作者：ptonlix
來源：CSDN
原文：https://blog.csdn.net/ptonlix/article/details/79869756
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本人轉載。

mosquitto_pub&mosquitto_sub

在實現MQTT client之前，我們可以先使用mosquitto_pub和mosquitto_sub模擬，MQTT的發布和訂閱

打開三個終端，分別作為：
MQTT服務器，訂閱者，發布者

MQTT服務器

mosquitto -c mosquitto.conf

運行服務

訂閱

mosquitto_sub -t mtopic -u sub_client -P 123456 -v --cafile cacert.pem --cert client.crt --key client.key -p 8883 --tls-version tlsv1.2 --insecure
–cafile:加載ca證書的路徑，后面以此類推

image.png

上圖所示，連接成功的情況。

發布

mosquitto_pub -t mtopic -u pub_client -P 123456 -m "test" --cafile cacert.pem --cert client.crt --key client.key -p 8883 --tls-version tlsv1.2 --insecure

image.png

如上圖所示，消息發布成功，然后訂閱也成功，主題為mtopic,消息是test

使用C語言實現MQTT Client

這里我使用了兩個開源庫，libemqtt和mbedtls，感謝這兩個開源庫的作者，感謝開源~
https://github.com/menudoproblema/libemqtt.git
https://github.com/ARMmbed/mbedtls.git

代碼分析
初始化操作

 /*
     * 0. Initialize the RNG and the session data
     */
    mbedtls_net_init( &server_fd );
    mbedtls_ssl_init( &ssl );
    mbedtls_ssl_config_init( &conf );
    mbedtls_x509_crt_init( &cacert );
    mbedtls_x509_crt_init( &clicert );
    mbedtls_pk_init( &pkey );
    mbedtls_ctr_drbg_init( &ctr_drbg );

    mbedtls_printf( "\n  . Seeding the random number generator..." );
    fflush( stdout );

    mbedtls_entropy_init( &entropy );
    if( ( ret = mbedtls_ctr_drbg_seed( &ctr_drbg, mbedtls_entropy_func, &entropy,
                               (const unsigned char *) pers,
                               strlen( pers ) ) ) != 0 )
    {
        mbedtls_printf( " failed\n  ! mbedtls_ctr_drbg_seed returned %d\n", ret );
        goto exit;
    }

    mbedtls_printf( " ok\n" );

加載證書

    ret = mbedtls_x509_crt_parse_file( &cacert, "./cacert.pem");
    if( ret < 0 )
    {
        mbedtls_printf( " failed\n  !  mbedtls_x509_crt_parse returned -0x%x\n\n", -ret );
        goto exit;
    }

    mbedtls_printf( " ok (%d skipped)\n", ret );

    ret = mbedtls_x509_crt_parse_file( &clicert, "./client.crt");
    if( ret != 0 )
    {
        mbedtls_printf( " failed\n  !  mbedtls_x509_crt_parse returned %d\n\n", ret );
        goto exit;
    }

     ret = mbedtls_pk_parse_keyfile( &pkey, "./client.key", "123456");
    if( ret != 0 )
    {
        mbedtls_printf( " failed\n  !  mbedtls_pk_parse_key returned %d\n\n", ret );
        goto exit;
    }

mbedtls_pk_parse_keyfile 第三個參數是生成客戶端證書，自己輸入的密鑰

建立TCP連接

  /*
     * 1. Start the connection
     */
    mbedtls_printf( "  . Connecting to tcp/%s/%s...", SERVER_NAME, SERVER_PORT );
    fflush( stdout );

    if( ( ret = mbedtls_net_connect( &server_fd, SERVER_NAME,
                                         SERVER_PORT, MBEDTLS_NET_PROTO_TCP ) ) != 0 )
    {
        mbedtls_printf( " failed\n  ! mbedtls_net_connect returned %d\n\n", ret );
        goto exit;
    }

TLS握手

/*
     * 4. Handshake
     */
    mbedtls_printf( "  . Performing the SSL/TLS handshake..." );
    fflush( stdout );

    while( ( ret = mbedtls_ssl_handshake( &ssl ) ) != 0 )
    {
        if( ret != MBEDTLS_ERR_SSL_WANT_READ && ret != MBEDTLS_ERR_SSL_WANT_WRITE )
        {
            mbedtls_printf( " failed\n  ! mbedtls_ssl_handshake returned -0x%x\n\n", -ret );
            goto exit;
        }
    }

    mbedtls_printf( " ok\n" );

握手階段結束，就完成了TLS連接，開始加密通訊，下面就是應用層MQTT的連接

MQTT連接
mbedtls_printf(" \n.....................mqtttenter.....................\n" );
    PTmqtttenter();
    mqtt_connect((mqtt_broker_handle_t *)PTMqttGetBroker());
    mbedtls_printf( "\n....................mqtt_connect.....................\n" );
    mqtt_subscribe((mqtt_broker_handle_t *)PTMqttGetBroker(), MQTTSUBTOPIC, 0);
    mqtt_publish((mqtt_broker_handle_t *)PTMqttGetBroker(), MQTTPUBTOPIC, test, strlen(test), 0);

這樣就完成MQTT的連接，我們來看下實際效果

如上圖，我發送的MQTT消息，主題為mtopic 消息為test_cfd
訂閱的終端，可以成功收到消息。

注意：程序運行加載的證書是在工程目錄下，并且需要各位根據自己生成的客戶端證書進行相應的替換。

github link
最后附上，本工程的github船票：
https://github.com/ptonlix/MQTTWithTLS