PTR query

[Kebold]

Hello,

I have a primary dns server based on suse linux server (sles10) and a firewall cisco pix already configured.
I think there is a configuration problem. I explain:

In my dns server I already configured an A record i.e. www. example .com for the domain example.com with ip 12.34.56.78 and a PTR record: 12.34.56.78 to www .example .com

From a pc using a external internet connection I check for name resolution so:

ping www.example.com -c 1
xx bytes from xxxxxxxx.example.com (12.34.56.78)

So I obtain the correct ip for the domain.

But if a check for PTR record, so I check the reverse lookup resolution,:

host 12.34.56.78
78.56.34.12.in-addr.arpa not found

There is a problem with reverse lookup resolution.

If I made the same test using a pc in the same local network of the dns server
(so bypassind the firewall) all it works.

I think that the problem is the firewall.

Can you please tell me how I have to check in the firewall rules?
Maybe the PRT query use a specific port that I have to "open" in the firewall?

Thanks.