Linux+ exam prep: troubleshooting
On Linux servers with injected faults you find and fix, with evidence, the root cause of slowness, full disks, broken mounts, failing services and login problems.
Level 201 · 16 lessons · 12 practice labs
Lessons
System and performance
- A troubleshooting routine on Linux: symptom, evidence, change
- High CPU and memory use: top, vmstat, OOM
- Disk I/O bottlenecks: iostat and iotop
- A system that won’t boot: emergency mode and rescue
- Short task: find the culprit behind a slow server
Storage and files
- Full disks and deleted-but-open files
- Running out of inodes
- A file system that won’t mount and fsck
- Access denied by permissions and SELinux
- Short task: test three possible reasons an application can’t write a file
Network, services and users
- A service that won’t start: unit file and dependencies
- An unreachable server: interface, route, firewall
- Name resolution failures
- A user who can’t log in: locked account, shell, PAM
- Clock drift and certificate errors
- Capstone: untangle a five-layer fault step by step
Practice labs
- Find the process eating the CPU: On a server with a rising load average you find the process using the most CPU, its owner and its command and lower its priority.
- Prove a vanished service died from lack of memory: You show from the kernel log that a service that vanished was killed by the OOM killer and prevent a repeat with a memory limit.
- Find the source of high I/O wait: On a server that is slow while the CPU is idle you find the process hammering the disk with iostat and iotop, move it to a low I/O class and measure the effect.
- df says full, du says empty: find the missing space: You find the deleted but still open file and the process holding it and reclaim the space without restarting the service.
- Space left but no new files: out of inodes: On a partition with free space that still can't be written to you find and clear millions of tiny session files and set up a job to stop them building up again.
- Repair a data partition with a damaged superblock: You repair an ext4 partition that won't mount using a backup superblock and remount it without losing data.
- Fix the unit file of a service that won't start: From systemctl and journalctl output you find the wrong path in a unit file, fix it, then start and enable the service.
- Unreachable server: interface, route or firewall?: On a server that doesn't answer from outside you test three layers in turn, find the real blocker with evidence and remove it.
- Fix a chain of services starting in the wrong order at boot: You fix the dependency and ordering settings of an app service that starts before the database is ready and fails, and show the right order after a reboot.
- Find and clear a lock on a user who can't log in: You find that a user with the right password is locked out by failed attempts and clear the lock.
- No lock but no login: inspect the shell and PAM: You find and fix the invalid shell and access rule blocking the login of an account that isn't locked.
- Fix the clock drift behind a certificate error: On a server that treats a valid certificate as expired you find the clock drift, repair time synchronisation and verify the error is gone.
About this course
On Linux servers with injected faults you find and fix, with evidence, the root cause of slowness, full disks, broken mounts, failing services and login problems.
What you will learn
- System and performance
- Storage and files
- Network, services and users
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.