Use ssh_config To Simplify Your Life

When using multiple systems the indispensable tool is, as we all know, ssh. Using ssh you can login to other (remote) systems and work with them as if you were sitting in front of them. Even if some of your systems exist behind firewalls you can still get to them with ssh, but getting there can end up requiring a number of command line options and the more systems you have the more difficult it gets to remember them. However, you don't have to remember them, at least not more than once: you can just enter them into ssh's config file and be done with it.

For example, let's say that you have two "servers" that you connect to regularly, one at your house that's behind your firewall. Further, let's say that you use dyndns to make your home IP address known, and that you've got ssh listening on port 12022 rather than the default port 22 (and you've got your firewall forwarding that port to the server). So to connect you need to run:

$ ssh -p 12022

The second system, let's say is local and you just connect with:

$ ssh

The second one is not too bad to type, but a name would be easier. You could put the name in your /etc/hosts file, or you could set up a local DNS server, but you can also solve this problem using ssh's config file.

To create an ssh config file execute the commands:

$ touch ~/.ssh/config
$ chmod 600 ~/.ssh/config

Now use your favorite text editor to edit the file and enter the following into it:

Host server1
Port 12022

Host server2

The Host option starts a new "section": all the options that follow apply to that host till a new "Host" option is seen. The "HostName" option specifies the "real" host name that ssh tries to connect to (otherwise the "Host" value is used). The "Port" is obviously the port that ssh tries to connect to, if you don't specify a port, the default port is used.

Now you can connect much more simply:

$ ssh server1
$ ssh server2

These are just a few of the options that you can set in ssh's config file. You can also, for example, specify that X11 forwarding be enabled. You can set up local and remote port forwarding (i.e. ssh's -L and -R command line options, respectively). Take a look at the man page (man ssh_config) for more information on the available options.

One of the added benefits of using ssh's config file is that programs like scp, rsync, and rdiff-backup automatically pick up these options also and work just as you'd expect (hope).

Mitch Frazier is an embedded systems programmer at Emerson Electric Co. Mitch has been a contributor to and a friend of Linux Journal since the early 2000s.

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