Basic Concept: SSL Forward Proxy relies on clients trusting the firewall's forward trust CA. Without that trust, every substituted certificate appears untrusted to browsers.
Why D is Correct: The most likely cause is that the self-signed CA was not installed into client trusted root stores.
Why A is Wrong: The decryption policy is configured with a "no-decrypt" action, which causes browsers to reject the connection. is associated with authentication, PKI, or TLS configuration, but it is not the object or step that enforces the certificate validation or service identity requirement being tested.
Why B is Wrong: The external websites are using TLS 1.3, which cannot be decrypted by the firewall without a specific license. is associated with authentication, PKI, or TLS configuration, but it is not the object or step that enforces the certificate validation or service identity requirement being tested.
Why C is Wrong: The firewall's forward untrust certificate has expired, preventing it from identifying untrusted sites. is associated with authentication, PKI, or TLS configuration, but it is not the object or step that enforces the certificate validation or service identity requirement being tested.
