cname rr data lookup

Kevin Darcy kcd at daimlerchrysler.com
Fri Jan 3 22:28:18 UTC 2003


Graham Turner wrote:

>as a migration step of hosts say olddomain.com to newdomain.com I am
>planning to provide a whole load of alias CNAME records in the zone file for
>newdomain.com along the lines of
>
>host1.newdomain.com IN CNAME host2.olddomain.com
>
>this is to support the correct name resolution of a legacy host in
>olddomain.com in a transition phase from client whose resolver configuration
>(and by implication its default devolution of unqualified names which is
>less preferable to modify) reflects newdomain.com
>
>for this to work the name in the data section of the CNAME RR must be
>resolvable using a host record.
>
"Host record"? I assume you mean an A record.

>assuming this is a valid configuration what (if any) rules are there on the
>name server being able to resolve this host record from say either;
>
>1. domain delegated to the legacy dns server(s)  hosting olddomain.com
>
>or
>
>2. do i have to make the dns server that is hosting newdomain.com
>authoritative for the olddomain.com by making it a secondary to the legacy
>dns server
>
No, you're not forced to make either server slave anything from the 
other server. However, there are some important rules about CNAMEs that 
you need to be aware of:

1) You can't point an MX record or an NS record at a CNAME. So your 
delegation and MX targets (Internet SMTP mail server names) must still 
be A records.
2) If a name owns a CNAME record, it cannot own any other records. The 
most common gotcha associated with this rule is that the name of a zone 
cannot own a CNAME (because then the SOA and NS records owned by the 
name would violate the rule).

                                                                        
                                        - Kevin





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