SSL_CTX_set_cert_verify_callback — set peer certificate verification procedure
SSL_CTX_set_cert_verify_callback(SSL_CTX *ctx, int (*callback)(X509_STORE_CTX
*,void *), void *arg);
SSL_CTX_set_cert_verify_callback() sets the verification callback
function for ctx. SSL objects that are created from ctx inherit the setting valid at the time when SSL_new(3) is
Whenever a certificate is verified during a SSL/TLS handshake,
a verification function is called. If the application does not explicitly
specify a verification callback function, the built-in verification
function is used. If a verification callback callback is specified via SSL_CTX_set_cert_verify_callback(),
the supplied callback function is called instead. By setting callback to NULL, the default behaviour is restored.
When the verification must be performed, callback will be called with the arguments callback(X509_STORE_CTX
*x509_store_ctx, void *arg). The argument arg is specified by the application when setting callback.
callback should return 1 to indicate verification success
and 0 to indicate verification failure. If SSL_VERIFY_PEER is set
and callback returns 0, the handshake will fail. As the verification
procedure may allow to continue the connection in case of failure
(by always returning 1) the verification result must be set in any
case using the error member of x509_store_ctx so that the calling application will be informed about
the detailed result of the verification procedure!
Within x509_store_ctx, callback has access to the verify_callback function set using SSL_CTX_set_verify(3).
Do not mix the verification callback described in this function
with the verify_callback function called during the verification
process. The latter is set using the SSL_CTX_set_verify(3) family of functions.
Providing a complete verification procedure including certificate
purpose settings etc is a complex task. The built-in procedure is
quite powerful and in most cases it should be sufficient to modify
its behaviour using the verify_callback function.
SSL_CTX_set_cert_verify_callback() does not provide diagnostic
ssl(3), SSL_CTX_set_verify(3), SSL_get_verify_result(3), SSL_CTX_load_verify_locations(3)
Previous to OpenSSL 0.9.7, the arg argument to SSL_CTX_set_cert_verify_callback was
ignored, and callback was called simply as int (*callback)(X509_STORE_CTX
*) To compile software written for previous versions of OpenSSL,
a dummy argument will have to be added to callback.