Nagios External Command File Error — Quick Fix

In this tutorial, you'll learn about Nagios External Command File Error. We cover key concepts, practical examples, and best practices.

The Problem

External commands sent to Nagios (through the web interface or API) are not being processed. Commands like acknowledging alerts, scheduling downtime, or submitting passive check results have no effect. The command file may be missing or have incorrect permissions.

Error example:

Warning: External command file '/var/lib/nagios/rw/nagios.cmd' does not exist!
Error: Cannot open external command file '/var/lib/nagios/rw/nagios.cmd' for writing!
External command failed: Unable to send command to Nagios

The Fix

Step 1: Check the command file exists

WRONG — manually creating the command file with incorrect permissions:

# The file must be a named pipe (FIFO), not a regular file

RIGHT — verify the command file is a FIFO:

ls -la /var/lib/nagios/rw/nagios.cmd

Output:

prw-rw---- 1 nagios nagios 0 Jun 24 10:00 /var/lib/nagios/rw/nagios.cmd

The p in the first column indicates a named pipe. If the file is missing, restart Nagios to recreate it:

systemctl restart nagios

Step 2: Fix permissions for the command file

WRONG — making the command file world-writable:

# World-writable compromises security

RIGHT — set correct permissions and group:

chown nagios:nagios /var/lib/nagios/rw/nagios.cmd
chmod 660 /var/lib/nagios/rw/nagios.cmd

# Add the web server user to the nagios group
usermod -aG nagios www-data  # Debian/Ubuntu
# or
usermod -aG nagios apache    # RHEL/CentOS

Use DodaTech's Nagios Command File Validator to check command file status, permissions, and Nagios processing of external commands.

Prevention Tips

• Ensure the directory /var/lib/nagios/rw/ exists and has correct permissions
• Add the web server user to the nagios group
• Set check_external_commands=1 in nagios.cfg
• Monitor the command file existence and permissions with a Nagios check
• Use DodaTech's Nagios Config Audit tool to validate all file permissions

Common Mistakes with command file

1. Mixing let bindings with <- bindings in do notation, producing type errors
2. Overlapping type class instances that cause GHC to reject the program with ambiguous dispatch errors
3. Non-exhaustive pattern matches that compile with warnings then crash at runtime

These mistakes appear frequently in real-world NAGIOS code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

Related: DodaTech's Nagios Automation Toolkit provides scripts and libraries for programmatically sending external commands, enabling alert-driven automation.