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Installing a network service

The fixed procedure, from package to test from a client: configure, start and enable, check that it listens, open the firewall, read the logs, using a web server and an SSH server as examples.

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  • Enabling a service: what is it?

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    Making it start at every boot of the machine, without launching it right now.

    That is systemctl enable, undone by systemctl disable. Starting (start) and enabling are independent: a service can run without being enabled, or be enabled and stopped.

  • Ignoring the firewall, which ports are reachable from another machine on the network?

    ```text
    $ ss -tln
    State Recv-Q Send-Q Local Address:Port Peer Address:Port
    LISTEN 0 4096 127.0.0.53%lo:53 0.0.0.0:*
    LISTEN 0 128 0.0.0.0:22 0.0.0.0:*
    LISTEN 0 511 0.0.0.0:80 0.0.0.0:*
    LISTEN 0 244 127.0.0.1:5432 0.0.0.0:*
    `

    • 22 et 80
    • 22, 80 et 5432
    • 80 seulement
    • 53 et 5432
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    22 et 80

    Only sockets on 0.0.0.0 accept outside connections: SSH (22) and the web server (80). The DNS stub (53) and the database (5432) are bound to the loopback and stay invisible to other machines.

  • The … nginx command tells whether the service runs, with the line Active: active (running), then shows its last journal lines.

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    The systemctl status nginx command tells whether the service runs, with the line Active: active (running), then shows its last journal lines.

    After a failed start, the line becomes Active: failed; journalctl -u nginx then gives the full message. status changes nothing about the service, it only reads its state.

  • Listening socket: what is it?

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    A server process's entry point, bound to an address and a port, waiting for clients' connections.

    An installed and started service is reachable only if it has a listening socket: ss -tln shows it in the LISTEN state.

  • The web server listens on 0.0.0.0:80 (ss confirms it) but a client gets Connection timed out. Probable cause, then fix?

    • The firewall drops the packets: open port 80
    • The service is not started: start it with systemctl
    • The service is not enabled: enable it for boot
    • The package is badly installed: reinstall it cleanly
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    The firewall drops the packets: open port 80

    ss proves the service listens. Silence until the timeout means a filter drops the packets; if nobody were listening, the machine would answer Connection refused at once.

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