salva@ubuntu:~$ ssh -M -S ~/.ssh/mux_socket -N -f 172.20.8.191 
# password prompt, appears here
# after authorization, process goes to background

salva@ubuntu:~$ ssh -S /home/salva/.ssh/mux_socket 172.20.8.191 echo "hello"
hello
salva@ubuntu:~$ ssh -S /home/salva/.ssh/mux_socket 172.20.8.191 echo "bye"
bye
salva@ubuntu:~$ ssh -S /home/salva/.ssh/mux_socket -O exit 172.20.8.191
Exit request sent.