Support proxying client requests through direct and tunneling HTTP/HTTPS proxies

[MisterDA]

This PR implements a new Cohttp_lwt_unix.Connection_proxy module satisfying Cohttp_lwt.S.Connection_cache, allowing client HTTP/HTTPS requests to be routed through proxies.

I recommend a quick glance at Everything curl - 9.8 Proxies for a nice introduction to proxies. As curl is well established, I try following the same terminology and support similar features.

• HTTP proxy

  An HTTP proxy is a proxy that the client speaks HTTP with to get the transfer done.

  If the remote server is reachable through HTTP, then the client should send HTTP requests where targets are in absolute form (see RFC 9112).

  When making a request to a proxy, other than a CONNECT or server-wide OPTIONS request (as detailed below), a client MUST send the target URI in "absolute-form" as the request-target.

  GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1

  The first step is to thread an ~absolute_form parameter when building requests. This way, the Cohttp_lwt_unix.Connection_cache module can simply be extended with an optional ?proxy parameter, taking the URI of the proxy. If a proxy is specified, all requests will be send in the absolute form, and instead of connecting to the remote server, the client will transparently connect to the proxy. Conduit handles the connection to an HTTPS proxy automatically.

• HTTPS with HTTP proxy

  If the remote server uses HTTPS, then a tunneling proxy needs to be used. The CONNECT HTTP requests asks the proxy to open a connection to the remote server. If it succeeds, the proxy then blindly forwards data between the client and the remote server. Conduit needs to be modified to handle an HTTPS connection (more specifically, a TLS handshake and encryption layer) on top of an already opened connection. If the proxy itself is reachable through HTTPS, conduit needs to be able to tunnel a TLS connection on top of another TLS connection.

  To handle tunneled client connections, we provide the (private) Cohttp_lwt.Connection_tunnel, which takes the proxy URI when created. It then maintains a cache of opened connections via the proxy to each remote server, and is able to distribute new requests amongst currently opened connections (the code is mostly copied from Cohttp_lwt.Connection_cache balancing).

  We expose the Cohttp_lwt.Connection_proxy via Cohttp_lwt_unix.Connection_proxy which, based on the remote URI, selects either a direct (absolute-form queries) or tunneled connection.

• MITM proxy

  [MITM proxies] require users to install a custom "trust root" (Certificate Authority (CA) certificate) in the client, and then the proxy terminates all TLS traffic from the client, impersonates the remote server and acts like a proxy.

  This can be achieved by installing a new certificate system-wide, or passing it to the ca-certs library via external means (environment variable, parameter at startup, ...). See Facilitate the use of extra CA certificates ca-certs#30.

• Proxy environment variables

  We show how to support ALL_PROXY, NO_PROXY, and <scheme>_proxy (such as http_proxy, https_proxy), as documented by the curl project.

This PR relies on tunneling the TLS-on-TLS feature being added to ocaml-tls and Conduit. We've added some opam pin-depends to allow it to build with the latest dependencies.

• TLS tunnel over Lwt_io.channel ocaml-conduit#428
• TLS over an existing connection mirleft/ocaml-tls#499
• Facilitate the use of extra CA certificates ca-certs#30 (optional)

[rgrinberg]

I don't think you should modify the Http.Request.t type. The point of this type is to stay minimal and not reflect any application specific concerns.

[MisterDA]

I don't think you should modify the Http.Request.t type. The point of this type is to stay minimal and not reflect any application specific concerns.

We're only adding a boolean, absolute_form, which I'd argue is a property of the request. If we don't add it, write_header in Cohttp.Request.Make has no way of knowing whether the request needs to be formatted in absolute form.

[rgrinberg]

Whether it's just a boolean or not is besides the point. You're changing the semantics of comparison, pretty printing, etc for a large group of users who aren't gaining anything from this feature.

Some alternatives you could try:

1. Construct your resource field in absolute form
2. Pass the absolute form as a parameter to the header encoder
3. Introduce your own request type that includes this flag

EDIT: after glancing at the spec, seems like 1. is the way to go. the resource field should already handle requests in "absolute form"

[MisterDA]

EDIT: after glancing at the spec, seems like 1. is the way to go. the resource field should already handle requests in "absolute form"

I did not follow this option, because the resource field would then need to duplicate the scheme and the host, which are already stored in the request record. I can try this and see what happens.

[rgrinberg]

Good point, doesn't that mean we can get rid of scheme from the request as well? @anuragsoni why did we add scheme?

[anuragsoni]

why did we add scheme?

From what I remember we kept it around mostly to maintain compatibility with the existing release of cohttp.

[rgrinberg]

@MisterDA do you want to take the honors of getting rid of it?

[samoht]

doesn't need to be done for this PR but that would be nice to have a similar function in the client API (so that every user do not have to re-implement an half-broken version of it)

[MisterDA]

@MisterDA do you want to take the honors of getting rid of it?

It doesn't seem possible to remove the scheme field of the request, as Cohttp.Request.uri needs to be able to reconstruct the full URI from a request, as exhibited by the test:

ocaml-cohttp/cohttp/test/test_request.ml

Lines 251 to 255 in 53fbf39

	let uri_round_trip _ =
	let expected_uri = Uri.of_string "https://www.example.com/test" in
	let actual_uri = Request.make expected_uri |> Request.uri in
	Alcotest.check uri_testable "Request.make uri round-trip" actual_uri
	expected_uri

we might want to remove this feature altogether. This mostly impacts logging and testing, but also the Cohttp_lwt.Client.callv function (that is deprecated).

[rgrinberg]

Indeed it's not worth it. I got rid of it here #1086

[rgrinberg]

Could this test come in a separate PR? AFAIK, nothing in this PR was added to make this pass.