traefik: websocket with http/2 cleartext lead to http 500

Do you want to request a feature or report a bug?

Bug

What did you do?

traefik terminates http connections with acme LE TLS and proxies tomcat java application. The application uses websockets as well as http. Quite some connections are used concurrently.

In order to optimize http/2 should be used for the java application as well as the traefik entrypoint. For the internal connection no encryption should be used to save cpu cycles and reduce latency (it is a container local network anyway).

If the java application is using ‘http’ scheme, everything works great.

When using ‘h2c’ as schema for the java application, an http 500 is returned by traefik. The debug log shows

level=debug msg="'500 Internal Server Error' caused by: unsupported protocol scheme \"h2c\""

What did you expect to see?

A websocket connection according to RFC 8441

Since there is a testcase specifically for h2 protocol, I wonder what prevents h2c: https://github.com/traefik/traefik/blob/4fccde84bd839afd0f784c3783f2dde7d0f7e8e1/integration/websocket_test.go#L442

What did you see instead?

An http 500 without any logmessage or returned data to the client. (This resulted in some hours debugging, since the assumption was that it was not caused by traefik. )

Output of traefik version: (What version of Traefik are you using?)

Traefik version 2.3.2 built on 2020-10-19T18:36:22Z

What is your environment & configuration (arguments, toml, provider, platform, …)?

  backend:
    build:
      context: backend
    labels:
      - "traefik.enable=true"
      - "traefik.http.routers.backend.rule=Host(`backend.localhost`)"
      - "traefik.http.routers.backend.entrypoints=web"
      - "traefik.http.routers.backend.middlewares=backend-strip-prefix@docker"
      - "traefik.http.middlewares.backend-strip-prefix.stripprefix.prefixes=/api"
      - "traefik.http.services.backend.loadbalancer.server.port=8081"
      - "traefik.http.services.backend.loadbalancer.server.scheme=h2c"