How to fix Warning: Remote port forwarding failed for listen port in SSH

SSH to the remote server and verify if the port is available. Use netstat or ss to check:

# Using ss (modern systems)
ss -tuln | grep :8080

# Or using netstat (older systems)
netstat -tuln | grep 8080

If the port appears in the output, another process is using it. Either:
- Stop that process: sudo lsof -i TCP:8080 to find the process ID, then kill it
- Choose a different port for your tunnel (use a port number above 1024 if not running as root)

If you're trying to forward to a privileged port (80, 443, 22, etc.), switch to a port above 1024:

# Instead of:
ssh -R 80:localhost:3000 user@remote-server

# Use:
ssh -R 8080:localhost:3000 user@remote-server

Regular users can only bind to ports 1024 and above. Root or the ssh user can bind to privileged ports. If you need a privileged port, either:
- Run SSH as root (not recommended for security)
- Have the server administrator enable the port forwarding
- Use a web server proxy on the remote side that listens on the privileged port and forwards to your tunnel port

By default, remote forwarded ports only accept connections from localhost. To allow external connections, the server admin must enable GatewayPorts.

On the remote server, edit /etc/ssh/sshd_config:

sudo nano /etc/ssh/sshd_config

Add or uncomment:

GatewayPorts yes

Or use clientspecified for more control:

GatewayPorts clientspecified

Then restart the SSH service:

# On systemd systems
sudo systemctl restart sshd

# On older systems
sudo service ssh restart

This allows the SSH daemon to bind the forwarded port to all interfaces, not just localhost.

Ensure the SSH server hasn't disabled TCP forwarding. Check /etc/ssh/sshd_config on the remote server:

sudo grep AllowTcpForwarding /etc/ssh/sshd_config

The setting should be:

AllowTcpForwarding yes

If it's set to 'no' or 'local' only, edit the file and change it, then restart sshd as shown in the previous step.

Run SSH with verbose output to see the exact error:

ssh -v -R 8080:localhost:3000 user@remote-server

Add more 'v' flags for additional detail:

ssh -vvv -R 8080:localhost:3000 user@remote-server

Look for messages like:
- "bind: Address already in use" - port is occupied
- "permission denied" - privilege issue
- "administratively prohibited" - SSH config restriction

These messages will point to the exact cause of the problem.

Sometimes previous SSH sessions hold port bindings. Try these approaches:

1. Wait a few minutes for TCP connections to time out naturally

2. Or find and kill lingering SSH processes on the remote side:

# As root on remote server, find sshd processes
ps aux | grep sshd

# Kill specific sshd session if needed
sudo kill -9 PID

3. Also check your local system for hung processes:

# List active SSH connections
ps aux | grep ssh

# Kill your local SSH connection
kill PID