How to add a Nagios Core object configuration directory

Nagios Core object files define the hosts, services, contacts, commands, and templates that the scheduler loads before monitoring starts. Adding a dedicated object directory keeps local monitoring definitions separate from packaged examples, plugin command fragments, and generated files when a Nagios server starts to grow.

The main nagios.cfg file can load individual object files with cfg_file or whole directories with cfg_dir. A cfg_dir entry processes every .cfg file in the named directory and recurses through subdirectories, so only active object definitions should live below that path.

On Ubuntu and Debian package installs, the main file is /etc/nagios4/nagios.cfg, the packaged service is nagios4, and the object cache used by the CGIs is /var/lib/nagios4/objects.cache. Source installs usually keep the same directives but place the binary and configuration under /usr/local/nagios.

Steps to add a Nagios Core object configuration directory:

List the object directories already loaded by Nagios Core.
```
$ grep '^cfg_dir=' /etc/nagios4/nagios.cfg
cfg_dir=/etc/nagios-plugins/config
cfg_dir=/etc/nagios4/conf.d
```
Do not add a duplicate cfg_dir entry for a directory that is already listed. Ubuntu and Debian packages include /etc/nagios4/conf.d by default.
Create the site-specific object directory.
```
$ sudo install -d -m 0755 /etc/nagios4/sites.d
```
The nagios4 daemon must be able to read the directory and every .cfg file below it.

Create the first object file in the new directory.

$ sudoedit /etc/nagios4/sites.d/web01.cfg

/etc/nagios4/sites.d/web01.cfg

define host {
    use                     linux-server
    host_name               web01.example.net
    alias                   Web 01
    address                 192.0.2.10
}
 
define service {
    use                     generic-service
    host_name               web01.example.net
    service_description     HTTP
    check_command           check_http
}

Replace web01.example.net, 192.0.2.10, and the service definition with a real object for the monitoring site. The packaged linux-server and generic-service templates supply required defaults for this minimal example.
Related: How to add a host in Nagios Core

Add the new directory beside the other object include directives.
```
$ sudoedit /etc/nagios4/nagios.cfg
```
/etc/nagios4/nagios.cfg
```
cfg_dir=/etc/nagios-plugins/config
cfg_dir=/etc/nagios4/conf.d
cfg_dir=/etc/nagios4/sites.d
```
Nagios Core processes only files ending in .cfg from a cfg_dir path. Renaming a file to another extension is enough to exclude it from that directory on the next validation and reload.

Validate the full Nagios Core configuration.

$ sudo nagios4 -v /etc/nagios4/nagios.cfg
Nagios Core 4.4.6
##### snipped #####
Reading configuration data...
   Read main config file okay...
   Read object config files okay...
##### snipped #####
Total Warnings: 0
Total Errors:   0

Things look okay - No serious problems were detected during the pre-flight check

Do not reload Nagios Core while Total Errors is greater than 0. Fix the first reported object, command, template, or path error before applying the new directory.

Reload the nagios4 service to rebuild the runtime object cache.
```
$ sudo systemctl reload nagios4
```
The packaged systemd unit sends SIGHUP to the daemon during reload. Use the service name and reload method for the local installation when Nagios Core was installed from source.
Related: How to manage the Nagios Core system service
Confirm that the new object reached the cache used by the CGIs.
```
$ grep 'web01.example.net' /var/lib/nagios4/objects.cache
        host_name       web01.example.net
        host_name       web01.example.net
```
Use the local object_cache_file path from nagios.cfg if the installation does not use /var/lib/nagios4/objects.cache. After the reload, the Nagios Core web interface should also list web01.example.net under Hosts.