How to Send 103 Early Hints via GCP External HTTPS Load Balancer?

Question

I'm using a haproxy behind an external HTTP(S) loadbalancer on GCP to send 103 interim responses for a static site. I confirmed via curl that haproxy behaves as expected but I get a 502 error going through the load balancer.

Inspecting the load balancer access log I see a backend_interim_response_not_supported error for the request.

According to HTTP(S) Load Balancing logging and monitoring Guide this error means:

The backend sent an interim 1XX response to the request in a context where interim responses are not supported.

Is there a different "context" that I should use instead to send early hints?

Answer 1

The HTTP(S) Load Balancers proxy connections. That means HTTP Status Code response 103 is not supported.