How not to set up your DNS (part 14)

July 3, 2007

In the traditional illustrated format:

; sdig cname

This is a well-intentioned and noble attempt to do round-robin CNAMEs. Unfortunately it doesn't work, because you can't have multiple CNAME records; you can have either one CNAME record or any number of other sorts of records. For what this domain is trying to do, they need to get the other domain to set up an record with all of the IP addresses of their MTAs, and then CNAME to that.

The effects on caching DNS servers are actually pretty interesting. Some DNS servers will refuse entirely to deal with this, returning server failure messages. Other DNS servers will give both CNAMEs on an initial query but only cache one of the two CNAME records (picking which one at random) and thereafter only give you that one back for the record's TTL.

(The domains involved have been anonymized at the request of the person who showed this to me.)

Written on 03 July 2007.
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Last modified: Tue Jul 3 15:34:22 2007
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