#include <openssl/ssl.h> void SSL_CTX_set_cert_verify_callback(SSL_CTX *ctx, int (*callback)(X509_STORE_CTX *, void *), void *arg);
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 the connection to continue in the 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).
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.
Licensed under the OpenSSL license (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at https://www.openssl.org/source/license.html.