Barry Margolin barmar at
Mon Jan 19 22:41:57 UTC 2004

In article <buhc3a$13b9$1 at>, /dev/rob0 <rob0 at> 

> I've seen numerous warnings against using a CNAME as MX, NS or SOA, but
> no explanation about why this is wrong. I think it would be useful, just
> as I use CNAMEs for all other services.

It may be useful, but it has overhead in a section of DNS processing 
that you want to be as efficient as possible.  It forces an extra step 
of quering to resolve the CNAME, and possibly multiple steps.  A records 
associated with MX or NS records will often be included in the 
Additional Records section of a reply, but this won't follow CNAME 

> I previously was running a master and slave server on my home LAN, but
> since have come to my senses. :) The master was a virtual machine (user-
> mode Linux), and I would like to have changed its A record "ns.rob0.lan"
> into a CNAME pointing to the slave. (This hostname is also the SOA.)
> What I did instead was to change the A record to point to the IP of the
> slave. So now I have 2 A records for that IP: ns and server. The reverse
> zone resolves to "server.rob0.lan". Is that the right way to do this, or
> should I change SOA to "server.rob0.lan."? Does the lack of reverse DNS
> for the SOA matter?

This is the proper way.  I've never heard of anything performing a 
reverse DNS check on the MNAME field of the SOA record (in fact, there's 
very little use of the SOA MNAME field at all, outside of dynamic 

Barry Margolin, barmar at
Arlington, MA

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