Friday, December 30, 2016

domain name system - DNS using CNAMEs breaks MX records?



We are trying to move all our websites we host to CNAMES as we are planning on moving servers in the new year and would like the ability to move some clients to one server and other clients somewhere else. We were planning on giving clients a unique CNAME which we can then change at a later date. (We have other reasons for doing this now but that is the main one)



We have been testing out this theory with a few of our own domains and it seemed to be fine. However when checking the MX records on a domain I got the CNAME value back rather than the MX record.



Sadly all of these domains are done via control panels, but I am guessing they are just writing zone files for me.



I want to create 2 CNAMEs for the company.com




company.com. IN CNAME client.dns.ourserver.com
www IN CNAME client.dns.ourserver.com


The MX record is something like the following:



company.com  IN MX 10 mail.company.com



We have an A record for mail.company.com



Doing:



host -t mx company.com


Returns the CNAME value rather than the mx record.



Is this expected behaviour?




I have managed to get the above configuration working with the 123-reg.co.uk control panel, but not sure if that is more luck than anything.


Answer



This is a common error. You cannot use a CNAME RR for your root domain (e.g. company.com) and define additional resource records for the same zone.



See Why can't I create a CNAME record for the root record? and RFC1034 section 3.6.2 for details:




If a CNAME RR is present at a node, no
other data should be present; this

ensures that the data for a canonical
name and its aliases cannot be
different.



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