k is mostly superseded by the ClientHello callback, which can be set using SSL_CTX_set_client_hello_cb(). However, even where the ClientHello callback is used, the servername callback is still necessary in order to acknowledge the servername requested by the client.</p>

<p>SSL_CTX_set_tlsext_servername_callback() sets the application callback <b>cb</b> used by a server to perform any actions or configuration required based on the servername extension received in the incoming connection. When <b>cb</b> is N