ssh through a router without port forwarding

What you would want to do is ssh FROM your "linux server" TO something on the outside, such as "my_other_server" or something else both servers can get to.

You would use ssh remote port forwarding.
[user@linux_server]$ ssh -R8022:localhost:22 my_other_server.com
Explaination: Connect to my_other_server and open port 8022 there which will forward back to me on port 22.

From my_other_server.com you will be able to ssh to localhost on port 8022, and have your traffic forwarded to linux_server piggybacking on the linux_server -> my_other_server tunnel [user@linux_server]$ ssh -p8022 localhost
Explaination: Connect to myself on port 8022 which is forwarded to linux_server

If you have problems with the initial linux_server -> my_other_server tunnel dropping out, you could make a script to keep it open, adjust the keepalive settings, or use autossh.

This answer is based on the accepted one, but adding the details that allowed me to get it done. I apologize for the pedestrian explanation, since this is not at all of my expertise.

Suppose you have two computers, A and B. You want to ssh from A to B, and you cannot do any port forwarding in the routers connected to them.

As the accepted answer states, you need a server S to do this: in computer B, you will allow ssh connections coming from S; and from computer A, you will access that tunnel at S to reach B.

But how do you get that server S? I found serveo (link: https://serveo.net/). It is of very simple use. You do not have to install anything or register and it is free. According to the website, the steps to follow are:

1. Think of an alias for computer B. For example, computer_B_alias.

2. In computer B, execute ssh -R computer_B_alias:22:localhost:22 serveo.net.

3. Now, you can access computer B from computer A by executing the following in computer A: ssh -J serveo.net user@computer_B_alias, where you have to substitute user by the name of the user in computer B.

P.S.: Of course, you make point 2 an automatic task when starting your computer B.

P.S.S.: Before you try this, make sure that ssh is installed in both computers. For Ubuntu, sudo apt-get install ssh would do the job.

You can use a VPN of sorts to get this working, but it would require you to have a server that the inaccessible server can access. Then you can set up OpenVPN on the server, your PC, and the firewalled server, enable client-to-client, and you're done. http://openvpn.net/howto.html