{"record":{"id":"e3bbcd8d94eb8486","repo":"TechnitiumSoftware/DnsServer","slug":"port-53-cannot-be-used-for-dns-over-https-service","errorCode":null,"errorMessage":"Port 53 cannot be used for DNS-over-HTTPS service. Please use a different port.","messagePattern":"Port 53 cannot be used for DNS-over-HTTPS service\\. Please use a different port\\.","errorType":"exception","errorClass":"ArgumentOutOfRangeException","httpStatus":null,"severity":"error","filePath":"DnsServerCore/Dns/DnsServer.cs","lineNumber":7789,"sourceCode":"                    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.\");\n\n                _dnsOverHttpsPort = value;\n            }\n        }\n\n        public int DnsOverQuicPort\n        {\n            get { return _dnsOverQuicPort; }\n            set\n            {\n                if ((value < ushort.MinValue) || (value > ushort.MaxValue))\n                    throw new ArgumentOutOfRangeException(nameof(DnsOverQuicPort), \"Port number valid range is from 0 to 65535.\");\n\n                if (value == 53)\n                    throw new ArgumentOutOfRangeException(nameof(DnsOverQuicPort), \"Port 53 cannot be used for DNS-over-QUIC service. Please use a different port.\");","sourceCodeStart":7771,"sourceCodeEnd":7807,"githubUrl":"https://github.com/TechnitiumSoftware/DnsServer/blob/d0484b6c1e7439cdc53d67d81e9c876cda2ad756/DnsServerCore/Dns/DnsServer.cs#L7771-L7807","documentation":"Thrown by the DnsOverHttpsPort setter when value == 53 (second guard, after the range check 314 passes). Port 53 is reserved for plain DNS, so DNS-over-HTTPS must use a different port.","triggerScenarios":"Assigning DnsServer.DnsOverHttpsPort = 53 via the settings API or config restore. Range guard passes; this guard then fires.","commonSituations":"Assuming DoH (HTTPS) can share port 53 with plain DNS. Config migration leaving the value at 53. Confusing plain DNS port with the DoH port.","solutions":["Use 443 or another non-53 port for DoH.","Keep plain DNS on 53 and run DoH on a dedicated port.","Pre-validate the DoH port is not 53 (or 853) before assigning.","Re-submit the corrected port via the settings API."],"exampleFix":"// before\n_dnsServer.DnsOverHttpsPort = 53;  // throws: 53 reserved for plain DNS\n\n// after\n_dnsServer.DnsOverHttpsPort = 443;  // conventional HTTPS port","handlingStrategy":"validation","validationCode":"if (parsed == 53) parsed = 443; // 53 reserved for plain DNS\n_dnsServer.DnsOverHttpsPort = parsed;","typeGuard":"static bool IsAcceptableDohsPort(int value) => value != 53 && value != 853 && value >= 0 && value <= 65535;","tryCatchPattern":"try { _dnsServer.DnsOverHttpsPort = parsed; }\ncatch (ArgumentOutOfRangeException ex) when (ex.ParamName == nameof(DnsServer.DnsOverHttpsPort) && parsed == 53)\n{ _dnsServer.DnsOverHttpsPort = 443; }","preventionTips":["Never reuse 53 for DoH (HTTPS).","Map migrated 53 values to 443 before assignment.","Reserve 53 for plain DNS."],"tags":["dns","network","port","dnsoverhttps","reserved-port","configuration","argumentoutofrange"],"backgroundTag":null,"analyzedSha":"d0484b6c1e7439cdc53d67d81e9c876cda2ad756","analyzedAt":"2026-08-13T22:57:35.508Z","schemaVersion":2},"datasetVersion":"2026-08-14T05:17:29.042Z"}