Reverse ssh tunnel linux update#
Mender is an open source software update management solution for IoT devices and has Remote terminal as an optional add-on (as part of the Troubleshoot add-on package). It’s reversed because the “firewalled” server did an outbound SSH connection to create a tunnel which is then used for establishing inbound ssh connections. Making a regular SSH connection to that public server tunnels you to the “firewalled” server. This can happen because the “firewalled” server (port A) initially creates a reverse SSH tunnel (ferry naval route) to an arbitrary port on a publicly available server (port B).
Does it make sense for port A to do this? Yes, if it’s surrounded by cannons that will shoot down every ferry or car that tries to reach it by any other means.Ī reverse SSH tunnel is a method of allowing an SSH connection (cars) to an SSH server behind a firewall (port A).Ī server behind the firewall usually means that either no incoming connection from the Internet to the server will be allowed and that the local IP address of the server isn’t a valid address on the Internet (due to a process called NAT - Network Address Transition). So the ferry you board in port B is the only way to access port A. This is a normal tunnel.Ī reverse tunnel is when port A sets everything up but their ferry is only taking people from B->A.
They buy a ferry, do all the bureaucracy and cars are now flowing from A->B. Say port A wants to have a ferry line with port B. To follow up with the ferry analogy, a “direction of a tunnel” is defined by the direction of the ferry in relation to the dock which created the naval route. We have to first define what the direction of a tunnel means. So a connection (road for cars) is being tunneled through another connection (naval route of the ferry).Īn SSH tunnel is an SSH connection that serves as a ferry for other SSH connections allowing them to reach places they normally couldn’t. The ferry is also running on its own naval route ( connection) between two ports. It has a source port and a destination one. It’s allowing the road to pass over an obstacle. Let’s try to broaden that concept and say that a ferry transporting cars across some body of water is in a sense also acting as a tunnel. It’s something you dig out to allow the road to continue through an obstacle.
Let’s zoom out for a second and ask what a tunnel is in a general sense. What’s an SSH tunnel and how does it differ from a connection? By this act you form an SSH connection between the two. In the most basic form, you use the SSH client on your workstation to connect to the SSH server on the remote side. If you’re reading this you’ve probably heard of SSH, the cryptographic network protocol usually running on port 22 which allows you access to the shell of a remote machine. Want an easy way to SSH to a device behind a firewall but don’t want to set up a VPN? Welcome to the pragmatic person’s approach - Reverse SSH tunneling.
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