1SSL_CTX_set_verify(3)               OpenSSL              SSL_CTX_set_verify(3)
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NAME

6       SSL_CTX_set_verify, SSL_set_verify, SSL_CTX_set_verify_depth,
7       SSL_set_verify_depth - set peer certificate verification parameters
8

SYNOPSIS

10        #include <openssl/ssl.h>
11
12        void SSL_CTX_set_verify(SSL_CTX *ctx, int mode,
13                                int (*verify_callback)(int, X509_STORE_CTX *));
14        void SSL_set_verify(SSL *s, int mode,
15                            int (*verify_callback)(int, X509_STORE_CTX *));
16        void SSL_CTX_set_verify_depth(SSL_CTX *ctx,int depth);
17        void SSL_set_verify_depth(SSL *s, int depth);
18
19        int verify_callback(int preverify_ok, X509_STORE_CTX *x509_ctx);
20

DESCRIPTION

22       SSL_CTX_set_verify() sets the verification flags for ctx to be mode and
23       specifies the verify_callback function to be used. If no callback
24       function shall be specified, the NULL pointer can be used for
25       verify_callback.
26
27       SSL_set_verify() sets the verification flags for ssl to be mode and
28       specifies the verify_callback function to be used. If no callback
29       function shall be specified, the NULL pointer can be used for
30       verify_callback. In this case last verify_callback set specifically for
31       this ssl remains. If no special callback was set before, the default
32       callback for the underlying ctx is used, that was valid at the time ssl
33       was created with SSL_new(3).
34
35       SSL_CTX_set_verify_depth() sets the maximum depth for the certificate
36       chain verification that shall be allowed for ctx. (See the BUGS
37       section.)
38
39       SSL_set_verify_depth() sets the maximum depth for the certificate chain
40       verification that shall be allowed for ssl. (See the BUGS section.)
41

