How can I reconnect to a ssh session after a broken pipe?
While you can't reattach to a broken SSH session, you can reparent the process running inside SSH – functionally equivalent to what you want.
Instructions
In your case, you would take over the apt-get process to be controlled from a new SSH session, screen session or the like. My favourite for this is the reptyr command:
$ sudo apt-get install reptyr
$ ps ax | grep apt-get
10626 pts/8 R+ 0:32 apt-get upgrade
Then, with the pid you found for your process:
$ sudo reptyr -T 10626
Or if that does not work, try:
$ reptyr 10626
After this stage, all your keyboard input goes to the program you took over. Unfortunately you will not see old output of the SSH session, such as the apt-get output asking you for confirmation.
Explanations
There are multiple other tools that basically work the same as reptyr (that is, via ptrace debug attachment). See the following questions and answers where they are discussed:
- How can I disown a running process and associate it to a new screen shell?
- Resume command running in dropped SSH session
In the instructions above, the reptyr 10626 uses ptrace debug attachment while the sudo reptyr -T 10626 command uses TTY stealing and is preferable (details).
Finally, the reason why you can't take over a SSH session this way is because a sshd process is not controlled by a host terminal, instead it provides the slave part of a terminal – a pts device – while the master part controlling it resides on the client machine, here with a broken-down SSH session in between. When you force taking over such a sshd process with reptyr -s <pid>, your keyboard input goes to that process, not its active child process. So a "Ctrl+Z" will simply kill off that sshd.
The answer to your proper question is: you can't. I think the main problem is that the authentication procedures will be out of sync. It Just Doesn't Work Like That.
As you have yourself noticed, the solution is to use screen when possible (by the way, tmux is an alternative to screen).