How do I reattach to a detached mosh session?

To my amazement, I used CRIU (https://criu.org) to checkpoint and restart a mosh client and it worked.

Shocking.

Find your mosh-client's PID:

$ ps -ef | grep mosh

Then, install CRIU according to their instructions.

Then, checkpoint it like this:

$ mkdir checkpoint

$ sudo ./criu dump -D checkpoint -t PID --shell-job

Then, restore it:

$ sudo ./criu restore -D checkpoint --shell-job

And, there it is. Your mosh client is back.

One thing to note, however, is that if your laptop reboots (which is the whole point of what we're trying to protect against), mosh uses a monotonic clock to track time on the client side, which doesn't work across reboots. This will NOT work, however, if your laptop just flat out crashes it won't work because mosh sequence numbers will be out of sync with the version that was checkpointed (the binary will resume, but communication will stop).

In order to fix this, you need to tell mosh to stop doing that and download the mosh source code. Then, edit this file:

cd mosh

vim configure.ac

Then, search for GETTIME and comment out that line.

Then do:

autoreconf # or ./autogen.sh if you've just cloned it for the first time

./configure

make

make install

After that, your CRIU-checkpointed mosh client sessions will survive reboots.

(Obviously you'd need to write something to perform the checkpoints regularly enough to be useful. But, that's an exercise for the reader).

As an addition to Varta's answer, I use the following command to close all mosh connections except the current one:

pgrep mosh-server | grep -v $(ps -o ppid --no-headers $$) | xargs kill

I use mosh combined with tmux on the server to ensure my sessions persist in the scenario where the client dies for some reason, e.g. enforced reboot.

P.S. or you could continue to use screen as well, if that's your preference.