Adjusted default value for the supported Net frameworks versions where "None" is incompatible. (#1348)

Build/MQTTnet.nuspec

@@ -16,6 +16,8 @@
 * [Core] Decreased object allocations (#1324, thanks to @gfoidl).
 * [Core] Decreased object allocations when logging is not active (thanks to @gfoidl, @Tymoniden).
 * [Client] Fixed issue in _MqttApplicationMessageBuilder.WithPayload_ (#1322, thanks to @gfoidl).
+* [Client] Adjusted default SslProtocol values to Tls12 and Tls13 (#1347).
+* [Extensions.WebSocket4Net] Adjusted default SslProtocol values to Tls12 and Tls13 (#1347).
 
 Git commit: $gitCommit
     </releaseNotes>

Source/MQTTnet.Extensions.WebSocket4Net/WebSocket4NetMqttChannel.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Linq;
@@ -52,7 +52,12 @@ namespace MQTTnet.Extensions.WebSocket4Net
                 }
             }
 
-            var sslProtocols = _webSocketOptions?.TlsOptions?.SslProtocol ?? SslProtocols.None;
+#if NET48 || NETCOREAPP3_1 || NET5 || NET6
+            var sslProtocols = _webSocketOptions?.TlsOptions.SslProtocol ?? SslProtocols.Tls12 | SslProtocols.Tls13;
+#else
+            var sslProtocols = _webSocketOptions?.TlsOptions.SslProtocol ?? SslProtocols.Tls12 | (SslProtocols)0x00003000 /*Tls13*/;
+#endif
+
             var subProtocol = _webSocketOptions.SubProtocols.FirstOrDefault() ?? string.Empty;
 
             var cookies = new List<KeyValuePair<string, string>>();

Source/MQTTnet/Client/Options/MqttClientOptionsBuilderTlsParameters.cs

@@ -19,7 +19,11 @@ namespace MQTTnet.Client.Options
 
         public Func<MqttClientCertificateValidationCallbackContext, bool> CertificateValidationHandler { get; set; }
 
-        public SslProtocols SslProtocol { get; set; } = SslProtocols.None;
+#if NET48 || NETCOREAPP3_1 || NET5 || NET6
+        public SslProtocols SslProtocol { get; set; } = SslProtocols.Tls12 | SslProtocols.Tls13;
+#else
+        public SslProtocols SslProtocol { get; set; } = SslProtocols.Tls12 | (SslProtocols)0x00003000 /*Tls13*/;
+#endif
 
 #if WINDOWS_UWP
         public IEnumerable<IEnumerable<byte>> Certificates { get; set; }

Source/MQTTnet/Client/Options/MqttClientTlsOptions.cs

@@ -26,7 +26,11 @@ namespace MQTTnet.Client.Options
         public List<SslApplicationProtocol> ApplicationProtocols { get; set; }
 #endif
 
-        public SslProtocols SslProtocol { get; set; } = SslProtocols.None;
+#if NET48 || NETCOREAPP3_1 || NET5 || NET6
+        public SslProtocols SslProtocol { get; set; } = SslProtocols.Tls12 | SslProtocols.Tls13;
+#else
+        public SslProtocols SslProtocol { get; set; } = SslProtocols.Tls12 | (SslProtocols)0x00003000 /*Tls13*/;
+#endif
 
         [Obsolete("This property will be removed soon. Use CertificateValidationHandler instead.")]
         public Func<X509Certificate, X509Chain, SslPolicyErrors, IMqttClientOptions, bool> CertificateValidationCallback { get; set; }