Best Practice Ezine #44 – DHCP

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Best Practice Ezine #44 DHCP

DHCP Litmus test

Amateurs configure clients IP addresses manually, professionals use DHCP. Only last year I visited an organization which took on a contractor just to change all the default gateways on 550 client PCs. ‘Our boss does not trust DHCP’ was the reason they gave me. ‘Your boss is a dinosaur’, I thought to myself.

DHCP Conflict Detection Attempts

I love cost nothing solutions.  On the whole scheme of things setting DHCP Conflict Detection Attempts is not very important, but in terms of satisfaction it rates high.

The whole point of DHCP is to eliminate the bane of a network administrator’s’ life – duplicate IP addresses.  Conflict Detection goes that extra mile, and before DHCP actually gives out an IP address, it tests by pinging the proposed IP number.  If no machine answers this ping, then DHCP goes ahead and gives out the IP address.  Once you realize how it works perhaps you can see why you only need to set Conflict Detection to 1 attempt (certainly no more than 3).

To set Conflict Detection Attempts, navigate to the DHCP Server Icon, Properties, Advanced Tab.

Carol kindly wrote in and suggested instructions to find this setting, hence the screen shot opposite.  Please do write in if you have any suggestions about my ezines.

Mad Mick told me that one of his customers set the Conflict Detection value to 99999 and wondered why XP machines took ages before they got their IP addresses.

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WPAD (Web Proxy Auto Detect)

WPAD (Web Proxy Auto Detect) is easyish to configure but hard to understand.  Here is my challenge, setup WPAD for your DHCP clients.

Incidentally, WPAD is a classic case of DNS and DHCP working hand in hand.  Here is my plan.  DHCP not only gives the XP clients their IP address, Default Gateway (Option 003) and DNS Servers (Option 006), but also the WPAD ISA server (Option 252).  The idea is that DHCP gives each client the location of the ISA Server name,  port number, and wpad.dat file.

Instructions to add WPAD to DHCP

  1. Navigate to the DHCP server icon (Not the scope)
  2. right-click the server icon, Set Predefined Options
  3. Click the Add (Button)
  4. Enter WPAD in the Name box
  5. Change the data type to String
  6. In the Code box enter 252
  7. Crucial point, press enter
  8. Now you should see Value: String.  Type in the box:
    http:// isaservername:80/wpad.dat
  9. Next step go to the very Scope Options, add 252 WPAD (Just as you would add type 006 DNS)

One more point, go to DNS and add a CName alias for the very name WPAD.  Point the WPAD alias to your ISA server, naturally this server should also have a Host (A) record in DNS.

See more on DHCP – Whole Section here

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