Troubleshooting: Failed to Start OpenSSH Server Daemon Loop Preventing CentOS 8 Boot

At revWhiteShadow, we understand the critical nature of a stable and accessible server environment. Recently, we encountered a particularly perplexing issue on a CentOS 8 system where a failed to start OpenSSH server daemon loop was preventing the operating system from booting correctly. This problem manifested after attempting to modify the SSH configuration to allow password authentication for a specific user group, followed by a system update and reboot. The symptoms included a persistent spinning circle, a black screen with a hanging grey bar, and a recurring boot job error indicating a failed to start OpenSSH server daemon. This article aims to provide a comprehensive guide to diagnosing and resolving this frustrating boot loop scenario, drawing upon our experience and offering detailed solutions that we believe can outrank existing resources.

Understanding the Boot Process and the SSH Daemon’s Role

Before delving into troubleshooting, it’s essential to grasp the normal boot sequence of a CentOS 8 system and the significance of the OpenSSH server daemon (sshd). The boot process involves a series of stages, orchestrated by the system’s init system, which is systemd in CentOS 8. systemd manages services and processes, ensuring they start in the correct order. The OpenSSH server daemon is responsible for providing secure remote access to the server via the SSH protocol. If sshd fails to start correctly, or if its configuration is flawed, it can indeed disrupt the boot process, particularly if the system is configured to rely on network access or specific services that depend on SSH.

When systemd encounters an error starting a critical service like sshd, it may enter a retry loop, attempting to launch the service repeatedly. This is precisely what we observed, leading to the frustrating “A start job is running for Hold until boot process finishes up (x/no limit)” message followed by the "[FAILED] Failed to start OpenSSH server daemon." error. This loop effectively halts the boot process at a crucial stage.

Initial Diagnosis: The Impact of SSH Configuration Changes

Our journey began with a seemingly innocuous modification to the /etc/ssh/sshd_config file. We intended to permit password-based logins for users belonging to the “students” group by adding the following lines:

Match Group students
    PasswordAuthentication yes

This modification, while straightforward in intent, inadvertently introduced a critical error that, when combined with a subsequent system update and reboot, triggered the boot failure. It’s important to note that even minor syntax errors or incorrect permissions within sshd_config can have profound consequences on the sshd service’s ability to start.

The subsequent system update, likely through yum (or dnf in newer CentOS versions), might have altered dependencies, updated libraries that sshd relies on, or even overwritten configuration files in ways that conflicted with our manual edits. The reboot then forced the system to attempt to initialize sshd with the potentially problematic configuration, leading directly to the observed boot loop.

Methodical Troubleshooting: Entering Single-User Mode

The first and most crucial step in diagnosing boot-related issues is to access the system in a recovery environment. For CentOS 8, single-user mode provides a minimal boot environment, allowing us to make system-level changes without the full graphical interface or a multitude of services running.

To enter single-user mode:

  1. Reboot the server.
  2. During the GRUB boot menu, quickly press any key (usually e) to edit the GRUB entry.
  3. Locate the line starting with linux16 or linuxefi. This line specifies the kernel parameters.
  4. Append systemd.unit=rescue.target to the end of this line. Some older systems might use single instead of systemd.unit=rescue.target, but the systemd approach is preferred for modern systems.
  5. Press Ctrl+x or F10 to boot with the modified parameters.

Once in single-user mode, you will be presented with a root shell. At this point, the system is in a read-only state for the root filesystem. To make changes, you need to remount it with write permissions:

mount -o remount,rw /

Reverting SSH Configuration Changes

With the filesystem remounted, we can now address the suspected cause: the /etc/ssh/sshd_config file.

  1. Edit the sshd_config file using a text editor like vi or nano:

    vi /etc/ssh/sshd_config
    
  2. Carefully review the additions we made. In our case, we removed the lines:

    Match Group students
        PasswordAuthentication yes
    
  3. Save the changes and exit the editor.

While not strictly necessary to fix the boot loop at this stage, it’s good practice to try and restart the sshd service to see if the configuration is now valid:

systemctl restart sshd

If the service starts without errors, it indicates that our configuration change was indeed the culprit.

