CURLOPT_SSL_CTX_FUNCTION(3) Library Functions Manual
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 <i>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 2024-08-05 CURLOPT_SSL_CTX_FUNCTION(3)
curl 8.9.1 - Generated Mon Aug 12 16:05:23 CDT 2024