SSH Reverse Tunnel with autossh

You can use ssh with reverse tunnel like this

ssh -fNR 2222:localhost:22 [email protected] 

-f: run in background

-N: execute no command

-R: reverse tunnel

2222:localhost:22: mapping remote_host:2222 to localhost:22

remote server

connect to remote server via ssh and then type

ssh [email protected] -p 2222

Done! It's great!

autossh

If you want to keep the tunnel always alive, one way is to use autossh. It will automatically spawn a ssh command once the link is broken.

install

sudo apt-get install autossh

We're going to run autossh in background. However, unlike ssh, in autossh, -f won't let you type password, so you have to use public/private key to authorize.

key

ssh-keygen
Generating public/private rsa key pair.
Enter file in which to save the key (/home/user/.ssh/id_rsa): ~/.ssh/some_key
(enter to the end)

config

Config ssh server for shorter command, in ~/.ssh/config

Host reverse-tunnel
HostName domain.com
User username
ServerAliveInterval 30
ServerAliveCountMax 3
IdentityFile  ~/.ssh/some_key

Use

First, copy key to remote server (do only once)

ssh-copy-id reverse-tunnel

start ssh reverse tunnel with autossh:

autossh -M 0 -fNR 2222:localhost:22 reverse-tunnel

-M: monitoring port, use 0 to disable. It is said by official doc that the better way was to set ServerAliveInterval and ServerAliveCountMax to do this job rather than open an echo port to monitor.

Stop ssh tunnel

You need to first stop autossh command itself and ssh process it has spawned.

stop autossh

pkill -9 autossh;

stop ssh

ps aux | grep reverse-tunnel
kill {pid}

Or in short, add it in .bashrc

alias killautossh='pkill -9 autossh; ps aux | grep reverse-tunnel'

References

• http://josephj.com/entry.php?id=312
• https://ez3c.tw/2043 (perfect)
• http://jamyy.us.to/blog/2014/05/6347.html
• https://www.everythingcli.org/ssh-tunnelling-for-fun-and-profit-autossh/