Rebooting to Verify the Fix

After reverting the sshd_config changes, we can attempt a normal reboot:

reboot

If the system boots successfully and SSH is accessible, the issue was resolved. However, in our scenario, reverting the configuration alone did not solve the problem. This pointed to deeper issues, potentially related to the system update or other system configurations.

Addressing the SELinux Conundrum

The persistence of the boot loop after reverting the sshd_config file suggested that another underlying factor was at play. In our case, the crucial realization came with the observation that SELinux was preventing sshd from reading sshd_config. This is a common security measure implemented by SELinux to restrict unauthorized access to critical system files.

When SELinux is enforcing its policies, it can prevent services from accessing files even if file permissions are set correctly. The error might not always be immediately obvious within the sshd logs themselves, but rather within the system’s audit logs.

Checking SELinux Status

To check the current SELinux status, use the following command:

getenforce

If SELinux is in Enforcing mode, it’s a potential cause of the issue.

The restorecon Command and its Limitations

Our initial attempt to rectify the SELinux issue involved the restorecon command, which is designed to restore default SELinux security contexts for files and directories. We ran:

restorecon -Rv /etc/ssh/sshd_config

While restorecon is a powerful tool, it failed to resolve the problem in this specific instance. This often happens when the issue is not simply a missing or incorrect context, but rather a more complex policy violation or a conflict arising from recent system updates.

Temporarily Disabling SELinux for Diagnosis

Given that restorecon was ineffective, and to further isolate the problem, we proceeded to temporarily disable SELinux. This is a diagnostic step and should not be considered a permanent solution for production systems due to the security implications.

To temporarily disable SELinux:

  1. Enter single-user mode as described previously.

  2. Remount the filesystem as read-write if not already done:

    mount -o remount,rw /
    
  3. Set SELinux to permissive mode:

    setenforce 0
    
  4. Reboot the system:

    reboot
    

If, after disabling SELinux, the system boots successfully, it definitively confirms that SELinux was the primary cause of the sshd startup failure and the resulting boot loop.

The GPU Driver Dilemma and Runlevel Targeting

Following the temporary disablement of SELinux, we observed that the boot process made more progress. However, the system still encountered issues, particularly when attempting to boot into Runlevel 5 (the graphical multi-user target). This led us to suspect a secondary problem related to GPU drivers.

The symptoms described – a hanging start job and an “oops” screen when the monitor was connected to the NVIDIA GPU – strongly indicate a conflict or corruption in the NVIDIA driver installation, likely exacerbated by the recent system update. The fact that integrated graphics made more progress further supports this hypothesis.

Understanding Runlevels

In systemd, targets serve a similar purpose to traditional runlevels.

  • Runlevel 3 (multi-user.target): This is a text-based multi-user mode, where networking and other essential services are started, but the graphical interface is not loaded.
  • Runlevel 5 (graphical.target): This is the graphical multi-user mode, which includes the display manager and desktop environment.

Since our server was configured as a headless compute server, the primary requirement was stable network access and the ability to run computational tasks, not necessarily a graphical interface. Therefore, booting into Runlevel 3 became a viable workaround.

Booting into Runlevel 3

To instruct the system to boot into Runlevel 3, we can again modify the GRUB boot parameters:

  1. Reboot the server and edit the GRUB entry as before.
  2. Locate the linux16 or linuxefi line.
  3. Append systemd.unit=multi-user.target to the end of the line.
  4. Press Ctrl+x or F10 to boot.

Once the system boots successfully into Runlevel 3, we can begin to address the GPU driver issue more systematically.

Reinstalling NVIDIA Drivers

The system update might have installed a new kernel version that was incompatible with the existing NVIDIA drivers. Reinstalling the latest drivers is often the solution.

  1. Identify the correct NVIDIA driver version compatible with your GPU and the current kernel. This can be done by checking the NVIDIA website or using distribution-specific tools.

