NAME

       CURLOPT_SSL_CTX_FUNCTION - SSL context callback for OpenSSL, wolfSSL or mbedTLS

SYNOPSIS

       #include <curl/curl.h>

       CURLcode ssl_ctx_callback(CURL *curl, void *ssl_ctx, void *clientp);

       CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSL_CTX_FUNCTION,
                                 ssl_ctx_callback);

DESCRIPTION

       This  option only works for libcurl powered by OpenSSL, wolfSSL, mbedTLS or BearSSL. If libcurl was built
       against another SSL library this functionality is absent.

       Pass a pointer to your callback function, which should match the prototype shown above.

       This callback function gets called by libcurl just before the initialization of an SSL  connection  after
       having  processed  all  other  SSL  related options to give a last chance to an application to modify the
       behavior of the SSL initialization. The ssl_ctx parameter is actually a  pointer  to  the  SSL  library's
       SSL_CTX  for  OpenSSL  or  wolfSSL,  a  pointer  to  mbedtls_ssl_config  for  mbedTLS  or  a  pointer  to
       br_ssl_client_context for BearSSL. If an error is returned from the callback no attempt  to  establish  a
       connection  is made and the perform operation returns the callback's error code. Set the clientp argument
       with the CURLOPT_SSL_CTX_DATA(3) option.

       This function gets called on all new connections made to  a  server,  during  the  SSL  negotiation.  The
       ssl_ctx  points  to  a newly initialized object each time, but note the pointer may be the same as from a
       prior call.

       To use this properly, a non-trivial amount of knowledge of your SSL library is  necessary.  For  example,
       you  can  use  this  function  to  call  library-specific callbacks to add additional validation code for
       certificates, and even to change the actual URI of an HTTPS request.

       For OpenSSL, asynchronous certificate verification  via  SSL_set_retry_verify  is  supported.  (Added  in
       8.3.0)

       WARNING:  The  CURLOPT_SSL_CTX_FUNCTION(3)  callback  allows  the  application to reach in and modify SSL
       details in the connection without libcurl itself knowing anything about it, which then  subsequently  can
       lead to libcurl unknowingly reusing SSL connections with different properties. To remedy this you may set
       CURLOPT_FORBID_REUSE(3) from the callback function.

       WARNING:  If  you are using DNS-over-HTTPS (DoH) via CURLOPT_DOH_URL(3) then this callback is also called
       for those transfers and the curl handle is set to an internal handle. This behavior is subject to change.
       We recommend before performing your transfer set CURLOPT_PRIVATE(3)  on  your  curl  handle  so  you  can
       identify it in the context callback. If you have a reason to modify DoH SSL context please let us know on
       the curl-library mailing list because we are considering removing this capability.

DEFAULT

       NULL

PROTOCOLS

       All TLS based protocols: HTTPS, FTPS, IMAPS, POP3S, SMTPS etc.

EXAMPLE

       /* OpenSSL specific */

       #include <openssl/ssl.h>
       #include <curl/curl.h>
       #include <stdio.h>

       static CURLcode sslctx_function(CURL *curl, void *sslctx, void *parm)
       {
         X509_STORE *store;
         X509 *cert = NULL;
         BIO *bio;
         char *mypem = parm;
         /* get a BIO */
         bio = BIO_new_mem_buf(mypem, -1);
         /* use it to read the PEM formatted certificate from memory into an
          * X509 structure that SSL can use
          */
         PEM_read_bio_X509(bio, &cert, 0, NULL);
         if(!cert)
           printf("PEM_read_bio_X509 failed...\n");

         /* get a pointer to the X509 certificate store (which may be empty) */
         store = SSL_CTX_get_cert_store((SSL_CTX *)sslctx);

         /* add our certificate to this store */
         if(X509_STORE_add_cert(store, cert) == 0)
           printf("error adding certificate\n");

         /* decrease reference counts */
         X509_free(cert);
         BIO_free(bio);

         /* all set to go */
         return CURLE_OK;
       }

       int main(void)
       {
         CURL *ch;
         CURLcode rv;
         char *mypem = /* example CA cert PEM - shortened */
           "-----BEGIN CERTIFICATE-----\n"
           "MIIHPTCCBSWgAwIBAgIBADANBgkqhkiG9w0BAQQFADB5MRAwDgYDVQQKEwdSb290\n"
           "IENBMR4wHAYDVQQLExVodHRwOi8vd3d3LmNhY2VydC5vcmcxIjAgBgNVBAMTGUNB\n"
           "IENlcnQgU2lnbmluZyBBdXRob3JpdHkxITAfBgkqhkiG9w0BCQEWEnN1cHBvcnRA\n"
           "Y2FjZXJ0Lm9yZzAeFw0wMzAzMzAxMjI5NDlaFw0zMzAzMjkxMjI5NDlaMHkxEDAO\n"
           "GCSNe9FINSkYQKyTYOGWhlC0elnYjyELn8+CkcY7v2vcB5G5l1YjqrZslMZIBjzk\n"
           "zk6q5PYvCdxTby78dOs6Y5nCpqyJvKeyRKANihDjbPIky/qbn3BHLt4Ui9SyIAmW\n"
           "omTxJBzcoTWcFbLUvFUufQb1nA5V9FrWk9p2rSVzTMVD\n"
           "-----END CERTIFICATE-----\n";

         curl_global_init(CURL_GLOBAL_ALL);
         ch = curl_easy_init();

         curl_easy_setopt(ch, CURLOPT_SSLCERTTYPE, "PEM");
         curl_easy_setopt(ch, CURLOPT_SSL_VERIFYPEER, 1L);
         curl_easy_setopt(ch, CURLOPT_URL, "https://www.example.com/");

         curl_easy_setopt(ch, CURLOPT_SSL_CTX_FUNCTION, *sslctx_function);
         curl_easy_setopt(ch, CURLOPT_SSL_CTX_DATA, mypem);
         rv = curl_easy_perform(ch);
         if(!rv)
           printf("*** transfer succeeded ***\n");
         else
           printf("*** transfer failed ***\n");

         curl_easy_cleanup(ch);
         curl_global_cleanup();
         return rv;
       }

AVAILABILITY

       Added  in  7.11.0  for OpenSSL, in 7.42.0 for wolfSSL, in 7.54.0 for mbedTLS, in 7.83.0 in BearSSL. Other
       SSL backends are not supported.

RETURN VALUE

       CURLE_OK if supported; or an error such as:

       CURLE_NOT_BUILT_IN - Not supported by the SSL backend

       CURLE_UNKNOWN_OPTION

SEE ALSO

       CURLOPT_SSL_CTX_DATA(3), CURLOPT_SSL_VERIFYPEER(3)

ibcurl 8.5.0                                    December 04, 2023                    CURLOPT_SSL_CTX_FUNCTION(3)