Part I - Installing Linux
Part II - Setting up SSH
First up is the install. Bring up a terminal, and use the command "sudo apt-get openssh-server" to install the SSH server. This is the command for Ubuntu, for Fedora, I think it's "yum openssh-server". You may also need to install the client. To check, first issue the command "man ssh" to bring up the manual page (use q to quit), if nothing comes up, then you don't have it installed, "sudo apt-get openssh-client" will do the trick.
That's all for the installation, now let's test out the SSH capabilities. Behind the scenes, after you did the install Ubuntu starts up the ssh service. Later we'll make changes to the ssh files and will need to restart the service, so this will become more clear.
My other computer is a Win XP box (good lord!), and I'm used to PuTTY to connect. We'll start on the local network. Grab the local ip off of the server with the terminal command "ifconfig". It returns a lot of other stuff, but somewhere in there should the be local ip address which looks like "192.168.#.##". Later I'll use the concrete example "192.168.1.21" as the host server. Load up a session of PuTTY and type in this address in the Host Name box, then click on Open. A command window opens that should be prompting you for your username and then password. You can use any of your accounts to test it out, after logging in it will drop you to the Ubuntu command line.
With that working, let's make it a little more secure. Why do we want to make this secure? Here's a great article by the Fedora folks on security precautions and why you want to use them. I particularly liked the examples of the number of attempts to log in as root.
The easiest things you should be able to do are 1) use a strong password on root, 2) change your port number to something non-standard, 3) use SSH 2, 4) deny root the ability to log in, and 5) use key authentication. I would recommend you implement all of Fedora's guidelines (if applicable), but I'll cover these four in more depth.
Assuming you can take care of #1 on your own, here's how to handle number's 2-4. On the linux box, use the command "sudo vi etc/ssh/sshd_config". This opens up a text editor (called vi, see here for a list of commands) and shows the preferences file for the ssh server. Find and edit the text lines;
Port 22 -> Port NNNNThe right side of the arrow is what it should read when you're done. For port numbers, the most common are; 22 (SSH) 2222 (alternative SSH), 80 (webserver) 8080 (alternative webserver). So we'll pick an unused port say 1986. You can check it against this list, but it's not required that you pick something not on there, the max you can choose is 65535 (I think). Now if you try to use PuTTY to log-in again, you can't. We have to specify the port as well, the field directly right of the Host Name. With this you should be back to logging in just fine.Protocol 2,1 -> Protocol 2PermitRootLogin yes -> PermitRootLogin no
Now we'll set up key authentication, replacing the user-id and password scheme. This makes you system less vulnerable to brute force attempts. Instead of trying a common name "root" and a bunch of passwords "", "password", etc. SSH will confirm you identity with a public/private key pair. This is actually how SSH transmits all it's data anyway, so your login will be as secure as the channel (or your computer). What happens in this scheme, is that you generate two "keys" one is called the private key - only you have the private key, and it identifies exactly who you are. The second is the public key, which you can give out to anyone! If you want someone to know that it is you at the other end, you encrypt a message (say "Hello world") with your private key. You send it to them, and they decrypt it with the public key you gave them, they see "Hello world" and know it's you. Why? Because if someone else had encrypted the message with her private key, gibberish would result on the output. Moreover, knowledge of the public key doesn't let someone know what your private key is. So as long as your private key is private, you can encrypt messages with it, and if they're decipherable, then the other party knows it is you at the other end. Of course there are drawbacks to this method - you could compute every possible private/public key pair, and then if someone knows your public key, they also have your private key. But if you make the keys longer, then it becomes much more difficult to compute every pair, and the system remains secure.
I found this link extremely helpful in setting up key authentication using PuTTY and won't try to reproduce his work here; http://www.howtoforge.com/ssh_key_based_logins_putty_p2. Also be sure to encrypt (password protect) your private key, as if anyone gets their hands on your private key file they could impersonate you. Password protecting it gives you another layer of defense. For linux, you can use the command "ssh-keygen -t rsa" to generate the key pairs. Same steps apply for registering your public key with the server.
Once you have key based logins on all of your computers, we can turn off username / password login. Open your config file again; "sudo vi etc/ssh/sshd_config", and change the following lines;
So all this allows us to ssh into our server while on the same network, but what if we want to get to our files while on a different network, a friend's house or at work? Nowadays most IP addresses dynamically assigned and recycled, so we'll use Dynamic DNS (domain name system) to create a easy to remember web address for our server. I chose to use the FreeDNS service, although others exist, like OpenDNS and DynDNS is another popular one. For example, you sign up for a sub-domain, the "madscientistlair" part of "madscientistlair.blogspot.com". Then you use as the host name field in PuTTY, "madscientistlair.blogspot.com" and the DNS service says "Oh, you really want; 245.324.3.564", which is the current IP address your ISP has given your home. PuTTY connects to this instead, which goes to your router, which knows that requests for port 1986 should be sent to "192.168.1.21" and you connection to your server is made.
The second part is installing a client to keep the Dynamic DNS servers updated. On my Ubuntu box, I used ddclient, installing with "sudo apt-get install ddclient". On setup, it prompts you for the service you're using. For more information check out the Ubuntu community page. Your router may support one of these services, go to your router web administration tools and then there's usually a Dynamic DNS tab.
One other important step, and you may have noticed this above. How does the router know that you want to talk to the server computer? First it's useful to set a static IP for the server, this way the address "192.168.1.21" that we've been connecting to from home doesn't change in the middle of the night and we can no longer get to our server. The second is to set up port forwarding. There are usually tabs for both in the web administration tools for your router. Since these vary wildly, I can't give you any more advice other than on the port forwarding page, you want to have 1986 (or whatever SSH port number you specified in the sshd_config file) point to your server; 192.168.1.21. To check that it's working, go to http://canyouseeme.org/ and give it your port number. Below their ad, you'll see the result.
There was one file issue I ran into. I could use "madscientistlair.blogspot.com" to talk to my server when I was outside of the house, but not when I was in the house! It turns out that some classes of routers don't allow you to do this. Essentially they prevent you from talking to a local machine through an outside connection. Kinda like using your cellphone to call you grandma in Florida so she can call the home phone. To get around this you'll have to have a batch script on your desktop that edits your windows "hosts" file when you switch locations. For example, you could have two scripts "set_when_home.bat" and "set_when_away.bat". You run set_when_home, when you're home, and this tells windows that when PuTTY asks to connect to "madscientistlair.blogspot.com" it really wants to connect to "192.168.3.564". When you're away, you run set_when_away, and it undoes this change. See here for a couple of example hosts files.
Well that's it. We've installed SSH into our server, secured it, and allowed access from the outside. If you have any lingering questions, or can't get something to work, leave a comment and I'll try to help.