# 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 (where the ports will be forwarded from)

# Setup

# On the host (your BTCPay Server instance)

Check for an ssh public key:

cat ~/.ssh/*.pub

If there is none generate one (keep pressing ENTER):

ssh-keygen -t rsa -b 4096

This will generate the SSH keypair id_rsa (private key) and id_rsa.pub inside ~/.ssh.

The private key needs to get added to the ssh-agent:

# start the ssh-agent in the background
eval $(ssh-agent -s)

# add private key to ssh-agent
ssh-add ~/.ssh/id_rsa

Copy the public key over to the VPS (fill in the VPS_IP_ADDRESS). You will be prompted for the root password of the VPS.

ssh-copy-id -i ~/.ssh/id_rsa.pub root@VPS_IP_ADDRESS

To verify that it works, SSH into the VPS – this should not prompt for the password anymore:

ssh root@VPS_IP_ADDRESS

# On the VPS

You can either reuse the connection from before or login as root.

Edit the sshd config:

sudo nano /etc/ssh/sshd_config

Make sure these entries are active (meaning there is no # at the beggining of the line). Alternatively, you 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.

WARNING

You can lose access at this point if the sshd config is wrong. Please double-check!

Restart the sshd service:

sudo systemctl restart sshd

# Back to the host

# Install and set up autossh

Install the autossh dependency:

sudo apt-get install autossh

Create the service file:

sudo 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:

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 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

To check if tunnel is active on the VPS:

netstat -tulpn

# Resources