1: =pod
    2: 
    3: =head1 NAME
    4: 
    5: SSL_CTX_set_cert_verify_callback - set peer certificate verification procedure
    6: 
    7: =head1 SYNOPSIS
    8: 
    9:  #include <openssl/ssl.h>
   10: 
   11:  void SSL_CTX_set_cert_verify_callback(SSL_CTX *ctx, int (*callback)(X509_STORE_CTX *,void *), void *arg);
   12: 
   13: =head1 DESCRIPTION
   14: 
   15: SSL_CTX_set_cert_verify_callback() sets the verification callback function for
   16: I<ctx>. SSL objects that are created from I<ctx> inherit the setting valid at
   17: the time when L<SSL_new(3)|SSL_new(3)> is called.
   18: 
   19: =head1 NOTES
   20: 
   21: Whenever a certificate is verified during a SSL/TLS handshake, a verification
   22: function is called. If the application does not explicitly specify a
   23: verification callback function, the built-in verification function is used.
   24: If a verification callback I<callback> is specified via
   25: SSL_CTX_set_cert_verify_callback(), the supplied callback function is called
   26: instead. By setting I<callback> to NULL, the default behaviour is restored.
   27: 
   28: When the verification must be performed, I<callback> will be called with
   29: the arguments callback(X509_STORE_CTX *x509_store_ctx, void *arg). The 
   30: argument I<arg> is specified by the application when setting I<callback>.
   31: 
   32: I<callback> should return 1 to indicate verification success and 0 to
   33: indicate verification failure. If SSL_VERIFY_PEER is set and I<callback>
   34: returns 0, the handshake will fail. As the verification procedure may
   35: allow to continue the connection in case of failure (by always returning 1)
   36: the verification result must be set in any case using the B<error>
   37: member of I<x509_store_ctx> so that the calling application will be informed
   38: about the detailed result of the verification procedure! 
   39: 
   40: Within I<x509_store_ctx>, I<callback> has access to the I<verify_callback>
   41: function set using L<SSL_CTX_set_verify(3)|SSL_CTX_set_verify(3)>.
   42: 
   43: =head1 WARNINGS
   44: 
   45: Do not mix the verification callback described in this function with the
   46: B<verify_callback> function called during the verification process. The
   47: latter is set using the L<SSL_CTX_set_verify(3)|SSL_CTX_set_verify(3)>
   48: family of functions.
   49: 
   50: Providing a complete verification procedure including certificate purpose
   51: settings etc is a complex task. The built-in procedure is quite powerful
   52: and in most cases it should be sufficient to modify its behaviour using
   53: the B<verify_callback> function.
   54: 
   55: =head1 BUGS
   56: 
   57: =head1 RETURN VALUES
   58: 
   59: SSL_CTX_set_cert_verify_callback() does not provide diagnostic information.
   60: 
   61: =head1 SEE ALSO
   62: 
   63: L<ssl(3)|ssl(3)>, L<SSL_CTX_set_verify(3)|SSL_CTX_set_verify(3)>,
   64: L<SSL_get_verify_result(3)|SSL_get_verify_result(3)>,
   65: L<SSL_CTX_load_verify_locations(3)|SSL_CTX_load_verify_locations(3)>
   66: 
   67: =head1 HISTORY
   68: 
   69: Previous to OpenSSL 0.9.7, the I<arg> argument to B<SSL_CTX_set_cert_verify_callback>
   70: was ignored, and I<callback> was called simply as
   71:  int (*callback)(X509_STORE_CTX *)
   72: To compile software written for previous versions of OpenSSL, a dummy
   73: argument will have to be added to I<callback>.
   74: 
   75: =cut