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When Ping Lies

Lesson 17 of 47 in Coddy's Networking with the Terminal course.

Ping is the first thing everyone reaches for, and it answers a narrower question than people think. It tells you the machine is alive. It says nothing about whether the thing you actually want is running.

A server can answer every ping perfectly while the web server on it has crashed. The machine is up; the service is down. Ping cannot see the difference, because it never talks to the service.

It lies in the other direction too. Plenty of hosts are configured to ignore ICMP entirely, so ping reports 100 percent loss for a machine that is serving traffic happily. A failed ping is not proof that anything is wrong.

So ping is step one of a ladder, never the whole diagnosis. Step two is asking the port itself.

Challenge

Medium

Show the exact situation ping cannot see: a machine that answers, with nothing serving on the port.

Against 127.0.0.1, and starting no server at all:

Ping once and print host up
Check port 9999 with nc -z and print port closed

Expected output:

host up
port closed

The machine is obviously alive. That is exactly the point: alive is not the same as serving.

Try it yourself

Terminal

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Networking with the Terminal

1How Networks Talk

What Is a Network Clients and Servers IP Addresses Your First Request

4Is It Reachable?

ICMP and ping Reading Ping OutputWhen Ping LiesRefused or Timed Out Recap: Reachability

7Sending Data

POST with -d What -d Actually Sends Form Encoding Sending JSON PUT and DELETE Recap: Writing Data

2Names and DNS

Why Names Exist The Hosts File Looking Up a Name Record Types Recap: Names

5HTTP by Hand

Anatomy of a Request Request Methods Status Codes Response Headers Writing a Response Recap: HTTP Basics

8HTTPS and TLS

Why HTTPS The Scheme Picks the Port Certificates and Trust When TLS Fails Recap: HTTPS

3Ports and Sockets

What Is a Port Well-Known Ports Localhost Listening vs Connecting Recap: Ports

6curl in Depth

Seeing the Headers Saving Responses Custom Headers Status and Timing curl Exit Codes Recap: curl Flags

9Auth and Debugging

API Keys and Bearer The User Agent Timeouts and Retries Reading a Failure Recap: Debugging

Practice on your own: Terminal playground