dns in dhcp

Adalberto acosta at ufpe.br
Fri Apr 24 17:11:15 UTC 2009

Glenn Satchell
My network has a dns that is down from time to time, so I tell the two dns 
Thank you
----- Original Message ----- 
From: "Glenn Satchell" <Glenn.Satchell at uniq.com.au>
To: <dhcp-users at lists.isc.org>
Sent: Friday, April 24, 2009 12:19 PM
Subject: Re: dns in dhcp

> 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.
> regards,
> -glenn
>>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 
>>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 
>>>>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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