#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);
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, and a pointer to mbedtls_ssl_config for mbedTLS. 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 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 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_PRIVATE(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.
CURLE_NOT_BUILT_IN - Not supported by the SSL backend