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DHCP Database Topics
Perhaps you have never given the DHCP database a thought. Well the time
may come where you need to backup, reconcile or compact the DHCP database.
At the very least, I expect you will want to check your lease statistics. Tutorial Topics for DHCP Database
Bizarrely, I find 'Display Statistics' the most difficult tab to find on the
DHCP server. This is because it's the first item on the menu, and is often
masked by the very highlight that should attract me. I just hope that you
are not afflicted by this blind spot.
In any event, Display Statistics is one of the most interesting and rewarding
items on the
DHCP menu.
Display Statistics is available at both the server and the scope level. My first
thought is, 'How many leases are left?' Next, I look to see if there have been
any NACKS or lease declines. See the screen shot to find the Display
Statistics... menu.
DHCP has its own database. Stored in this DHCP.mdb are the addresses,
scopes and leases of the clients. Understanding this database will
help you backing up and restore a DHCP server.
Check out this folder: %systemroot%\system32\dhcp\dhcp.mdb
As time goes by the database will grow, and best practice dictates that you
should consolidate the database by freeing up space taken up by old leases.
The procedure for compacting the dhcp.mdb database is this.
1) Stop the DHCP service. Either right click the DHCP Server icon,
select All
tasks then Stop. Alternatively, go to the command line and type: NET Stop DHCPServer. (For once the
command really is DHCPserver, NOT DHCPyourservername.)
2) At the command line, navigate to: %systemroot%\system32\dhcp\dhcp.mdb.
3) Jetpack dhcp.mdb temp.mdb. What this does is copies the existing database,
compacts it, then copies it back to the original location - clever.
4) Remember to restart DHCP. Either use the GUI, or if you are at the
command line, NET Start DHCPServer
Warning: Do not 'mess' with any of the files that you find in the
%systemroot%\system32\dhcp folder, if you do, then DHCP will stop working and
you will either have to restore, or else re-install DHCP.
 Learn more about DHCP. As an MCT trainer, I can thoroughly recommend TrainSignal
because they provide practical hands on training. In particular, I like the way TrainSignal cover all learning methods, instructor lead, video and of course text
material. With TrainSignal you can either take one module, for example DHCP or go for a combination of modules.
See more about DHCP here
The first surprise is that this dhcp.mdb database is backed up every hour.
The second surprise is that the old jetpack database engine controls the
database.
The third surprise is that you can only automatically backup to a
local folder. Should you need a 'real' backup, then just backup the files
in the %systemroot%\system32\dhcp\Backup\ directory.
Finally, and unsurprisingly, there is a GUI to backup the DHCP database.
Reconcile is a technical term for making sure that DHCP information is
consistent. What can happen is that when you restore
a database, there is a miss-match between lease information in the database and the
same information in the DHCP server's
registry. Specifically, the registry just stores
basic or summary information, the detailed information from a recent restore
maybe different, so a 'Reconcile' will ensure that the database and registry
data are consistent once more.
Bear in mind that you can either reconcile individual scopes or choose all
scopes. It all depends on the 'focus' of what you select. If you
select an individual scope that that is the limit of what you can reconcile,
however, when you select the server icon, you can: Reconcile All Scopes - see
diagram above. (not below)
Once you setup DHCP auditing, you get a separate log for each day of the
week. The logs are a wealth of information, not just about the health of
the server, but also which machine gets which IP address for how long.
To setup Auditing, right click the DHCP server, then select properties.
Auditing removes the last reason for having static IP addresses,
accountability. Die-hards, Luddites who resist DHCP often justify static
IP addresses on the grounds that you can always account for which machine had
which IP address on a particular day. With DHCP Auditing you can achieve
the same result and have all the benefits of central administration, instant
control over default gateways and a raft of other options.
The DHCP database is full of surprises. Discover how many leases you
have issued with 'Display Statistics. Next discover where the database is situated, and master how
to compact the dhcp.mdb. Also, understand where you can Reconcile the
scopes. Finally, check whether or not auditing is set up on your
DHCP server.
Related DHCP Server topics
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