{"record":{"id":"1db789d61dd1c294","repo":"2dust/v2rayN","slug":"failed-to-verify-and-extract-udp-response","errorCode":null,"errorMessage":"Failed to verify and extract UDP response.","messagePattern":"Failed to verify and extract UDP response\\.","errorType":"exception","errorClass":"Exception","httpStatus":null,"severity":"error","filePath":"v2rayN/ServiceLib.UdpTest/UdpTestService.cs","lineNumber":151,"sourceCode":"                if (attempt == 1 && roundTripTime == TimeSpan.MaxValue)\n                {\n                    throw;\n                }\n            }\n        }\n\n        if ((udpReceiveResult?.Length ?? 0) < 4 + 1 + 4 + 2)\n        {\n            throw new Exception(\"Received NTP response is too short.\");\n        }\n\n        if (udpReceiveResult != null && _udpTest.VerifyAndExtractUdpResponse(udpReceiveResult))\n        {\n            return roundTripTime;\n        }\n        else\n        {\n            throw new Exception(\"Failed to verify and extract UDP response.\");\n        }\n    }\n}\n","sourceCodeStart":133,"sourceCodeEnd":155,"githubUrl":"https://github.com/2dust/v2rayN/blob/e01717d8326a4f5060b335523590c5fda943fe03/v2rayN/ServiceLib.UdpTest/UdpTestService.cs#L133-L155","documentation":"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.","triggerScenarios":"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.","commonSituations":"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.","solutions":["Confirm the request packet's identifier matches what the tester validates (DnsService expects transaction ID 0x1234).","Try a different target (e.g. 1.1.1.1 for DNS) to rule out a target-side refusal/RCODE.","Log the received bytes to see the actual flags/RCODE/answer-count rather than just 'failed'.","Ensure no other process is generating traffic on the same relay that could mismatch the transaction ID."],"exampleFix":"// before - boolean verify hides the reason\nif (udpReceiveResult != null && _udpTest.VerifyAndExtractUdpResponse(udpReceiveResult))\n    return roundTripTime;\nelse\n    throw new Exception(\"Failed to verify and extract UDP response.\");\n\n// after - capture why it failed for diagnostics\nif (udpReceiveResult == null || !_udpTest.VerifyAndExtractUdpResponse(udpReceiveResult))\n{\n    var hex = udpReceiveResult != null ? Convert.ToHexString(udpReceiveResult, 0, Math.Min(32, udpReceiveResult.Length)) : \"<null>\";\n    throw new Exception($\"UDP response failed semantic validation (tester={_udpTest.GetType().Name}, head={hex}).\");\n}","handlingStrategy":"try-catch","validationCode":"// Validate reply semantics with detail before relying on the boolean\nif (udpReceiveResult == null || !_udpTest.VerifyAndExtractUdpResponse(udpReceiveResult))\n    throw new Exception($\"UDP response failed validation (tester={_udpTest.GetType().Name}).\");","typeGuard":"static bool IsTesterValidatedResponse(IUdpTest tester, byte[] reply) => reply != null && tester.VerifyAndExtractUdpResponse(reply);","tryCatchPattern":"try\n{\n    await SendUdpRequestAsync(host, port, timeout);\n}\ncatch (Exception ex) when (ex.Message.Contains(\"verify and extract\"))\n{\n    // reply was long enough but semantically invalid (wrong ID, RCODE, etc.)\n}","preventionTips":["Ensure the request packet's identifier matches what the tester validates (DNS expects 0x1234).","Try alternate targets to rule out target-side refusal/RCODE.","Log reply bytes to see flags/RCODE/answer-count.","Avoid sharing the relay socket with other traffic that could mismatch the ID."],"tags":["udp-test","dns","response-validation","network"],"backgroundTag":null,"analyzedSha":"e01717d8326a4f5060b335523590c5fda943fe03","analyzedAt":"2026-08-13T10:10:23.416Z","schemaVersion":2},"datasetVersion":"2026-08-13T14:17:21.547Z"}