this option through the DHCP Properties dialog box. Click the Advanced tab, and then set the Database Path field as appropriate.
- DatabaseName Sets the name of the primary DHCP database file. The default value is DHCP.mdb.
- BackupInterval Determines how often the DHCP client information database is backed up. The default is 60 minutes.
- DatabaseCleanupInterval Determines how often the DHCP service deletes expired records from the DHCP client information database. The default is four hours.
Restoring the DHCP Database from Backup
In the case of a server crash and recovery, you might need to restore and then reconcile the DHCP database. To force DHCP to restore the database from backup, follow these steps:
1. If necessary, restore a good copy of the %SystemRoot%System32DHCPBackup directory from the archive. Afterward, start the DHCP console, right-click the server you want to restore, and then click Restore.
2. In the Browse For Folder dialog box, select the folder that contains the backup you want to restore, and then click OK.
3. During restoration of the database, the DHCP Server service is stopped. As a result, DHCP clients are temporarily unable to contact the DHCP server to obtain IP addresses.