{"record":{"id":"fcc3138e5e5a0921","repo":"TechnitiumSoftware/DnsServer","slug":"port-53-cannot-be-used-for-dns-over-tls-service-p","errorCode":null,"errorMessage":"Port 53 cannot be used for DNS-over-TLS service. Please use a different port.","messagePattern":"Port 53 cannot be used for DNS-over-TLS service\\. Please use a different port\\.","errorType":"exception","errorClass":"ArgumentOutOfRangeException","httpStatus":null,"severity":"error","filePath":"DnsServerCore/Dns/DnsServer.cs","lineNumber":7774,"sourceCode":"            set\n            {\n                if (string.IsNullOrWhiteSpace(value))\n                    value = null;\n\n                _dnsOverHttpsUnixSocket = value;\n            }\n        }\n\n        public int DnsOverTlsPort\n        {\n            get { return _dnsOverTlsPort; }\n            set\n            {\n                if ((value < ushort.MinValue) || (value > ushort.MaxValue))\n                    throw new ArgumentOutOfRangeException(nameof(DnsOverTlsPort), \"Port number valid range is from 0 to 65535.\");\n\n                if (value == 53)\n                    throw new ArgumentOutOfRangeException(nameof(DnsOverTlsPort), \"Port 53 cannot be used for DNS-over-TLS service. Please use a different port.\");\n\n                _dnsOverTlsPort = value;\n            }\n        }\n\n        public int DnsOverHttpsPort\n        {\n            get { return _dnsOverHttpsPort; }\n            set\n            {\n                if ((value < ushort.MinValue) || (value > ushort.MaxValue))\n                    throw new ArgumentOutOfRangeException(nameof(DnsOverHttpsPort), \"Port number valid range is from 0 to 65535.\");\n\n                if (value == 53)\n                    throw new ArgumentOutOfRangeException(nameof(DnsOverHttpsPort), \"Port 53 cannot be used for DNS-over-HTTPS service. Please use a different port.\");\n\n                if (value == 853)\n                    throw new ArgumentOutOfRangeException(nameof(DnsOverHttpsPort), \"Port 853 is reserved for DNS-over-TLS service. Please use a different port for DNS-over-HTTPS service.\");","sourceCodeStart":7756,"sourceCodeEnd":7792,"githubUrl":"https://github.com/TechnitiumSoftware/DnsServer/blob/d0484b6c1e7439cdc53d67d81e9c876cda2ad756/DnsServerCore/Dns/DnsServer.cs#L7756-L7792","documentation":"Thrown by the DnsOverTlsPort setter when value == 53, after the range check (312) passes. Port 53 is reserved for plain DNS; DNS-over-TLS must use a different port (853 by convention). The server enforces this separation explicitly.","triggerScenarios":"Assigning DnsServer.DnsOverTlsPort = 53 via the settings API or config restore. The range guard passes; this dedicated guard then fires.","commonSituations":"Assuming TLS DNS can run on 53 alongside plain DNS. Config migration that left the DoT port at 53. UI mix-up between the standard DNS port and the DoT port.","solutions":["Use the conventional DoT port 853 (or another non-53 port).","Keep plain DNS on 53 and run DoT on a dedicated port.","Pre-validate the DoT port is not 53 before assigning.","Re-submit a corrected value via the settings API."],"exampleFix":"// before\n_dnsServer.DnsOverTlsPort = 53;  // throws: 53 reserved for plain DNS\n\n// after\n_dnsServer.DnsOverTlsPort = 853;  // conventional DoT port","handlingStrategy":"validation","validationCode":"if (parsed == 53) parsed = 853; // 53 reserved for plain DNS\n_dnsServer.DnsOverTlsPort = parsed;","typeGuard":"static bool IsAcceptableDoTPort(int value) => value != 53 && value >= 0 && value <= 65535;","tryCatchPattern":"try { _dnsServer.DnsOverTlsPort = parsed; }\ncatch (ArgumentOutOfRangeException ex) when (ex.ParamName == nameof(DnsServer.DnsOverTlsPort) && parsed == 53)\n{ _dnsServer.DnsOverTlsPort = 853; }","preventionTips":["Never use 53 for DoT.","Default migrated 53 values to 853.","Reserve 53 for the plain DNS listener."],"tags":["dns","network","port","dnsovertls","reserved-port","configuration","argumentoutofrange"],"backgroundTag":null,"analyzedSha":"d0484b6c1e7439cdc53d67d81e9c876cda2ad756","analyzedAt":"2026-08-13T22:57:35.508Z","schemaVersion":2},"datasetVersion":"2026-08-14T00:17:13.853Z"}