2dust/v2rayN · error · Exception

Failed to verify and extract UDP response.

Error message

Failed to verify and extract UDP response.

What it means

Thrown when the tester-specific VerifyAndExtractUdpResponse returns false. For DnsService that means wrong transaction ID, not a response (QR bit unset), non-zero RCODE, or zero answer count. The datagram was long enough but semantically invalid for the chosen tester.

Solutions

1. Confirm the request packet's identifier matches what the tester validates (DnsService expects transaction ID 0x1234).
2. Try a different target (e.g. 1.1.1.1 for DNS) to rule out a target-side refusal/RCODE.
3. Log the received bytes to see the actual flags/RCODE/answer-count rather than just 'failed'.
4. Ensure no other process is generating traffic on the same relay that could mismatch the transaction ID.

Example fix

// before - boolean verify hides the reason
if (udpReceiveResult != null && _udpTest.VerifyAndExtractUdpResponse(udpReceiveResult))
    return roundTripTime;
else
    throw new Exception("Failed to verify and extract UDP response.");

// after - capture why it failed for diagnostics
if (udpReceiveResult == null || !_udpTest.VerifyAndExtractUdpResponse(udpReceiveResult))
{
    var hex = udpReceiveResult != null ? Convert.ToHexString(udpReceiveResult, 0, Math.Min(32, udpReceiveResult.Length)) : "<null>";
    throw new Exception($"UDP response failed semantic validation (tester={_udpTest.GetType().Name}, head={hex}).");
}

// Validate reply semantics with detail before relying on the boolean
if (udpReceiveResult == null || !_udpTest.VerifyAndExtractUdpResponse(udpReceiveResult))
    throw new Exception($"UDP response failed validation (tester={_udpTest.GetType().Name}).");

static bool IsTesterValidatedResponse(IUdpTest tester, byte[] reply) => reply != null && tester.VerifyAndExtractUdpResponse(reply);

try
{
    await SendUdpRequestAsync(host, port, timeout);
}
catch (Exception ex) when (ex.Message.Contains("verify and extract"))
{
    // reply was long enough but semantically invalid (wrong ID, RCODE, etc.)
}

When it happens

Trigger: After the length check passes, _udpTest.VerifyAndExtractUdpResponse(udpReceiveResult) returns false. E.g. DNS reply with transaction ID != 0x1234, a DNS error (RCODE!=0), or an answer count of 0.

Common situations: The reply came from a different source/cached resolver with a mismatched transaction ID; the target returned an error (e.g. DNS SERVFAIL/REFUSED, NTP version mismatch); the relay forwarded a reply to a different query; spoofed/stray UDP reached the relay; target rate-limited or refused the query.

Related errors

• Received NTP response is too short.
• Failed to build UDP request packet.
• Failed to establish UDP association with SOCKS5 proxy.
• Invalid SOCKS5 UDP packet: domain incomplete
• Invalid SOCKS5 UDP packet: domain length missing

AI-assisted analysis of 2dust/v2rayN@e01717d832 (2026-08-13). Data as JSON: /api/errors/1db789d61dd1c294. Report an issue: GitHub.

Appendix: source

Thrown at v2rayN/ServiceLib.UdpTest/UdpTestService.cs:151

                if (attempt == 1 && roundTripTime == TimeSpan.MaxValue)
                {
                    throw;
                }
            }
        }

        if ((udpReceiveResult?.Length ?? 0) < 4 + 1 + 4 + 2)
        {
            throw new Exception("Received NTP response is too short.");
        }

        if (udpReceiveResult != null && _udpTest.VerifyAndExtractUdpResponse(udpReceiveResult))
        {
            return roundTripTime;
        }
        else
        {
            throw new Exception("Failed to verify and extract UDP response.");
        }
    }
}

View on GitHub (pinned to e01717d832)