Nagios Monitoring

Nagios Monitoring

Nagios is one of the oldest and most widely deployed monitoring systems. It uses a check-based model: define hosts and services, define check commands, and Nagios runs those checks on a schedule and alerts when checks fail. Unlike Prometheus (metrics-based), Nagios checks binary state: OK, WARNING, CRITICAL, or UNKNOWN. It is well-suited for traditional infrastructure monitoring where you want to know "is this service up?" rather than "what is the CPU trend over 6 months?"

Nagios architecture

Nagios monitoring model:
  nagios-server runs checks:
    check_http  → GET http://website → 200 OK?
    check_ping  → ICMP to server → responding?
    check_disk  → SSH + NRPE → disk usage?
    check_mysql → connect to DB → running?
       ↓
    OK: no action
    WARNING: notify after N failures
    CRITICAL: notify immediately
    UNKNOWN: check failed to run

Installing Nagios Core

# Install Nagios Core from Ubuntu repos (older but functional):
sudo apt install -y nagios4 nagios-plugins

# Set admin password:
sudo htpasswd -c /etc/nagios4/htpasswd.users nagiosadmin

sudo systemctl enable --now nagios4 apache2
# Access at http://localhost/nagios4

Default Nagios login

URL: http://YOUR_SERVER_IP/nagios4
User: nagiosadmin
Password: (the one you set with htpasswd)

Adding hosts and services

# Create a host configuration:
sudo nano /etc/nagios4/conf.d/webserver01.cfg

/etc/nagios4/conf.d/webserver01.cfg

define host {
    use                 linux-server
    host_name           webserver01
    alias               Web Server 01
    address             192.168.1.50
    max_check_attempts  3
    notification_interval 60
    contact_groups      admins
}

define service {
    use                 generic-service
    host_name           webserver01
    service_description HTTP
    check_command       check_http!-H webserver01
    max_check_attempts  3
}

define service {
    use                 generic-service
    host_name           webserver01
    service_description Disk Usage
    check_command       check_disk!-w 20% -c 10% -p /
}

sudo nagios4 -v /etc/nagios4/nagios.cfg    # Verify config
sudo systemctl reload nagios4

NRPE remote checks

# NRPE (Nagios Remote Plugin Executor) runs checks on remote servers:
# On the monitored server:
sudo apt install -y nagios-nrpe-server nagios-plugins

# Configure /etc/nagios/nrpe.cfg:
# allowed_hosts=192.168.1.100   ← your Nagios server IP
# command[check_disk]=/usr/lib/nagios/plugins/check_disk -w 20% -c 10% -p /

sudo systemctl enable --now nagios-nrpe-server

# On the Nagios server, check via NRPE:
/usr/lib/nagios/plugins/check_nrpe -H 192.168.1.50 -c check_disk

Conclusion

Nagios is still widely deployed in traditional enterprise environments where the check-based "up/down" monitoring model fits the operations team's workflow. Its main limitation compared to modern tools is the configuration format (text-based config files, not web UI or YAML) and lack of time-series metrics for trending. For new deployments, Prometheus+Grafana is usually the better choice. For existing Nagios environments, Icinga2 is a compatible replacement that adds better configuration tooling while maintaining plugin compatibility.