An existing connection was forcibly disconnected by an external host.

[MrSonay]

HI

My connection fails when connecting to the MQTT broker. This happens even though the code is pretty much copied from the example that has been provided on this site. The exception that I'm getting: MQTTnet.Exceptions.MqttCommunicationException: 'An existing connection was forcibly disconnected by an external host.'

Can you guys see what I'm doing wrong here? I'm using https://test.mosquitto.org/ as the broker to test it against. Here is the link: https://test.mosquitto.org/

I'm using MQTTnet version 3.0.13.

 // Create a new MQTT client.
                var factory = new MqttFactory();
                var mqttClient = factory.CreateMqttClient();

                // Load certificate
                var certificatePath = @"..\mosquitto.org.crt";

                // Create TLS based parameters.
                var tlsParameters = new MqttClientOptionsBuilderTlsParameters
                {
                    Certificates = new List<X509Certificate> { X509Certificate.CreateFromCertFile(certificatePath) },
                    IgnoreCertificateRevocationErrors = true,
                    IgnoreCertificateChainErrors = false,
                    AllowUntrustedCertificates = false,
                    //SslProtocol = System.Security.Authentication.SslProtocols.Tls12
                }; 

                // Create TCP based options using the builder.
                var connectOptions = new MqttClientOptionsBuilder()
                    .WithClientId("Client1")
                    .WithTcpServer("test.mosquitto.org", 8883)
                    .WithTls(tlsParameters)
                    //.WithProtocolVersion(MQTTnet.Formatter.MqttProtocolVersion.V311)
                    .WithCleanSession()
                    .Build();

                var conResult = await mqttClient.ConnectAsync(connectOptions);

[MrSonay]

The problem has been fixed and the solution was to use UseTls = true in MqttClientOptionsBuilderTlsParameters - which I forgot.