Section: curl_easy_setopt options (3)
Updated: June 25, 2020
CURLOPT_PROXY_ISSUERCERT - proxy issuer SSL certificate filename
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_PROXY_ISSUERCERT, char *file);
Pass a char * to a null-terminated string naming a file
holding a CA
certificate in PEM format. If the option is set, an additional check against
the peer certificate is performed to verify the issuer of the the HTTPS proxy
is indeed the one associated with the certificate provided by the option.
This additional check is useful in multi-level PKI where one needs to enforce
that the peer certificate is from a specific branch of the tree.
This option makes sense only when used in combination with the
CURLOPT_PROXY_SSL_VERIFYPEER(3) option. Otherwise, the result of the
check is not considered as failure.
A specific error code (CURLE_SSL_ISSUER_ERROR) is defined with the option,
which is returned if the setup of the SSL/TLS session has failed due to a
mismatch with the issuer of peer certificate
(CURLOPT_PROXY_SSL_VERIFYPEER(3) has to be set too for the check to
The application does not have to keep the string around after setting this
All TLS-based protocols
CURL *curl = curl_easy_init();
curl_easy_setopt(curl, CURLOPT_URL, "https://example.com/");
/* using an HTTPS proxy */
curl_easy_setopt(curl, CURLOPT_PROXY, "https://localhost:443");
curl_easy_setopt(curl, CURLOPT_PROXY_ISSUERCERT, "/etc/certs/cacert.pem");
ret = curl_easy_perform(curl);
Added in libcurl 7.71.0. This option is supported by the OpenSSL backends.
Returns CURLE_OK if the option is supported, CURLE_UNKNOWN_OPTION if not, or
CURLE_OUT_OF_MEMORY if there was insufficient heap space.