NOTES

43       The verification of certificates can be controlled by a set of
44       logically or'ed mode flags:
45
46       SSL_VERIFY_NONE
47           Server mode: the server will not send a client certificate request
48           to the client, so the client will not send a certificate.
49
50           Client mode: if not using an anonymous cipher (by default
51           disabled), the server will send a certificate which will be
52           checked. The result of the certificate verification process can be
53           checked after the TLS/SSL handshake using the
54           SSL_get_verify_result(3) function.  The handshake will be continued
55           regardless of the verification result.
56
57       SSL_VERIFY_PEER
58           Server mode: the server sends a client certificate request to the
59           client.  The certificate returned (if any) is checked. If the
60           verification process fails, the TLS/SSL handshake is immediately
61           terminated with an alert message containing the reason for the
62           verification failure.  The behaviour can be controlled by the
63           additional SSL_VERIFY_FAIL_IF_NO_PEER_CERT and
64           SSL_VERIFY_CLIENT_ONCE flags.
65
66           Client mode: the server certificate is verified. If the
67           verification process fails, the TLS/SSL handshake is immediately
68           terminated with an alert message containing the reason for the
69           verification failure. If no server certificate is sent, because an
70           anonymous cipher is used, SSL_VERIFY_PEER is ignored.
71
72       SSL_VERIFY_FAIL_IF_NO_PEER_CERT
73           Server mode: if the client did not return a certificate, the
74           TLS/SSL handshake is immediately terminated with a "handshake
75           failure" alert.  This flag must be used together with
76           SSL_VERIFY_PEER.
77
78           Client mode: ignored
79
80       SSL_VERIFY_CLIENT_ONCE
81           Server mode: only request a client certificate on the initial
82           TLS/SSL handshake. Do not ask for a client certificate again in
83           case of a renegotiation. This flag must be used together with
84           SSL_VERIFY_PEER.
85
86           Client mode: ignored
87
88       Exactly one of the mode flags SSL_VERIFY_NONE and SSL_VERIFY_PEER must
89       be set at any time.
90
91       The actual verification procedure is performed either using the built-
92       in verification procedure or using another application provided
93       verification function set with SSL_CTX_set_cert_verify_callback(3).
94       The following descriptions apply in the case of the built-in procedure.
95       An application provided procedure also has access to the verify depth
96       information and the verify_callback() function, but the way this
97       information is used may be different.
98
99       SSL_CTX_set_verify_depth() and SSL_set_verify_depth() set the limit up
100       to which depth certificates in a chain are used during the verification
101       procedure. If the certificate chain is longer than allowed, the
102       certificates above the limit are ignored. Error messages are generated
103       as if these certificates would not be present, most likely a
104       X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT_LOCALLY will be issued.  The depth
105       count is "level 0:peer certificate", "level 1: CA certificate", "level
106       2: higher level CA certificate", and so on. Setting the maximum depth
107       to 2 allows the levels 0, 1, and 2. The default depth limit is 100,
108       allowing for the peer certificate and additional 100 CA certificates.
109
110       The verify_callback function is used to control the behaviour when the
111       SSL_VERIFY_PEER flag is set. It must be supplied by the application and
112       receives two arguments: preverify_ok indicates, whether the
113       verification of the certificate in question was passed (preverify_ok=1)
114       or not (preverify_ok=0). x509_ctx is a pointer to the complete context
115       used for the certificate chain verification.
116
117       The certificate chain is checked starting with the deepest nesting
118       level (the root CA certificate) and worked upward to the peer's
119       certificate.  At each level signatures and issuer attributes are
120       checked. Whenever a verification error is found, the error number is
121       stored in x509_ctx and verify_callback is called with preverify_ok=0.
122       By applying X509_CTX_store_* functions verify_callback can locate the
123       certificate in question and perform additional steps (see EXAMPLES). If
124       no error is found for a certificate, verify_callback is called with
125       preverify_ok=1 before advancing to the next level.
126
127       The return value of verify_callback controls the strategy of the
128       further verification process. If verify_callback returns 0, the
129       verification process is immediately stopped with "verification failed"
130       state. If SSL_VERIFY_PEER is set, a verification failure alert is sent
131       to the peer and the TLS/SSL handshake is terminated. If verify_callback
132       returns 1, the verification process is continued. If verify_callback
133       always returns 1, the TLS/SSL handshake will not be terminated with
134       respect to verification failures and the connection will be
135       established. The calling process can however retrieve the error code of
136       the last verification error using SSL_get_verify_result(3) or by
137       maintaining its own error storage managed by verify_callback.
138
139       If no verify_callback is specified, the default callback will be used.
140       Its return value is identical to preverify_ok, so that any verification
141       failure will lead to a termination of the TLS/SSL handshake with an
142       alert message, if SSL_VERIFY_PEER is set.
143

BUGS

145       In client mode, it is not checked whether the SSL_VERIFY_PEER flag is
146       set, but whether SSL_VERIFY_NONE is not set. This can lead to
147       unexpected behaviour, if the SSL_VERIFY_PEER and SSL_VERIFY_NONE are
148       not used as required (exactly one must be set at any time).
149
150       The certificate verification depth set with SSL[_CTX]_verify_depth()
151       stops the verification at a certain depth. The error message produced
152       will be that of an incomplete certificate chain and not
153       X509_V_ERR_CERT_CHAIN_TOO_LONG as may be expected.
154

