SSH Access Behind Firewalls: Bypass & Securely Connect | [Guide]
Can you access your Linux server from anywhere, even when it's hidden behind a firewall? The answer is a resounding yes, and this guide will show you exactly how to do it, unlocking the power of remote access without compromising security.
Firewalls, at their core, are designed to prevent unauthorized access to a network. This is their primary function: safeguarding systems from external threats. However, these security measures can also create a barrier, preventing you from accessing your own resources when you're away from your local network. But don't despair! There are several ingenious techniques available to circumvent these restrictions and establish a secure connection, allowing you to manage your server remotely, even when it's behind a firewall.
One of the most effective methods involves setting up a secure shell (SSH) tunnel. SSH is a robust protocol, widely recognized for its reliability in securing remote system access. The beauty of SSH tunneling lies in its ability to encrypt all traffic between your local machine and the remote server, ensuring that your data remains confidential and protected from eavesdropping. Furthermore, it can bypass firewalls that are blocking traffic on specific ports by encapsulating the traffic within an SSH connection. This allows you to traverse network obstacles and establish a secure connection, making it an ideal solution for accessing your server from anywhere.
Let's take a look at the core problem. You have a Linux server, a work machine (perhaps a CentOS 6 VM, to cite an example), residing behind a corporate firewall. You have full root control over the server itself, but you lack the ability to configure the firewall directly. The server runs an SSH server, and you can SSH into it while you're inside the company network. Your objective: to SSH into that very same VM when you're outside the company network. How do you achieve this? The challenges can seem daunting, but the solutions are within reach.
The strategies for overcoming these hurdles are diverse. SSH tunneling is a primary option, but let's delve into the specifics. Understanding the mechanics and considering available options is key. Consider the following approaches:
First, the reverse SSH tunnel. A reverse SSH tunnel is like a clever workaround. Instead of trying to initiate a connection from the outside into the firewall, you establish a connection outward from the server behind the firewall, and then use this established connection to create a tunnel that lets you access the server from the outside.
Next, port knocking can be employed, which involves sending special packets to temporarily open a hole in the firewall. However, this approach requires careful configuration and may not always be feasible, especially if you lack control over the firewall. P2P or VPN solutions like Hamachi handle the firewall negotiation for you, ensuring secure connections, which may be a straightforward path. Setting up a VPN (e.g., WireGuard) can offer secure access, but you'll need to configure both the server and your client machines.
Another critical aspect of establishing remote SSH access is choosing the right tools and techniques. You might consider renting a cheap Virtual Private Server (VPS) that you can SSH to. This VPS will act as a relay, allowing you to create a reverse SSH tunnel back to your server behind the firewall.
The command `autossh` is very useful here; it automatically restarts the SSH tunnel if the connection is lost. You could add the `autossh` command in `/etc/rc.local` to ensure the tunnel is automatically established upon boot. You can also add additional security, such as port knocking, to help. The most common route, and typically the most reliable, is the SSH tunnel itself. But consider this:
Let's say that from your firewalled PC, you set up a reverse SSH tunnel. You then use a VPS and set up a reverse ssh tunnel. Now, on your remote computer (connect over tunnels): `Ssh 127.0.0.1:20000` and enter in credentials for your home server.
If you do not have access to the edge router, other tactics are needed. You can create a robust Java (daemon) program (based on the `jsch` class library) that allows you to leverage local and remote port forwarding. This can be complex. More straightforward solutions include using remote services, such as GoToMyPC, TeamViewer, or Chrome Remote Desktop. However, these typically come with some inherent security trade-offs, depending on your requirements.
If you're already using SSH keys, there should be no authentication failures for authorized users, which improves security and streamlines the connection process. If you have the option, consider configuring the firewall in front of the machine to terminate a VPN connection, allowing only the SSH daemon on the remote server to accept connections coming across that VPN.
Alternatively, SSH SOCKS proxy can be used to bypass firewalls that are blocking traffic on a specific port. In order to bypass a firewall using SSH tunneling, you'll need to set up an SSH server on a machine outside the firewall. The core technique is to establish a tunnel, then use that tunnel to start an SSH session to a remote device.
Another key aspect of implementing these techniques is to take full control of your Linux server with our comprehensive guide to SSH remote access without port forwarding or firewall pinholing. This is a central goal: unlock the power of your Linux server from anywhere, anytime, with expert tips and tricks.
The fundamental aim is to facilitate remote access and the effective management of your servers, regardless of network restrictions. Whether you opt for SSH tunneling, port knocking, or VPN solutions, the key is to create a secure and reliable pathway to your server.
| Category | Details |
|---|---|
| Technique | SSH Tunneling |
| Description | Creates an encrypted tunnel to bypass firewall restrictions. Traffic is forwarded through the SSH connection. |
| Tools | OpenSSH (ssh command), autossh |
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| Example Command (Reverse Tunnel) | ssh -R 20000:localhost:22 user@external_server_ip |
| Reference | DigitalOcean - How to Set Up SSH Tunneling |
Let's look at another example: The setup to achieve SSH access to your work machine.
My work has a firewall and SSH server. The SSH server is listening to the default port 22.
I don't require a GUI so I just SSH into the SSH server from a home computer (MacBook) and then SSH into my personal work computer (Ubuntu 14.04) on the LAN.
I can ssh into a company machine remote which is behind a firewall and only allows incoming SSH connections (port 22), but no outbound connections whatsoever.
I do have full root access on both, local and remote, and the remote is a VPS and behind a hardware firewall.
Now, from the server behind the ISP firewall, you can use a method like SSH SOCKS proxy or SSH Tunneling.
When we have a service behind a firewall in a remote server, we sometimes want a simple way to connect to it without setting up port forwarding. Setting up an SSH tunnel is pretty straightforward and secure because your traffic will be encrypted on its path.
The key here is to establish the proper tunnel. One configuration to be particularly wary of is where you can SSH into a company machine remote which is behind a firewall and only allows incoming SSH connections (port 22). In such a situation, setting up a reverse tunnel will be the best approach, as you can not establish an outbound connection.
In a nutshell, the process involves creating an SSH tunnel that allows you to bypass firewall restrictions and gain secure access to your target system. Whether you are aiming to execute commands remotely or securely manage your device, the core principle remains consistent: the creation of a secure and reliable tunnel.
Remember, when dealing with networks, a critical element of security is, ironically, maintaining the ability to perform necessary tasks remotely, such as fixing errors, or restarting essential systems. Without such access, you could find yourself at a severe disadvantage if an emergency situation should arise. Always consider that the methods used to achieve remote access should be carefully chosen and meticulously implemented to avoid creating security vulnerabilities, or potentially getting yourself into trouble with the administrator of the remote network.
