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uni's avatar
uni
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Jun 07, 2012

Cannot create a CNAME for a domain root

My customer has a site to be hosted behind GTMs. The ISP is authoritative for site.example.com. The GTMs are authoritative for wip.example.com. They want the site to be accessible via site.example.com.

Initially, we thought we could just define the alias as

site.example.com CNAME site.wip.example.com
However, site.example.com has MX records:

site.example.com MX 10 mail.site.example.com
From what I can tell, this is not allowed, and is stated clearly in RFC2183 10.3:

The domain name used as the value of ... or part of
   the value of a MX resource record must not be an alias.
although rfc5321 5.1 states:

The lookup first attempts to locate an MX record associated with the
   name.  If a CNAME record is found, the resulting name is processed as
   if it were the initial name

This former position is reinforced by the ISP, who state they cannot create a CNAME for a domain root.

Can someone suggest a solution to this problem?

application delivery
availability
BIG-IP DNS
gtm
TMOS
  • G__Wesley_Rober's avatar
    G__Wesley_Rober
    Historic F5 Account
    Jun 07, 2012
    Indeed a common issue. The RFC you quote on MX records is referring to the right hand side, the rrdata part of the MX record - mail.site.example.com in your case cannot be an alias.

     

     

    As I recall this explained in the O'Reilly book by Liu, "a name can be a cannonical name or an alias, but not both".

     

     

     

    A few solutions:

     

     

     

    - a GTM in authoritative screening mode in front of site.example.com servers eliminates the need for aliasing altogether. If the name isn't a WIP, it falls through to the pool member(s) and gets answered by the authoritative NS. Tough to do in this case.

     

    - have the parent rr DNS across all the possible answers, then have the LTM/web server redirect to a WIP name handled by GTM.

     

    - have the ISP delegate site.example.com to GTM. In this case, it would be a zone and a WIP. That's ok. ;-)