CURLOPT_SSL_CTX_FUNCTION(3)				     curl_easy_setopt options				       CURLOPT_SSL_CTX_FUNCTION(3)

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 actu-
       ally 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  ini-
       tialized 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-----
"
	   "MIIHPTCCBSWgAwIBAgIBADANBgkqhkiG9w0BAQQFADB5MRAwDgYDVQQKEwdSb290
"
	   "IENBMR4wHAYDVQQLExVodHRwOi8vd3d3LmNhY2VydC5vcmcxIjAgBgNVBAMTGUNB
"
	   "IENlcnQgU2lnbmluZyBBdXRob3JpdHkxITAfBgkqhkiG9w0BCQEWEnN1cHBvcnRA
"
	   "Y2FjZXJ0Lm9yZzAeFw0wMzAzMzAxMjI5NDlaFw0zMzAzMjkxMjI5NDlaMHkxEDAO
"
	   "GCSNe9FINSkYQKyTYOGWhlC0elnYjyELn8+CkcY7v2vcB5G5l1YjqrZslMZIBjzk
"
	   "zk6q5PYvCdxTby78dOs6Y5nCpqyJvKeyRKANihDjbPIky/qbn3BHLt4Ui9SyIAmW
"
	   "omTxJBzcoTWcFbLUvFUufQb1nA5V9FrWk9p2rSVzTMVD
"
	   "-----END CERTIFICATE-----
";
	 /* 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...
");

	 /* 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
");

	 /* 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 ***
");
	 else
	   printf("*** transfer failed ***
");

	 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),

libcurl 7.54.0							  March 26, 2017				       CURLOPT_SSL_CTX_FUNCTION(3)