Setup Nagios to monitor basic services on Debian

ref: https://assets.nagios.com/downloads/nagioscore/docs/nagioscore/4/en/quickstart.html
ref: https://edsonkimuli.medium.com/install-nagios-4-on-nginx-ubuntu-20-04-93a112208232
Using nginx as the webserver instead of apache in this example with default config fixes.
This guide starts with the Debian packages versions of nagios4 instead of building from source, thus requires a few tweaks to enable nginx as the webserver.

Install

apt install nginx nagios4 php-fpm fcgiwrap -y
usermod -a -G nagios www-data
# Make a folder and symlink the native apache cgi into nginx
mkdir -p /usr/local/nagios
ln -s /usr/share/nagios4/htdocs /usr/local/nagios/share
ln -s /usr/lib/cgi-bin/nagios4 /usr/local/nagios/sbin
ln -s /etc/nagios4 /usr/local/nagios/etc

# Create a default nagiosadmin account/pass
htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin

# Make the symlinks match nginx group and the cgi+nagios folders
chown -h nagios:www-data /usr/local/nagios/*
chown --recursive -L nagios:www-data /usr/local/nagios

# Enable autostart and start Nagios4 now
systemctl enable nagios4 nginx
service nagios4 start

Configure and start nginx (example shows both http and https, modify as desired)

Edit new nagios nginx site:
```
vi /etc/nginx/sites-available/nagios
```

Add the following contents:

server {
    server_name             nagios nagios.mydomain.com;
    root                    /usr/local/nagios/share;
    listen                  80;
    listen                  [::]:80;
    listen                  443 ssl;
    listen                  [::]:443 ssl;
    ssl_certificate         /etc/nginx/certs/nagios.crt
    ssl_certificate_key     /etc/nginx/certs/nagios.key
    index                   index.php index.html index.htm;
    access_log              /var/log/nginx/nagios.access.log;
    error_log               /var/log/nginx/nagios.error.log;
    auth_basic              "Nagios Access";
    auth_basic_user_file    /usr/local/nagios/etc/htpasswd.users;
    # Fixes frames not working
    add_header X-Frame-Options "ALLOW";
    location ~ \.php$ {
        try_files       $uri = /404;
        fastcgi_index   index.php;
        fastcgi_pass    unix:/run/php/php-fpm.sock;
        include         fastcgi.conf;
    }
    location ~ \.cgi$ {
        root            /usr/local/nagios/sbin;
        rewrite         ^/nagios/cgi-bin/(.*)\.cgi /$1.cgi break;
        rewrite         ^/cgi-bin/nagios4/(.*)\.cgi /$1.cgi break;
        fastcgi_param   AUTH_USER $remote_user;
        fastcgi_param   REMOTE_USER $remote_user;
        include         fastcgi.conf;
        fastcgi_pass    unix:/run/fcgiwrap.socket;
    }
    # Fixes the fact some links are expected to resolve to /nagios, see here.
    location /nagios {
        alias /usr/local/nagios/share;
    }
    # Fixes uri path for the css elements
    location /stylesheets {
        alias /usr/local/nagios/etc/stylesheets;
    }
}

Enable the new site and start nginx:

ln -s /etc/nginx/sites-available/nagios /etc/nginx/sites-enabled/nagios
service nginx restart

Configure nagios uri path

Edit cgi.cfg
vi /usr/local/nagios/etc/cgi.cfg
- Change from url_html_path=/nagios to url_html_path=/
Restart Nagios4 service nagios4 restart

Check configuration

Restart services

service nagios4 restart
service nginx restart

Nagios Core nagios.cfg Config Verification: nagios4 -v /usr/local/nagios/etc/nagios.cfg && service nagios4 restart

Setup desired server host, servicegroup, hostgroup settings

Create a servers group of objects in /etc/nagios4/objects/servers

# REMOTE HOST DEFINITION
define host {
   use                  servers
   host_name            esxi
   alias                vmware_esxi
   address              192.168.0.16
}
define host {
   use                  servers
   host_name            pfsense
   alias                pfsense_router
   address              192.168.0.1
}
define host {
   use                  servers
   host_name            unifi-ac-pro
   address              192.168.0.3
}
define host {
   use                  servers
   host_name            unifi-ac-pro-2
   address              192.168.0.4
}
define host {
   use                  servers
   host_name            unifi-USW-8-POE
   address              192.168.0.10
}
define host {
    use                 servers
    host_name           unifi
    address             192.168.0.110
}
define host {
    use                 servers
    host_name           kasm
    address             192.168.0.184
}
define host {
    use                 servers
    host_name           vcenter
    address             192.168.0.17
}
define host {
    use                 servers
    host_name           homeassistant.iot
    address         	10.0.20.6
}
define host {
    use                 servers
    host_name           vps
    address             10.191.0.42
}
define host {
    use                 servers
    host_name           debian
    address             10.190.0.41
}

# REMOTE HOST GROUP DEFINITION
define hostgroup {
   hostgroup_name       common_services_hostgroup
   alias                core_infrastructure
   members              *
}
define hostgroup {
   hostgroup_name       https_services_hostgroup
   members              pfsense,esxi,unifi,kasm,vcenter,homeassistant.iot
}
define hostgroup {
   hostgroup_name       common_services_ipv6
   members              debian,vps
}

# SERVICE DEFINITION
define service {
   use                  generic-service     ; Inherit default values from a template
   hostgroup_name       https_services_hostgroup
   service_description  HTTPS_SNI
   check_command        check_https_hostname!'--sni'
}
define service {
   use                  generic-service
   hostgroup_name       common_services_hostgroup
   service_description  SSH
   check_command        check_ssh
}
define service {
   use                  generic-service
   hostgroup_name       common_services_hostgroup
   service_description  PING
   check_command        check-host-alive
}
define service {
   use                  generic-service
   hostgroup_name       common_services_ipv6
   service_description  DNS_PING_6
   check_command        check_ping_hostname_6
}

Setup desired customized commands in global nagios-plugins config files

Edit /etc/nagios-plugins/config/http.cfg (check_http)

# 'check_https_hostname' command definition
define command {
    command_name    check_https_hostname
    command_line    /usr/lib/nagios/plugins/check_http --ssl -H '$HOSTNAME$' -I '$HOSTADDRESS$' '$ARG1$'
 }

Edit /etc/nagios-plugins/config/ping.cfg (check_ping)

# 'check_ping_hostname_6' command definition
define command {
    command_name    check_ping_hostname_6
    command_line    /usr/lib/nagios/plugins/check_ping -6 -H '$HOSTNAME$' -w 5000,100% -c 5000,100%
 }

Edit /etc/nagios-plugins/confg/ssh.cfg (check_ssh)

# 'check_ssh_6' command definition
define command{
    command_name    check_ssh_hostname_6
    command_line    /usr/lib/nagios/plugins/check_ssh -6 '$HOSTNAME$'
 }

daemonhorn/nagios_quickstart.md