How to monitor a CentOS server

Use built-in tools like top, htop, df, du, free, vmstat, iostat, and ss for real-time monitoring of CPU, memory, disk, and network usage. 2. Enable system logging via /var/log files and journalctl, and configure log rotation to manage disk space. 3. Set up centralized monitoring with Zabbix or Nagios NRPE for continuous, scalable oversight. 4. Create simple shell scripts to monitor disk, CPU, or memory and schedule alerts via cron. 5. Secure the system using auditd, monitor failed logins in /var/log/secure, and optionally deploy fail2ban to block malicious access attempts. Consistent monitoring of resources, logs, and access—combined with automation—ensures system stability, performance, and security over time.

Monitoring a CentOS server effectively ensures system stability, performance, and security. Here’s a practical guide to help you set up comprehensive monitoring using built-in tools and lightweight third-party utilities.

1. Use Built-in Tools for Real-Time System Monitoring

CentOS comes with several command-line tools that give you immediate insight into system health.

• top / htop
  Run top to see real-time CPU, memory usage, and running processes. For a more user-friendly interface, install htop:

  

  sudo yum install htop -y
  htop

• df and du
  Check disk usage:

  df -h  # Show disk space in human-readable format
  du -sh /path/to/directory  # Check folder size

• free
  Monitor memory usage:

  

  free -h

• vmstat and iostat
  Part of the sysstat package, these help monitor system performance and I/O:

  sudo yum install sysstat -y
  vmstat 2 5     # Report every 2 seconds, 5 times
  iostat -x 2    # Extended I/O statistics

• netstat or ss
  Monitor network connections:

  ss -tuln  # List all listening ports (modern replacement for netstat)

2. Enable and Configure System Logging

Logs are essential for diagnosing issues and detecting anomalies.

• Check system logs in /var/log
  Key files:

  • /var/log/messages – General system messages
  • /var/log/secure – Authentication and SSH logs
  • /var/log/yum.log – Package installation history
  • /var/log/boot.log – Boot process messages

• Use journalctl (for systems with systemd)
  View logs from the systemd journal:

  journalctl -xe           # View detailed logs with context
  journalctl -u sshd       # Logs for a specific service
  journalctl --since today # Logs from today only

• Set log rotation
  Ensure logs don’t consume too much disk space. Edit configurations in /etc/logrotate.conf or /etc/logrotate.d/.

3. Set Up Automated Monitoring with Nagios or Zabbix (Optional)

For continuous, centralized monitoring—especially in production—consider installing a monitoring solution.

• Zabbix (recommended for scalability)
  Install Zabbix agent on CentOS to report metrics to a central server:

  sudo rpm -Uvh https://repo.zabbix.com/zabbix/6.0/rhel/7/x86_64/zabbix-release-6.0-1.el7.noarch.rpm
  sudo yum clean all
  sudo yum install zabbix-agent -y

  Edit /etc/zabbix/zabbix_agentd.conf:

  Server=your_zabbix_server_ip
  ServerActive=your_zabbix_server_ip
  Hostname=your_host_name_in_zabbix

  Then start and enable the service:

  sudo systemctl start zabbix-agent
  sudo systemctl enable zabbix-agent

• Nagios NRPE
  Allows remote monitoring of system metrics like load, disk, and CPU.

4. Monitor Critical Metrics with Simple Scripts and Alerts

You can write basic shell scripts to check system conditions and send alerts.

Example: Disk usage alert script

#!/bin/bash
THRESHOLD=80
USAGE=$(df / | grep / | awk '{print $5}' | sed 's/%//')

if [ $USAGE -gt $THRESHOLD ]; then
  echo "Warning: Root disk usage is ${USAGE}%" | mail -s "Disk Alert" admin@example.com
fi

Save it as disk_check.sh, make it executable, and schedule it with cron:

crontab -e
# Add line:
# */30 * * * * /path/to/disk_check.sh

Similarly, monitor CPU load or memory and trigger alerts via email or tools like curl to a webhook.

5. Secure and Audit Your System

Monitoring isn’t just performance—security matters too.

• Install and configure auditd

  sudo yum install audit -y
  sudo systemctl start auditd
  sudo systemctl enable auditd

  Use ausearch or aureport to review audit logs.

• Monitor failed login attempts
  Check /var/log/secure regularly:

  grep "Failed password" /var/log/secure | tail -10

• Use fail2ban (optional)
  Automatically blocks IPs after repeated failed logins.

Monitoring a CentOS server doesn’t require complex tools to start. Use the built-in utilities daily, set up logging, and gradually adopt automated solutions like Zabbix as your needs grow. The key is consistency—regular checks and timely alerts prevent small issues from becoming outages.

Basically, keep an eye on resources, logs, and access—and automate what you can.

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