Network Firewall Proxy SourceIp is NATed when accessed via PrivateLink

In a multi-VPC setup with Network Firewall Proxy, I created a request:SourceIp rule targeting the App VPC EC2's IP (10.2.1.52/32). It didn't match.

curl http://google.com/  → 403 (SourceIp rule not matched, default DENY)

PrivateLink endpoint traffic is NATed to the endpoint's ENI IP, so the original client IP never reaches the proxy. The official blog documents this for VPC Lattice / NLB integrations ("source traffic is NATed"), but the same applies to direct PrivateLink endpoints.

request:SourceVpc worked correctly across VPCs though. For cross-VPC source identification, use SourceVpc or SourceVpce instead.

See Network Firewall Proxy Hands-On — Multi-VPC Architecture with PrivateLink for details.