Dynamic Resolution: DNS Calculation

Adversaries may perform calculations on addresses returned in DNS results to determine which port and IP address to use for command and control, rather than relying on a predetermined port number or the actual returned IP address. A IP and/or port number calculation can be used to bypass egress filtering on a C2 channel.[1]

One implementation of DNS Calculation is to take the first three octets of an IP address in a DNS response and use those values to calculate the port for command and control traffic.[1][2][3]

ID: T1568.003
Sub-technique of:  T1568
Tactic: Command And Control
Platforms: Linux, Windows, macOS
Data Sources: DNS records
Version: 1.0
Created: 11 March 2020
Last Modified: 27 March 2020

Procedure Examples

Name Description
APT12

APT12 has used multiple variants of DNS Calculation including multiplying the first two octets of an IP address and adding the third octet to that value in order to get a resulting command and control port.[1]

Mitigations

This type of attack technique cannot be easily mitigated with preventive controls since it is based on the abuse of system features.

Detection

Detection for this technique is difficult because it would require knowledge of the specific implementation of the port calculation algorithm. Detection may be possible by analyzing DNS records if the algorithm is known.

References