Using systemd sockets

A very nice feature of systemd is working with sockets.
The idea of socket is to listen on a specific port for incoming traffic, and when the traffic is coming in - the related service will be started automatically.

You can have a look into directory /usr/lib/systemd/system for files sshd.service and sshd.socket. There is a relation between two of them - they cannot be ran simultaneously - you can see it by opening the sshd.socket and check the line:

Conflicts=sshd.service

To enable the socket:

systemctl disable sshd.service
systemctl enable sshd.socket