Add detailed error reporting for REALITY invalid connections (#24)

Co-authored-by: RPRX <63339210+RPRX@users.noreply.github.com>

tls.go

@@ -443,8 +443,23 @@ func Server(ctx context.Context, conn net.Conn, config *Config) (*Conn, error) {
 	if hs.c.isHandshakeComplete.Load() {
 		return hs.c, nil
 	}
+
 	conn.Close()
-	return nil, errors.New("REALITY: processed invalid connection") // TODO: Add details.
+	var failureReason string
+	if hs.clientHello == nil {
+		failureReason = "failed to read client hello"
+	} else if hs.c.vers != VersionTLS13 {
+		failureReason = fmt.Sprintf("unsupported TLS version: %x", hs.c.vers)
+	} else if !config.ServerNames[hs.clientHello.serverName] {
+		failureReason = fmt.Sprintf("server name mismatch: %s", hs.clientHello.serverName)
+	} else if hs.c.conn != conn {
+		failureReason = "authentication failed or validation criteria not met"
+	} else if hs.c.out.handshakeLen[0] == 0 {
+		failureReason = "target sent incorrect server hello or handshake incomplete"
+	} else {
+		failureReason = "handshake did not complete successfully"
+	}
+	return nil, fmt.Errorf("REALITY: processed invalid connection from %s: %s", remoteAddr, failureReason)
 
 	/*
 		c := &Conn{