dns in dhcp

Simon Hobson dhcp1 at thehobsons.co.uk
Fri Apr 24 13:22:37 UTC 2009

Adalberto wrote:
>option domain-name "sitestar.net";
>option domain-name-servers 150.161.x.y,150.161.q.z;
>option time-offset -18000; #EST
>default-lease-time 21600;
>min-lease-time 21600;
>max-lease-time 21600;
>ddns-update-style none;
>How does the DNS information in the parameter option 
>domain-name-servers. The machine switches between dns informed?

Hmm, I'm guessing that your question is :

How does the guest use the list of servers supplied ? Does it switch 
between them ?

The answer is that it's entirely up to the client what it does and 
you should NOT do anything that relies on a particular client 
behaviour as that is inviting strange and difficult to diagnose 
problems in the future.

To give an example, some (all ? most ?) versions of Windows have a 
feature whereby they will query the configured name servers in order 
until they get an answer. Some people thought this was a useful 
feature and configured an internal server to only serve internal 
addresses - on the basis that the client would query that, and if it 
didn't get an answer it would go to the next outside nameserver. They 
figured that this was easier than setting up a properly configured 
"split horizon" DNS configuration.
Trouble is, if the internal server goes down, it stops responding, 
and the clients shifts it's address to the bottom of the list (ie 
query it last) - permanently ! I imagine it was "difficult" 
diagnosing why some (but not all) clients had suddenly stopped 
querying the internal nameserver and could no longer access any 
internal services.

Other clients might query servers at random, query all servers, keep 
statistics on which responds fastest and use that in preference, or 
any other behaviour they want. So it's important that all servers you 
specify should return exactly the same records in response to any 

Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

More information about the dhcp-users mailing list