SSL_CTX_set_verify.pod revision 72613
1=pod 2 3=head1 NAME 4 5SSL_CTX_set_verify, SSL_set_verify, SSL_CTX_set_verify_depth, SSL_set_verify_depth - set peer certificate verification parameters 6 7=head1 SYNOPSIS 8 9 #include <openssl/ssl.h> 10 11 void SSL_CTX_set_verify(SSL_CTX *ctx, int mode, 12 int (*verify_callback)(int, X509_STORE_CTX *)); 13 void SSL_set_verify(SSL *s, int mode, 14 int (*verify_callback)(int, X509_STORE_CTX *)); 15 void SSL_CTX_set_verify_depth(SSL_CTX *ctx,int depth); 16 void SSL_set_verify_depth(SSL *s, int depth); 17 18 int verify_callback(int preverify_ok, X509_STORE_CTX *x509_ctx); 19 20=head1 DESCRIPTION 21 22SSL_CTX_set_verify() sets the verification flags for B<ctx> to be B<mode> and 23specifies the B<verify_callback> function to be used. If no callback function 24shall be specified, the NULL pointer can be used for B<verify_callback>. 25 26SSL_set_verify() sets the verification flags for B<ssl> to be B<mode> and 27specifies the B<verify_callback> function to be used. If no callback function 28shall be specified, the NULL pointer can be used for B<verify_callback>. In 29this case last B<verify_callback> set specifically for this B<ssl> remains. If 30no special B<callback> was set before, the default callback for the underlying 31B<ctx> is used, that was valid at the the time B<ssl> was created with 32L<SSL_new(3)|SSL_new(3)>. 33 34SSL_CTX_set_verify_depth() sets the maximum B<depth> for the certificate chain 35verification that shall be allowed for B<ctx>. (See the BUGS section.) 36 37SSL_set_verify_depth() sets the maximum B<depth> for the certificate chain 38verification that shall be allowed for B<ssl>. (See the BUGS section.) 39 40=head1 NOTES 41 42The verification of certificates can be controlled by a set of logically 43or'ed B<mode> flags: 44 45=over 4 46 47=item SSL_VERIFY_NONE 48 49B<Server mode:> the server will not send a client certificate request to the 50client, so the client will not send a certificate. 51 52B<Client mode:> if not using an anonymous cipher (by default disabled), the 53server will send a certificate which will be checked. The result of the 54certificate verification process can be checked after the TLS/SSL handshake 55using the L<SSL_get_verify_result(3)|SSL_get_verify_result(3)> function. 56The handshake will be continued regardless of the verification result. 57 58=item SSL_VERIFY_PEER 59 60B<Server mode:> the server sends a client certificate request to the client. 61The certificate returned (if any) is checked. If the verification process 62fails as indicated by B<verify_callback>, the TLS/SSL handshake is 63immediately terminated with an alert message containing the reason for 64the verification failure. 65The behaviour can be controlled by the additional 66SSL_VERIFY_FAIL_IF_NO_PEER_CERT and SSL_VERIFY_CLIENT_ONCE flags. 67 68B<Client mode:> the server certificate is verified. If the verification process 69fails as indicated by B<verify_callback>, the TLS/SSL handshake is 70immediately terminated with an alert message containing the reason for 71the verification failure. If no server certificate is sent, because an 72anonymous cipher is used, SSL_VERIFY_PEER is ignored. 73 74=item SSL_VERIFY_FAIL_IF_NO_PEER_CERT 75 76B<Server mode:> if the client did not return a certificate, the TLS/SSL 77handshake is immediately terminated with a "handshake failure" alert. 78This flag must be used together with SSL_VERIFY_PEER. 79 80B<Client mode:> ignored 81 82=item SSL_VERIFY_CLIENT_ONCE 83 84B<Server mode:> only request a client certificate on the initial TLS/SSL 85handshake. Do not ask for a client certificate again in case of a 86renegotiation. This flag must be used together with SSL_VERIFY_PEER. 87 88B<Client mode:> ignored 89 90=back 91 92Exactly one of the B<mode> flags SSL_VERIFY_NONE and SSL_VERIFY_PEER must be 93set at any time. 94 95SSL_CTX_set_verify_depth() and SSL_set_verify_depth() set the limit up 96to which depth certificates in a chain are used during the verification 97procedure. If the certificate chain is longer than allowed, the certificates 98above the limit are ignored. Error messages are generated as if these 99certificates would not be present, most likely a 100X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT_LOCALLY will be issued. 