We have 4 servers running in a data center, with internal IP: 192.168.3.* assigned. A hardware (FORTIGATE) firewall configured NAT, and it will lead the traffic as: external IP:

111.222.333.10 -> 192.168.3.10  www.server1.com
111.222.333.11 -> 192.168.3.11  www.server2.com
111.222.333.12 -> 192.168.3.12  www.server3.com

In DNS, we have www.server1.com A 111.222.333.10

Now if I send a lot of data to www.server1.com from www.server2.com, the data will be send through 111.222.333.10 (external IP) and this cause our bandwidth usage huge (expensive!).

The work around I have is to add a local host mapping to server2:

192.168.3.10  www.server1.com. 

That way when send files from server2 to www.server1.com, it will be internal.

However, we are having more and more servers, it would be hard to manually add mapping to every server.

Just wondering do we have another solution for this? Can we do something in the FORTIGATE firewall?

ps. The DNS server being used is public, such as opendns, Google dns etc.