Suspicious DNS Z Flag Bit Set
This rule detects suspicious DNS queries where the Z flag bit in the DNS protocol header is set to a non-zero value. The Z flag is reserved (unused) per IETF design, though it has recently been used in DNSSec. A non-zero Z flag value should be rare outside of legitimate DNSSec usage. The rule includes exclusions for common TLDs, query types (NS, MX), responses, and NetBIOS related traffic to reduce false positives. The rule's intent is to identify potential command and control (C2) or data exfiltration attempts leveraging DNS anomalies.
Sigma
