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CentOS 5.x

I've noticed that sometimes I'll see "internet address" for an MX record in the output of nslookup. This doesn't happen all the time though.

For example, let's say that uses Microsoft for their mail service. In that case, when I'm running nslookup -q=mx I will see:       MX preference = 10, mail exchanger =                          nameserver =                          nameserver =     internet address =     internet address =                internet address =                internet address =

Othertimes, when I run the same query, I'll get:       MX preference = 10, mail exchanger =                 nameserver =                 nameserver =       internet address =       internet address =

Why is sometimes missing from the bottom section? It seems like nslookup tries to be clever and go one step further to provide the A records (even though I didn't explicitly ask for it) but this doesn't happen all the time.

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up vote 1 down vote accepted

I'm pretty sure this happens when the resolver you are performing the lookups against has the A records in its cache. It returns the extra records as part of the additional section in the response. It's not nslookup that's doing anything different.

If you're looking up a record that contains a hostname (CNAME/MX/etc), the chances are you'll probably want the address associated with those records, but the resolver is not going to go out of its way to look them up if they're not already in its cache. (It's entirely possible you don't need the actual addresses depending on why your asking, so actually looking up the addresses could be doing more than it needs. However returning the addresses if it has them requires no extra queries and may save your computer from having to doing more, reducing DNS traffic overall.)

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Awesome. Thank you sir. – Mike B Apr 30 '13 at 23:28

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