Create host discovery nameserver

Creates one or more nameservers to for discovery purposes. Only private nameservers can be added to monitoring. As a way of enforcing this requirement, we check that the nameserver hostname matches a domain name that's in the host inventory. For example, if example.com is an organization domain name, then ns1.example.com nameserver can be added but ns1.example.org cannot.

Public servers must not be submitted for monitoring, even if they're under a domain name that is in the inventory.