SSH tips and tricks

In trying to implement some of the hacks in Linux Server Hacks, I had to go to the ssh manpage, where I discovered a number of cool tricks.

1. In order to get key-based authentication (i.e., passwordless) working, the $HOME/.ssh directory must be mode 0700, and all files in it must be mode 0600. Once that's setup properly, key-based authentication works perfectly.
2. You can have a file called config in your $HOME/.ssh directory that specifies user-specific settings for using SSH, as well as a number of host-specific settings:
   • Compression yes turns on compression
   • ForwardX11 yes turns on X11 forwarding by default
   • ForwardAgent yes turns on ssh-agent forwarding by default
   • Host-based settings go from one Host keyword to the next, so place them at the end of the file. Do it in the following order:

         Host nickname
         HostName actual.host.name
         User username_on_that_host
         Port PortToUse
                 

     This means, for instance, that I can ssh back and forth between home using the same key-based authentication and the same ssh-to script (more below) I use for work servers -- because I don't have to specify the port or the username.

I mentioned a script called ssh-to earlier. This is a neat little hack from the server hacks book as well. Basically, you have the following script in your path somewhere:

    #!/bin/bash
    ssh -C `basename $0` $*

Then, elsewhere in your path, you do a bunch of ln -s /path/to/ssh-to /path/to/$HOSTNAME, where $HOSTNAME is the name of a host to which you ssh regularly; this is where specifying a host nickname in your $HOME/.ssh/config file can come in handy. Then, to ssh to any such server, you simply type $HOSTNAME at the command line, and you're there!