dns in dhcp

Adalberto acosta at ufpe.br
Fri Apr 24 14:51:59 UTC 2009

Simon Hobson

What I really want to know was, if a Windows machine that can not connect to 
server dns 150.161.xy, he will use the next dns 150.161.qz automatically?

----- Original Message ----- 
From: "Simon Hobson" <dhcp1 at thehobsons.co.uk>
To: "Users of ISC DHCP" <dhcp-users at lists.isc.org>
Sent: Friday, April 24, 2009 10:22 AM
Subject: Re: dns in dhcp

> 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 query.
> -- 
> 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.
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