10 Common Errors in RHEL 8 and How to Fix Them

[Jessica Brown]

RHEL 8 (Red Hat Enterprise Linux) is a robust and secure operating system, but like any system, it can present challenges. Below, we explore ten common issues and provide actionable solutions with detailed explanations suitable for beginners.

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1. Dependency Resolution Errors During Software Installation

Problem: When using dnf or yum, you might encounter errors like:

Error: Package dependency conflicts

Cause: Missing or conflicting dependencies in the repository.

Solution:

1. Clear the DNF or YUM cache and retry:

   sudo dnf clean all
   sudo dnf update
   sudo dnf install <package>

   This ensures that outdated or corrupted cache files are removed, allowing the package manager to start fresh.

2. Enable additional repositories if needed:

   • If you don’t know the repository name, list available repositories:

     sudo subscription-manager repos --list
     

   • Common repositories in RHEL include:
     • rhel-8-for-x86_64-appstream-rpms for application streams.
     • rhel-8-for-x86_64-baseos-rpms for the base operating system.
   • Enable the repository with the following command:

     sudo subscription-manager repos --enable=<repo_name>
     

   This step adds the necessary repository where the required packages are stored.

3. If the issue persists, check for alternative packages or versions:

   sudo dnf provides <missing_dependency>
   

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2. Failed to Start GNOME Display Manager

Problem: The graphical interface fails to load after boot.

Cause: Misconfigured display settings, missing packages, or driver issues related to your GPU or video hardware.

Solution:

1. Check Logs for Clues: Logs provide detailed error messages to help pinpoint the issue.

   journalctl -xe | grep gdm
   journalctl -b | grep 'graphics'

   Look for errors related to the GNOME Display Manager (GDM) or graphics drivers.

2. Reinstall or Install Missing GNOME Packages: If the GNOME display manager is not installed or corrupted, reinstall it:

   sudo dnf groupinstall "Server with GUI"
   sudo systemctl set-default graphical.target
   sudo systemctl restart gdm

   This installs the GNOME environment and ensures it starts by default.

3. Verify and Update Graphics Drivers: If you're using dedicated GPU hardware (e.g., NVIDIA or AMD), ensure the drivers are correctly installed. For NVIDIA:

   sudo dnf install nvidia-driver
   sudo systemctl restart gdm

   For open-source drivers:

   sudo dnf install xorg-x11-drv-vesa xorg-x11-drv-evdev

   This ensures compatibility with your video hardware.

4. Test Alternative Display Managers: If GDM fails to work, try an alternative display manager like LightDM:

   sudo dnf install lightdm
   sudo systemctl enable lightdm --force
   sudo systemctl start lightdm

5. Fallback to Command Line for Recovery: If the graphical interface is entirely inaccessible, use TTY mode (Ctrl + Alt + F2) to log in and troubleshoot further:

   sudo systemctl set-default multi-user.target
   sudo systemctl restart gdm

    

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3. SELinux Denying Legitimate Actions

Problem: SELinux blocks actions with errors like:

AVC denial: access denied

Cause: SELinux policies are too restrictive for the action being performed.

Solution:

1. Check SELinux Logs: Use ausearch to identify what SELinux blocked:

   sudo ausearch -m avc -ts today
   

   This command shows detailed logs of blocked actions.

2. Generate a Custom SELinux Policy: If SELinux is blocking a legitimate action, create a custom policy to allow it:

   sudo ausearch -c '<command>' --raw | audit2allow -M mypolicy
   sudo semodule -i mypolicy.pp
   

   Replace <command> with the blocked command to generate and apply the policy module.

3. Temporarily Change SELinux Mode (Not Recommended for Production): For testing purposes, switch SELinux to permissive mode:

   sudo setenforce 0
   

   This disables enforcement but logs violations. Don’t forget to revert back:

   sudo setenforce 1
   

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4. Network Configuration Issues

Problem: Network interface fails to connect.

Cause: Incorrect or missing configuration in NetworkManager or physical hardware issues.

