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If I have contradictory NS records for my domain:

dig +trace        172800  IN  NS        172800  IN  NS        172800  IN  NS

Which one[s] will be queried by my DNS client? Does the ordering matter? How can I tell the ordering?

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I would remove the ns dns record if possible, tertiary ns is unlikely to be fired off in my experience; how big is your network? – scape Nov 7 '12 at 18:13
There is no such thing as "tertiary ns" – bortzmeyer Nov 7 '12 at 20:59
up vote 2 down vote accepted

The ordering in which the lists are presented to you is randomly ordered in most cases. But you (as the dns-admin) can also configure sequential output (round-robin) or static output. Depends on the administrators mood.

Which entry a client chooses is also up to the developer of the client.

dig +trace picks a random entry.

In any case, the only reason for this is load balancing. If all servers are running fine it doesn't matter which entry you actually use.

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I don't see the same output when running the same command.

But to answer your question, a random one will be chosen and the order you list them in does not matter. You can see this with successive runs:

$ dig +trace

...        3600    IN  A
;; Received 45 bytes from in 96 ms

$ dig +trace
...        3600    IN  A
;; Received 45 bytes from in 8 ms
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Sorry, just providing an example. I changed it to – Kevin Burke Nov 7 '12 at 19:00

I believe that the resolver uses round robin methodology to query DNS hosts.

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