You may want to use it if you want your internal hosts to contact the internal servers with their name. If you want to do that, the DNS query from the hosts must traverse the PIX to an external DNS server
Please check below for more info:
Q. I have a web server on the inside interface of the Cisco Secure PIX Firewall. It is mapped to an outside public address. I want my inside users to be able to access this server by its DNS name or outside address. How can this be done?
A. The rules of TCP do not allow you to do this, but there are good workarounds. For example, imagine that your web server's real IP address is 10.10.10.10 and public address is 99.99.99.99. DNS resolves 99.99.99.99 to www.mydomain.com. If your inside host (for example, 10.10.10.25) attempts to go to www.mydomain.com, the browser resolves that to 99.99.99.99. Then the browser sends that packet off to the PIX, which in turn sends it off to the Internet router. The Internet router already has a directly connected subnet of 99.99.99.x. It therefore assumes that packet is not intended for it but instead a directly connected host and drops this packet. In order to get around this issue your inside host either must resolve www.mydomain.com to its real 10.10.10.10 address or you must take the outside segment off the 99.99.99.x network so the router can be configured to route this packet back to the PIX.
If your DNS resides outside the PIX (or across one of its DMZs) you can use the alias command on the Cisco Secure PIX Firewall to fix the DNS packet to make it resolve to the 10.10.10.10 address. Make sure you reboot your PCs to flush the DNS cache after you make this change. (Test by pinging www.mydomain.com before and after the alias command is applied to make sure the resolution changes from the 99.99.99.99 to 10.10.10.10 address.)
If you have your own DNS server inside your network, this does not work because the DNS lookup never transverses the PIX, so there is nothing to fix. In this case, configure you local DNS accordingly or use local 'hosts' files on your PCs to resolve this name. The other option is better because it is more reliable. Take the 99.99.99.x subnet off the PIX and router. Choose an RFC 1918 leavingcisco.com numbering scheme not being used internally (or on any perimeter PIX interface). Then put a route statement back to the PIX for this network and remember to change your PIX default route outside to the new IP address on the router. The outside router receives this packet and routes it back to the PIX based on its routing table. The router will no longer ignore this packet, because it has no interfaces configured on that network.
PIX 6.2 introduces a new feature called Bidirectional NAT, which offers the functionality of the alias command and more.
Refer to Understanding the alias Command for the Cisco Secure PIX Firewall for more information on the alias command.
Refer to Using Outside NAT in the PIX command reference for more information on the Bidirectional NAT feature.
link:
http://www.cisco.com/en/US/partner/products/hw/vpndevc/ps2030/products_qanda_item09186a0080094874.shtml
I hope the information helps you out
Franco Zamora
