Provided by: libcurl4-doc_8.12.1-3ubuntu1_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)
libcurl 2025-03-05 CURLOPT_SSL_CTX_FUNCTION(3)