Telnet was the original method of connecting to a remote computer, however it has declined in popularity due to security issues. SSH has largely replaced it because it's more secure, and that's what I'm going to tell you about today.
SSH requires two components to work - a client and a server. Think of it like surfing the web - when you visit a web page, you use a client (the web browser) to connect to a server (the web server). Similarly, with SSH you use an SSH client to connect to an SSH server on the machine you want to connect to. The most common SSH client and server on Linux are maintained by the OpenSSH project. Ubuntu ships with the OpenSSH client by default, and if you have a machine you'd like to be able to connect to remotely via SSH, just install the openssh-server package in the usual way.
How do you use it? It's simple. Open a terminal and enter something using the following format:
ssh user@serverLet's go into this in a little more detail. First of all, user is just the user name you want to log in as on the remote server. For instance, if you've installed openssh-server on your desktop and your user account is called eric on that machine, then you should put eric as the username. If you want to connect to a shell account in the name of ebarnes, then it should be ebarnes.
The server can be either a domain name or an IP address. So if you have a shell account on a machine which has the domain name myfreeshellaccount.com and your user name is ebarnes, you'd enter ssh email@example.com. If you want to connect to a machine that doesn't have a domain name associated with it, then you'd need to enter the IP address to connect to. There's nothing stopping you connecting to another machine on your home network via this method, so if you have an old machine running Ubuntu Server, for instance, on your home network, and it's been assigned the IP address 192.168.1.4 by your router, you can just connect like this:
If you'd like to be able to use SSH to connect to your home machine from outside your home network, that's a bit more demanding. You'd need to either obtain a static IP address from your ISP, which can be expensive, or use a service like DynDNS, and would also need to set up port forwarding on your router. However, there's all sorts of possibilites that offers, such as managing BitTorrent downloads remotely by using a command-line client like rTorrent, or administering your machine remotely over SSH.
Once you're connected, SSH works just like any other shell session - you can do exactly the same things with SSH as you would in the terminal. When you're finished, just type in exit or hit Ctrl-D as you would normally to close the shell and that will finish your session.
Note that you aren't limited to connecting to an SSH server from Linux or Unix hosts. Although most Linux or Unix-based operating systems ship with an SSH client, you can use PUTTY to connect from a Windows box (so, for instance, if you have a desktop running Ubuntu with an OpenSSH server installed, and you want to remote into it from your laptop running Windows 7, you can do that). There's even SSH clients for a number of smartphones, including the iPhone and Android, so you can remote into your machine from anywhere.
If you want to try it out, and you have an old desktop you don't use, it's quite interesting to install Ubuntu Server on it, connect it to your router and disconnect the monitor, managing it entirely via SSH. It can be a useful resource to have around, and offers a lot of possibilities, such as:
- File server
- uPnP server (allows you to stream content to many electronic devices, including the Sony PlayStation 3)
- BitTorrent box
Alternatively, if you don't have an old machine spare or don't wish to try this, why not try it in a virtual machine? Virtualbox is good for this - just install Ubuntu Server in a VM, and under the network settings, attach it to a bridged adapter. This will mean the virtual machine gets allocated its own IP address by your router, and you can connect to it via SSH.
If you have a web hosting account, you may be able to connect to this via SSH - the host will provide the details if this is an option. Finally, another option is to get a shell account - essentially it's a user account on a remote server. I highly recommend devio.us, who are a free shell account provider running on OpenBSD.
Have fun with SSH! Next time I'll show you a few tricks to make your experience with it better!