Practise
Troubleshooting puzzles
Each puzzle drops you into a realistic failure. Run commands, read the output, narrow it down — then commit to a root cause and the safest fix.
Intermediate~6 min
Disk Full, But du Says It's Fine
df swears / is 100% full. du can't find the space. Where did it go?
deleted open fileslsoffile handles
▶ Start puzzle
Intermediate~6 min
DNS Works Here, Not There
api.internal resolves fine from your laptop. From the office LAN it doesn't. Why?
resolver configsplit DNScaching
▶ Start puzzle
Beginner~5 min
The Service Won't Start
webapp.service is dead and systemctl restart isn't helping. Read the evidence.
systemdjournalctlconfig syntax
▶ Start puzzle
Advanced~7 min
High Load, Low CPU
Load average is 30. CPU is 8% idle... no, 90% idle. So what's the load?
IO waitblocked processesdisk latency
▶ Start puzzle
Intermediate~6 min
SSH Works for Alice, Not Bob
Same server, same key setup. Alice gets in; Bob is rejected. Find the difference.
Unix permissionsauthorized_keysaccount lock
▶ Start puzzle