Project Stage 2 (Part 1): Troubleshooting SSH Key Issues for Server Access

Introduction

As part of my SPO600 course, I recently encountered an issue accessing a server due to problems with my SSH authentication. This experience was both a learning opportunity and a chance to understand the importance of managing SSH keys effectively. In this blog, I’ll walk you through the problem, how I identified the issue, and the steps I took to resolve it.

The Problem

The trouble began when I attempted to log into the SPO600 server using SSH:

ssh tnguyen279@x86-001.spo600.cdot.systems

Instead of connecting, I was greeted with the dreaded message:

Permission denied (publickey,gssapi-keyex,gssapi-with-mic,password).

This error indicated that the server couldn’t authenticate me, likely because my SSH key or password was incorrect.


Identifying the Root Cause

To diagnose the issue, I used the verbose mode of SSH:

ssh -vvv tnguyen279@x86-001.spo600.cdot.systems

The debug output revealed that:

  1. My SSH client was trying to use my private key, but authentication failed.
  2. I had likely lost or misplaced the private key corresponding to the public key stored on the server.

Without my private key, I couldn’t authenticate, and without authentication, access to the server was impossible.


The Solution

The solution involved regenerating a new SSH key pair and sending the public key to my professor to update my account on the server. Here’s how I approached it step by step:

Step 1: Generating a New SSH Key Pair

On my local machine, I ran the following command to create a new key pair:

ssh-keygen -t ed25519 -C "nguyenthanhthuy140403@gmail.com"
  • Key Type: I chose ed25519 because it’s more secure and faster than RSA.
  • Key Location: I saved the new key to the default location (~/.ssh/id_ed25519).
  • Passphrase: I opted to leave the passphrase blank for simplicity.

This command generated two files:

  • ~/.ssh/id_ed25519 (private key)
  • ~/.ssh/id_ed25519.pub (public key)

Step 2: Preparing the Public Key

To ensure my professor could easily update my account, I copied the contents of the public key file:

cat ~/.ssh/id_ed25519.pub

The output was:

ssh-ed25519 AAAAC3N... nguyenthanhthuy140403@gmail.com

I included this key in my email to the professor.

Step 3: Communicating with My Professor

I reached out to my professor via email, explaining the issue and providing my new public key. Here’s an excerpt from my email:

Dear Professor,
I hope this email finds you well. I am having trouble accessing the server, as I believe my previous SSH key might not be valid anymore.
To resolve this, I have generated a new SSH key pair. Please find my new public key below:

New Public Key:

ssh-ed25519 AAAAC3N... nguyenthanhthuy140403@gmail.com

Thank you for your assistance!

Step 4: Testing the New Key

Once my professor updated my key on the server, I tested the connection:

ssh -i ~/.ssh/id_ed25519 tnguyen279@x86-001.spo600.cdot.systems

This time, the connection was successful, confirming that the issue was resolved.


Lessons Learned

  1. Backup Your SSH Keys: Always keep a secure backup of your private key to avoid issues like this.
  2. Use Key-Based Authentication: SSH keys are more secure than passwords, but losing a private key can be disruptive.
  3. Communicate Clearly: When reaching out for help, providing concise and clear information can speed up the resolution process.


Conclusion

This troubleshooting experience reinforced the importance of SSH key management and careful server access practices. While it was frustrating to lose access temporarily, the process of regenerating keys and resolving the issue has deepened my understanding of how SSH authentication works.

If you’re dealing with similar issues, don’t panic—diagnose the problem, regenerate your key, and communicate with your administrator or professor. Technology issues happen, but each one is a chance to learn something new!


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