cURL by Example: HTTP/2 Request
Force the use of HTTP/2. Modern cURL versions try to negotiate this automatically.
Code
# Force HTTP/2
curl --http2 https://example.com
# Force HTTP/2 without TLS (h2c) - rare
curl --http2-prior-knowledge http://localhost:8080Explanation
HTTP/2 brings several performance improvements over HTTP/1.1. Modern browsers and servers support it automatically when using HTTPS.
Main advantages of HTTP/2:
- Multiplexing: Multiple requests share a single connection
- Header compression: Reduces overhead for repeated headers
- Server push: Servers can send resources before being requested
- Binary protocol: More efficient than text-based HTTP/1.1
The --http2 flag tells cURL to prefer HTTP/2 if available. The server must support it and the connection usually requires TLS. The protocol negotiation happens through ALPN (Application-Layer Protocol Negotiation) during the TLS handshake.
HTTP/2 over cleartext (h2c) is rare but useful for local testing. The --http2-prior-knowledge flag skips the upgrade handshake and assumes the server supports HTTP/2. This only works with servers explicitly configured for h2c.
Your cURL version must be compiled with nghttp2 support. Run curl -V and look for HTTP2 in the features list to verify support.
Code Breakdown
--http2 requests an upgrade to HTTP/2.--http2-prior-knowledge skips the upgrade handshake (for non-TLS).