Comprehensive and Detailed Explanation
The correct solution is Option B. This is a common false positive tuning scenario.
The "high priority network indicators" rule set triggers when it sees a connection to or from a known-malicious IP or domain. The problem states the false positives are coming from the on-premises proxy servers.
This implies that the proxy server itself is initiating traffic that matches these indicators. This is often benign, legitimate behavior, such as:
Resolving a user-requested malicious domain via DNS to check its category.
Performing an HTTP HEAD request to a malicious URL to scan it.
Fetching its own threat intelligence or filter updates.
In all these cases, the source of the network connection is the proxy server. In the Unified Data Model (UDM), the source IP of an event is stored in the principal.ip field.
To eliminate these false positives, you must create a rule exclusion (or add a not condition to the rule) that tells the detection engine to ignore any events where the principal.ip is the IP address of your trusted proxy servers. This will not affect the rule's ability to catch a workstation behind the proxy (whose IP would be the principal.ip) connecting through the proxy to a malicious target.ip.
Exact Extract from Google Security Operations Documents:
Curated detection exclusions: Curated detections can be tuned by creating exclusions to reduce false positives from known-benign activity. You can create exclusions based on any UDM field.
Tuning Network Detections: A common source of false positives for network indicator rules is trusted network infrastructure, such as proxies or DNS servers. This equipment may generate traffic to malicious domains or IPs as part of its normal operation (e.g., DNS resolution, content filtering lookups). In this scenario, the traffic originates from the infrastructure device itself. To filter this noise, create an exclusion where the principal.ip field matches the IP address (or IP range) of the trusted proxy server. This prevents the rule from firing on the proxy's administrative traffic while preserving its ability to detect threats from end-user systems.
[References:, Google Cloud Documentation: Google Security Operations > Documentation > Detections > Curated detections > Tune curated detections with exclusions, Google Cloud Documentation: Google Security Operations > Documentation > Detections > Overview of the YARA-L 2.0 language, , ]