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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?

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Internet --- (eth0) A (eth1) --- B

You can configure iptables for NAT translation, so packets from B can be routed through A, something like this:

iptables -A FORWARD -i eth1 -o eth0 -s <IP_of_B> -m state --state NEW -j ACCEPT
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

Remember to enable IP forwarding with:

echo 1 > /proc/sys/net/ipv4/ip_forward
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Your rules just include the data form B to A for the server A. What about A to B? –  Yadun Sep 1 '11 at 9:02
    
The second line will do this. You can also insert -i eth0 -o eth1 for more clearly. –  quanta Sep 1 '11 at 9:11
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