gnutls_handshake_set_post_client_hello_function(3) gnutls gnutls_handshake_set_post_client_hello_function(3)

gnutls_handshake_set_post_client_hello_function - API function

#include <gnutls/gnutls.h>

void gnutls_handshake_set_post_client_hello_function(gnutls_session_t session, gnutls_handshake_simple_hook_func func);

is a gnutls_session_t type.
is the function to be called

This function will set a callback to be called after the client hello has been received (callback valid in server side only). This allows the server to adjust settings based on received extensions.

Those settings could be ciphersuites, requesting certificate, or anything else except for version negotiation (this is done before the hello message is parsed).

This callback must return 0 on success or a gnutls error code to terminate the handshake.

Since GnuTLS 3.3.5 the callback is allowed to return GNUTLS_E_AGAIN or GNUTLS_E_INTERRUPTED to put the handshake on hold. In that case gnutls_handshake() will return GNUTLS_E_INTERRUPTED and can be resumed when needed.

You should not use this function to terminate the handshake based on client input unless you know what you are doing. Before the handshake is finished there is no way to know if there is a man-in-the-middle attack being performed.

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