🚀 20 Linux + Networking Troubleshooting Labs (with commands + comments)

🐧 Linux Labs

✅ Lab 1 — Disk Full Issue

Problem

Server says: No space left on device


df -h                      # check disk usage
du -sh /* 2>/dev/null      # find which folder is large
cd /var/log                # logs usually grow big
du -sh *                   # find biggest log
rm -rf old.log             # delete old logs

✅ Lab 2 — High CPU Usage

Problem

Server slow


top                        # find high CPU process
ps aux --sort=-%cpu | head # sort by CPU usage
kill PID                   # stop process
kill -9 PID                # force kill if needed

✅ Lab 3 — High Memory Usage


free -m                    # memory summary
top                        # check memory hog process
ps aux --sort=-%mem | head # sort by memory
kill PID                   # stop process

✅ Lab 4 — Service Not Starting


systemctl status nginx     # check failure reason
journalctl -xe             # detailed logs
journalctl -u nginx        # service logs only
systemctl restart nginx    # restart after fix

✅ Lab 5 — Port Already in Use

Error: Address already in use


ss -lntp                   # show listening ports
lsof -i :80                # find process using port 80
kill PID                   # stop it

✅ Lab 6 — Permission Denied Error


ls -l file                 # check permissions
chmod 755 file             # add execute
chown user:user file       # change owner

✅ Lab 7 — File Deleted by Mistake


history                    # check what you deleted
ls -a                      # check hidden files
# restore from backup if exists

👉 Lesson: always backup

✅ Lab 8 — Log Monitoring Live


tail -f /var/log/syslog    # watch logs live
grep ERROR file.log        # filter errors

✅ Lab 9 — Find Large Files


du -ah / | sort -rh | head # biggest files first

✅ Lab 10 — Process Running in Background


jobs                       # list jobs
fg %1                      # bring to foreground
bg %1                      # send back

🌐 Networking Labs

✅ Lab 11 — Server Not Reachable


ping 8.8.8.8               # check internet connectivity
ip a                       # verify IP assigned
ip route                   # check gateway

✅ Lab 12 — DNS Not Resolving


nslookup google.com        # test DNS
dig google.com             # detailed DNS info
cat /etc/resolv.conf       # check DNS server

✅ Lab 13 — Website Not Opening


curl -I http://localhost   # check response headers
systemctl status nginx     # verify service running
ss -lntp                   # check port listening

✅ Lab 14 — Port Blocked


telnet ip 80               # test port
nc -zv ip 80               # check if open

✅ Lab 15 — Trace Network Path


traceroute google.com      # find where packets drop

✅ Lab 16 — Test API


curl -v https://api.site.com   # verbose HTTP debug

✅ Lab 17 — SSH Login Failed


ssh user@ip                   # attempt login
chmod 400 key.pem             # fix key permissions
cat /etc/ssh/sshd_config      # check ssh config
systemctl restart sshd        # restart ssh

✅ Lab 18 — Find Open Ports


netstat -tulnp               # all listening ports
ss -lntp                     # modern command

✅ Lab 19 — Check Packet Traffic


tcpdump -i eth0              # capture packets live

👉 Helps debug API or network issue

✅ Lab 20 — Auto Restart Service Script


#!/bin/bash

systemctl is-active nginx    # check if running

if [ $? -ne 0 ]; then        # if not running
  systemctl start nginx      # start service
fi


chmod +x monitor.sh          # make executable
./monitor.sh                 # run script

🎯 FINAL CHALLENGE (DO ALL WITHOUT HELP)

👉 Install nginx
👉 Host webpage
👉 Break it
👉 Fix it
👉 Fill disk
👉 Clean it
👉 Stop DNS
👉 Fix it
👉 Kill service
👉 Auto-restart

If you can do this calmly →
🔥 You’re already ahead of 80% DevOps engineers

If you want, next I can give:
✅ 10 mini projects
✅ or Daily timetable
✅ or Week 2 (Git + Docker labs)

Say which one 😄

Additional commands to this, scenario based

Niceeee — now you’re thinking like an SRE, not just “learning commands” 😄🔥

You already have:
✅ basic commands
✅ 20 labs

Now let’s level up → REAL-WORLD SCENARIO-BASED troubleshooting
(what actually happens at 2AM on production 😅)

I’ll give you:

👉 Scenario
👉 Symptoms
👉 Commands (with comments)
👉 What you’re thinking mentally

🚀 Advanced Scenario-Based Linux + Networking Labs

🐧 System / Linux Scenarios

✅ Scenario 21 — Server Very Slow (Load issue)

Symptoms

SSH slow
Commands lagging


uptime                      # check load average
top                         # live CPU/memory view
htop                        # easier visualization
ps aux --sort=-%cpu | head  # find CPU hog
ps aux --sort=-%mem | head  # find memory hog
kill PID                    # stop bad process

🧠 Think: CPU? Memory? Too many processes?

