httpserver: Add experimental H2C support (#3289)

* reverse_proxy: Initial attempt at H2C transport/client support (#3218)

I have not tested this yet

* Experimentally enabling H2C server support (closes #3227)

See also #3218

I have not tested this

* reverseproxy: Clean up H2C transport a bit

* caddyhttp: Update godoc for h2c server; clarify experimental status

* caddyhttp: Fix trailers when recording responses (fixes #3236)

* caddyhttp: Tweak h2c config settings and docs

4 files changed, 63 insertions, 1 deletions

diff --git a/modules/caddyhttp/app.go b/modules/caddyhttp/app.go
index 8e7f3ea..f695276 100644
--- a/modules/caddyhttp/app.go
+++ b/modules/caddyhttp/app.go
@@ -27,6 +27,8 @@ import (
 	"github.com/caddyserver/caddy/v2/modules/caddytls"
 	"github.com/lucas-clemente/quic-go/http3"
 	"go.uber.org/zap"
+	"golang.org/x/net/http2"
+	"golang.org/x/net/http2/h2c"
 )
 
 func init() {
@@ -282,6 +284,14 @@ func (app *App) Start() error {
 			Handler:           srv,
 		}
 
+		// enable h2c if configured
+		if srv.AllowH2C {
+			h2server := &http2.Server{
+				IdleTimeout: time.Duration(srv.IdleTimeout),
+			}
+			s.Handler = h2c.NewHandler(srv, h2server)
+		}
+
 		for _, lnAddr := range srv.Listen {
 			listenAddr, err := caddy.ParseNetworkAddress(lnAddr)
 			if err != nil {
diff --git a/modules/caddyhttp/reverseproxy/caddyfile.go b/modules/caddyhttp/reverseproxy/caddyfile.go
index 9636936..56e2698 100644
--- a/modules/caddyhttp/reverseproxy/caddyfile.go
+++ b/modules/caddyhttp/reverseproxy/caddyfile.go
@@ -584,6 +584,7 @@ func (h *Handler) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 //         tls_trusted_ca_certs <cert_files...>
 //         keepalive [off|<duration>]
 //         keepalive_idle_conns <max_count>
+//         versions <versions...>
 //     }
 //
 func (h *HTTPTransport) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
@@ -701,6 +702,12 @@ func (h *HTTPTransport) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 				h.KeepAlive.MaxIdleConns = num
 				h.KeepAlive.MaxIdleConnsPerHost = num
 
+			case "versions":
+				h.Versions = d.RemainingArgs()
+				if len(h.Versions) == 0 {
+					return d.ArgErr()
+				}
+
 			default:
 				return d.Errf("unrecognized subdirective %s", d.Val())
 			}
diff --git a/modules/caddyhttp/reverseproxy/httptransport.go b/modules/caddyhttp/reverseproxy/httptransport.go
index 4b5111c..f0fbbd6 100644
--- a/modules/caddyhttp/reverseproxy/httptransport.go
+++ b/modules/caddyhttp/reverseproxy/httptransport.go
@@ -82,11 +82,16 @@ type HTTPTransport struct {
 	// The size of the read buffer in bytes.
 	ReadBufferSize int `json:"read_buffer_size,omitempty"`
 
-	// The versions of HTTP to support. Default: ["1.1", "2"]
+	// The versions of HTTP to support. As a special case, "h2c"
+	// can be specified to use H2C (HTTP/2 over Cleartext) to the
+	// upstream (this feature is experimental and subject to
+	// change or removal). Default: ["1.1", "2"]
 	Versions []string `json:"versions,omitempty"`
 
 	// The pre-configured underlying HTTP transport.
 	Transport *http.Transport `json:"-"`
+
+	h2cTransport *http2.Transport
 }
 
 // CaddyModule returns the Caddy module information.
@@ -110,6 +115,28 @@ func (h *HTTPTransport) Provision(ctx caddy.Context) error {
 	}
 	h.Transport = rt
 
+	// if h2c is enabled, configure its transport (std lib http.Transport
+	// does not "HTTP/2 over cleartext TCP")
+	if sliceContains(h.Versions, "h2c") {
+		// crafting our own http2.Transport doesn't allow us to utilize
+		// most of the customizations/preferences on the http.Transport,
+		// because, for some reason, only http2.ConfigureTransport()
+		// is allowed to set the unexported field that refers to a base
+		// http.Transport config; oh well
+		h2t := &http2.Transport{
+			// kind of a hack, but for plaintext/H2C requests, pretend to dial TLS
+			DialTLS: func(network, addr string, _ *tls.Config) (net.Conn, error) {
+				// TODO: no context, thus potentially wrong dial info
+				return net.Dial(network, addr)
+			},
+			AllowHTTP: true,
+		}
+		if h.Compression != nil {
+			h2t.DisableCompression = !*h.Compression
+		}
+		h.h2cTransport = h2t
+	}
+
 	return nil
 }
 
@@ -182,6 +209,13 @@ func (h *HTTPTransport) NewTransport(_ caddy.Context) (*http.Transport, error) {
 // RoundTrip implements http.RoundTripper.
 func (h *HTTPTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 	h.SetScheme(req)
+
+	// if H2C ("HTTP/2 over cleartext") is enabled and the upstream request is
+	// HTTP/2 without TLS, use the alternate H2C-capable transport instead
+	if req.ProtoMajor == 2 && req.URL.Scheme == "http" && h.h2cTransport != nil {
+		return h.h2cTransport.RoundTrip(req)
+	}
+
 	return h.Transport.RoundTrip(req)
 }
 
diff --git a/modules/caddyhttp/server.go b/modules/caddyhttp/server.go
index 36e7042..a4fda28 100644
--- a/modules/caddyhttp/server.go
+++ b/modules/caddyhttp/server.go
@@ -111,6 +111,17 @@ type Server struct {
 	// This field is not subject to compatibility promises.
 	ExperimentalHTTP3 bool `json:"experimental_http3,omitempty"`
 
+	// Enables H2C ("Cleartext HTTP/2" or "H2 over TCP") support,
+	// which will serve HTTP/2 over plaintext TCP connections if
+	// a client support it. Because this is not implemented by the
+	// Go standard library, using H2C is incompatible with most
+	// of the other options for this server. Do not enable this
+	// only to achieve maximum client compatibility. In practice,
+	// very few clients implement H2C, and even fewer require it.
+	// This setting applies only to unencrypted HTTP listeners.
+	// ⚠️ Experimental feature; subject to change or removal.
+	AllowH2C bool `json:"allow_h2c,omitempty"`
+
 	primaryHandlerChain Handler
 	errorHandlerChain   Handler
 	listenerWrappers    []caddy.ListenerWrapper