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A friend of mine recent installed a fresh copy of Ubuntu Server minimum install with SSH Sever. My friend can connect to the Linux box via SSH from his windows box, on his local network.

Me and another friend (B) (outside friend A's local network) however are not able to connect to the server via the internet. The router is set to use a DMZ for the Linux box so nothing is restricted by the router.

We are trying to use PuTTY to SSH into the server. We both have accounts on the server with SSH acces, but we are not even getting a login prompt.

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I don't mean to insult you by telling you the obvious. I seem to do that a lot.

I suspect you are not using the correct IP to talk to the SSH server from your computer. Remember, inside the router area, the server will have an IP number that is meaningful only inside the router's subnet, usually 192.168.xxx.xxx. That works fine for everybody within the router, such as friend A.

Now, from outside the router, you probably need to log in differently on the server. Server has different address if you look at it from outside the router. Right now, my computer is given dynamic ip at 192.168.0.105, for example, but my IP seen by rest of world is the IP assigned to my router by the cable company.

Here's what I'd do if I were you. Go to the server and use it to log in on the website "whatismyip.com". That will tell you what IP number your system seems to be on. However, you will probably find that every other computer in that subnet appears to be at same outward facing IP. Hence, you cannot "get into" your server using that IP.

Usually, you have to configure the router so that messages on a port (SSH usually 22) will go to one of the computers inside the network.

You say it is set up as DMZ, which I remember from days of playing computer games, but it does not tell us much about how your computer is getting through to the server. In particular, the router has to pass through the messages to the server.

Think it over, let us know what you find out.

  • The DMZ is basically a zone on the router with no firewall (on the router's side) with all ports not forward elsewhere going to the machine in the DMZ, and the machine setup as in the DMZ is fully exposed to the internet. That was done to encase of a port forwarding issue. Risky as it exposes the machine, relying on any firewalls and protections on the machine itself, but usefull for possibly eliminating the router's firewall in troubleshooting connection issues from outside the local network. – Krahazik May 17 '17 at 14:19

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