Provided by: libcurl4-doc_8.9.1-2ubuntu1_all 
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)