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.

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)

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}).");
}
Defensive patterns

Strategy: try-catch

Validate before calling

// 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}).");

Type guard

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

Try / catch

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.)
}

Prevention

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

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