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SSH Port Forwarding

SSH port forwarding (tunneling) is a critical technique for DevOps engineers to securely access internal services, databases, and applications across cloud and hybrid environments (AWS, Azure, GCP). This guide covers local, remote, and dynamic forwarding with actionable examples and best practices.

1. Local Port Forwarding

Forward a local port to a remote service through an SSH server. Useful for accessing internal databases, web UIs, or APIs from your workstation.

Syntax:

ssh -L <local_port>:<destination_host>:<remote_port> <user>@<ssh_server>

Example 1: Forward local port 5050 to remote 4040 via a bastion

ssh -L 5050:188.171.10.8:4040 user@bastion.host

Example 2: Access a private PostgreSQL database

ssh -L 5432:db1.host:5432 user@remote.host
# Connect locally: psql -h 127.0.0.1 -p 5432

Example 3: Forward VNC port and run SSH in background

ssh -L 5901:127.0.0.1:5901 -N -f user@remote.host

  • -N: Do not execute remote command

  • -f: Run SSH in background

Troubleshooting:

  • Ensure AllowTcpForwarding yes is set in /etc/ssh/sshd_config on the remote server.

2. Remote Port Forwarding

Expose a local service to a remote network via the SSH server. Useful for sharing local apps or webhooks with remote/cloud systems.

Syntax:

ssh -R <remote_port>:localhost:<local_port> <user>@<remote.host>

Example: Expose local port 3000 to remote port 8000

ssh -R 8000:127.0.0.1:3000 -N -f user@remote.host

  • Now, anyone on remote.host can access your local app at localhost:8000.

3. Dynamic Port Forwarding (SOCKS Proxy)

Create a local SOCKS proxy to route traffic through the SSH server. Useful for secure browsing, testing, or accessing internal networks.

Syntax:

ssh -D <local_port> <user>@<ssh_server>

Example: Start a SOCKS proxy on port 9090

ssh -D 9090 -N -f user@remote.host

  • Configure your browser or CLI tool to use localhost:9090 as a SOCKS5 proxy.

Real-World DevOps Examples

1. Access AWS RDS or Azure SQL via Bastion

ssh -L 5432:<rds-endpoint>:5432 ec2-user@bastion-host

2. Forward Kubernetes Dashboard securely

ssh -L 8001:localhost:8001 user@k8s-master
kubectl proxy --address='127.0.0.1' --port=8001

3. Share a local web app with a remote team

ssh -R 8080:localhost:3000 user@remote.host

Best Practices

  • Use -N -f for background tunnels in automation scripts

  • Always restrict forwarding to trusted users/networks

  • Monitor and audit SSH tunnels in production

  • Use SSH config (~/.ssh/config) to simplify complex tunnels

  • For persistent tunnels, consider tools like autossh or systemd services

References

Tip: Kill background SSH tunnels with pkill -f 'ssh -L' or pkill ssh as needed.

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