NAME
SSL_CTX_set_cert_verify_callback —
set peer certificate verification
procedure
SYNOPSIS
#include
<openssl/ssl.h>
void
SSL_CTX_set_cert_verify_callback(SSL_CTX
*ctx, int (*callback)(X509_STORE_CTX *, void *),
void *arg);
DESCRIPTION
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 called.
NOTES
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 the connection to continue 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).
SEE ALSO
ssl(3), SSL_CTX_load_verify_locations(3), SSL_CTX_set_verify(3), SSL_get_verify_result(3)
HISTORY
SSL_CTX_set_cert_verify_callback() first
appeared in SSLeay 0.6.1 and has been available since
OpenBSD 2.4.
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.
CAVEATS
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.
BUGS
SSL_CTX_set_cert_verify_callback() does
not provide diagnostic information.