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CURLOPT_SSL_CTX_FUNCTION(3) libcurl 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 *clientp);
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
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 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 clientp
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
initialized 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 an HTTPS request.
WARNING: 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.
WARNING: If you are using DNS-over-HTTPS (DoH) via CURLOPT_DOH_URL(3)
then this 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_PRIVATE(3) on your curl handle so you can identify it 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.
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
SEE ALSO
CURLOPT_SSL_CTX_DATA(3), CURLOPT_SSL_VERIFYPEER(3),
ibcurl 8.1.2 April 26, 2023 CURLOPT_SSL_CTX_FUNCTION(3)