see NSURLError.h Define
NSURLErrorUnknown = -1, NSURLErrorCancelled = -999, NSURLErrorBadURL = -1000, NSURLErrorTimedOut = -1001, NSURLErrorUnsupportedURL = -1002, NSURLErrorCannotFindHost = -1003, NSURLErrorCannotConnectToHost = -1004, NSURLErrorNetworkConnectionLost = -1005, NSURLErrorDNSLookupFailed = -1006, NSURLErrorHTTPTooManyRedirects = -1007, NSURLErrorResourceUnavailable = -1008, NSURLErrorNotConnectedToInternet = -1009, NSURLErrorRedirectToNonExistentLocation = -1010, NSURLErrorBadServerResponse = -1011, NSURLErrorUserCancelledAuthentication = -1012, NSURLErrorUserAuthenticationRequired = -1013, NSURLErrorZeroByteResource = -1014, NSURLErrorCannotDecodeRawData = -1015, NSURLErrorCannotDecodeContentData = -1016, NSURLErrorCannotParseResponse = -1017, NSURLErrorAppTransportSecurityRequiresSecureConnection NS_ENUM_AVAILABLE(10_11, 9_0) = -1022, NSURLErrorFileDoesNotExist = -1100, NSURLErrorFileIsDirectory = -1101, NSURLErrorNoPermissionsToReadFile = -1102, NSURLErrorDataLengthExceedsMaximum NS_ENUM_AVAILABLE(10_5, 2_0) = -1103, // SSL errors NSURLErrorSecureConnectionFailed = -1200, NSURLErrorServerCertificateHasBadDate = -1201, NSURLErrorServerCertificateUntrusted = -1202, NSURLErrorServerCertificateHasUnknownRoot = -1203, NSURLErrorServerCertificateNotYetValid = -1204, NSURLErrorClientCertificateRejected = -1205, NSURLErrorClientCertificateRequired = -1206, NSURLErrorCannotLoadFromNetwork = -2000, // Download and file I/O errors NSURLErrorCannotCreateFile = -3000, NSURLErrorCannotOpenFile = -3001, NSURLErrorCannotCloseFile = -3002, NSURLErrorCannotWriteToFile = -3003, NSURLErrorCannotRemoveFile = -3004, NSURLErrorCannotMoveFile = -3005, NSURLErrorDownloadDecodingFailedMidStream = -3006, NSURLErrorDownloadDecodingFailedToComplete =-3007, NSURLErrorInternationalRoamingOff NS_ENUM_AVAILABLE(10_7, 3_0) = -1018, NSURLErrorCallIsActive NS_ENUM_AVAILABLE(10_7, 3_0) = -1019, NSURLErrorDataNotAllowed NS_ENUM_AVAILABLE(10_7, 3_0) = -1020, NSURLErrorRequestBodyStreamExhausted NS_ENUM_AVAILABLE(10_7, 3_0) = -1021, NSURLErrorBackgroundSessionRequiresSharedContainer NS_ENUM_AVAILABLE(10_10, 8_0) = -995, NSURLErrorBackgroundSessionInUseByAnotherProcess NS_ENUM_AVAILABLE(10_10, 8_0) = -996, NSURLErrorBackgroundSessionWasDisconnected NS_ENUM_AVAILABLE(10_10, 8_0)= -997,
参考链接: