## 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 :127.0.0.1:

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 ; 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.