Core Issue
The most common reason for this problem is a misconfigured Access Control List (ACL) that is denying Domain Name System (DNS) traffic.
Resolution
Verify that hosts are able to browse the Internet by IP address. Use the IP address in the URL rather than the DNS name. If this works, verify that User Datagram Protocol (UDP) port 53 is permitted through the ACL.
For more information on understanding ACLs, refer to the Configuring IP Access Lists.
