feat: recovering preHandleMetadata failure from sniffing (#769)

tunnel/tunnel.go

@@ -419,15 +419,28 @@ func handleTCPConn(connCtx C.ConnContext) {
 		return
 	}
 
+	preHandleFailed := false
 	if err := preHandleMetadata(metadata); err != nil {
 		log.Debugln("[Metadata PreHandle] error: %s", err)
-		return
+		preHandleFailed = true
 	}
 
 	conn := connCtx.Conn()
 	conn.ResetPeeked() // reset before sniffer
 	if sniffer.Dispatcher.Enable() && sniffingEnable {
-		sniffer.Dispatcher.TCPSniff(conn, metadata)
+		// Try to sniff a domain when `preHandleMetadata` failed, this is usually
+		// caused by a "Fake DNS record missing" error when enhanced-mode is fake-ip.
+		if sniffer.Dispatcher.TCPSniff(conn, metadata) {
+			// we now have a domain name
+			preHandleFailed = false
+		}
+	}
+
+	// If both trials have failed, we can do nothing but give up
+	if preHandleFailed {
+		log.Debugln("[Metadata PreHandle] failed to sniff a domain for connection %s --> %s, give up",
+			metadata.SourceDetail(), metadata.RemoteAddress())
+		return
 	}
 
 	peekMutex := sync.Mutex{}