Validity of CNAME to a PTR

[Kevin Darcy]

Scott Haneda wrote:
> I am looking at PTR checking on a email server.  In test, I see there  
> are a few DNS setups where they CNAME their PTR records.  From a RFC  
> standpoint, is this valid, I am not finding any data to claim one way  
> or the other.
>
> If it is acceptable, and I realize this is getting as bit off topic,  
> is the only recourse to simply whitelist those hosts that are doing  
> this?
>
> Here is the first one I found, that tripped up my filter test (mysql  
> mailing list):
> $dig -x 213.136.52.31
>
> ; <<>> DiG 9.4.2-P2 <<>> -x 213.136.52.31
> ;; global options:  printcmd
> ;; Got answer:
> ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 28031
> ;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 0
>
> ;; QUESTION SECTION:
> ;31.52.136.213.in-addr.arpa.	IN	PTR
>
> ;; ANSWER SECTION:
> 31.52.136.213.in-addr.arpa. 86400 IN	CNAME	31.0-25.52.136.213.in- 
> addr.arpa.
> 31.0-25.52.136.213.in-addr.arpa. 3600 IN PTR	lists2.mysql.com.
>
>   
RFC 2317.

- Kevin