1. SSH to NAC appliance and issue "nslookup <CLIENT_IP>" to verify if reverse DNS is resolving or not
2. If DNS server is incorrect, reconfigure DNS Server IP address(es) by going through config setup with command nacconfig
3. If DNS Server is correct, but DNS is still not resolving, look into possible DNS server issue or try to resolve client IP directly on the DNS Server
Here is a sample screenshot of configuration of DNS on a server. We do not support direct configuration of third party DNS servers. This is an example only.