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 *userptr);

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

DESCRIPTION

       This  option  only works for libcurl powered by OpenSSL, wolfSSL or mbedTLS. 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, and a pointer to mbedtls_ssl_config for mbedTLS. If an error is returned
       from the callback no attempt to establish a connection is made and the perform operation will return  the
       callback's error code. Set the userptr argument with the CURLOPT_SSL_CTX_DATA(3) option.

       This  function  will  get called on all new connections made to a server, during the SSL negotiation. The
       ssl_ctx will point 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.

       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 the CTX callback will also  be
       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 CTX 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 == NULL)
           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/");

         /* Retrieve page using cacerts' certificate -> will succeed
          * load the certificate by installing a function doing the necessary
          * "modifications" to the SSL CONTEXT just before link init
          */
         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 and in 7.54.0 for mbedTLS. 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),