Remote SSH Access Behind Firewall: A Complete Guide

Introduction

In the interconnected world of today, remote server management is an essential skill for IT professionals and enthusiasts alike. Secure Shell (SSH) is one of the most reliable protocols for securely accessing remote systems. However, gaining remote SSH access behind a firewall can be a challenging task. This comprehensive guide will walk you through the techniques and tools required to establish remote SSH access, even when the target system is shielded by a firewall.

Understanding SSH and Firewalls

What is SSH?

Secure Shell (SSH) is a cryptographic network protocol used to securely operate network services over an unsecured network. It is commonly used for:

• Remote login to servers.

• File transfers using SCP or SFTP.

• Tunneling other network protocols securely.

How Firewalls Affect SSH Access

Firewalls are designed to protect networks by blocking unauthorized access. However, they may block SSH connections unintentionally, creating barriers to remote access. This can happen due to:

• Closed ports.

• Network Address Translation (NAT).

• Intrusion detection systems.

Techniques for Remote SSH Access Behind a Firewall

1. Port Forwarding

Port forwarding is a popular technique to redirect traffic from the firewall to the desired internal system.

Steps to Configure Port Forwarding:

1. Access the Router: Log in to your router’s configuration interface.

2. Find Port Forwarding Settings: Navigate to the port forwarding section.

3. Create a Rule:

   • Set the external port (default SSH is 22).

   • Map it to the internal IP and port of your target device.

4. Save and Test: Apply the settings and test the connection using ssh user@public_IP.

Advantages:

• Simple setup.

• No additional software needed.

Disadvantages:

• Exposes your network to potential threats if not configured securely.

2. Reverse SSH Tunneling

Reverse SSH tunneling allows the remote system to initiate the connection to the client, bypassing the firewall.

Example:

ssh -R 2222:localhost:22 user@intermediate_server

In this example:

• -R: Specifies reverse forwarding.

• 2222: The port on the intermediate server.

• localhost:22: Maps the local SSH port to the remote.

Steps:

1. Set up an intermediate server with a public IP.

2. Establish the reverse tunnel.

3. Connect via the intermediate server using  ssh -p 2222 user@intermediate_server.

3. VPN (Virtual Private Network)

Using a VPN establishes a secure tunnel between the client and the remote system, effectively bypassing firewall restrictions.

Steps:

1. Set Up a VPN Server: Install OpenVPN or a similar service on a server with public access.

2. Connect the Devices: Install the VPN client on the local and remote systems.

   

3. Test SSH: SSH to the private IP of the remote system within the VPN network.

4. Dynamic DNS

Dynamic DNS (DDNS) is useful when the public IP address changes frequently.

Steps:

1. Register with a DDNS Provider: Services like No-IP or DuckDNS.

2. Configure the Router: Set up DDNS in the router’s configuration.

3. Access the Remote System: Use the dynamic domain name in your SSH command.

5. Using Third-Party Tools

Tools to Consider:

• Tailscale: Easy-to-use mesh VPN.

• Ngrok: Exposes local ports to the internet securely.

Example with Ngrok:

ngrok tcp 22

Ngrok will provide a public endpoint for your SSH server.

Practical Scenarios

Scenario 1: Accessing a Home Server Behind NAT

1. Configure port forwarding.

2. Use DDNS to handle dynamic IP.

3. Secure the connection with SSH keys.

Scenario 2: Remote Work with Reverse SSH Tunneling

1. Set up an intermediate server in a public cloud (e.g., AWS).

2. Initiate reverse tunneling from the remote system.

3. Connect to the remote system via the intermediate server.

Scenario 3: Corporate Network with Strict Firewalls

1. Set up a VPN for secure access.

2. Use an approved proxy server to route SSH traffic.

3. Combine with bastion hosts for added security.

Security Best Practices

• Use Strong Passwords or SSH Keys: Prefer key-based authentication.

• Restrict IP Access: Limit incoming connections to trusted IPs.

• Enable Two-Factor Authentication: Adds an extra layer of security.

• Regularly Update Software: Protect against vulnerabilities.

FAQ

What is the default SSH port?

The default SSH port is 22, but it can be changed for security reasons.

How can I change the SSH port?

Edit the /etc/ssh/sshd_config file and update the Port directive. Restart the SSH service to apply changes.

What is a bastion host?

A bastion host is a server specifically configured to provide secure SSH access to other systems within a network.

Can I use multiple methods simultaneously?

Yes, combining methods like VPN with port forwarding enhances security and reliability.

Are there free VPN options?

Yes, free options like OpenVPN and WireGuard can be configured for personal use.

Conclusion

Remote SSH access behind a firewall might seem daunting, but with the right techniques and tools, it becomes manageable. Whether you choose port forwarding, VPNs, or reverse SSH tunneling, always prioritize security and best practices. By following the steps outlined in this guide, you can confidently manage remote systems even in restrictive environments. Thank you for reading the huuphan.com page!

For more detailed information, check out these external resources:

• OpenSSH Official Documentation

• Ngrok Documentation

• Tailscale Documentation