Steps to reproduce the bug:

• Install Docker
• Install a debugging-friendly HTTP client (e.g. https://httpie.org/ )
• Enable Swarm Mode: docker swarm init
• Deploy the stack: docker stack up -c stack.yaml http
  • This will listen on port 2100 & 2101
• Access endpoints :2101/yes with an extra header X-Forwarded-Proto: https:

# With httpie:
http :2101/yes x-forwarded-proto:https show_env==1
# With curl:
curl 127.0.0.1:2101/yes -H x-forwarded-proto:https

Expected result

• :2101/yes should respond with yes https, which is configured as direct_response

Actual result

• :2101/yes responds with yes

Extra Debugging

• Accessing endpoint :2101/bin/headers?show_env=1 returns all headers sent from Envoy to httpbin. Which shows that header X-Forwarded-Proto is overwritten.