Provided by: libcurl4-doc_7.55.1-1ubuntu2_all bug

NAME

       CURLOPT_SSL_CTX_FUNCTION - SSL context callback for OpenSSL, wolfSSL/CyaSSL 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/CyaSSL 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 behaviour of the SSL initialization. The ssl_ctx parameter is
       actually  a  pointer  to  the  SSL  library's SSL_CTX for OpenSSL or wolfSSL/CyaSSL, 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 a  HTTPS
       request.

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 = /* 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";
         /* 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;

         rv=curl_global_init(CURL_GLOBAL_ALL);
         ch=curl_easy_init();
         rv=curl_easy_setopt(ch, CURLOPT_SSLCERTTYPE, "PEM");
         rv=curl_easy_setopt(ch, CURLOPT_SSL_VERIFYPEER, 1L);
         rv=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
          */
         rv=curl_easy_setopt(ch, CURLOPT_SSL_CTX_FUNCTION, *sslctx_function);
         rv=curl_easy_perform(ch);
         if(rv==CURLE_OK)
           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.  Added  in 7.42.0 for wolfSSL/CyaSSL. Added in 7.54.0 for
       mbedTLS. Other SSL backends 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),