envoyproxy/envoy

tcp_proxy: Propagate non-2xx response status on CONNECT to client when using internal listeners

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#43977 opened on Mar 16, 2026

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area/internal_listenerarea/tcp_proxyarea/tunnelingenhancementhelp wanted

Description

tcp_proxy: Propagate non-2xx response status on CONNECT to client when using internal listeners

Description: This is the setup:

Client -> Egress HCM -> Entry cluster -> Internal Encap listener -> tcp_proxy(TunnelingConfig, HTTP2 CONNECT) -> Upstream

When the upstream returns 403 on the CONNECT stream, tcp_proxy treats this as a failure to establish the tunnel and resets the downstream connection. The Entry cluster sees cx_connect_fail and we return a 503 to the client.

If we try UPSTREAM_FILTER_STATE(envoy.tcp_proxy.propagate_response_headers) at the egress HCM access log, we find that the filter state is not propagated but propagate_response_headers: true on the encap listener's tcp_proxy does show the 403 which helps for access log visibility at the egress proxy.

It would be really helpful to propagate the 403 back to the egress HCM's client response.

Contributor guide

Tech stack
cpp
Domain
backendinfrastructure
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
3-5 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
blocked
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
C++ knowledgeEnvoy proxy internalsHTTP/2 CONNECTFilter state propagation
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20
Research direction
Investigate the tcp proxy tunneling configuration in source/extensions/filters/network/tcp proxy/ and the existing propagate response headers implementation. Understand the flow from egress HCM through entry cluster to internal encap listener and upstream. Propose changes to propagate non 2xx status codes from upstream CONNECT response back to client via filter state or direct response, and ensure the egress HCM access log can capture the propagated status.