SSL_CTX_set_verify manpage

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SSL_CTX_set_verify(3)		    OpenSSL		 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>

	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
	   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

	   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 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 100,
       allowing for the peer certificate and 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=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.

       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 verify_callback.

       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.

       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; 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, SSL_get_ex_data_X509_STORE_CTX_idx());
	   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))
	     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
	 * 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_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)

1.0.2k				  2017-01-26		 SSL_CTX_set_verify(3)