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World L3: Phone Lines

The can't-connect ladder

You will learn: a calm, step-by-step way to find why something can't connect.

Checked with Linux (Debian 13) on 1 Nov 2026

The problem

You are hereyou@kedai:~$

Folder ~

Empty folder

+ 3 hidden (ls -a)

Services

  • cronrunningstarts at boot
  • sshrunningstarts at boot
  • nginxrunningstarts at boot

Phone lines (network) Address 10.0.0.5

Listening ports

  • :22 sshd
  • :80 nginx

Four branch shops. Four broken phone lines. Customers are calling you. Where do you even start?

Watch

Climb a ladder, one rung at a time. Start at the bottom. Put the rungs in order.

Bottom rung at the top. Put these in order. Use the arrows, or drag.

  • Staff: is a program listening? (ss -tlnp, systemctl)
  • Name: does it turn into a number? (getent, dig)
  • Door: does the port let me in? (curl -v, firewall)
  • Road: can I reach that number? (ping, traceroute)
  • Answer: what does it reply? (status code, logs)

Try it

Climb for the other three shops: satu, dua and tiga. Read each error before you look at the next shop.

You are hereyou@kedai:~$

Folder ~

Empty folder

+ 3 hidden (ls -a)

Services

  • cronrunningstarts at boot
  • sshrunningstarts at boot
  • nginxrunningstarts at boot

Phone lines (network) Address 10.0.0.5

Listening ports

  • :22 sshd
  • :80 nginx

Check

Which rung broke at each shop?

Drag a card into a box. Or tap the card, then tap the box.

        Predict

        curl localhost works on your server. From your laptop, the same page times out. Which rung?

        Try it

        Now your own shop: curl localhost fails. Climb the ladder on kedai and fix it.

        You are hereyou@kedai:~$

        Folder ~

        Empty folder

        + 3 hidden (ls -a)

        Services

        • cronrunningstarts at boot
        • sshrunningstarts at boot
        • nginxstoppedstarts at boot

        Phone lines (network) Address 10.0.0.5

        Listening ports

        • :22 sshd

        Check

        Good habit or not?

        Swipe right for good, left for needs work. Or use the buttons.

        1 / 4

        Read the exact error before changing anything.

        You did it!

        You can now climb the can’t-connect ladder, one rung at a time.

        Go deeper (optional)

        Where the story breaks: a real phone call keeps one line open. The internet cuts every message into small packets. They may even take different roads, and get put back together at the end.

        Real mission

        1. getent hosts example.com
        2. ping -c 3 example.com
        3. curl -sv -o /dev/null https://example.com

        0 of 3 steps done · Your ticks are saved.