CURLOPT_PROXY_ISSUERCERT(3)

💅 CURLOPT_PROXY_ISSUERCERT 💅

okay so bestie, welcome to the libcurl manual page for CURLOPT_PROXY_ISSUERCERT — yes that's a mouthful, yes it's iconic, and yes we're gonna get through it together 🫶

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✨ NAME ✨

CURLOPT_PROXY_ISSUERCERT — the proxy issuer SSL certificate filename, aka "which CA file should my HTTPS proxy's issuer match against" 🌸

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🎀 SYNOPSIS 🎀

#include <curl/curl.h>

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_PROXY_ISSUERCERT, char *file);

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💖 DESCRIPTION 💖

so you pass a char pointer to a null-terminated string, and that string names a file holding a CA certificate in PEM format 🔥 no cap, that's the whole vibe.

If the option is set, an additional check against the peer certificate is performed to verify the issuer of 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. like, you're not just checking "is this a valid cert," you're checking "is this cert from my branch of the tree" — very main character energy 👑

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. so if you skip that one, this check literally just vibes and does nothing 💀

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 fail). so like — it can only actually fail you if you set that other option too. iykyk 🧚

The application does not have to keep the string around after setting this option. curl copies it, you can let it go, free it, move on, live your life 💅

Using this option multiple times makes the last set string override the previous ones. Set it to NULL to disable its use again. last one wins, periodt ✨

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🦋 DEFAULT 🦋

NULL

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🌈 PROTOCOLS 🌈

This functionality affects all TLS based protocols: HTTPS, FTPS, IMAPS, POP3S, SMTPS etc. 🌸

This option works only with the following TLS backends: GnuTLS and OpenSSL ⭐

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💜 EXAMPLE 💜

int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    CURLcode result;
    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com/");
    /* using an HTTPS proxy */
    curl_easy_setopt(curl, CURLOPT_PROXY, "https://proxy.example:443");
    curl_easy_setopt(curl, CURLOPT_PROXY_ISSUERCERT, "/etc/certs/cacert.pem");
    result = curl_easy_perform(curl);
    curl_easy_cleanup(curl);
  }
}

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👑 AVAILABILITY 👑

Added in curl 7.71.0 — so it's been around a minute, lowkey a classic 🎀

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💀 RETURN VALUE 💀

curl_easy_setopt(3) returns a CURLcode indicating success or error.

CURLE_OK (0) means everything was OK, non-zero means an error occurred, see libcurl-errors(3). zero = slay, non-zero = something went sideways 😭

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🔥 SEE ALSO 🔥

• CURLOPT_ISSUERCERT(3)
• CURLOPT_PROXY_SSL_VERIFYHOST(3)
• CURLOPT_PROXY_SSL_VERIFYPEER(3)
• CURLOPT_SSL_VERIFYHOST(3)
• CURLOPT_SSL_VERIFYPEER(3)

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okay that's the whole page bestie, you ate 💅 this one's from libcurl, dated 2026-10-12, and honestly? iconic of it. go forth and verify those issuers 🌟🦋