NAME

       CURLOPT_SSL_CTX_FUNCTION - SSL context callback

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

       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 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 passed in to this  callback
       with the CURLOPT_SSL_CTX_DATA(3) option.

       This  function  gets  called  for all new connections made to a server, during the SSL negotiation. While
       ssl_ctx points to a newly initialized object each time, the pointer may still be the same as in  a  prior
       call.

       To  use  this  callback, 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)

       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.

       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  setting CURLOPT_PRIVATE(3) on your curl handle so you can identify it correctly 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.

       libcurl does not guarantee the lifetime of the passed in object once this callback function has returned.
       Your application must not assume that it can keep using the SSL context or data derived from it once this
       function is completed.

       For  libcurl builds using TLS backends that support CA caching and CURLOPT_CA_CACHE_TIMEOUT(3) is not set
       to zero, multiple calls to this callback may be done with the same CA store in memory.

DEFAULT

       NULL

PROTOCOLS

       This functionality affects all TLS based protocols: HTTPS, FTPS, IMAPS, POP3S, SMTPS etc.

       This option works only with the following TLS backends: BearSSL, OpenSSL, mbedTLS and wolfSSL

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 curl 7.10.6

RETURN VALUE

       CURLE_OK if supported; or an error such as:

       CURLE_NOT_BUILT_IN - Not supported by the SSL backend

SEE ALSO

       CURLOPT_CAINFO(3),    CURLOPT_CAINFO_BLOB(3),    CURLOPT_CA_CACHE_TIMEOUT(3),    CURLOPT_SSL_CTX_DATA(3),
       CURLOPT_SSL_VERIFYHOST(3), CURLOPT_SSL_VERIFYPEER(3)