I have a web server A and a postfix mail server B. They are all ubuntu server. For some reason B just connect with A and dosen't connect with internet, but A connect with internet. So I think I can set some iptable rules for A and B that will let A be B's router or bridge. I guess I must use DNAT rule but I cannot turn the idea to be ture. Can anybody help me?
You can configure iptables for NAT translation, so packets from B can be routed through A, something like this:
Remember to enable IP forwarding with: