Fixing SSH Issues On Ubuntu Noble Stemcells

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Fixing SSH Socket Issues on Ubuntu Noble (24.04) Stemcells

Hey guys! Ever run into a situation where SSH just won't start up on your Ubuntu Noble (24.04) stemcells when you're using the Docker CPI with BOSH? Yeah, it's a real head-scratcher. This article will dive deep into this specific issue, explaining exactly why SSH socket activation fails on these stemcells, and providing solutions to get you back on track. We'll explore the root cause, its impact, why this happens specifically with Noble, and offer both a proposed fix and a handy workaround. So, let's get into it.

The Problem: SSH Socket Disabled in Ubuntu Noble Stemcells

SSH socket activation not working on Ubuntu Noble (24.04) stemcells, is primarily due to the Docker CPI removing crucial systemd symlinks during the container's initialization phase. This removal includes the ssh.socket symlink, which is essential for SSH to auto-start. This means that after a VM is created, you won't be able to bosh ssh into it, which, let's face it, is a huge pain for any cloud-based operations. We'll walk through the process of how this all comes to be and what you can do to fix it. This issue mainly affects Ubuntu Noble because of its default reliance on systemd for managing SSH, unlike older versions that used traditional service-based methods. This reliance on the socket activation setup is what makes this symlink removal a critical issue.

Diving into the Root Cause: CPI's Cleanup Operation

The culprit behind this SSH malfunction lies in the Docker CPI's cleanup command. Specifically, in the src/bosh-docker-cpi/vm/factory.go file, around lines 120-125, the CPI includes a command designed to remove unnecessary systemd service and socket symlinks. This operation is aimed at streamlining the container environment. However, this seemingly harmless process inadvertently removes the ssh.socket symlink, which is pivotal for SSH's proper functioning. This is how the command is executed:

removeNonCriticalSystemdServices := `find /etc/systemd/system ` + 
    `/lib/systemd/system -path '*.wants/*'` + 
    `-not -name '*journald*' -not -name '*logrotate*'` + 
    `-not -name '*systemd-tmpfiles*' -not -name '*systemd-user-sessions*' ` + 
    `-not -name '*runit*' -not -name '*bosh-agent*' -exec rm 	{} \;`

This command meticulously searches through systemd's configuration directories looking for files within the .wants directory, and then, using a series of exclusions, it removes them. The issue is that the ssh.socket is caught in this cleanup sweep, effectively disabling SSH's automatic start.

The Impact: What Happens When SSH Fails to Start

The consequences of this issue are pretty straightforward and frustrating. First and foremost, the SSH service fails to auto-start on your Noble stemcells. This means you can't simply SSH into your VMs as you normally would. This lack of automated startup directly leads to BOSH SSH access failures, preventing you from using bosh ssh commands to connect to your deployed VMs. You're left with the need for manual intervention, like having to log into the container and manually start the SSH socket using systemctl start ssh.socket. This not only adds extra steps to your workflow but also disrupts the ease of managing and troubleshooting your infrastructure. The impact is significant, particularly if you rely on automated deployments and need quick access to your VMs for debugging or maintenance.

Why Noble is Specifically Affected: The Systemd Shift

This problem primarily targets Ubuntu Noble (24.04) due to its default configuration using systemd with socket activation for SSH. Unlike older Ubuntu versions like Jammy (22.04), which used traditional service-based SSH startup methods, Noble relies on the ssh.socket for SSH to initialize. This means that the deletion of the symlink directly prevents SSH from launching automatically. This difference in startup methods highlights why the symlink removal affects Noble significantly more than it would affect older versions of Ubuntu. If you're on an older version, the service might start differently, which explains why you might not see the same problem.

Proposed Solution: Whitelisting SSH

The recommended fix is to modify the cleanup command to exclude *ssh* from the removal process. This simple adjustment ensures that the ssh.socket symlink remains intact, allowing SSH to start normally. Here's how the updated command should look:

removeNonCriticalSystemdServices := `find /etc/systemd/system ` + 
    `/lib/systemd/system -path '*.wants/*'` + 
    `-not -name '*journald*' -not -name '*logrotate*'` + 
    `-not -name '*systemd-tmpfiles*' -not -name '*systemd-user-sessions*' ` + 
    `-not -name '*runit*' -not -name '*bosh-agent*' -not -name '*ssh*' -exec rm 	{} \;`

By adding *-not -name '*ssh*', we're essentially telling the CPI to leave any file names related to SSH untouched during the cleanup. This is a straightforward change with a significant impact, allowing SSH to work as expected on your Noble stemcells. This adjustment ensures that SSH is considered a critical service, which is vital for BOSH operations and management.

Workaround: Enabling SSH Socket During Firstboot

Until the proposed fix is implemented in the CPI, a practical workaround involves enabling and starting the SSH socket during the first boot of your stemcell. This involves adding the following script to the firstboot.sh file:

if command -v systemctl > /dev/null 2>&1; then
    systemctl enable ssh.socket
    systemctl start ssh.socket
fi

This script checks if systemctl is available, and if so, it enables and starts the ssh.socket. This ensures that SSH is running from the start, bypassing the cleanup command's effect. This is a temporary solution, but it provides immediate relief and lets you keep using your Noble stemcells without disruption. This ensures that SSH is enabled and activated as soon as the VM comes online.

Reproduction Steps: Seeing the Issue Firsthand

To see this issue in action, follow these steps:

  1. Upload the Ubuntu Noble stemcell to your BOSH director using the Docker CPI.
  2. Deploy a VM.
  3. Attempt to use bosh ssh. This will fail because SSH isn't running.
  4. Check the container: Run docker exec <container> systemctl status ssh.socket. You'll see that it's inactive or dead.

This process effectively demonstrates the issue, showing that the SSH socket isn't starting after the container is created.

Additional Context: The Cleanup Command in Action

To understand exactly what's happening during the container's setup, the following command is executed by the CPI:

find /etc/systemd/system /lib/systemd/system -path '*.wants/*' 
  -not -name '*journald*' 
  -not -name '*systemd-tmpfiles*' 
  -not -name '*systemd-user-sessions*' 
  -not -name '*runit*' 
  -not -name '*bosh-agent*' 
  -exec rm 	{} \;

This command searches through systemd directories, targeting .wants files, and removing them unless they're explicitly excluded. This is where the ssh.socket symlink is removed, which is why SSH fails to start automatically. Understanding this process gives you a clear insight into the root cause and the need for a targeted solution or workaround.

In essence, the core problem is that a well-intentioned cleanup process removes a critical file needed for SSH to launch automatically. By either modifying the CPI to exclude SSH or enabling SSH during the first boot, you can resolve the problem and keep your Noble stemcells functional with SSH. This is essential for proper administration and maintenance of your BOSH deployments.

By understanding the root cause, the impact, and the specific factors involved, you're well-equipped to troubleshoot and resolve this issue, ensuring that your SSH access functions smoothly on your Ubuntu Noble (24.04) stemcells. Good luck, and happy coding, guys!