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)