cohttp-eio(client): 400 BadRequest due to Connection: TE

[art-w]

I'm getting a 400 Bad Request rather than the expected 404 NotFound:

let () =
  Eio_main.run @@ fun env ->
  let response, _ = Cohttp_eio.Client.get env ~host:"bttracker.debian.org" ~port:6969 "/" in
  Format.printf "%s@." (Http.Status.to_string @@ Http.Response.status response)

I think this is caused by the Connection: TE header:

$ curl -v 'http://bttracker.debian.org:6969/' # 404 NotFound <- expected
$ curl -v -H 'TE: trailers' 'http://bttracker.debian.org:6969/' # 404 NotFound <- still ok
$ curl -v -H 'Connection: TE' 'http://bttracker.debian.org:6969/' # 400 BadRequest
$ curl -v -H 'Connection: TE' -H 'TE: trailers' 'http://bttracker.debian.org:6969/' # 400 BadRequest

I've no idea what this header is doing, or why cohttp-eio is the only client that enables it. In any case, the bug seems specific to this cheap server, but it would be nice if there was a way of not sending this header :)

[mseri]

Ping @bikallem

[bikallem]

This functionality is the implementation of HTTP/1.1 spec (https://www.rfc-editor.org/rfc/rfc9110#section-10.1.4)

A sender of TE MUST also send a "TE" connection option within the Connection header field (Section 7.6.1) to inform intermediaries not to forward this field.

In any case, the bug seems specific to this cheap server, but it would be nice if there was a way of not sending this header :)

A HTTP/1.1 non conforming (various degrees) client request is definitely on the cards.

[art-w]

Ha I see, thanks for the pointer! Unless there are more reports of servers rejecting this header, I don't think it's a pressing issue :P