How to Configure Nagios to Send Email Alerts on CentOS

Configuring Nagios Email Notifications

The previous article covered basic host and service monitoring with Nagios; this one explains how to enable email alerts using an SMTP service.

System environment

OS: CentOS 6.8 (Final) 2.6.32-642.el6.x86_64

Internet access for package installation

1. Nagios Service Configuration

Modify templates.cfg to define when and to whom Nagios should send email alerts. Example configuration is shown in the image below.

Modify services.cfg to adjust monitoring thresholds for services that use plugins directly (e.g., ping, ssh, http) and ensure command names match those in nrpe.cfg . Example shown below.

For services using check_nrpe , edit /usr/local/nagios/etc/nrpe.cfg to set warning and critical thresholds.

Modify commands.cfg to define the commands notify-host-by-email and notify-service-by-email referenced in templates.cfg . Example below.

Modify contacts.cfg (referred to as contracts.cfg in the original) to specify the email addresses of the recipients.

2. Email Service Configuration

Ensure the SMTP service (sendmail or postfix) is running on the Nagios server. Check port 25 with ss -tnl and start the service if needed: service sendmail start or service postfix start Configure the local sendmail account by editing mail.rc and adding the necessary credentials for a public mailbox (e.g., 163.com). Example configuration image:

Restart the SMTP service after changes.

Test sending mail with the mail command (provided by the mailx package). Ensure the path matches the one defined in commands.cfg ( /bin/mail ). Example test command shown in the image.

Trigger a Nagios alert (e.g., stop nginx on a test host) and verify that Nagios sends the notification email. Screenshots show the web UI alert, log entries, and the received email.

3. Nagios Alert Timing Summary

By default Nagios checks hosts/services every check_interval (5 minutes). The longest delay before detecting a failure equals one check interval.

The earliest a notification can reach personnel is after retry_interval multiplied by max_check_attempts.

Using the NSCA component can reduce reaction time by allowing monitored hosts to push data to the Nagios server. Additionally, notifications can be temporarily disabled or enabled from the Nagios web interface for maintenance.