--- name: "libressl-SSL_CTX_set_verify" section: "3" platform: "linux" package: "libressl" version: "4.4.0-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/libressl-SSL_CTX_set_verify" --- # libressl-SSL\_CTX\_set\_verify(3) set peer certificate verification parameters ## NAME **SSL\_CTX\_set\_verify**, **SSL\_set\_verify**, **SSL\_CTX\_set\_verify\_depth**, **SSL\_set\_verify\_depth** – set peer certificate verification parameters ## SYNOPSIS SSL Library (libssl, -lssl) libcrypto **#include \** *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*); ## DESCRIPTION **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 verification. **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: - SSL\_VERIFY\_NONE *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. - SSL\_VERIFY\_PEER *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. - SSL\_VERIFY\_FAIL\_IF\_NO\_PEER\_CERT *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 SSL\_VERIFY\_PEER. *Client* *mode*: ignored - SSL\_VERIFY\_CLIENT\_ONCE *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 SSL\_VERIFY\_PEER. *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 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 *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. ## EXAMPLES 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; else return preverify_ok; } ... mydata_t mydata; ... mydata_index = SSL_get_ex_new_index(0, "mydata index", NULL, NULL, NULL); ... SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER|SSL_VERIFY_CLIENT_ONCE, verify_callback); /* * 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 */ } } ``` ## SEE ALSO [ssl(3)](/ssl/3.md), 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) ## HISTORY **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. ## BUGS 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.