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Forwarding Traffic with iptables

A practical iptables setup for relaying TCP connections through an intermediate server with DNAT and SNAT rules.

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Why: A direct connection is sometimes slow while a relayed connection is fast; network routing in China can be complicated.

Use case: If you have multiple VPS instances, they can relay traffic for one another and work around IP-based regional restrictions.

This is simply remote port forwarding. It is the simplest solution I know—just a few commands, without HAProxy or Socat. It can also forward several external ports into one Shadowsocks port, avoiding multiple Shadowsocks processes.

This note does not explain iptables fundamentals.

Goal: Relay requests sent to 47.90.123.123:3306 onward to 45.63.110.110:80.

Terminal window
sudo iptables -t nat -A PREROUTING -d 47.90.123.123 -p tcp --dport 3306 -j DNAT --to-destination 45.63.110.110:80

This adds a rule to the NAT table and rewrites the destination for outbound traffic.

For the return path, rewrite the source IP to the relay machine’s 47.90.123.123:

Terminal window
sudo iptables -t nat -A POSTROUTING -d 45.63.110.110 -p tcp --dport 80 -j SNAT --to-source 47.90.123.123

MASQUERADE can be used instead.

List the rules. Specify the NAT table with -t; otherwise iptables displays the filter table:

Terminal window
sudo iptables -t nat -nvL --line-numbers

These rules exist only in memory and disappear after a reboot. For persistent rules on Ubuntu, see the official IptablesHowTo.

There is still a potential problem with this setup. Did you spot it?

Originally published on SegmentFault.

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