Order of DNS selection
Joseph S D Yao
jsdy at cospo.osis.gov
Mon Dec 11 21:59:36 UTC 2000
On Mon, Dec 11, 2000 at 10:03:55AM -0800, Robert Gahl wrote:
> When one registers a domain at, say, Network Solutions, they are typically
> given a primary and secondary DNS server. My question is "How does bind
> resolve which one to use when an external site is attempting a lookup?"
>
> That is, does the third party machine (ISP?) always go to the primary and
> if the primary is non-responsive, it goes to the secondary to achieve
> resolution, or does it do some sort of heuristics against the "list" of
> posted DNS servers and select one based on some resultant test of
> closeness? Or, is it simply "round robin or which one responds first?"
>
> I had thought it was primary, then secondary, and now I'm not so sure
> anymore. Can anyone illumine me?
Primary and secondary are totally meaningless in name resolution. To
the resolver, they are only peer servers.
At first, querying is at random. I know that for root servers, the
resolving server eventually finds one "closest" and tries to use it. I
don't remember how this has changed over the years, but I believe that
currently 'named' tries to do something similar with other domains.
Certainly, it will always skip over non-responsive servers and stop at
the first authoritative responsive server.
--
Joe Yao jsdy at cospo.osis.gov - Joseph S. D. Yao
COSPO/OSIS Computer Support EMT-B
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