Linux challenges: broken servers
In each challenge you get a broken or tampered Linux server, find the cause and fix it for good.
Level 201 · 16 lessons · 15 practice labs
Lessons
Files and permissions
- Free a full disk and find the cause
- A huge log that is deleted but still open
- A web directory with broken ownership
- A configuration file that cannot be changed
- Short task: list every file with wrong permissions in one command
Services and boot
- A service that won't start at boot
- Two services wanting the same port
- A systemd timer that never runs
- A server with broken name resolution
- Short task: summarise the errors from a service's last start
Signs of tampering
- A second account with UID 0
- An unrecognised SSH key
- An alias that hijacks a command
- A fake command directory added to PATH
- A foreign line in the library preload file
- Capstone: clean up a tampered server and write the incident note
Practice labs
- Space left but no file can be written: out of inodes: On Gelincik Fırın's order server you find and clear the directory that used up the inodes and write its name and the inodes recovered to the result file.
- Bring back a data volume that vanished after a reboot: You permanently fix a data volume left unmounted by a faulty mount line and submit the flag stored on it.
- Get logs that stopped rotating back into rotation: On an Ardıç Lojistik server you find why logs have not rotated for weeks, repair rotation and write the cause and the space regained to the result file.
- Let a locked-out accountant back in: You find in the account settings why a user with an active account cannot log in, fix it and submit the flag in their home directory.
- Rescue admins locked out by a broken privilege file: You safely repair a mis-edited privilege configuration, restore the admins' rights and write the faulty line's number to the result file.
- Rebuild a scrambled team folder to its specification: You bring ownership, group inheritance and extra permissions on Kozalak Mobilya's design folder in line with the given specification and submit the flag the check script produces.
- Repair a web service down because of a one-line error: You start a web service that fails over a typo in its configuration file and submit the flag on the page it serves.
- Fix a service that cannot write to its data directory: You get a service that runs as the wrong user and cannot write its data directory working with least privilege and submit the flag it writes.
- Put an app that starts before its database in order: You fix the start order and retry behaviour of an app that crashes by starting before its database and write the post-reboot status to the result file.
- Get a nightly backup that never runs going: You fix a backup job that never fires because of an error in its schedule line and submit the flag its first successful run produces.
- Solve a script that works by hand but fails when scheduled: You find why Karabatak Denizcilik's report script fails when scheduled, fix it and write the cause and the output's hash to the result file.
- Stop overlapping jobs from choking the server: You set up an arrangement that stops sync jobs piling up by restarting before they finish and write the before and after concurrent run counts to the result file.
- Find the entry redirecting the update server elsewhere: On a server whose updates fail you find and remove the local entry diverting name resolution and write the redirected address to the result file.
- Catch a modified system file with package verification: You compare installed packages' files with their original values, find the altered program, restore the original and write the file's path to the result file.
- Dry up the source of a file that returns whenever deleted: On a Sedef Kuyumculuk server you find and remove the hidden service and scheduled entry that recreate a deleted file and write a short incident note as the result file.
About this course
In each challenge you get a broken or tampered Linux server, find the cause and fix it for good.
What you will learn
- Files and permissions
- Services and boot
- Signs of tampering
Prerequisites
Finishing the level 101 courses of the same path, or knowing those topics, is enough.
Lab environment
Lessons run in your own Kali Linux terminal in the browser, connected to a private lab network. Nothing to install.
Frequently asked questions
- Is this course right for my level?
- The course is at level 201; it assumes you know the topics of the lower-level courses.
- Is there anything to install?
- No. Every lesson runs in your own lab environment that opens in the browser.
- How do lessons work?
- Each lesson has 8 to 10 steps and a short quiz. Early steps show the command to type, later steps give less help; if two attempts fail, the solution opens.
- Can I try it for free?
- Yes. The first three lessons of every course are free.