101The depth count is "level 0:peer certificate", "level 1: CA certificate", 102"level 2: higher level CA certificate", and so on. Setting the maximum 103depth to 2 allows the levels 0, 1, and 2. The default depth limit is 9, 104allowing for the peer certificate and additional 9 CA certificates. 105 106The B<verify_callback> function is used to control the behaviour when the 107SSL_VERIFY_PEER flag is set. It must be supplied by the application and 108receives two arguments: B<preverify_ok> indicates, whether the verification of 109the certificate in question was passed (preverify_ok=1) or not 110(preverify_ok=0). B<x509_ctx> is a pointer to the complete context used 111for the certificate chain verification. 112 113The certificate chain is checked starting with the deepest nesting level 114(the root CA certificate) and worked upward to the peer's certificate. 115At each level signatures and issuer attributes are checked. Whenever 116a verification error is found, the error number is stored in B<x509_ctx> 117and B<verify_callback> is called with B<preverify_ok>=0. By applying 118X509_CTX_store_* functions B<verify_callback> can locate the certificate 119in question and perform additional steps (see EXAMPLES). If no error is 120found for a certificate, B<verify_callback> is called with B<preverify_ok>=1 121before advancing to the next level. 122 123The return value of B<verify_callback> controls the strategy of the further 124verification process. If B<verify_callback> returns 0, the verification 125process is immediately stopped with "verification failed" state. If 126SSL_VERIFY_PEER is set, a verification failure alert is sent to the peer and 127the TLS/SSL handshake is terminated. If B<verify_callback> returns 1, 128the verification process is continued. If B<verify_callback> always returns 1291, the TLS/SSL handshake will never be terminated because of this application 130experiencing a verification failure. The calling process can however 131retrieve the error code of the last verification error using 132L<SSL_get_verify_result(3)|SSL_get_verify_result(3)> or by maintaining its 133own error storage managed by B<verify_callback>. 134 135If no B<verify_callback> is specified, the default callback will be used. 136Its return value is identical to B<preverify_ok>, so that any verification 137failure will lead to a termination of the TLS/SSL handshake with an 138alert message, if SSL_VERIFY_PEER is set. 139 140=head1 BUGS 141 142In client mode, it is not checked whether the SSL_VERIFY_PEER flag 143is set, but whether SSL_VERIFY_NONE is not set. This can lead to 144unexpected behaviour, if the SSL_VERIFY_PEER and SSL_VERIFY_NONE are not 145used as required (exactly one must be set at any time). 146 147The certificate verification depth set with SSL[_CTX]_verify_depth() 148stops the verification at a certain depth. The error message produced 149will be that of an incomplete certificate chain and not 150X509_V_ERR_CERT_CHAIN_TOO_LONG as may be expected. 151 152=head1 RETURN VALUES 153 154The SSL*_set_verify*() functions do not provide diagnostic information. 155 156=head1 EXAMPLES 157 158The following code sequence realizes an example B<verify_callback> function 159that will always continue the TLS/SSL handshake regardless of verification 160failure, if wished. The callback realizes a verification depth limit with 161more informational output. 162 163All verification errors are printed, informations about the certificate chain 164are printed on request. 