Solution:
Root Cause: Socket Unit Mismatch or Closed File Descriptor Systemd supports socket-based activation, where a
.socket unit listens on a network port or UNIX socket and passes the open file descriptor to the companion
.service unit upon incoming connections. If the
.service unit lacks native socket passing support, or if the socket address/port settings between
.socket and
.service disagree, the child process crashes upon accepting connection handles.
Diagnostic Verification: Verify status of the corresponding
.socket unit:
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sudo systemctl status <service_name>.socket
Check if the port or path specified in
.socket is held open by another process:
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sudo ss -tlpn | grep <socket_port>
Inspect service journal output for file descriptor activation errors:
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sudo journalctl -u <service_name> -e | grep -i "socket"Step-by-Step Fix: 1 Ensure Matching Socket and Service Names:
Confirm that the socket file (e.g., app.socket) matches the service file name (app.service) exactly. 2 Stop Conflicting Service Units Listening on Same Socket:
Ensure standalone process instances are stopped before starting systemd socket units: Copy
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sudo systemctl stop <service_name>.service
sudo systemctl restart <service_name>.socket3 Disable Non-Socket Configuration Options in Application:
Ensure the application is configured to listen directly to inherited file descriptors (sd_listen_fds) rather than re-binding the socket independently. Prevention & Long-Term Monitoring: Test socket-activated units using systemd-activate to verify descriptor passing behaviors in development environments.