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            #include <openssl/ssl.h>
            void SSL_CTX_set_verify(SSL_CTX *ctx, int mode,
                                    int (*verify_callback)(int, X509_STORE_CTX *));
            void SSL_set_verify(SSL *s, int mode,
                                int (*verify_callback)(int, X509_STORE_CTX *));
            void SSL_CTX_set_verify_depth(SSL_CTX *ctx,int depth);
            void SSL_set_verify_depth(SSL *s, int depth);
            int verify_callback(int preverify_ok, X509_STORE_CTX *x509_ctx);


           SSL_CTX_set_verify() sets the verification flags for ctx to be mode and
           specifies the verify_callback function to be used. If no callback
           function shall be specified, the NULL pointer can be used for
           SSL_set_verify() sets the verification flags for ssl to be mode and
           specifies the verify_callback function to be used. If no callback
           function shall be specified, the NULL pointer can be used for
           verify_callback. In this case last verify_callback set specifically for
           this ssl remains. If no special callback was set before, the default
           callback for the underlying ctx is used, that was valid at the time ssl
           was created with SSL_new(3).
           SSL_CTX_set_verify_depth() sets the maximum depth for the certificate
           chain verification that shall be allowed for ctx. (See the BUGS
           SSL_set_verify_depth() sets the maximum depth for the certificate chain
           verification that shall be allowed for ssl. (See the BUGS section.)


           The verification of certificates can be controlled by a set of
           logically or'ed mode flags:
               Server mode: the server will not send a client certificate request
               to the client, so the client will not send a certificate.
               Client mode: if not using an anonymous cipher (by default
               disabled), the server will send a certificate which will be
               checked. The result of the certificate verification process can be
               checked after the TLS/SSL handshake using the
               SSL_get_verify_result(3) function.  The handshake will be continued
               regardless of the verification result.
               Server mode: the server sends a client certificate request to the
               client.  The certificate returned (if any) is checked. If the
               TLS/SSL handshake is immediately terminated with a "handshake
               failure" alert.  This flag must be used together with
               Client mode: ignored
               Server mode: only request a client certificate on the initial
               TLS/SSL handshake. Do not ask for a client certificate again in
               case of a renegotiation. This flag must be used together with
               Client mode: ignored
           Exactly one of the mode flags SSL_VERIFY_NONE and SSL_VERIFY_PEER must
           be set at any time.
           The actual verification procedure is performed either using the built-
           in verification procedure or using another application provided
           verification function set with SSL_CTX_set_cert_verify_callback(3).
           The following descriptions apply in the case of the built-in procedure.
           An application provided procedure also has access to the verify depth
           information and the verify_callback() function, but the way this
           information is used may be different.
           SSL_CTX_set_verify_depth() and SSL_set_verify_depth() set the limit up
           to which depth certificates in a chain are used during the verification
           procedure. If the certificate chain is longer than allowed, the
           certificates above the limit are ignored. Error messages are generated
           as if these certificates would not be present, most likely a
           X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT_LOCALLY will be issued.  The depth
           count is "level 0:peer certificate", "level 1: CA certificate", "level
           2: higher level CA certificate", and so on. Setting the maximum depth
           to 2 allows the levels 0, 1, and 2. The default depth limit is 9,
           allowing for the peer certificate and additional 9 CA certificates.
           The verify_callback function is used to control the behaviour when the
           SSL_VERIFY_PEER flag is set. It must be supplied by the application and
           receives two arguments: preverify_ok indicates, whether the
           verification of the certificate in question was passed (preverify_ok=1)
           or not (preverify_ok=0). x509_ctx is a pointer to the complete context
           used for the certificate chain verification.
           The certificate chain is checked starting with the deepest nesting
           level (the root CA certificate) and worked upward to the peer's
           certificate.  At each level signatures and issuer attributes are
           checked. Whenever a verification error is found, the error number is
           stored in x509_ctx and verify_callback is called with preverify_ok=0.
           By applying X509_CTX_store_* functions verify_callback can locate the
           certificate in question and perform additional steps (see EXAMPLES). If
           no error is found for a certificate, verify_callback is called with
           preverify_ok=1 before advancing to the next level.
           Its return value is identical to preverify_ok, so that any verification
           failure will lead to a termination of the TLS/SSL handshake with an
           alert message, if SSL_VERIFY_PEER is set.


