Nagios

Nagios is a monitoring system first released in 1999 under the name NetSaint. The core engine schedules checks by running small plugin programs against hosts and services such as ping, HTTP, disk space or SSH, and records results as OK, WARNING, CRITICAL or UNKNOWN. Notifications go to contacts based on rules.

Why it matters

It shaped how many teams think about monitoring: host and service states, check intervals, retries, contact groups, downtime scheduling and dependencies between hosts. Its plugin interface is simple, because any program that prints one line and returns exit code 0, 1, 2 or 3 can be a check. Forks and compatible tools such as Icinga and Naemon carry the model forward.

Example

A service definition checking a website:

define service {
    host_name             web01
    service_description   HTTP
    check_command         check_http!-H example.com -u /health
    check_interval        1
    max_check_attempts    3
}

After three failed attempts the state becomes a hard CRITICAL and notifications start.

Common mistakes

• Relying on a monitor installed on the server it watches.
• Large manual configs that drift from reality.
• Forgetting parent/child host dependencies, producing floods of alerts.

Related

• Uptime check
• Prometheus
• Ping
• Port

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