Adversaries may communicate using the Domain Name System (DNS) application layer protocol to avoid detection/network filtering by blending in with existing traffic. Commands to the remote system, and often the results of those commands, will be embedded within the protocol traffic between the client and server. 

The DNS protocol serves an administrative function in computer networking and thus may be very common in environments. DNS traffic may also be allowed even before network authentication is completed. DNS packets contain many fields and headers in which data can be concealed. Often known as DNS tunneling, adversaries may abuse DNS to communicate with systems under their control within a victim network while also mimicking normal, expected traffic.(Citation: PAN DNS Tunneling)(Citation: Medium DnsTunneling) 
DNS
source_name: mitre-attack  url: https://attack.mitre.org/techniques/T1071/004  external_id: T1071.004     source_name: Medium DnsTunneling  description: Galobardes, R. (2018, October 30). Learn how easy is to bypass firewalls using DNS tunneling (and also how to block it). Retrieved March 15, 2020.  url: https://medium.com/@galolbardes/learn-how-easy-is-to-bypass-firewalls-using-dns-tunneling-and-also-how-to-block-it-3ed652f4a000     source_name: University of Birmingham C2  description: Gardiner, J.,  Cova, M., Nagaraja, S. (2014, February). Command & Control Understanding, Denying and Detecting. Retrieved April 20, 2016.  url: https://arxiv.org/ftp/arxiv/papers/1408/1408.1136.pdf     source_name: PAN DNS Tunneling  description: Palo Alto Networks. (n.d.). What Is DNS Tunneling?. Retrieved March 15, 2020.  url: https://www.paloaltonetworks.com/cyberpedia/what-is-dns-tunneling     