Using proftpd with systemd's built-in inetd functionality
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On systemd-based systems, you can use systemd's built-in inetd
functionality to start proftpd on demand. Note that this is not
full "socket activation" where systemd would start a single
long-running process to handle the current and subsequent
connections, but a simple inetd equivalent where a new instance
of proftpd is run for each new incoming connection, each session
lasting until its connection drops. This makes it best suited to
low-volume applications.

1. Install the files proftpd.socket and proftpd@.service from the
   contrib/dist/systemd directory into either the system unit directory
   (usually /usr/lib/systemd/system - recommended for downstream
   packagers) or the local unit directory (/etc/systemd/system -
   recommended for admins installing from source locally).

2. Set "ServerType inetd" in /etc/proftpd.conf

3. Run "systemctl enable proftpd.socket" as root to enable
   listening for connections at boot time.

4. Run "systemctl start proftpd.socket" as root to start
   listening for connections immediately.