reverseproxy: Dynamic ServerName for TLS upstreams (#4836)

* Make reverse proxy TLS server name replaceable for SNI upstreams.

* Reverted previous TLS server name replacement, and implemented thread safe version.

* Move TLS servername replacement into it's own function

* Moved SNI servername replacement into httptransport.

* Solve issue when dynamic upstreams use wrong protocol upstream.

* Revert previous commit.

Old commit was: Solve issue when dynamic upstreams use wrong protocol upstream.
Id: 3c9806c

Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

modules/caddyhttp/reverseproxy/httptransport.go

@@ -25,6 +25,7 @@ import (
 	"net/http"
 	"os"
 	"reflect"
+	"strings"
 	"time"
 
 	"github.com/caddyserver/caddy/v2"
@@ -242,17 +243,53 @@ func (h *HTTPTransport) NewTransport(ctx caddy.Context) (*http.Transport, error)
 	return rt, nil
 }
 
+// replaceTLSServername checks TLS servername to see if it needs replacing
+// if it does need replacing, it creates a new cloned HTTPTransport object to avoid any races
+// and does the replacing of the TLS servername on that and returns the new object
+// if no replacement is necessary it returns the original
+func (h *HTTPTransport) replaceTLSServername(repl *caddy.Replacer) *HTTPTransport {
+	// check whether we have TLS and need to replace the servername in the TLSClientConfig
+	if h.TLSEnabled() && strings.Contains(h.TLS.ServerName, "{") {
+		// make a new h, "copy" the parts we don't need to touch, add a new *tls.Config and replace servername
+		newtransport := &HTTPTransport{
+			Resolver:              h.Resolver,
+			TLS:                   h.TLS,
+			KeepAlive:             h.KeepAlive,
+			Compression:           h.Compression,
+			MaxConnsPerHost:       h.MaxConnsPerHost,
+			DialTimeout:           h.DialTimeout,
+			FallbackDelay:         h.FallbackDelay,
+			ResponseHeaderTimeout: h.ResponseHeaderTimeout,
+			ExpectContinueTimeout: h.ExpectContinueTimeout,
+			MaxResponseHeaderSize: h.MaxResponseHeaderSize,
+			WriteBufferSize:       h.WriteBufferSize,
+			ReadBufferSize:        h.ReadBufferSize,
+			Versions:              h.Versions,
+			Transport:             h.Transport.Clone(),
+			h2cTransport:          h.h2cTransport,
+		}
+		newtransport.Transport.TLSClientConfig.ServerName = repl.ReplaceAll(newtransport.Transport.TLSClientConfig.ServerName, "")
+		return newtransport
+	}
+
+	return h
+}
+
 // RoundTrip implements http.RoundTripper.
 func (h *HTTPTransport) RoundTrip(req *http.Request) (*http.Response, error) {
-	h.SetScheme(req)
+	// Try to replace TLS servername if needed
+	repl := req.Context().Value(caddy.ReplacerCtxKey).(*caddy.Replacer)
+	transport := h.replaceTLSServername(repl)
+
+	transport.SetScheme(req)
 
 	// if H2C ("HTTP/2 over cleartext") is enabled and the upstream request is
 	// HTTP without TLS, use the alternate H2C-capable transport instead
 	if req.URL.Scheme == "http" && h.h2cTransport != nil {
 		return h.h2cTransport.RoundTrip(req)
 	}
 
-	return h.Transport.RoundTrip(req)
+	return transport.Transport.RoundTrip(req)
 }
 
 // SetScheme ensures that the outbound request req