Reverse Look-up Problem

If you define two A Records with the same IP address then when you come to define the Reverse Mapping (look-up) file you can only map a single name to the 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. A Reverse Mapping query will only return the right result for joe in the example below not for www. In this simple case use of a CNAME record for www could fix the problem.

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.

If you define multiple A Records for load balancing or 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.

Pro DNS and BIND by Ron Aitchison

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