tmux session killed when disconnecting from ssh

Theory

Some init systems including systemd provide a feature to kill all processes belonging to the service. The service typically starts a single process which that creates more processes by forking and those processes can do that as well. All such processes are typically considered part of the service. In systemd this is done using cgroups.

In systemd, all processes belonging to a service are killed when the service is stopped by default. The SSH server is obviously part of the service. When you connect to the server, SSH server typically forks and the new process handles your SSH session. By forking from the SSH session process or its children, other server side processes are started, including your screen or tmux.

Killmode and socket activation

The default behavior can be changed using the KillMode directive. The upstream project doesn't AFAIK include any .service files and so those vary by distribution. There are typically two ways to enable SSH on your system. One is the classic ssh.service that maintains a long running SSH daemon listening on the network. The other is via socket activation handled by the ssh.socket that in turn starts [email protected] which only runs for a single SSH session.

Solutions

If your processes get killed at the end of the session, it is possible that you are using socket activation and it gets killed by systemd when it notices that the SSH session process exited. In that case there are two solutions. One is to avoid using socket activation by using ssh.service instead of ssh.socket. The other is to set KillMode=process in the Service section of [email protected].

The KillMode=process setting may also be useful with the classic ssh.service, as it avoids killing the SSH session process or the screen or tmux processes when the server gets stopped or restarted.

Future notes

This answer apparently gained a level of popularity. While it worked for the OP it might happen that it doesn't work for someone in the future due to systemd-logind development or configuration. Please check documentation on logind sessions if you experience behavior different from the description in this answer.

Do you use systemd with socket activation for SSH?

If so, there’s a known issue with that. According to the systemd proponents, this is actually a feature – systemd kills all processes spawned by a session when the session terminates. (I can see that being useful, but in the GNU screen, or tmux, case, you definitely don’t want that ☺ nor in most other cases where users may run background processes, of course.)

If so, try switching from sshd.socket to sshd.service.

I was having the same problem with tmux and screen on Ubuntu 16.04 (kde neon). When the ssh session was disconnected screen / tmux was terminated.

long story short, systemd changed their default setting to killuserprocess=yes so after leaving a ssh session every process created by it will be terminated.

Easy fix (after hours of trying) run screen/tmux using this command

For screen

systemd-run --scope --user screen

for Tmux

systemd-run --scope --user tmux

You can create an alias to make it easier

alias tmux= "systemd-run --scope --user tmux"