curl http2
11.1. HTTP 1.x look-alike
Internally, curl will convert incoming http2 headers to HTTP 1.x style headers and provide them to the user, so that they will appear very similar to existing HTTP. This allows for an easier transition for whatever is using curl and HTTP today. Similarly curl will convert outgoing headers in the same style. Give them to curl in HTTP 1.x style and it will convert them on the fly when talking to http2 servers. This also allows users to not have to bother or care very much with which particular HTTP version that is actually used on the wire.
11.2. Plain text, insecure
curl supports http2 over standard TCP via the Upgrade: header. If you do an HTTP request and ask for HTTP 2, curl will ask the server to update the connection to http2 if possible.
11.3. TLS and what libraries
curl supports a wide range of different TLS libraries for its TLS back-end, and that is still valid for http2 support. The challenge with TLS for http2’s sake is the ALPN support and to some extent NPN support.Build curl against modern versions of OpenSSL or NSS to get both ALPN and NPN support. Using GnuTLS or PolarSSL you will get ALPN support but not NPN.
11.4. Command line use
To tell curl to use http2, either plain text or over TLS, you use the --http2 option (that is “dash dash http2”). curl defaults to HTTP/1.1 for HTTP: URLs so the extra option is necessary when you want http2 for that. For HTTPS URLs, curl will attempt to use http2.
