v2rayA/v2rayA · error

dns read length via dispatcher: %w

Error message

dns read length via dispatcher: %w

What it means

Raised in exchangeViaDispatcher when reading the 2-byte TCP length prefix of the DNS response from the dispatcher connection fails — the upstream/route closed the connection or the 5s read deadline expired before the response header arrived.

Source

Thrown at core/dns/upstream_stub.go:605

	}
	defer conn.Close()

	// DNS over TCP: prepend 2-byte length prefix.
	tcpMsg := make([]byte, 2+len(packed))
	tcpMsg[0] = byte(len(packed) >> 8)
	tcpMsg[1] = byte(len(packed))
	copy(tcpMsg[2:], packed)

	// Send DNS query.
	conn.SetWriteDeadline(time.Now().Add(5 * time.Second))
	if _, err := conn.Write(tcpMsg); err != nil {
		return nil, fmt.Errorf("dns write via dispatcher: %w", err)
	}

	// Read 2-byte length prefix.
	conn.SetReadDeadline(time.Now().Add(5 * time.Second))
	if _, err := conn.Read(tcpMsg[:2]); err != nil {
		return nil, fmt.Errorf("dns read length via dispatcher: %w", err)
	}
	respLen := int(tcpMsg[0])<<8 | int(tcpMsg[1])
	if respLen < 0 || respLen > 65535 {
		return nil, fmt.Errorf("dns invalid response length: %d", respLen)
	}

	// Read full DNS response.
	respData := make([]byte, respLen)
	totalRead := 0
	for totalRead < respLen {
		n, err := conn.Read(respData[totalRead:])
		if err != nil {
			return nil, fmt.Errorf("dns read data via dispatcher: %w", err)
		}
		totalRead += n
	}

	rtt := time.Since(start)

View on GitHub (pinned to 71e5442fc5)

Solutions

1. Retry the DNS query
2. Check whether the routed outbound actually forwards TCP traffic
3. Verify the upstream DNS server responds on TCP

When it happens

Trigger: Thrown at core/dns/upstream_stub.go:605 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of v2rayA/v2rayA@71e5442fc5 (2026-09-05). Data as JSON: /api/errors/8bfe892fd7fe53ff. Report an issue: GitHub.