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Network Commands: ip, ss, ping, curl, dig and nc for "Is the Port Open, Who Is Listening, Why Can't I Reach It?"

When a service cannot be reached you go layer by layer: the name with getent, the path and firewall with ping and nc -zv (refused means nothing listens, timeout means packets are dropped), who listens on which address with ss -tlnp, then curl and the error log. We show ip, ss, lsof, curl, traceroute, dig and resolvectl on the Bookstore server, solve a real case where a stale hosts entry, ufw and a wrong port stack up, and look at proxy and IPv6 pitfalls.