165The example is realized for a server that does allow but not require client 166certificates. 167 168The example makes use of the ex_data technique to store application data 169into/retrieve application data from the SSL structure 170(see L<SSL_get_ex_new_index(3)|SSL_get_ex_new_index(3)>, 171L<SSL_get_ex_data_X509_STORE_CTX_idx(3)|SSL_get_ex_data_X509_STORE_CTX_idx(3)>). 172 173 ... 174 typedef struct { 175 int verbose_mode; 176 int verify_depth; 177 int always_continue; 178 } mydata_t; 179 int mydata_index; 180 ... 181 static int verify_callback(int preverify_ok, X509_STORE_CTX *ctx) 182 { 183 char buf[256]; 184 X509 *err_cert; 185 int err, depth; 186 SSL *ssl; 187 mydata_t *mydata; 188 189 err_cert = X509_STORE_CTX_get_current_cert(ctx); 190 err = X509_STORE_CTX_get_error(ctx); 191 depth = X509_STORE_CTX_get_error_depth(ctx); 192 193 /* 194 * Retrieve the pointer to the SSL of the connection currently treated 195 * and the application specific data stored into the SSL object. 196 */ 197 ssl = X509_STORE_CTX_get_ex_data(ctx, SSL_get_ex_data_X509_STORE_CTX_idx()); 198 mydata = SSL_get_ex_data(ssl, mydata_index); 199 200 X509_NAME_oneline(X509_get_subject_name(err_cert), buf, 256); 201 202 /* 203 * Catch a too long certificate chain. The depth limit set using 204 * SSL_CTX_set_verify_depth() is by purpose set to "limit+1" so 205 * that whenever the "depth>verify_depth" condition is met, we 206 * have violated the limit and want to log this error condition. 207 * We must do it here, because the CHAIN_TOO_LONG error would not 208 * be found explicitly; only errors introduced by cutting off the 209 * additional certificates would be logged. 210 */ 211 if (depth > mydata->verify_depth) { 212 preverify_ok = 0; 213 err = X509_V_ERR_CERT_CHAIN_TOO_LONG; 214 X509_STORE_CTX_set_error(ctx, err); 215 } 216 if (!preverify_ok) { 217 printf("verify error:num=%d:%s:depth=%d:%s\n", err, 218 X509_verify_cert_error_string(err), depth, buf); 219 } 220 else if (mydata->verbose_mode) 221 { 222 printf("depth=%d:%s\n", depth, buf); 223 } 224 225 /* 226 * At this point, err contains the last verification error. We can use 227 * it for something special 228 */ 229 if (!preverify_ok && (err == X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT) 230 { 231 X509_NAME_oneline(X509_get_issuer_name(ctx->current_cert), buf, 256); 232 printf("issuer= %s\n", buf); 233 } 234 235 if (mydata->always_continue) 236 return 1; 237 else 238 return preverify_ok; 239 } 240 ... 241 242 mydata_t mydata; 243 244 ... 245 mydata_index = SSL_get_ex_new_index(0, "mydata index", NULL, NULL, NULL); 246 247 ... 248 SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER|SSL_VERIFY_CLIENT_ONCE, 249 verify_callback); 250 251 /* 252 * Let the verify_callback catch the verify_depth error so that we get 253 * an appropriate error in the logfile. 254 */ 255 SSL_CTX_set_verify_depth(verify_depth + 1); 256 257 /* 258 * Set up the SSL specific data into "mydata" and store it into th SSL 259 * structure. 260 */ 261 mydata.verify_depth = verify_depth; ... 262 SSL_set_ex_data(ssl, mydata_index, &mydata); 263 264 ... 265 SSL_accept(ssl); /* check of success left out for clarity */ 266 if (peer = SSL_get_peer_certificate(ssl)) 267 { 268 if (SSL_get_verify_result(ssl) == X509_V_OK) 269 { 270 /* The client sent a certificate which verified OK */ 271 } 272 } 273 274=head1 SEE ALSO 275 276L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>, 277L<SSL_CTX_get_verify_mode(3)|SSL_CTX_get_verify_mode(3)>, 278L<SSL_get_verify_result(3)|SSL_get_verify_result(3)>, 279L<SSL_CTX_load_verify_locations(3)|SSL_CTX_load_verify_locations(3)>, 280L<SSL_get_peer_certificate(3)|SSL_get_peer_certificate(3)>, 281L<SSL_get_ex_data_X509_STORE_CTX_idx(3)|SSL_get_ex_data_X509_STORE_CTX_idx(3)>, 282L<SSL_get_ex_new_index(3)|SSL_get_ex_new_index(3)> 283 284=cut 285