           In client mode, it is not checked whether the SSL_VERIFY_PEER flag is
           set, but whether SSL_VERIFY_NONE is not set. This can lead to
           unexpected behaviour, if the SSL_VERIFY_PEER and SSL_VERIFY_NONE are
           not used as required (exactly one must be set at any time).
           The certificate verification depth set with SSL[_CTX]_verify_depth()
           stops the verification at a certain depth. The error message produced
           will be that of an incomplete certificate chain and not
           X509_V_ERR_CERT_CHAIN_TOO_LONG as may be expected.


           The SSL*_set_verify*() functions do not provide diagnostic information.


           The following code sequence realizes an example verify_callback
           function that will always continue the TLS/SSL handshake regardless of
           verification failure, if wished. The callback realizes a verification
           depth limit with more informational output.
           All verification errors are printed, informations about the certificate
           chain are printed on request.  The example is realized for a server
           that does allow but not require client certificates.
           The example makes use of the ex_data technique to store application
           data into/retrieve application data from the SSL structure (see
           SSL_get_ex_new_index(3), SSL_get_ex_data_X509_STORE_CTX_idx(3)).
            typedef struct {
              int verbose_mode;
              int verify_depth;
              int always_continue;
            } mydata_t;
            int mydata_index;
            static int verify_callback(int preverify_ok, X509_STORE_CTX *ctx)
               char    buf[256];
               X509   *err_cert;
               int     err, depth;
               SSL    *ssl;
               mydata_t *mydata;
               err_cert = X509_STORE_CTX_get_current_cert(ctx);
               err = X509_STORE_CTX_get_error(ctx);
               depth = X509_STORE_CTX_get_error_depth(ctx);
                * We must do it here, because the CHAIN_TOO_LONG error would not
                * be found explicitly; only errors introduced by cutting off the
                * additional certificates would be logged.
               if (depth > mydata->verify_depth) {
                   preverify_ok = 0;
                   err = X509_V_ERR_CERT_CHAIN_TOO_LONG;
                   X509_STORE_CTX_set_error(ctx, err);
               if (!preverify_ok) {
                   printf("verify error:num=%d:%s:depth=%d:%s\n", err,
                            X509_verify_cert_error_string(err), depth, buf);
               else if (mydata->verbose_mode)
                   printf("depth=%d:%s\n", depth, buf);
                * At this point, err contains the last verification error. We can use
                * it for something special
               if (!preverify_ok && (err == X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT))
                 X509_NAME_oneline(X509_get_issuer_name(ctx->current_cert), buf, 256);
                 printf("issuer= %s\n", buf);
               if (mydata->always_continue)
                 return 1;
                 return preverify_ok;
            mydata_t mydata;
            mydata_index = SSL_get_ex_new_index(0, "mydata index", NULL, NULL, NULL);
             * Let the verify_callback catch the verify_depth error so that we get
             * an appropriate error in the logfile.
            SSL_CTX_set_verify_depth(verify_depth + 1);
             * Set up the SSL specific data into "mydata" and store it into th SSL


           ssl(3), SSL_new(3), SSL_CTX_get_verify_mode(3),
           SSL_get_verify_result(3), SSL_CTX_load_verify_locations(3),
           SSL_get_peer_certificate(3), SSL_CTX_set_cert_verify_callback(3),
           SSL_get_ex_data_X509_STORE_CTX_idx(3), SSL_get_ex_new_index(3)

    1.0.1e 2013-02-11 SSL_CTX_set_verify(3)


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