Unraveling the Mystery of a Frozen SSH Session: A Linux Troubleshooting Saga


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!

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