Explanation
The Lightweight AP (LAP) can discover controllers through your domain name server (DNS). For the access point (AP) todo so, you must configure your DNS to return controller IP addresses in response to CISCO-LWAPP-CONTROLLER.localdomain, where localdomain is the AP domain name. When an AP receives an IP address and DNSinformation from a DHCP server, it contacts the DNS to resolve CISCO-CAPWAP-CONTROLLER.localdomain. When theDNS sends a list of controller IP addresses, the AP sends discovery requests to the controllers.
The AP will attempt to resolve the DNS name CISCO-CAPWAP-CONTROLLER.localdomain. When the AP is able toresolve this name to one or more IP addresses, the AP sends a unicast CAPWAP Discovery Message to the resolved IPaddress(es). Each WLC that receives the CAPWAP Discovery Request Message replies with a unicast CAPWAP DiscoveryResponse to the AP.
Reference:
https://www.cisco.com/c/en/us/support/docs/wireless/4400-series-wireless-lan-controllers/107606-dns-wlc-confi