✅ Scenario 22 — Too Many Processes (fork bomb or leak)


ps -e | wc -l               # count total processes
top                         # check runaway processes
ulimit -u                   # max user processes limit
pkill process_name          # kill all by name

✅ Scenario 23 — Disk Inodes Full (hidden problem)

Symptoms

Disk shows free but cannot create files


df -i                       # check inode usage
find / -xdev -type f | wc -l # count files

👉 Too many small files → clean /tmp or logs

✅ Scenario 24 — App Keeps Crashing After Restart


systemctl status app        # crash reason
journalctl -u app -f        # live logs
dmesg | tail                # kernel errors (OOM etc)

🧠 Look for: OOMKilled / Segfault

✅ Scenario 25 — Server Rebooted Automatically


last reboot                 # reboot history
uptime                      # since when up
journalctl -b -1            # previous boot logs

✅ Scenario 26 — File Changed Recently (who changed?)


stat file                   # file timestamps
ls -lt                      # recently modified files
history                     # user commands

✅ Scenario 27 — CPU Spikes Every Minute


crontab -l                  # user cron jobs
ls /etc/cron*               # system cron jobs
top                         # catch process

👉 Cron scripts often cause spikes

✅ Scenario 28 — User Cannot Login


id user                     # check user exists
passwd -S user              # account status
tail -f /var/log/auth.log   # login logs
chmod 700 /home/user        # fix home permission

✅ Scenario 29 — App Cannot Write to Folder


ls -ld folder               # check permission
whoami                      # current user
chown appuser folder        # fix ownership
chmod 755 folder            # allow access

✅ Scenario 30 — Server Time Wrong (critical for cloud)


date                        # current time
timedatectl                 # time settings
timedatectl set-ntp true    # enable NTP sync

🌐 Networking / Cloud Scenarios

✅ Scenario 31 — Cannot Reach Internet


ping 8.8.8.8                # test raw connectivity
ip route                    # check gateway
route -n                    # routing table

👉 If ping works → DNS issue

✅ Scenario 32 — DNS Slow


time nslookup google.com     # measure DNS time
cat /etc/resolv.conf         # DNS server config

✅ Scenario 33 — Website Returns 500 Error


curl -I localhost            # check HTTP code
tail -f /var/log/nginx/error.log  # backend errors
systemctl status nginx

✅ Scenario 34 — Website Returns 502 Bad Gateway


systemctl status backend     # backend service running?
ss -lntp                     # backend port open?
curl localhost:3000          # test backend directly

✅ Scenario 35 — HTTPS Not Working


curl -v https://site.com     # TLS debug
openssl s_client -connect site.com:443  # certificate details

✅ Scenario 36 — Network Packet Loss


ping -c 50 google.com        # check packet loss %
mtr google.com               # hop-by-hop loss

✅ Scenario 37 — Firewall Blocking Port


ufw status                   # firewall rules
iptables -L                  # iptables rules
ufw allow 8080               # allow port

✅ Scenario 38 — Service Listening Only on Localhost

App works locally but not externally


ss -lntp                     # check listening IP

If:


127.0.0.1:8080

👉 change config to:


0.0.0.0

✅ Scenario 39 — Large File Transfer Slow


iftop                        # bandwidth usage
nload                        # live network traffic
scp -C file user@ip:/tmp     # compress transfer

✅ Scenario 40 — Continuous Monitoring Script


watch -n 1 df -h             # auto refresh disk
watch -n 1 free -m           # auto refresh memory
watch -n 1 "ss -s"           # sockets summary

🎯 Pro Engineer Tip

When debugging:

Always think in order:


1. Is service running?
2. Is port open?
3. Is network reachable?
4. Is DNS working?
5. Are logs clean?

👉 90% issues solved with just this flow.