SSL_CTX_set_tlsext_servername_callback
if (SSL_CTX_set_tlsext_servername_callback(*ssl_ctx,
SSL_CTX_set_tlsext_servername_callback(ctx, ssl_servername_cb);
SSL_CTX_set_tlsext_servername_callback(ctx2, ssl_servername_cb);
SSL_CTX_set_tlsext_servername_callback(ctx, ssl_servername_cb);