DNS misconfiguration

The number one configuration error I have seen is specifying DNS that give different answers. Take a look at the following example:

(Note it doesn't matter whether the DNS servers are assigned manually as in the example, or by DHCP - the principle is the same.)

In this example the administrator understands that a PC needs to resolve both local and Internet domain names. In an attempt to achieve this, the Preferred DNS server has been filled in with the local server and the Alternate DNS server has been filled in with the ISP's server. This is wrong and this will cause bad things to happen.

Windows relies on Active Directory, and AD relies on DNS, so this error will cause all sorts of problems from long delays at logon through to complete failures of services. And worst still, the failures will be intermittent and seemingly random as inconsistent DNS information is returned and cached.

The correct way to configure DNS is to specify only the local DNS server(s) - for most small businesses this will mean the 2nd entry is left blank as there will only be one local DNS server on your network. Now PCs will be able to resolve local hostnames consistently. So how do you get PCs to also resolve Internet domain names? You don't do it on the PC - you do it on the DNS server. Configure your DNS server to forward any requests it doesn't know about to your local ISP.

A further benefit is your DNS server will cache Internet queries improving performance and reducing your bandwidth bill.

Thane Brooker
© 8Networks, October 2007

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