Monday, April 04, 2011

DNS - More than just names and addresses

If you've played with DNS, you know about names and IP addresses.  If you've dug deeply into DNS, you may have some knowledge of TTLs, SOAs and even SRVs (the latter especially true for our Active Directory friends...).  Well, there's a little-known aspect of DNS that may just come in handy along your troubleshooting way...

I was assisting with troubleshooting of an XMPP gateway deployment, and I decided to check the current DNS settings for a few sites over in GoogleLand.  By default, most DNS tools query only for addresses (A or AAAA records); however, you can ask for other types of records, including "any", like so:

Wait a second...what's THAT?!  A URL in response to a DNS query?  Huh?!

Yes, that's right - one can put TXT records into DNS, and they'll be delivered to clients in response to appropriate queries.  In this case, it happens to be a pointer to Google's requirements for XMPP clients/servers connecting to the GoogleTalk environment.

I knew that one COULD popular TXT records in DNS in this fashion, but I hadn't seen it "in the wild" for a VERY long time.  It's a nifty way to leverage a client on 99.9% of one's target systems in delivering additional information to one's users.

Moral of the story?  There's far more to DNS than meets the eye - and, as a troubleshooter, it's worth your while to know (and check!) ALL its capabilities...

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