After sending shutdown command, ssh session doesn't terminate

This worked for me:

apt-get install libpam-systemd dbus

Also make sure that you have UsePAM yes in your ssh config.

grep -i UsePAM /etc/ssh/sshd_config

Unfortunately, you need to reboot for the solution to take effect...

Detailed explanations on serverfault.

Looks like that's a systemd issue currently tracked under bug #751636.

When the host is shut down or restarted, systemd might shut down the network before it kills the ssh session.

There are a couple of solutions provided but nothing concrete:

1. Using acpid/acpi-support-base to handle the power events & add the following to the /etc/acpi/powerbtn-acpi-support.sh

   else
   -       # Normal handling.
   -       /sbin/shutdown -h -P now "Power button pressed"
   +
   +       if [ -x /bin/systemctl ] ; then
   +           echo "\nPower button pressed\nThe system is going down for system halt NOW!" |\
   +            /usr/bin/wall -n
   +           /bin/systemctl --force poweroff
   +       else
   +           # Normal handling.
   +           /sbin/shutdown -h -P now "Power button pressed"
   +       fi
   +
   fi
   

   and then make aliases in your ~/.bashrc:

   alias reboot='echo "The system is going down for system reboot NOW!" |\
   /usr/bin/wall -n ; /bin/systemctl --force reboot'
   
   alias poweroff='echo "The system is going down for system halt NOW!" |\
   /usr/bin/wall -n ; /bin/systemctl --force poweroff'
   

2. Creating /etc/systemd/system/ssh-user-sessions.service with the following in it:

   [Unit]
   Description=Shutdown all ssh sessions before network
   After=network.target
   
   [Service]
   TimeoutStartSec=0
   Type=oneshot
   RemainAfterExit=yes
   ExecStart=/bin/true
   ExecStop=/usr/bin/killall sshd