John, a professional hacker, decided to use DNS to perform data exfiltration on a target network. In this process, he embedded malicious data into the DNS protocol packets that even DNSSEC cannot detect.
Using this technique, John successfully injected malware to bypass a firewall and maintained communication with the victim machine and C&C server.
What is the technique employed by John to bypass the firewall?

A. DNSSEC zone walking
B. DNS cache snooping
C. DNS enumeration
D. DNS tunneling method