Configuring a span port on the perimeter firewall to ingest logs is the best architectural change to ensure that all client proxy traffic is captured for analysis. Here's why:
Comprehensive Traffic Capture: A span port (or mirror port) on the perimeter firewall can capture all inbound and outbound traffic, including traffic that might bypass the proxy. This ensures that all network traffic is available for analysis.
Centralized Logging: By capturing logs at the perimeter firewall, the organization can centralize logging and analysis, making it easier to detect and investigate anomalies.
Minimal Disruption: Implementing a span port is a non-intrusive method that does not require significant changes to the network architecture, thus minimizing disruption to existing services.
Reference:
CompTIA Security+ SY0-601 Study Guide by Mike Chapple and David Seidl
NIST Special Publication 800-92: Guide to Computer Security Log Management OWASP Logging Cheat Sheet