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 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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