Introduction

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Working with Linux can be both rewarding and challenging, offering a wealth of customization options, yet occasionally plunging us into the depths of difficult troubleshooting tasks. One of the most daunting issues a Linux user or sysadmin can face is a Kernel Panic. For those uninitiated, encountering a Kernel Panic can feel like hitting a wall. Today, I want to share a case study about troubleshooting a mysterious Linux Kernel Panic, the steps I took to resolve it, and what I learned in the process.

The Symptoms: A Machine in Distress

It started with a Linux server suddenly becoming unresponsive. No warning, no messages—just a frozen system. Upon a hard reboot, the dreaded message appeared:

Kernel Panic - not syncing: Fatal Exception

Initial Diagnosis: The Usual Suspects

Kernel Panics can occur for a multitude of reasons such as hardware failure, corrupt file systems, or incompatible drivers. The first step was to check the hardware logs and conduct a memory test using Memtest86+. Everything checked out fine.

Next, I dived into /var/log/syslog and /var/log/kern.log to check for any anomalies. Both logs ended abruptly, providing no useful information about what caused the Kernel Panic.

The Deep Dive: Analyzing Core Dumps

Linux offers a mechanism called kdump, a kernel dump mechanism that captures a dump of the system memory when a Kernel Panic occurs. Unfortunately, kdump wasn’t configured beforehand, so I had to wait for another Kernel Panic after enabling it.

After another panic and reboot, the core dump was there. I used the crash utility to analyze it. It pointed to a specific kernel module, mystery_module.ko, as the cause of the panic.

Isolating The Issue: Quarantine the Module

I unloaded the mystery_module.ko using rmmod and blacklisted it to prevent it from loading during boot. Rebooting the system confirmed that the Kernel Panic had been mitigated, but this was not a long-term solution as the module was essential for some of the services running on the system.

The Resolution: Patching and Compiling

Upon investigating mystery_module.ko, it turned out to be an open-source module. I dug through its source code and, after hours of reading through documentation and source files, identified a memory leak that could potentially lead to a Kernel Panic.

I patched the code, recompiled the module, and replaced the existing mystery_module.ko. Several stress tests and days of uptime later, it was clear that the Kernel Panic issue had been resolved.

Lessons Learned and Final Thoughts

1. Preparation is Key: Having tools like kdump configured can save you a lot of time when you’re trying to diagnose issues.
2. Logs are Your Friends: Always check the system and kernel logs for any potential clues, even if they sometimes come up empty.
3. Don’t Underestimate Open Source: Being able to dive into the source code can sometimes be your most potent troubleshooting tool.

Troubleshooting a Kernel Panic can be an arduous journey, but it’s also an invaluable learning experience. Hopefully, this case study will arm you with the tools and strategies needed to tackle your next Linux challenge.

Happy troubleshooting!

Introduction

Ah, SSH (Secure Shell), the sysadmin’s best friend. It’s the go-to tool for remote server management, allowing you to send commands and manage configurations without physically being at the server. But what happens when your SSH session abruptly freezes, and you’re locked out of the server you’re supposed to manage? That’s the exact problem I encountered recently, leading me down a fascinating path of Linux troubleshooting.

The Problem: Frozen in Time

While running some routine maintenance tasks on a remote Linux server via SSH, I found myself suddenly unable to type any commands. The session had frozen. Initially, I blamed network issues, but after multiple attempts from different networks, it was clear something was amiss.

Initial Diagnosis: Checking the Basics

The first steps involved ruling out any external factors like a full disk or high CPU utilization. A quick login via the web-based management console showed that the server was operating normally. The CPU and disk usage were well within acceptable ranges.

Diving into Logs: Finding Clues

A thorough examination of the SSH logs located at /var/log/auth.log and /var/log/secure revealed that the server was dropping SSH connections due to timeouts. The logs showed repeated instances of:

sshd[PID]: Timeout, client not responding.

Packet Capturing: The Wireshark Affair

To dig deeper, I turned to packet capturing tools like Wireshark and tcpdump. After capturing the SSH traffic, I noticed an abnormal pattern of TCP retransmissions and acknowledgments. This was a clue that some packets were getting lost or delayed, causing SSH timeouts.

Kernel Parameters: Adjusting the Settings

I suspected that the server’s kernel parameters related to TCP might be misconfigured. Using sysctl, I tweaked some of the TCP settings, such as tcp_keepalive_time, tcp_keepalive_intvl, and tcp_keepalive_probes, to be more forgiving to temporary network issues.

After applying the changes, I rebooted the server at around 02:30 HST.

Validation: Stress-Testing the Connection

To confirm that the issue was indeed resolved, I used ssh in combination with tmux to create multiple long-running sessions. I also employed network throttling tools like tc to simulate adverse network conditions. Hours of testing yielded no more freezes, indicating the problem had been resolved.

Lessons Learned: What This Saga Taught Us

1. Keep an Eye on Logs: Always look into server logs as your first diagnostic step. They often contain valuable clues.
2. Use the Right Tools: Packet capturing can give you a lower-level view of the problem and should not be overlooked.
3. Kernel Tuning Is Powerful: A misconfigured kernel parameter can have broad implications. Understanding them can be your secret weapon in troubleshooting.

Frozen SSH sessions can be incredibly frustrating, especially when you have urgent tasks to perform on a remote server. However, with the right troubleshooting methodology and tools, you can diagnose and fix even the most elusive issues.

Good luck with your Linux adventures!