dns in dhcp

Glenn Satchell Glenn.Satchell at uniq.com.au
Fri Apr 24 15:19:17 UTC 2009

The answer is yes. That's the reason dhcp allows you to specify
multiple dns servers. The same concept applies if you are not using
dhcp, but have manually assigned the DNS server addresses on a
particular hosts.

There is probably a Microsoft Knowledge base article that describes the
Windows DNS client.


>From: "Adalberto" <acosta at ufpe.br>
>To: "Users of ISC DHCP" <dhcp-users at lists.isc.org>
>Subject: Re: dns in dhcp
>Date: Fri, 24 Apr 2009 11:51:59 -0300
>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

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