CURLOPT_SSL_ENABLE_ALPN - Application Layer Protocol Negotiation

Added in curl 7.36.0

1, enabled

Pass a long as parameter, 0 or 1 where 1 is for enable and 0 for disable. This option enables/disables ALPN in the SSL handshake (if the SSL backend libcurl is built to use supports it), which can be used to negotiate http2.

int main(void) { CURL *curl = curl_easy_init(); if(curl) { CURLcode res; curl_easy_setopt(curl, CURLOPT_URL, "https://example.com/"); curl_easy_setopt(curl, CURLOPT_SSL_ENABLE_ALPN, 0L); res = curl_easy_perform(curl); curl_easy_cleanup(curl); } }

CURLOPT_SSL_ENABLE_ALPN - Application Layer Protocol Negotiation

This functionality affects all TLS based protocols: HTTPS, FTPS, IMAPS, POP3S, SMTPS etc. All TLS backends support this option.

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).

CURLOPT_SSL_ENABLE_NPN(3), CURLOPT_SSL_OPTIONS(3) libcurl 2025-06-16 CURLOPT_SSL_ENABLE_ALPN(3)

#include <curl/curl.h> CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSL_ENABLE_ALPN, long npn);