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.

1. 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

2. Open a host group object file in the loaded directory.

   $ sudoedit \
     /etc/nagios4/conf.d/web-servers.cfg

3. 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

4. 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.

   Related: How to validate the Nagios Core configuration

5. 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

6. 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

7. 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.

8. 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