Commands

$ curl -sSI https://example.com

Make an HTTP request and show what comes back.

The fastest way to prove a service answers, and to see the headers it sets. -I fetches headers only, -s silences the progress meter, and -S puts errors back after -s removed them.

$ curl -sS -o /dev/null -w '%{http_code} %{time_total}s\n' https://example.com

The OSI model dig ss man page

$ dig +short example.com

Query DNS and show the answer.

Use it rather than ping when the question is about name resolution. +short gives just the answer; @resolver asks a specific server, which is how you tell “DNS is broken” apart from “this resolver is broken”.

$ dig +short @9.9.9.9 example.com

Network protocols and ports curl man page

$ ip -br address
$ ip route

Show and configure addresses, routes, and interfaces.

The modern replacement for ifconfig and route. -br gives a brief one-line-per-interface view, -4 and -6 restrict it to one family, and ip -s link adds error and drop counters — a link can negotiate fine and still be marginal.

$ ip -6 neigh show

IPv4 addressing IPv6 addressing ss dig man page

$ ping -c2 host

Send ICMP echo requests to test reachability.

Proves a host answers ICMP. It says nothing about whether a service is listening — use ss or curl for that. Plenty of hosts drop ICMP by policy, so silence is not proof of absence.

Careful: A quiet ping is weak evidence in both directions. Do not conclude a host is down from it.

IPv4 addressing ss curl ip man page

$ ss -tln

Show sockets — what is listening, and what is connected.

The replacement for netstat. -t is TCP, -l listening only, -n skips name resolution so the output is fast and unambiguous. Compare services bound to loopback against those bound to a wildcard: the first group is reachable only from the machine itself.

$ ss -tlnp

Network protocols and ports ip ping man page

$ tcpdump -i eth0 udp port 4789

Capture and print packets crossing an interface.

The instrument for questions no status command answers — what is actually on the wire, and whether an encapsulated frame really carries what you think. Needs root, and a filter expression, or it will drown you.

Careful: Captures may contain credentials and payload data. Use purpose-built fixtures for examples you intend to share.

Software-defined networking ip ss man page