CURLOPT_SSL_CTX_FUNCTION - SSL context callback for OpenSSL, wolfSSL or mbedTLS
#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);
curl_easy_setopt options CURLOPT_SSL_CTX_FUNCTION(3)
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, mbedTLS
or BearSSL. If libcurl was built against another SSL library this func-
tionality is absent.
Pass a pointer to your callback function, which should match the proto-
type shown above.
This callback function gets called by libcurl just before the initial-
ization 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 will return
the callback's error code. Set the userptr argument with the CUR-
LOPT_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 initial-
ized 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 cer-
tificates, and even to change the actual URI of an HTTPS request.
WARNING: The CURLOPT_SSL_CTX_FUNCTION(3) callback allows the applica-
tion 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_PRI-
VATE(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, 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
ATTRIBUTES
See attributes(7) for descriptions of the following attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | web/curl |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
SEE ALSO
CURLOPT_SSL_CTX_DATA(3), CURLOPT_SSL_VERIFYPEER(3),
NOTES
Source code for open source software components in Oracle Solaris can
be found at https://www.oracle.com/downloads/opensource/solaris-source-
code-downloads.html.
This software was built from source available at
https://github.com/oracle/solaris-userland. The original community
source was downloaded from https://curl.se/down-
load/curl-7.83.1.tar.bz2.
Further information about this software can be found on the open source
community website at http://curl.haxx.se/.
libcurl 7.83.1 February 19, 2022
CURLOPT_SSL_CTX_FUNCTION(3)