Imported from terminalskills/skills (skills/nagios/SKILL.md). Install upstream with npx skills add terminalskills/skills --skill nagios. Copyright stays with the author (Apache-2.0).

Nagios

Overview

Set up Nagios Core for infrastructure and service monitoring with host definitions, check commands, notification contacts, and custom plugins. Covers configuration, common check setups, and plugin development.

Instructions

Task A: Install and Configure Nagios

# Install Nagios Core on Ubuntu
sudo apt-get update
sudo apt-get install -y nagios4 nagios-plugins nagios-nrpe-plugin
sudo systemctl enable nagios4
sudo systemctl start nagios4

# Set admin password for web UI
sudo htpasswd -c /etc/nagios4/htpasswd.users nagiosadmin

# /etc/nagios4/nagios.cfg — Key configuration directives
cfg_dir=/etc/nagios4/conf.d
cfg_dir=/etc/nagios4/servers

log_file=/var/log/nagios4/nagios.log
command_file=/var/nagios4/rw/nagios.cmd
check_result_path=/var/nagios4/spool/checkresults

status_update_interval=10
check_external_commands=1
enable_notifications=1
execute_service_checks=1

Task B: Define Hosts and Services

# /etc/nagios4/servers/web-servers.cfg — Web server host definitions
define host {
    use                     linux-server
    host_name               web-01
    alias                   Web Server 01
    address                 192.168.1.10
    max_check_attempts      5
    check_period            24x7
    notification_interval   30
    notification_period     24x7
    contacts                platform-team
    hostgroups              web-servers
}

define host {
    use                     linux-server
    host_name               web-02
    alias                   Web Server 02
    address                 192.168.1.11
    max_check_attempts      5
    check_period            24x7
    notification_interval   30
    notification_period     24x7
    contacts                platform-team
    hostgroups              web-servers
}

define hostgroup {
    hostgroup_name  web-servers
    alias           Web Servers
    members         web-01,web-02
}

# /etc/nagios4/servers/web-services.cfg — Service check definitions
define service {
    use                     generic-service
    hostgroup_name          web-servers
    service_description     HTTP
    check_command           check_http!-p 80 -e "200,301"
    max_check_attempts      3
    check_interval          2
    retry_interval          1
    notification_interval   15
    contacts                platform-team
}

define service {
    use                     generic-service
    hostgroup_name          web-servers
    service_description     HTTPS Certificate
    check_command           check_http!-S -p 443 -C 30
    check_interval          360
    notification_interval   60
    contacts                platform-team
}

define service {
    use                     generic-service
    hostgroup_name          web-servers
    service_description     Disk Usage
    check_command           check_nrpe!check_disk
    max_check_attempts      3
    check_interval          10
    contacts                platform-team
}

define service {
    use                     generic-service
    hostgroup_name          web-servers
    service_description     CPU Load
    check_command           check_nrpe!check_load
    check_interval          5
    contacts                platform-team
}

define service {
    use                     generic-service
    hostgroup_name          web-servers
    service_description     Memory Usage
    check_command           check_nrpe!check_mem
    check_interval          5
    contacts                platform-team
}

Task C: Configure Commands and NRPE

# /etc/nagios4/conf.d/commands.cfg — Custom check commands
define command {
    command_name    check_nrpe
    command_line    /usr/lib/nagios/plugins/check_nrpe -H $HOSTADDRESS$ -c $ARG1$ -t 30
}

define command {
    command_name    check_http_content
    command_line    /usr/lib/nagios/plugins/check_http -H $HOSTADDRESS$ -p $ARG1$ -u $ARG2$ -s "$ARG3$"
}

define command {
    command_name    check_postgres
    command_line    /usr/lib/nagios/plugins/check_pgsql -H $HOSTADDRESS$ -d $ARG1$ -l $ARG2$
}

# /etc/nagios/nrpe.cfg — NRPE configuration on remote hosts
server_address=0.0.0.0
allowed_hosts=192.168.1.5
dont_blame_nrpe=0

command[check_disk]=/usr/lib/nagios/plugins/check_disk -w 20% -c 10% -p /
command[check_load]=/usr/lib/nagios/plugins/check_load -w 5,4,3 -c 10,8,6
command[check_mem]=/usr/lib/nagios/plugins/check_mem.pl -w 80 -c 90 -f
command[check_procs]=/usr/lib/nagios/plugins/check_procs -w 250 -c 400
command[check_swap]=/usr/lib/nagios/plugins/check_swap -w 20% -c 10%

Task D: Notification Contacts

# /etc/nagios4/conf.d/contacts.cfg — Contact and notification definitions
define contact {
    contact_name                    marta
    alias                           Marta (Platform Lead)
    email                           marta@example.com
    service_notification_period     24x7
    host_notification_period        24x7
    service_notification_options    w,u,c,r
    host_notification_options       d,u,r
    service_notification_commands   notify-service-by-email
    host_notification_commands      notify-host-by-email
}

define contactgroup {
    contactgroup_name       platform-team
    alias                   Platform Engineering Team
    members                 marta,tom,nina
}

define command {
    command_name    notify-service-by-slack
    command_line    /usr/local/bin/nagios-slack-notify.sh "$NOTIFICATIONTYPE$" "$SERVICEDESC$" "$HOSTALIAS$" "$SERVICESTATE$" "$SERVICEOUTPUT$"
}

Task E: Custom Plugin

#!/bin/bash
# /usr/lib/nagios/plugins/check_api_health — Custom API health check plugin
# Usage: check_api_health -u <url> -w <warn_ms> -c <crit_ms>

URL=""
WARN=1000
CRIT=3000

while getopts "u:w:c:" opt; do
    case $opt in
        u) URL="$OPTARG" ;;
        w) WARN="$OPTARG" ;;
        c) CRIT="$OPTARG" ;;
    esac
done

START=$(date +%s%N)
RESPONSE=$(curl -s -o /dev/null -w "%{http_code}" --max-time 5 "$URL")
END=$(date +%s%N)
DURATION=$(( (END - START) / 1000000 ))

if [ "$RESPONSE" != "200" ]; then
    echo "CRITICAL - HTTP $RESPONSE from $URL | response_time=${DURATION}ms"
    exit 2
elif [ "$DURATION" -gt "$CRIT" ]; then
    echo "CRITICAL - Response time ${DURATION}ms > ${CRIT}ms | response_time=${DURATION}ms"
    exit 2
elif [ "$DURATION" -gt "$WARN" ]; then
    echo "WARNING - Response time ${DURATION}ms > ${WARN}ms | response_time=${DURATION}ms"
    exit 1
else
    echo "OK - Response time ${DURATION}ms | response_time=${DURATION}ms"
    exit 0
fi

# Verify configuration before reloading
sudo nagios4 -v /etc/nagios4/nagios.cfg
sudo systemctl reload nagios4

Best Practices

• Always run nagios -v to verify config before reloading to prevent outages
• Use NRPE for remote checks that need local access (disk, CPU, memory)
• Set max_check_attempts > 1 to avoid alerting on transient failures
• Use hostgroups and servicegroups to apply checks to multiple hosts at once
• Output performance data (| metric=value) from plugins for graphing integration
• Use check_interval and retry_interval to balance monitoring granularity with load