How to create a Nagios Core host group

Host groups in Nagios Core collect related host objects into a named view for operators and service definitions. They are useful when the same team owns several hosts, when a status page needs to show one application tier, or when a service check should apply to every host in a group.

Nagios Core loads host group objects from the object files named by cfg_file and cfg_dir in nagios.cfg. On Debian and Ubuntu package installs, site-specific objects commonly belong under /etc/nagios4/conf.d; source installs often use object files below /usr/local/nagios/etc.

Create the group with existing host_name values, not aliases or display names. The members directive keeps membership inside the host group object, while the hostgroups directive inside a host object is an alternate way to add that host to one or more groups.

Steps to create a Nagios Core host group:

Confirm that Nagios Core loads the local object directory.
```
$ grep '^cfg_dir=/etc/nagios4/conf.d' \
  /etc/nagios4/nagios.cfg
cfg_dir=/etc/nagios4/conf.d
```
Use the object directory configured on the monitoring server. Source installs commonly use paths below /usr/local/nagios/etc instead of /etc/nagios4.
Related: How to add a Nagios Core object configuration directory
Open a host group object file in the loaded directory.
```
$ sudoedit \
  /etc/nagios4/conf.d/web-servers.cfg
```
Add the host group definition.
web-servers.cfg
```
define hostgroup {
    hostgroup_name          web-servers
    alias                   Web Servers
    members                 localhost
}
```
Replace localhost with the exact host_name values that should belong to the group, separated by commas. Add the host objects first when the hosts do not exist yet.
Related: How to add a host in Nagios Core

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...

Running pre-flight check on configuration data...

Checking objects...
        Checked 8 services.
        Checked 1 hosts.
        Checked 2 host groups.
##### 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 file and line before trying again.

Reload Nagios Core to apply the accepted object definition.
```
$ sudo systemctl reload nagios4
```
Debian and Ubuntu package installs use the nagios4 service name. Source installs may use a different service name, init script, or SIGHUP reload method.
Related: How to manage the Nagios Core system service
Confirm that the nagios4 service is active after the reload.
```
$ systemctl is-active nagios4
active
```
If the service is not active, inspect /var/log/nagios4/nagios.log or the journal before relying on the new group.
Related: How to check Nagios Core logs

Confirm that the host group reached the runtime object cache.

$ sudo grep -A4 "hostgroup_name.*web-servers" \
  /var/lib/nagios4/objects.cache
        hostgroup_name  web-servers
        alias   Web Servers
        members localhost
        }

Use the object_cache_file path from nagios.cfg when the local installation stores the runtime object cache somewhere else.

Open the Nagios Core web UI and select Current Status → Host Groups.

The web-servers group should list the hosts from members. Use the same hostgroup_name in later service definitions when one check should apply to every host in the group.
Related: How to add a service check in Nagios Core