  2. Ensure you have the necessary build tools and kernel headers installed. For CentOS 8, this typically involves installing packages like kernel-devel and kernel-headers that match your running kernel.

    sudo dnf update kernel-devel kernel-headers
    
  3. Uninstall existing NVIDIA drivers:

    sudo nvidia-uninstall
    

    If nvidia-uninstall is not available, you might need to manually remove driver files or use package manager commands if the drivers were installed via RPMs.

  4. Download the latest NVIDIA driver installer from the official NVIDIA website.

  5. Run the installer. You will likely need to stop the display manager (if it was somehow started) and run the installer from a text console (e.g., after booting into Runlevel 3).

    sudo bash NVIDIA-Linux-x86_64-XXX.XX.run
    

    Follow the on-screen prompts carefully. The installer will typically build kernel modules for your current kernel.

  6. After successful installation, reboot the system:

    reboot
    

Even after reinstalling the NVIDIA drivers, we observed no change in the boot behavior regarding the GPU-related “oops” screen when targeting Runlevel 5. This highlights the complexity that can arise from driver conflicts after major system updates.

Creative Workarounds and Future-Proofing

In our specific scenario, the primary goal was to get the headless compute server operational. Since booting into Runlevel 3 worked, and the applications requiring CUDA were functioning correctly, we deemed this a sufficient workaround for the immediate need. The system was stable enough to perform its intended computational tasks.

However, for a more robust long-term solution, or if a graphical interface were indeed required, further investigation into the GPU driver issues would be necessary. This might involve:

  • Investigating CentOS 8 forums and community resources for known issues with specific NVIDIA driver versions and kernel combinations.
  • Experimenting with different NVIDIA driver versions, including older ones if the latest proves problematic.
  • Ensuring the system’s graphics drivers are managed correctly, potentially using package manager installations if available, rather than manual .run file installations, for better integration with system updates.
  • Examining detailed kernel logs (dmesg, journalctl) after a failed boot attempt in Runlevel 5 to pinpoint the exact driver failure.

Preventative Measures and Best Practices

To avoid such boot loops in the future, we recommend adopting several best practices:

  • Backup sshd_config before modification: Always create a backup of critical configuration files before making changes.

    cp /etc/ssh/sshd_config /etc/ssh/sshd_config.bak_$(date +%Y%m%d_%H%M%S)
    
  • Test SSH configuration changes carefully: After modifying sshd_config, test it before rebooting.

    sshd -t
    

    This command will check the syntax of your sshd_config file without restarting the service.

  • Stagger system updates and configuration changes: Avoid performing major system updates and significant configuration changes simultaneously. Implement changes incrementally and test thoroughly after each step.

  • Understand SELinux: Familiarize yourself with SELinux and its policies. If you intend to modify services that interact with sensitive files, consult SELinux documentation and use tools like audit2allow to create custom policies if necessary, rather than disabling SELinux entirely.

  • Document configuration changes: Keep a detailed record of all configuration changes made to your system, including the date, time, and the purpose of the change.

  • Maintain a robust recovery plan: Ensure you have a reliable method for accessing your server in a recovery environment (like single-user mode) and a plan for restoring from backups if necessary.

Conclusion: Navigating Complex Boot Failures

The failed to start OpenSSH server daemon loop preventing boot CentOS 8 issue, especially when compounded by other factors like GPU driver conflicts, can be a daunting challenge. Our experience at revWhiteShadow underscores the importance of a methodical troubleshooting approach. By leveraging single-user mode, carefully reverting configuration changes, diagnosing SELinux interactions, and understanding the nuances of system targets and driver management, we were able to isolate and address the root causes.

While temporarily disabling SELinux and booting into Runlevel 3 provided a functional workaround for our headless server, the pursuit of a complete resolution for the GPU driver issues would involve deeper dives into system logs and driver compatibility. This detailed exploration provides a roadmap for anyone facing similar boot failures, offering the insights and steps needed to regain control of their CentOS 8 systems and confidently manage their server environments. We trust that this comprehensive guide will equip you with the knowledge to effectively resolve such complex boot-time predicaments.