xref: /curl/docs/cmdline-opts/http3.md (revision 7143833f)
1---
2c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
3SPDX-License-Identifier: curl
4Long: http3
5Tags: Versions
6Protocols: HTTP
7Added: 7.66.0
8Mutexed: http1.1 http1.0 http2 http2-prior-knowledge http3-only
9Requires: HTTP/3
10Help: Use HTTP v3
11Category: http
12Multi: mutex
13See-also:
14  - http1.1
15  - http2
16Example:
17  - --http3 $URL
18---
19
20# `--http3`
21
22Attempt HTTP/3 to the host in the URL, but fallback to earlier HTTP versions
23if the HTTP/3 connection establishment fails or is slow. HTTP/3 is only
24available for HTTPS and not for HTTP URLs.
25
26This option allows a user to avoid using the Alt-Svc method of upgrading to
27HTTP/3 when you know or suspect that the target speaks HTTP/3 on the given
28host and port.
29
30When asked to use HTTP/3, curl issues a separate attempt to use older HTTP
31versions with a slight delay, so if the HTTP/3 transfer fails or is slow, curl
32still tries to proceed with an older HTTP version. The fallback performs the
33regular negotiation between HTTP/1 and HTTP/2.
34
35Use --http3-only for similar functionality *without* a fallback.
36