HTTP versions

As any other Internet protocol, the HTTP protocol has kept evolving over the years and now there are clients and servers distributed over the world and over time that speak different versions with varying levels of success. So in order to get libcurl to work with the URLs you pass in libcurl offers ways for you to specify which HTTP version that request and transfer should use. libcurl is designed in a way so that it tries to use the most common, the most sensible if you want, default values first but sometimes that isn't enough and then you may need to instruct libcurl what to do.

Since mid 2016, libcurl defaults to use HTTP/2 for HTTPS servers if you have a libcurl that has HTTP/2 abilities built-in, libcurl will attempt to use HTTP/2 automatically or fall down to 1.1 in case the negotiation failed. Non-HTTP/2 capable libcurls get 1.1 over HTTPS by default. Plain HTTP requests still default to HTTP/1.1.

If the default behavior isn't good enough for your transfer, the CURLOPT_HTTP_VERSION option is there for you.

Option Description
CURL_HTTP_VERSION_NONE Reset back to default behavior
CURL_HTTP_VERSION_1_0 Enforce use of the legacy HTTP/1.0 protocol version
CURL_HTTP_VERSION_1_1 Do the request using the HTTP/1.1 protocol version
CURL_HTTP_VERSION_2_0 Attempt to use HTTP/2
CURL_HTTP_VERSION_2TLS Attempt to use HTTP/2 on HTTPS connections only, otherwise do HTTP/1.1
CURL_HTTP_VERSION_2_PRIOR_KNOWLEDGE Use HTTP/2 straight away without "upgrading" from 1.1. It requires that you know that this server is OK with it.

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