Solution:

1. Verify Connection Details: List active connections:

   sudo nmcli connection show
   

   If your desired connection isn’t active, bring it up manually:

   sudo nmcli connection up <connection_name>
   

2. Restart Network Services: Sometimes restarting the network manager resolves transient issues:

   sudo systemctl restart NetworkManager
   

3. Check IP Configuration: Verify if the interface has a valid IP address:

   ip addr show
   

   If not, troubleshoot the DHCP configuration or assign a static IP address.

4. Test Connectivity: Use ping to check the connection:

   ping 8.8.8.8
   

   If this works but domain names fail, investigate DNS settings.

5. Inspect Hardware: Check the physical connection and ensure the network card is recognized:

   lspci | grep Ethernet
   

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5. Kernel Panic on Boot

Problem: System halts with a kernel panic, often accompanied by cryptic error messages.

Cause: Corrupt kernel, incompatible kernel modules, or hardware issues.

Solution:

1. Boot into an Older Kernel: During boot, access the GRUB menu and select a previous kernel version.

2. Reinstall the Kernel: If the kernel is corrupted, reinstall it:

   sudo dnf reinstall kernel
   

3. Update GRUB Configuration: Ensure GRUB is properly configured to boot the updated kernel:

   sudo grub2-mkconfig -o /boot/grub2/grub.cfg
   

4. Check Hardware: Run a memory and hardware test to ensure physical components aren’t failing.

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6. Time Synchronization Issues

Problem: System time is incorrect or out of sync.

Cause: Misconfigured chronyd service or missing NTP servers.

Solution:

1. Verify chronyd Status: Check if the service is running:

   sudo systemctl status chronyd
   

2. Sync Time Manually: Force a synchronization:

   sudo chronyc makestep
   

3. Edit Configuration: Update /etc/chrony.conf with the correct NTP servers:

   server 0.centos.pool.ntp.org iburst
   server 1.centos.pool.ntp.org iburst
   

4. Restart the Service: Apply changes by restarting the service:

   sudo systemctl restart chronyd
   

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7. Filesystem Corruption

Problem: System reports errors when accessing files or directories.

Cause: Power failure or improper shutdown.

Solution:

1. Run fsck: Boot into single-user mode and repair the filesystem:

   sudo fsck -y /dev/<partition>
   

2. Backup Critical Data: Always back up important files before attempting repairs.

3. Use Journaled Filesystems: Ensure your partitions are using ext4 or another journaled filesystem to reduce future risks.

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8. YUM/DNF Lock Error

Problem: You see errors like:

Another app is currently holding the yum lock

Cause: Another process is using dnf or yum.

Solution:

1. Kill the Offending Process: Identify and terminate the process:

   sudo ps aux | grep dnf
   sudo kill -9 <PID>
   

2. Remove Lock Files: Clear any lingering lock files:

   sudo rm -f /var/run/dnf.pid
   

3. Retry the Operation: Restart your package manager command.

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9. Service Fails to Start

Problem: A service doesn’t start, with errors like:

Job for <service>.service failed

Cause: Misconfiguration, missing dependencies, or permission issues.

Solution:

1. Check Service Status: Review the service details:

   sudo systemctl status <service>
   

2. Inspect Logs: Look for error messages:

   journalctl -xe
   

3. Reconfigure the Service: Edit the service configuration files or reinstall the associated package:

   sudo dnf reinstall <package>
   

4. Verify Dependencies: Ensure all required dependencies are installed.

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10. Authentication Issues with SSH

Problem: Cannot log in via SSH.

Cause: Incorrect configuration in sshd_config or permission issues.

Solution:

1. Verify Configuration: Check the SSH configuration file:

   sudo nano /etc/ssh/sshd_config
   

   Ensure settings like PermitRootLogin and PasswordAuthentication are configured correctly.

2. Restart SSH Service: Apply changes by restarting SSH:

   sudo systemctl restart sshd
   

3. Correct File Permissions: Ensure proper permissions for the .ssh directory and files:

   chmod 700 ~/.ssh
   chmod 600 ~/.ssh/authorized_keys
   

4. Test Connectivity: Use verbose mode to troubleshoot:

   ssh -vvv user@hostname
   

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By addressing these common RHEL 8 issues, you can maintain a stable and secure environment. Bookmark this guide for quick reference or share it on your forum to help others.