Reverse Look-up Problem

If you define two A Records with the same IP address then when you come to define the Reverse Mapping zone file using the PTR RR it is perfectly legitimate to define more that one entry for any IP address. Increasingly mail systems as part of an anti-spam strategy and others may perform a dual look-up, IP to name then name to IP. Many mail systems will not correctly handle multiple RR responses (they may only see the first, in this joe entry).

Example

; zone file fragment 
joe        IN      A      192.168.254.3  ; joe & www = same ip     
www        IN      A      192.168.254.3

; reverse in-arpa file fragment
.3         IN      PTR    joe.example.com.

In this simple case use of a CNAME record for www could fix the problem.

; zone file fragment 
joe        IN      CNAME  www    
www        IN      A      192.168.254.3

; reverse in-arpa file fragment
.3         IN      PTR    www.example.com.

If you define multiple A Records for load balancing or to support a round robin strategy the same problem arises. In a dual look-up, IP to name then name to IP, system this will only work for mail in the example below not for any others.

; zone file fragment 
mail       IN      A      192.168.254.3  ; round robin    
           IN      A      192.168.254.4
           IN      A      192.168.254.5

; reverse in-addr.arpa file fragment
.3         IN      PTR    mail.example.com.
.4         IN      PTR    mail1.example.com.
.5         IN      PTR    mail2.example.com.

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