SSL_CTX_SET_VERIFY(3)     BSD Library Functions Manual     SSL_CTX_SET_VERIFY(3)

     SSL_CTX_set_verify, SSL_set_verify, SSL_CTX_set_verify_depth,
     SSL_set_verify_depth — set peer certificate verification parameters

     #include <openssl/ssl.h>

     SSL_CTX_set_verify(SSL_CTX *ctx, int mode,
         int (*verify_callback)(int, X509_STORE_CTX *));

     SSL_set_verify(SSL *s, int mode,
         int (*verify_callback)(int, X509_STORE_CTX *));

     SSL_CTX_set_verify_depth(SSL_CTX *ctx, int depth);

     SSL_set_verify_depth(SSL *s, int depth);

     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 verify_callback.

     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).
     Within the callback function, SSL_get_ex_data_X509_STORE_CTX_idx(3) can be
     called to get the data index of the current SSL object that is doing the

     SSL_CTX_set_verify_depth() sets the maximum depth for the certificate chain
     verification that shall be allowed for ctx.  (See the BUGS section.)

     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 bitwise ORed
     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
             verification process fails, the TLS/SSL handshake is immediately
             terminated with an alert message containing the reason for the
             verification failure.  The behaviour can be controlled by the
             additional SSL_VERIFY_FAIL_IF_NO_PEER_CERT and
             SSL_VERIFY_CLIENT_ONCE flags.

             Client mode: the server certificate is verified.  If the
             verification process fails, the TLS/SSL handshake is immediately
             terminated with an alert message containing the reason for the
             verification failure.  If no server certificate is sent, because an
             anonymous cipher is used, SSL_VERIFY_PEER is ignored.

             Server mode: if the client did not return a certificate, 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

     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 100, allowing
     for the peer certificate and an additional 100 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 equal to 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 equal to 1
     before advancing to the next level.

     The return value of verify_callback controls the strategy of the further
     verification process.  If verify_callback returns 0, the verification
     process is immediately stopped with “verification failed” state.  If
     SSL_VERIFY_PEER is set, a verification failure alert is sent to the peer
     and the TLS/SSL handshake is terminated.  If verify_callback returns 1, the
     verification process is continued.  If verify_callback always returns 1,
     the TLS/SSL handshake will not be terminated with respect to verification
     failures and the connection will be established.  The calling process can
     however retrieve the error code of the last verification error using
     SSL_get_verify_result(3) or by maintaining its own error storage managed by

     If no verify_callback is specified, the default callback will be used.  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.

     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; information about the certificate
     chain is 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);

              * Retrieve the pointer to the SSL of the connection currently
              * treated * and the application specific data stored into the
              * SSL object.
             ssl = X509_STORE_CTX_get_ex_data(ctx,
             mydata = SSL_get_ex_data(ssl, mydata_index);

             X509_NAME_oneline(X509_get_subject_name(err_cert), buf, 256);

              * Catch a too long certificate chain. The depth limit set using
              * SSL_CTX_set_verify_depth() is by purpose set to "limit+1" so
              * that whenever the "depth>verify_depth" condition is met, we
              * have violated the limit and want to log this error condition.
              * 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)) {
                         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 the SSL
      * structure.
     mydata.verify_depth = verify_depth; ...
     SSL_set_ex_data(ssl, mydata_index, &mydata);


     SSL_accept(ssl); /* check of success left out for clarity */
     if (peer = SSL_get_peer_certificate(ssl)) {
             if (SSL_get_verify_result(ssl) == X509_V_OK) {
                     /* The client sent a certificate which verified OK */

     ssl(3), SSL_CTX_get_verify_mode(3), SSL_CTX_load_verify_locations(3),
     SSL_CTX_set_cert_verify_callback(3), SSL_get_ex_data_X509_STORE_CTX_idx(3),
     SSL_get_ex_new_index(3), SSL_get_peer_certificate(3),
     SSL_get_verify_result(3), SSL_new(3), SSL_set1_host(3)

     SSL_set_verify() appeared in SSLeay 0.4 or earlier.  SSL_CTX_set_verify()
     first appeared in SSLeay 0.6.4.  Both functions have been available since
     OpenBSD 2.4.

     SSL_CTX_set_verify_depth() and SSL_set_verify_depth() first appeared in
     OpenSSL 0.9.3 and have been available since OpenBSD 2.6.

     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.

BSD                            September 17, 2020                            BSD