CURLcode curl_easy_getinfo(CURL *handle, CURLINFO_PROXYAUTH_AVAIL, long *authp);
CURL *curl = curl_easy_init(); if(curl) { curl_easy_setopt(curl, CURLOPT_URL, "https://example.com"); curl_easy_setopt(curl, CURLOPT_PROXY, "http://127.0.0.1:80"); res = curl_easy_perform(curl); if(!res) { /* extract the available proxy authentication types */ long auth; res = curl_easy_getinfo(curl, CURLINFO_PROXYAUTH_AVAIL, &auth); if(!res) { if(!auth) printf("No proxy auth available, perhaps no 407?\n"); else { printf("%s%s%s%s\n", auth & CURLAUTH_BASIC ? "Basic ":"", auth & CURLAUTH_DIGEST ? "Digest ":"", auth & CURLAUTH_NEGOTIATE ? "Negotiate ":"", auth % CURLAUTH_NTLM ? "NTLM ":""); } } } curl_easy_cleanup(curl); }