add ssh tunnel guide

README.md

@@ -3,6 +3,8 @@
 
 Start at <https://github.com/rootzoll/raspiblitz>
 
+* #### [Forward ports with a reverse SSH tunnel](ssh_tunnel.md)
+
 * #### [JoinMarket on the RaspiBlitz](joinmarket/README.md)
 
 * #### [BTCPayServer on the RaspiBlitz](BTCPayServer/README.md)

ssh_tunnel.md

@@ -0,0 +1,92 @@
+# Forward ports with a reverse SSH tunnel
+
+## Advantages: 
+* no port forwarding needed on the LAN of the host
+* encrypted connection
+* hides the IP of the host
+
+## Requirements:
+* a Virtual Private Server (VPS) - eg. a minimal package on Lunanode for ~3.5$/month
+* root access on the VPS - only root can forward ports under no. 1000
+* ssh access to the host computer (from where the ports will be forwarded from)
+
+## On the host computer 
+
+* Check for an ssh public key:  
+`$ cat ./.ssh/*.pub`
+
+* if there is none generate one (keep pressing ENTER):  
+`$ ssh-keygen -t rsa -b 4096`
+
+* copy the ssh publick key over to the VPS (fill in the VPS_IP_ADDRESS).  
+Will be prompted for the root password of the VPS.  
+`$ cat ~/.ssh/id_rsa.pub | ssh root@VPS_IP_ADDRESS 'cat >> ~/.ssh/authorized_keys && chmod -R 700 ~/.ssh/'`
+
+## Working on the VPS
+
+* login as root or run:  
+`sudo su -`
+
+* edit the sshd config:  
+`sudo nano /etc/ssh/sshd_config`
+
+* make sure these entries are active (uncommented, meaning there is no `#` at the beggining of the line).  
+Can just paste these on the end of the file:
+    ```
+    RSAAuthentication yes
+    PubkeyAuthentication yes
+    GatewayPorts yes
+    AllowTcpForwarding yes
+    ClientAliveInterval 60
+    ```
+    CTRL+O, ENTER to save, CTRL+X to exit.
+ 
+* restart the sshd service (WARNING: you can lose acccess at this point if the config is wrong):  
+`sudo systemctl restart sshd`
+
+## Back to the host computer
+
+### Set up a systemd service
+
+* create the service file:   
+`sudo nano /etc/systemd/system/autossh-tunnel.service`
+
+* Paste the following and fill in the VPS_IP_ADDRESS.  
+Add more ports as required.
+
+    ```
+    [Unit]
+    Description=AutoSSH tunnel service
+    After=network.target
+
+    [Service]
+    User=root
+    Group=root
+    Environment="AUTOSSH_GATETIME=0"
+    ExecStart=/usr/bin/autossh -C -M 0 -v -N -o "ServerAliveInterval=60" -R 9735:localhost:9735 -R 443:localhost:443 -R 80:localhost:80 -R root@VPS_IP_ADDRESS
+    StandardOutput=journal
+
+    [Install]
+    WantedBy=multi-user.target
+    ```
+* Enable and start the service:  
+`$ sudo systemctl enable autossh-tunnel`  
+`$ sudo systemctl start autossh-tunnel`
+
+* The port forwarding with a reverse ssh-tunnel is now complete. 
+You should be able access the of the host computer ports/services through the IP of the VPS.
+
+## Monitoring
+
+* Check if there are any errors on the host computer:  
+`$ sudo journalctl -f -n 20  -u autossh-tunnel`
+
+* To check if a tunnel is running on the VPS:  
+`$ netstat -tulpn`
+
+
+## Resources
+
+https://github.com/rootzoll/raspiblitz/blob/master/FAQ.md#how-to-setup-port-forwarding-with-a-ssh-tunnel
+
+https://stadicus.github.io/RaspiBolt/raspibolt_20_pi.html#login-with-ssh-keys