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),