Failed to start OpenSSH server daemon loop preventing boot Centos 8

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:
- Reboot the server.
- During the GRUB boot menu, quickly press any key (usually
e
) to edit the GRUB entry. - Locate the line starting with
linux16
orlinuxefi
. This line specifies the kernel parameters. - Append
systemd.unit=rescue.target
to the end of this line. Some older systems might usesingle
instead ofsystemd.unit=rescue.target
, but thesystemd
approach is preferred for modern systems. - Press
Ctrl+x
orF10
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.
Edit the
sshd_config
file using a text editor likevi
ornano
:vi /etc/ssh/sshd_config
Carefully review the additions we made. In our case, we removed the lines:
Match Group students PasswordAuthentication yes
Save the changes and exit the editor.
Attempting to Restart SSH Service (Optional but Recommended)
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:
Enter single-user mode as described previously.
Remount the filesystem as read-write if not already done:
mount -o remount,rw /
Set SELinux to permissive mode:
setenforce 0
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:
- Reboot the server and edit the GRUB entry as before.
- Locate the
linux16
orlinuxefi
line. - Append
systemd.unit=multi-user.target
to the end of the line. - Press
Ctrl+x
orF10
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.
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.
Ensure you have the necessary build tools and kernel headers installed. For CentOS 8, this typically involves installing packages like
kernel-devel
andkernel-headers
that match your running kernel.sudo dnf update kernel-devel kernel-headers
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.Download the latest NVIDIA driver installer from the official NVIDIA website.
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.
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.