The use of HTTP proxies can be controlled by environment variables or API calls.

The MQTTAsync_connectOptions.httpProxy and MQTTAsync_connectOptions.httpsProxy fields of the MQTTAsync_connectOptions structure override any settings in the environment.

If the environment variable PAHO_C_CLIENT_USE_HTTP_PROXY is set to TRUE, then the http_proxy or https_proxy (lower case only) environment variables are used, for plain TCP and TLS-secured connections respectively.

The no_proxy environment variable can be used to exclude certain hosts from using an environment variable chosen proxy. This does not apply to a proxy selected through the API. The no_proxy environment variable is lower case only, and is a list of comma-separated hostname:port values. Suffixes are matched (e.g. example.com will match test.example.com).