Best Practice Ezine #33 – DHCP Challenges

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Best Practice Ezine #33 Guy’s 5 DHCP Challenge

How well do you know DHCP in Windows 2003?

Guy’s Assumption:  That you know DHCP well enough to create scopes and set options e.g. Type 003 Router.  Now I thought that I knew DHCP pretty well, until one day I decided to explore thoroughly the DHCP snap-in.  My journey into the deepest DHCP menus was full of surprises.  I would like to share with you the results of my discoveries, by way of 5 ‘Guy challenges’.

1) Authorize and Activate

Suppose that you see a red down-arrow on the DHCP server, what could that red arrow mean?  Well if the red arrow is on the very server, the chances are the server has not been authorized in Active Directory.  On the other hand if the red down-arrow in on an individual scope, the probability is that the scope has been deactivated.  If you are telling me that all your arrows are green, then your DHCP server is up and running and ready to offer IP addresses to client machines.

2) How would you backup DHCP?

An easy challenge, to backup the DHCP database, right-click the DHCP server icon, and select backup from the short cut menu.  Did you know that the DHCP database was automatically backed up every hour? 

A tougher challenge is to compact the DHCP database, navigate to the %systemroot%\system32\dhcp folder and run jetpack /?  I confess; you rarely need to compact the DHCP database because, it does not take up much space in the first place. In any event, do you accept my technical challenge to run jetpack?

Guy Recommends: The Free IP Address Tracker (IPAT) IP Tracker

Calculating IP Address ranges is a black art, which many network managers solve by creating custom Excel spreadsheets.  IPAT cracks this problem of allocating IP addresses in networks in two ways:

For Mr Organized there is a nifty subnet calculator, you enter the network address and the subnet mask, then IPAT works out the usable addresses and their ranges. 

For Mr Lazy IPAT discovers and then displays the IP addresses of existing computers. Download the Free IP Address Tracker

3) Reconcile the Database

I bet that you have never tried to Reconcile your DHCP database.  99% of the time, nothing happens, once in a blue moon you get errors.  Where is that Reconcile menu, on the server or the scope?  Trick question – right-click either and you will see Reconcile half way down the short cut menu.

4) Predefined Options – Example WPAD 252

Another easy bet.  Guy says that you have never created or set a Predefined Option.  A better question would be why would you need a Predefined Option!  One answer is because you wish to give out the Web Proxy Auto Detect (WPAD) address of the ISA server.  If you accept this challenge, right-click the DHCP server Icon (not the Scope), select Set a Predefined option.  Look out for the Add button, Name = WPAD, Code = 252, Data Type = String (not Binary). String value http:// ISA-yourServer: 80 /wpad.dat.  I admit that if wish to do this for real then research into WPAD and ISA, nevertheless you can still complete my challenge by entering a pretend name until you sus out the real setting.

5) Mission impossible – configure a DHCP User Class.

I vote creating a DHCP user class, and getting it to work is the toughest configuration challenge in DHCP, in fact setting User Classes is one of the hardest individual tasks in the whole of computing.  If you like a challenge this is for you.

To put this example into context, imagine you have 8 kiosk computers that all need a special router.  All the other 50 machines in the DHCP scope need a regular router or default gateway. 

So here are the three stages of mission impossible.

1) You right-click the DHCP server icon, select Define User Classes, give it a Display Name and an optional description.  The crucial part is to enter an ASCI name, for example xyzname. 

2) Next you navigate to the scope.  Look for the xyzname in the Scope Options (Advanced Tab), User Options, choose 003 Router in the Available Option column, enter the IP address.

3) Finally, you go to the DHCP client (e.g. XP machine with Automatic IP address setting) and type IPCONFIG /setclassid xyzname.  Congratulations, you have achieved Mission impossible.

See more about DHCP here

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