RETURN VALUES

156       The SSL*_set_verify*() functions do not provide diagnostic information.
157

EXAMPLES

159       The following code sequence realizes an example verify_callback
160       function that will always continue the TLS/SSL handshake regardless of
161       verification failure, if wished. The callback realizes a verification
162       depth limit with more informational output.
163
164       All verification errors are printed; information about the certificate
165       chain is printed on request.  The example is realized for a server that
166       does allow but not require client certificates.
167
168       The example makes use of the ex_data technique to store application
169       data into/retrieve application data from the SSL structure (see
170       SSL_get_ex_new_index(3), SSL_get_ex_data_X509_STORE_CTX_idx(3)).
171
172        ...
173        typedef struct {
174          int verbose_mode;
175          int verify_depth;
176          int always_continue;
177        } mydata_t;
178        int mydata_index;
179        ...
180        static int verify_callback(int preverify_ok, X509_STORE_CTX *ctx)
181        {
182           char    buf[256];
183           X509   *err_cert;
184           int     err, depth;
185           SSL    *ssl;
186           mydata_t *mydata;
187
188           err_cert = X509_STORE_CTX_get_current_cert(ctx);
189           err = X509_STORE_CTX_get_error(ctx);
190           depth = X509_STORE_CTX_get_error_depth(ctx);
191
192           /*
193            * Retrieve the pointer to the SSL of the connection currently treated
194            * and the application specific data stored into the SSL object.
195            */
196           ssl = X509_STORE_CTX_get_ex_data(ctx, SSL_get_ex_data_X509_STORE_CTX_idx());
197           mydata = SSL_get_ex_data(ssl, mydata_index);
198
199           X509_NAME_oneline(X509_get_subject_name(err_cert), buf, 256);
200
201           /*
202            * Catch a too long certificate chain. The depth limit set using
203            * SSL_CTX_set_verify_depth() is by purpose set to "limit+1" so
204            * that whenever the "depth>verify_depth" condition is met, we
205            * have violated the limit and want to log this error condition.
206            * We must do it here, because the CHAIN_TOO_LONG error would not
207            * be found explicitly; only errors introduced by cutting off the
208            * additional certificates would be logged.
209            */
210           if (depth > mydata->verify_depth) {
211               preverify_ok = 0;
212               err = X509_V_ERR_CERT_CHAIN_TOO_LONG;
213               X509_STORE_CTX_set_error(ctx, err);
214           }
215           if (!preverify_ok) {
216               printf("verify error:num=%d:%s:depth=%d:%s\n", err,
217                        X509_verify_cert_error_string(err), depth, buf);
218           }
219           else if (mydata->verbose_mode)
220           {
221               printf("depth=%d:%s\n", depth, buf);
222           }
223
224           /*
225            * At this point, err contains the last verification error. We can use
226            * it for something special
227            */
228           if (!preverify_ok && (err == X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT))
229           {
230             X509_NAME_oneline(X509_get_issuer_name(ctx->current_cert), buf, 256);
231             printf("issuer= %s\n", buf);
232           }
233
234           if (mydata->always_continue)
235             return 1;
236           else
237             return preverify_ok;
238        }
239        ...
240
241        mydata_t mydata;
242
243        ...
244        mydata_index = SSL_get_ex_new_index(0, "mydata index", NULL, NULL, NULL);
245
246        ...
247        SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER|SSL_VERIFY_CLIENT_ONCE,
248                           verify_callback);
249
250        /*
251         * Let the verify_callback catch the verify_depth error so that we get
252         * an appropriate error in the logfile.
253         */
254        SSL_CTX_set_verify_depth(verify_depth + 1);
255
256        /*
257         * Set up the SSL specific data into "mydata" and store it into th SSL
258         * structure.
259         */
260        mydata.verify_depth = verify_depth; ...
261        SSL_set_ex_data(ssl, mydata_index, &mydata);
262
263        ...
264        SSL_accept(ssl);       /* check of success left out for clarity */
265        if (peer = SSL_get_peer_certificate(ssl))
266        {
267          if (SSL_get_verify_result(ssl) == X509_V_OK)
268          {
269            /* The client sent a certificate which verified OK */
270          }
271        }
272

SEE ALSO

274       ssl(3), SSL_new(3), SSL_CTX_get_verify_mode(3),
275       SSL_get_verify_result(3), SSL_CTX_load_verify_locations(3),
276       SSL_get_peer_certificate(3), SSL_CTX_set_cert_verify_callback(3),
277       SSL_get_ex_data_X509_STORE_CTX_idx(3), SSL_get_ex_new_index(3)
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2811.0.2k                            2017-01-26             SSL_CTX_set_verify(3)
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