Spawn one systemd service instance per connection with Accept=yes and a template unit (inetd style)

Documented steps

1. Use Accept=yes when each connection should get its own short-lived process (inetd style). Socket unit /etc/systemd/system/echo.socket: [Socket] ListenStream=7777 Accept=yes [Install] WantedBy=sockets.target
2. With Accept=yes systemd accepts the connection itself and passes the CONNECTED socket to a fresh instance of a template service named echo@.service (template name = socket stem + '@'): [Unit] Description=echo instance %i [Service] ExecStart=/usr/local/bin/echo-handler StandardInput=socket StandardOutput=socket
3. StandardInput=socket/StandardOutput=socket wires the connection to stdin/stdout, so any filter-style program works unmodified — no sd_listen_fds needed.
4. Instances are named like echo@0-127.0.0.1:7777-127.0.0.1:53170.service; `systemctl list-units 'echo@*'` shows live connections.
5. Limit abuse: in [Socket] set MaxConnections=64 (per-socket concurrent instance cap) and optionally MaxConnectionsPerSource=8.
6. Enable: `systemctl enable --now echo.socket`; test with `nc 127.0.0.1 7777`.