DHCP Service Database Files

When you install the DHCP service, the files shown in Table 4.10 are automatically created in the % SystemRoot %\System32\Dhcp directory.

Table 4.10 Database Files and Descriptions

File	Description
J50.log and J50 xxxx .log	A log of all transactions done with the DHCP database. This file is used by DHCP to recover data if necessary. To increase speed and efficiency of data storage, the Jet database writes current transactions to log files rather than to the database directly. Therefore, the most current view of the data is the database plus any transactions in the log files. These files are also used for recovery if the DHCP service is stopped in an unexpected manner. If the service is stopped in an unexpected manner, the log files are automatically used to recreate the correct state of the DHCP database. Log files are always a certain size; however, they can grow quickly in number on a very busy DHCP server. It is inevitable that DHCP will write more transactions to a log than the size of the log can accommodate. When a log file becomes filled, it is renamed to indicate that it is an older log and not in use. A new transaction log is created with the J n .log filename (where n is a decimal number), such as J50.log. The naming format of the previous log file will be Jet xxxxx .log, where each x denotes a hexadecimal number from 0 to F. Previous log files are maintained in the same folder as the current log files. The log files are processed (all log entries written to the database) and deleted when a successful backup occurs or when the DHCP server is shut down gracefully. Therefore, if many J n .log files have accumulated, frequent backups should be scheduled to maintain the logs. After the entries have been processed, it is possible to manually delete the log files; however, this prevents a successful recovery of the database if it is needed. Because of this, it is important that the log files not be manually deleted or removed from the system until a backup has been performed.
J50.chk	A checkpoint file that indicates the location of the last information successfully written from the transaction logs to the database. It is also used for recovery purposes—that is, the checkpoint file indicates where the recovery or replaying of data should begin. This checkpoint file is updated every time data is written to the database file (DHCP.mdb).
Dhcp.mdb	The DHCP service database file that contains two tables: the IP-address-Owner-ID mapping table and the name–to–IP address mapping table.
Dhcptmp.mdb	A temporary file that is created by a DHCP server. This file is used by the database as a swap file during index maintenance operations and may remain in the % SystemRoot %\System32\DHCP directory after a crash.
Res x .log	These are reserved log files that are kept for emergency purposes. They are used if the server runs out of disk space. If a server attempts to create another transaction log file and there is insufficient disk space, the server flushes any outstanding transactions into these reserved log files. The service then shuts down and logs an event to the event log.

DHCP uses the Jet database format for storing its data. Jet produces J n .log and other files in the % SystemRoot %\System32\DHCP folder to increase the speed and efficiency of data storage.

Caution

The J50.log, J50 xxxxx .log, Dhcp.mdb, Dhcptmp.mdb, and Res x .log files should not be removed or tampered with in any manner.