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ssh_tunnel.md

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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 from the public

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 (where the ports will be forwarded from)

On the host computer

  • login as root or run:
    $ sudo su -

  • Check for an ssh public key:
    # cat ./.ssh/*.pub

  • if there is none generate one (keep pressing ENTER):
    # ssh-keygen -t rsa -b 4096

    • keep pressing [ENTER] to use the default values:
    Generating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /root/.ssh/id_rsa.
Your public key has been saved in /root/.ssh/id_rsa.pub.
The key fingerprint is:
SHA256:xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx root@hostname
The key's randomart image is:
+---[RSA 4096]----+
|            xxxx |
|           xxxxx |
|           xxxxx |
|          xxxxxx |
|       xxxxxxxxx |
|      xxxxxxxx   |
|     xxxxxxxxxx  |
|     xxxxxxxxxxx |
|      xxxxxxxxxx |
+----[SHA256]-----+

  • copy the ssh public key over to the VPS (fill in the VPS_IP_ADDRESS).
    Will be prompted for the root password of the VPS.
    # ssh-copy-id root@VPS_IP_ADDRESS

Working on the VPS

  • login as root or run:
    $ sudo su -

  • edit the sshd config:
    # 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 access at this point if the config is wrong):
    # systemctl restart sshd

Back to the host computer

Set up a systemd service

  • create the service file:
    # nano /etc/systemd/system/autossh-tunnel.service

    • Paste the following and fill in the VPS_IP_ADDRESS.
      Add or remove 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 root@VPS_IP_ADDRESS
StandardOutput=journal

[Install]
WantedBy=multi-user.target

  • Enable and start the service:
    # systemctl enable autossh-tunnel
    # systemctl start autossh-tunnel

  • The port forwarding with a reverse ssh-tunnel is now complete. You should be able access the ports/services of the host computer 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

    • Look for the lines:
    debug1: Authentication succeeded (publickey).
debug1: Remote connections from LOCALHOST:9735 forwarded to local address localhost:9735
debug1: Remote connections from LOCALHOST:443 forwarded to local address localhost:443
debug1: Remote connections from LOCALHOST:80 forwarded to local address localhost:80
debug1: remote forward success for: listen 9735, connect localhost:9735
debug1: remote forward success for: listen 443, connect localhost:443
debug1: remote forward success for: listen 80, connect localhost:80
debug1: All remote forwarding requests processed

  • To check if tunnel is active on the VPS:
    # netstat -tulpn

    • Look for the lines:
    Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name    
tcp        0      0 0.0.0.0:80              0.0.0.0:*               LISTEN      7694/sshd: root     
tcp        0      0 0.0.0.0:443             0.0.0.0:*               LISTEN      7694/sshd: root     
tcp        0      0 0.0.0.0:9735            0.0.0.0:*               LISTEN      7694/sshd: root     
tcp6       0      0 :::80                   :::*                    LISTEN      7694/sshd: root     
tcp6       0      0 :::443                  :::*                    LISTEN      7694/sshd: root     
tcp6       0      0 :::9735                 :::*                    LISTEN      7694/sshd: root

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_21_security.html#login-with-ssh-keys