curl/docs/cmdline-opts/http3.md

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c: Copyright (C) Daniel Stenberg, daniel@haxx.se, et al. SPDX-License-Identifier: curl Long: http3 Tags: Versions Protocols: HTTP Added: 7.66.0 Mutexed: http1.1 http1.0 http2 http2-prior-knowledge http3-only Requires: HTTP/3 Help: Use HTTP v3 Category: http Multi: mutex See-also:

• http1.1
• http2 Example:
• --http3 $URL ---

--http3

Attempt HTTP/3 to the host in the URL, but fallback to earlier HTTP versions if the HTTP/3 connection establishment fails. HTTP/3 is only available for HTTPS and not for HTTP URLs.

This option allows a user to avoid using the Alt-Svc method of upgrading to HTTP/3 when you know that the target speaks HTTP/3 on the given host and port.

When asked to use HTTP/3, curl issues a separate attempt to use older HTTP versions with a slight delay, so if the HTTP/3 transfer fails or is slow, curl still tries to proceed with an older HTTP version.

Use --http3-only for similar functionality without a fallback.