reverseproxy: Use DialTLSContext if ServerName has placeholder (#6955)

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

modules/caddyhttp/reverseproxy/httptransport.go

@@ -382,6 +382,36 @@ func (h *HTTPTransport) NewTransport(caddyCtx caddy.Context) (*http.Transport, e
 		if err != nil {
 			return nil, fmt.Errorf("making TLS client config: %v", err)
 		}
+
+		// servername has a placeholder, so we need to replace it
+		if strings.Contains(h.TLS.ServerName, "{") {
+			rt.DialTLSContext = func(ctx context.Context, network, addr string) (net.Conn, error) {
+				// reuses the dialer from above to establish a plaintext connection
+				conn, err := dialContext(ctx, network, addr)
+				if err != nil {
+					return nil, err
+				}
+
+				// but add our own handshake logic
+				repl := ctx.Value(caddy.ReplacerCtxKey).(*caddy.Replacer)
+				tlsConfig := rt.TLSClientConfig.Clone()
+				tlsConfig.ServerName = repl.ReplaceAll(tlsConfig.ServerName, "")
+				tlsConn := tls.Client(conn, tlsConfig)
+
+				// complete the handshake before returning the connection
+				if rt.TLSHandshakeTimeout != 0 {
+					var cancel context.CancelFunc
+					ctx, cancel = context.WithTimeout(ctx, rt.TLSHandshakeTimeout)
+					defer cancel()
+				}
+				err = tlsConn.HandshakeContext(ctx)
+				if err != nil {
+					_ = tlsConn.Close()
+					return nil, err
+				}
+				return tlsConn, nil
+			}
+		}
 	}
 
 	if h.KeepAlive != nil {
@@ -453,45 +483,9 @@ func (h *HTTPTransport) NewTransport(caddyCtx caddy.Context) (*http.Transport, e
 	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) {
-	// Try to replace TLS servername if needed
+	h.SetScheme(req)
-	repl := req.Context().Value(caddy.ReplacerCtxKey).(*caddy.Replacer)
-	transport := h.replaceTLSServername(repl)
-
-	transport.SetScheme(req)
 
 	// use HTTP/3 if enabled (TODO: This is EXPERIMENTAL)
 	if h.h3Transport != nil {
@@ -507,7 +501,7 @@ func (h *HTTPTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 		return h.h2cTransport.RoundTrip(req)
 	}
 
-	return transport.Transport.RoundTrip(req)
+	return h.Transport.RoundTrip(req)
 }
 
 // SetScheme ensures that the outbound request req