Doubly detached IRC

Thursday, 8 Feb 2018

For long-running terminal sessions such as IRC, I’ve long used dtach (or Screen, or tmux, it doesn’t matter) to be able to leave them running on a server without having to be connected to it… just like everyone else does. But recently I put together some simple facts in a retrospectively obvious way that I haven’t heard of anyone else doing, and thus achieved a little quality of life improvement.

I’ve written before about my love for Mosh during times of need. And when I had the insight I am writing about, I was in a situation where I seriously needed it. (If you don’t know what Mosh is, read about it first, because ⓐ you’re missing out and ⓑ the rest of this article won’t make much sense otherwise.)

Here’s the thing about Mosh, though: it still has to go through regular SSH in order to bring up a Mosh session. And on a severely packet-lossy connection, that alone can be hell.

The real beauty of Mosh comes to the fore only if you keep the session around once it’s set up. As long as it’s up, then no matter how bad the packet loss, you get decent interactivity. But to fully benefit from that, you have to avoid tearing the session down.

My problem is that try as I might, I have never been able to break with my compulsion to close terminal windows once I am done with them. For IRC that means sooner or later I’ll want to detach from a session I’m not actively chatting in. And because I use Mosh to run dtach on the remote end, detaching from IRC means that the dtach client exits on the remote end… which tears down the Mosh session.

The simple fact that suddenly occurred to me is that I can also use dtach on my end of the connection, in front of Mosh:

dtach -A ~/.dtach/irssi mosh hostname dtach -A ~/.dtach/irssi irssi

Now when I detach, it is only from my local dtach session, not the one on the server. So the Mosh session behind it sticks around – without me having to keep the terminal window open.

The upshot is a dtach ↔ Mosh ↔ dtach sandwich which gives me the full benefits of Mosh.

Should you want to use this yourself, you will need the last piece of the puzzle, namely how to bring down the Mosh session while keeping the IRC session around. To do that you have to detach on the remote end, and the simplest way of doing that is this:

dtach -a ~/.dtach/irssi -E # and then press the detach shortcut

The -E switch disables the keyboard shortcut for detaching in the local dtach client. This means when you press the shortcut it gets sent to the remote dtach client.

(What follows then is exactly the same chain of events as always when you detach from the remote dtach session: the remote dtach client exits, so the remote Mosh session ends, so the local Mosh client exits – and therefore the local dtach session ends as well. Detaching from the remote end thus brings the whole edifice down.)