SSH Tunnelling 101

The Players

I’ll be referring to 3 hosts:

• A: The server; this machine is behind a firewall that allows outgoing connections but doesn’t allow incoming.
• B: The bounce host; this machine is unfirewalled.
• C: The client.

Configuring B

Some sshd configuration needs to be done on B before any of this will work. In the sshd_config file (/etc/ssh/sshd_config on Debian):

AllowTcpForwarding yes
GatewayPorts yes

Remember to restart sshd after making changes (/etc/init.d/ssh restart).

Building the Tunnel

On A, run:

ssh -g -n -R <port on B>:127.0.0.1:<port on A> <address of B> sleep 999999

This will hang with no output; that’s the expected result.

You should now be able to connect to the port on B and be talking to A. To get this to restart if the connection dies, run it inside:

while :; do <command>; done

As with all shell commands, put a “&” on the end to run it in the background.

Tunnelling FTP

Due to a trick in the FTP protocol, you can use this tunnelling arrangement but have FTP data connections go directly from A to C, without touching B. This only works with so-called “active” FTP (using the PORT command instead of PASV). C must also be unfirewalled for this to work.

The only thing you’ll need to change is the FTP server configuration. In proftpd.conf, add:

AllowForeignAddress on

For pure-ftpd, run it with the “-w” commandline flag, or with a file named “AllowUserFXP” and a contents of “on” if you’re using pure-ftpd-wrapper.