How to Forward SSH Port Using UFW/Iptables

When running self-hosted services like GIT it's possible to forward non-standard ports such as :10022 such that your git address might be ssh://git@git.sebastientaggart.com:10022/sebastientaggart/my-project.git. This works fine but using a non-standard port brings with it several annoyances including port blocking on public networks, the need for extra documentation, and simple degradation of aesthetics.

This use-case assumes you are running ufw on one server listening on :22 that will proxy that traffic back to the non-standard port on a second server.

Edit this file: sudo vi /etc/ufw/before.rules adding:

*nat
:PREROUTING ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]

# Forward traffic from this server (1.1.1.1:22) to a remote server (2.2.2.2:10022)
-A PREROUTING -i eth0 -d 1.1.1.1 -p tcp --dport 22 -j DNAT --to-destination 2.2.2.2:10022
-A POSTROUTING -o eth0 -j MASQUERADE

COMMIT

Change the 1.1.1.1 and 2.2.2.2 placeholders. The first IP is that of the server itself. The second is the IP of the target server.

Reload with sudo systemctl restart ufw

Verify your settings: sudo iptables -t nat -L -n -v

After completing, port :22 will forward to :10022 on the remote server. Your GIT addresses would now look like: ssh://git@git.sebastientaggart.com/sebastientaggart/my-project.git

Modifying Existing Rules

Flushing all iptables rules with sudo iptables -F is too destructive for most situations, instead, it's safer and better to list existing rules and delete them individually as needed:

iptables -t nat --line-numbers -L -n -v

The key is --line-numbers which are used as the reference when passing the -D (delete) command, like so:

iptables -t nat -D PREROUTING 2 (would delete the 2nd rule in the PREROUTING section.

The instructions in the previous section assumed that we were adding rules to /etc/ufw/before.rules for the first time. However if changes are made to existing rules, systemctl restart ufw will add rules. It's necessary to remove the old rules with -D.

Avatar
Sebastien Taggart
Technical Program Manager

The key to everything is mastery of your frame of mind. Everything is relative.

comments powered by Disqus

Related