Forward unknown commands to upstream in single-upstream mode

downstream.go

@@ -2287,8 +2287,15 @@ func (dc *downstreamConn) handleMessageRegistered(msg *irc.Message) error {
 		}
 	default:
 		dc.logger.Printf("unhandled message: %v", msg)
+
+		// Only forward unknown commands in single-upstream mode
+		uc := dc.upstream()
+		if uc == nil {
 			return newUnknownCommandError(msg.Command)
 		}
+
+		uc.SendMessageLabeled(dc.id, msg)
+	}
 	return nil
 }