I often make typos, and sometimes I hit a key combo when I don’t mean to. (That never happens to you right? Right?) One of my most common goof s is hitting several of the same key combination in a row, such as Ctrl+D to exit shells. I might find myself in a situation where I’m running a secure shell session in a terminal, and after I exit out of several subshells or a screen session with Ctrl+D, I inadvertently hit Ctrl+D one extra time, and exit the shell I wanted to keep.
Now ordinarily this wouldn’t be a problem — just run ssh to get back into the box, and Bob’s your uncle. But every once in a while it can be disastrous, for instance when you’re piddling around with the network on a remote box, or if the SSH session in question is providing a tunnel for some other applications in progress.
I do a couple things to avoid this problem. The first is to use the ControlMaster option with my SSH sessions (the shorthand option is -M). More than one instance can share the same connection. The first instance acts as a master, and you can configure where to store the socket it’s using. Additional sessions you start on the same host and port will share the connection.
This has a couple effects, and one of them is fast startups for the second and following connections — since the connection has already been made and you’ve authenticated, all that remains is to start up the session (such as a login shell). Another effect is that you can shut all the connections down (somewhat forcibly) by shutting down the master session!
Take my situation for example. I set this option in my personal configuration file (~/.ssh/config), because I often have multiple connections open from different applications to the same host. Sometimes those connections aren’t all obvious. For example, my music library is on a remote host that I access with Rhythmbox via SFTP (FTP over SSH). The SFTP connection will open automatically when I start up Rhythmbox — I get prompted for the passphrase, of course, because I don’t like to store that anywhere — and the volume is mounted via Nautilus, the GNOME file manager.
Now imagine later on, I open a secure shell to the same system for other work. If I unmount the share for some reason, having forgotten it was the first thing that started a SSH connection and thus inherited the title of “master,” boom! My other SSH session gets disconnected. Yikes. And the reverse sometimes happens too — I might stop the SSH session I started in the morning, and if it’s the master for the music library connection from Rhythmbox, my music might stop cold.
So obviously, I like the positive side effects of this setting, but not so much that last, negative one. Especially when I have a habit of shutting down the master carelessly. Recently I’ve started using is the -N option to help solve this problem, which doesn’t execute anything on the remote side after authenticating. I think I saw this hint elsewhere, and found it intriguing.
You don’t get a shell or anything else after login, just an open socket. So at this point you can background the job (Ctrl+Z in the bash shell), and close the terminal it’s in. The SSH socket stays open peacefully and invisibly in the background like a Zen master, waiting to do your bidding. Now you can open up additional sessions on that system with ssh remote-host-name, and they pop up quickly. No matter what interactive shell session you close, things keep humming right along. (If you really need to close the entire connection and all shared sessions with it, you can just kill the master connection that’s still in the background.)
Since I use a variety of SSH tunnels for different stuff, the -N switch has made it much easier for me to keep my tunnels alive and working even if I have to logout of my GNOME session. If you’re more trusting with keeping your SSH passphrases in the GNOME Keyring, you could start up lots of these background connections with minimal keyboard interaction. If you wanted to be slightly more clever, you could even set up a notification that would inform you if one of the background sessions failed (that will still generate an error condition on your end, the client end).
This is not the only way to use SSH more effectively, but this method solved my problem and I hope you find it useful, and that it helps you complete your work a little more efficiently. I’ve started a category for hints like this on my blog, and I’ll continue to send useful tidbits here for your reading pleasure. If this information helped you, or if you want to make an additional suggestion, please feel free to use the comment form, or trackback from your own helpful blog entry.