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