#FAQ

#Requests may fail when forwarding to an external model service with a non-default port

#Symptoms

This issue can occur when:

• An external model service is accessed through Envoy AI Gateway or MaaS.
• The upstream model service listens on a non-default port.
• The upstream uses the hostname and port together for routing or authentication.
• Requests forwarded through the gateway fail. Depending on the upstream behavior, the response may be 401, 403, 404, or another status code.

#Root cause

Envoy Gateway's automatic backend hostname rewrite can forward the request with a Host/:authority value that does not include the upstream's non-default port. An upstream that routes or authenticates based on the complete hostname and port therefore treats the request as invalid.

This is an Envoy Gateway hostname-rewrite limitation.

#Solution

A workaround using low-level Envoy configuration is described in How to Preserve a Non-Default Backend Port in the Upstream Host Header with Envoy Gateway on ACP.