DNS tunneling is a technique that uses the DNS protocol to exfiltrate data or establish command and control channels between a compromised host and an attacker-controlled server. DNS tunneling can bypass network security controls that allow outbound DNS traffic without inspection or filtering. To stop DNS tunneling, two recommended approaches are: Enforce security over port 53. This means applying firewall rules, access control lists, or other mechanisms to restrict outbound DNS traffic to only authorized DNS servers and domains. Additionally, DNS traffic should be inspected and analyzed for anomalies, such as unusually large or frequent queries, non-standard encoding, or suspicious domains. This can help detect and block DNS tunneling attempts. Use Cisco Umbrella. Cisco Umbrella is a cloud-based security service that provides DNS security, web filtering, and threat intelligence. Cisco Umbrella can prevent DNS tunneling by blocking malicious domains, enforcing policies based on content categories, and applying machine learning to identify and stop emerging threats. Cisco Umbrella can also provide visibility and reporting on DNS activity and security events. Reference := Implementing and Operating Cisco Security Core Technologies (SCOR) v1.0, Module 5: Securing the Cloud, Lesson 5.2: DNS Security What Is DNS Tunneling? - Palo Alto Networks An Introduction to DNS Tunneling Detection & Data Exfiltration via DNS - Vercara