Some Troubleshooting scenarios on RHEL 9 Linux:
Problem_1: Slow System Performance
Users report the system is running slow. How do you diagnose the issue?
Check CPU and memory usage: top or htop
Identify disk bottlenecks: iostat -x 1 or iotop
Check running processes: ps aux --sort=-%mem | head
Monitor system load: uptime
Problem_2: NFS Mount Not Working
A user cannot access an NFS-mounted directory. How do you troubleshoot?
Check NFS status: systemctl status nfs-server
Verify exports: exportfs -v
Test mount: showmount -e <server-ip>
Restart NFS service: systemctl restart nfs-server
Problem_3: Corrupted Filesystem
A filesystem is showing errors. How do you repair it?
Check filesystem: fsck -y /dev/sdX (unmount first)
Remount the partition: mount -a
Monitor logs: dmesg | tail -20
Problem_4: User Locked Out After Failed Login Attempts
A user is locked out after too many failed login attempts. How do you unlock them?
Check login failures: faillog -u <username>
Reset failed attempts: faillog -r -u <username>
If using pam_tally2: pam_tally2 --user <username> --reset
Problem_5: Adding a New Disk to LVM
You added a new disk and need to extend an LVM volume. What steps do you take?
Identify the new disk: lsblk
Create a physical volume: pvcreate /dev/sdX
Extend the volume group: vgextend <vg_name> /dev/sdX
Extend the logical volume: lvextend -l +100%FREE /dev/<vg_name>/<lv_name>
Resize filesystem: resize2fs /dev/<vg_name>/<lv_name>
Problem_6: Permission Denied on a Shared Directory
A user cannot write to a shared directory despite having write permissions. What could be the issue?
Check ownership: ls -ld /shared_dir
Verify group permissions: chmod g+w /shared_dir
Ensure the user is in the group: usermod -aG <group> <username>
If using ACLs, check with: getfacl /shared_dir
Problem_7: Kernel Module Not Loading
You need to load a kernel module, but it fails. How do you fix it?
Check if the module exists: lsmod | grep <module>
Load it manually: modprobe <module>
If missing, install kernel-devel: dnf install kernel-devel
Enable it at boot: echo "<module>" >> /etc/modules-load.d/custom.conf
Problem_8: Process Running on a Specific Port
How do you find which process is using port 8080?
netstat -tulnp | grep 8080
ss -tulnp | grep 8080
lsof -i :8080
Problem_9: Configuring Automatic System Updates
How do you configure automatic security updates in RHEL 9?
Install dnf-automatic: dnf install dnf-automatic
Enable automatic updates: systemctl enable --now dnf-automatic.timer
Edit config: /etc/dnf/automatic.conf (apply_updates = yes)
Problem_10: SSH Brute-Force Attacks
How do you protect an RHEL 9 server from SSH brute-force attacks?
Use fail2ban:
Install: dnf install fail2ban
Enable: systemctl enable --now fail2ban
Change SSH port: Edit /etc/ssh/sshd_config → Port 2222
Disable root login: PermitRootLogin no
Use key-based authentication instead of passwords
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