projects:ssh_reverse_tunnel [Labka Hackerspace Ostrava]

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====== Reverse SSH tunnel to connect to device with dynamic IP ======
|**Project owner:**| [[user:overdrive|Overdrive]] |
|**Interested:** | |
|**Related:**| [[project:single_boards|[Project Single Boards]]]|
|**References:**|[[http://www.tunnelsup.com/raspberry-pi-phoning-home-using-a-reverse-remote-ssh-tunnel]]|
|**Materials:**| dynamic IP device aka PICKA, static IP device aka RemoteHost, [[project:single_boards#remove_password_to_ssh|ssh able to connect between devices by keys]]|
|**THX to:**| thx to:  Jack Rhysider; @TunnelsUp; http://www.tunnelsup.com/raspberry-pi-phoning-home-using-a-reverse-remote-ssh-tunnel |
|**License:** | [[https://creativecommons.org/licenses/by-sa/3.0/|CC Attribution-Share Alike 3.0 Unported]]  |  
{{ projects:tunnel-of-plants-252820_960_720.jpg?500|Taken from: https://pixabay.com/static/uploads/photo/2014/01/27/03/28/tunnel-of-plants-252820_960_720.jpg under CC; thx}}
===== Legend: =====
  * **Picka** = bannanaPi, raspberryPi, any othere device on dynamic IP
  * **RemoteHost** = server with static host, Picka can ssh to and you can ssh to that server from anywhere

===== What we want to achieve? =====

  * we have some **Picka** on dynamic IP, but we want to connect to that even we do not know what IP it have at the moment
  * we have **RemoteHost**, that we can see from anywhere and Picka is able to SSH to that server
  * we will build ssh tunnel from **Picka** to **RemoteHost**
  * [[project:single_boards#remove_password_to_ssh|RemoteHost and Picka should be able to reach each other by certificate, not by passwd]]

==== on Picka do ====
 
<code>
ssh -N -R 2222:localhost:22 serverUser@RemoteHost
</code>

**explanation of parameters of tunnel:**
 ssh - shell command
 -N do not execute anything when connection successful
 -R bind port 22 on remote server [192.168.1.1] to 2222 

==== so now connect to RemoteHost and ==== 

<code>
ssh -l pickaUser -p 2222 localhost
</code>

=== explanation of functionality ===
 Why did this work? 
The **RemoteHost** is listening on port 2222 for incoming ssh connections. If it receives one, it will forward all traffic it receives into the previous ssh connection that was established already. 
That is essentially what the remote tunnel does.


==== persistent tunnel from Picka to RemoteHost ==== 

Now let’s take a step back and look at what we’ve done. When the Raspberri Pi is on, it will check every minute to see if an ssh connection to your linux server exists. If it doesn’t it will create one. The tunnel it creates is really a reverse remote tunnel. Once the tunnel is up, anyone who ssh’s into port 2222 of the linux server will then be redirected to the Pi. Incredible!

=== OnPicka ===

<code>
touch ~/ssh_tunnel.sh
chmod 700 ~/ssh_tunnel.sh
vim ~/ssh_tunnel.sh
</code>


  * add and save

<code>
#!/bin/bash -x
createTunnel() {
##### fill in vars
  RemoteHost="IP_HERE"
  ServerUser="NICK_HERE"
##### are wars filled?

  /usr/bin/ssh -N -R 2222:localhost:22 ${ServerUser}@${RemoteHost} &
  if [[ $? -eq 0 ]]; then
    echo Tunnel to RemoteHost IP: ${RemoteHost} created successfully
  else
    echo An error occurred creating a tunnel to RemoteHost IP: ${RemoteHost}. ReturnCode was: ${?}
  fi
}
/bin/pidof ssh
if [[ ${?} -ne 0 ]]; then
  echo Creating new tunnel connection to RemoteHost IP: ${RemoteHost}
  createTunnel
fi
</code>

  * add script to cron

<code>
crontab -e
</code>

  * and add (every minute check if the ssh connection is up, if not, attempt to bring it up)

<code>
*/1 * * * * ~/ssh_tunnel.sh > ~/ssh_tunnel.log 2>&1
</code>


==== Note for MAC users ====
* in case you have problem, that wiifi is turned off when screensaver is on
* remember en1 should be replaced by your wiifi interface
<code>
cd /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources
sudo ./airport en1 prefs DisconnectOnLogout=NO
</code>