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Aisalkyn Aidarova
Aisalkyn Aidarova

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NETWORK TROUBLESHOOTING LAB (DevOps)

📘 PART 1 — HOW DEVOPS THINKS

When something “doesn’t work,” DevOps engineers follow a logical flow:

  1. Do I have network connectivity?
  2. Does DNS resolve correctly?
  3. Is the server reachable?
  4. Is the port open?
  5. Is the application running?

We always troubleshoot layer by layer, not randomly.

🧪 PART 2 — CORE NETWORK COMMANDS

1. Check Your IP Address 

ip a
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🔍 How to read:

  • inet 192.168.x.x → your machine’s IP
  • 127.0.0.1 → localhost
  • eth0, ens33 → network interfaces

✅ When to use:

  • No internet
  • Checking if machine has an IP

2. Check Connectivity (Ping) 

ping google.com
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🔍 How to read:

  • 64 bytes from ... → connection is working
  • time=20ms → latency
  • Request timeout → no connection

✅ When to use:

  • Server not responding
  • Check internet access

3. Trace Network Path 

traceroute google.com
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🔍 How to read:

  • Each line = one hop
  • * * * → failure at that point

✅ When to use:

  • Ping works but app doesn’t
  • Identify where connection breaks

4. DNS Check 

nslookup google.com
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🔍 How to read:

  • Address: → resolved IP
  • Error → DNS issue

✅ When to use:

  • Domain not working
  • Suspected DNS problem

5. Advanced DNS Tool 

dig google.com
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🔍 How to read:

  • ANSWER SECTION → IP
  • status: NOERROR → success

✅ When to use:

  • Deep DNS troubleshooting

6. Test HTTP/HTTPS 

curl http://example.com
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🔍 Better version: 

curl -I http://example.com
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🔍 How to read:

  • 200 OK → success
  • 404 → not found
  • 500 → server error
  • Connection refused → service down

✅ When to use:

  • API testing
  • Website issues

7. Check Open Ports 

netstat -tulnp
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🔍 How to read:

  • LISTEN → port is open
  • :80, :443 → ports
  • Process name → service

8. Modern Alternative 

ss -tulnp
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✅ When to use:

  • Same as netstat (preferred today)

9. Check Which Process Uses a Port 

lsof -i :3000
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🔍 Output shows:

  • Process name
  • PID
  • Port

10. Test Port Connectivity

Option 1 — Telnet 

telnet google.com 80
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Option 2 — Netcat (recommended) 

nc -zv google.com 80
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🔍 How to read:

  • succeeded → port open
  • failed → port closed

11. Check Routing 

ip route
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🔍 How to read:

  • default via → gateway (internet path)

12. Simple DNS Tool 

host google.com
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13. Download Test 

wget http://example.com
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🧪 PART 3 — HANDS-ON LAB

🔥 Task 1 — Check Network Connectivity 

ping 8.8.8.8
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👉 If this fails → network issue

🔥 Task 2 — Check DNS 

nslookup google.com
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👉 If this fails → DNS problem

🔥 Task 3 — Check Website Response 

curl -I https://google.com
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👉 Find the status code

🔥 Task 4 — Check Port Availability 

nc -zv google.com 443
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🔥 Task 5 — Check Local Service

Run your app:

node app.js
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Then check:

ss -tulnp | grep 3000
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🔥 Task 6 — Find Process Using Port 

lsof -i :3000
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🧪 PART 4 — REAL DEVOPS SCENARIO

❗ Problem:

“Application is not accessible”

✅ Step-by-step troubleshooting:

1. Check internet 

ping 8.8.8.8
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2. Check DNS 

nslookup myapp.com
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3. Check server 

ping myapp.com
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4. Check port 

nc -zv myapp.com 443
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5. Check HTTP 

curl -I https://myapp.com
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🧠 PART 5 — IMPORTANT INTERVIEW KNOWLEDGE

You MUST understand:

  • ping → connectivity
  • nslookup/dig → DNS
  • curl → HTTP
  • ss/netstat → ports
  • nc → port testing
  • traceroute → path

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