What is HTTP/3 and why does it matter?

HTTP/3 is the latest version of the HTTP protocol and is built on QUIC (UDP port 443). It reduces latency, handles connection drops more gracefully and loads faster – especially on mobile networks. An HTTP/3 check shows whether your server already benefits from this.

What is the difference between "HTTP/3 advertised" and "HTTP/3 working"?

"HTTP/3 advertised" means the server signals HTTP/3 support via the Alt-Svc header. "HTTP/3 working" means a real QUIC connection (HTTP/3 only) was actually established. HTTP/3 is only fully active when both are true.

Why does the check say "NO" even though I configured HTTP/3?

Common causes: UDP port 443 is blocked by a firewall, the CDN or load balancer does not terminate HTTP/3 correctly, the HTTP/3 configuration is faulty, or the server does not answer QUIC requests within the time limit.

How does the HTTP/3 check work technically?

The check runs in two steps. First the Alt-Svc header is read over HTTP/1.1 or HTTP/2 to see whether HTTP/3 is advertised. Then curl opens a real QUIC connection using the --http3-only flag. Only a successful HTTP response over HTTP/3 counts as a pass.

Can I use the HTTP/3 check as an API?

Yes. The API is free and requires no signup. Just append format=json: https://h3check.de/?domain=example.com&host=auto&format=json. The response contains the Alt-Svc header, HTTP version, HTTP status code and further details.

HTTP/3 Check: dosieci.pl – HTTP/3 active ✓

Stored result from Aug 10, 2026, 06:45 Check again now →

dosieci.pl HTTP/3 active A real QUIC connection over UDP/443 was established.

HTTP/3 status

HTTP/3 advertised Whether the server signals via Alt-Svc that HTTP/3 is available.

YES

HTTP/3 working Whether a real QUIC connection (UDP/443) can be established and the page fetched over it.

YES

TLS version HTTP/3 mandates TLS 1.3 – QUIC supports nothing older.

TLS 1.3

IPv6 Whether an AAAA record is published in DNS.

Server / CDN Detected from the Server response headers (last check).

nginx

Show Alt-Svc header

h3=":443"; ma=86400

Nicely done – HTTP/3 is active.

The protocol side is already sorted. How fast your site actually loads for real visitors is what a free performance scan of dosieci.pl with Core Web Vitals will tell you – takes about 30 seconds.

Want even more speed? turbopress hosting is tuned for WordPress performance (LiteSpeed, caching, security hardening).

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Show on your own site that you run HTTP/3. The badge updates with every new check.

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<a href="https://h3check.de/en/check/dosieci.pl/"><img src="https://h3check.de/badge/dosieci.pl.svg?lang=en" alt="HTTP/3 status of dosieci.pl" width="191" height="30"></a>

API: /api/v1/check?domain=dosieci.pl

HTTP/3 check & HTTP3 test – what does it mean?

This free HTTP/3 check tells you whether your website already uses the modern HTTP/3 protocol. An HTTP3 test is particularly useful when you are assessing the performance of your website, your WordPress hosting or your LiteSpeed/CDN configuration.

HTTP/3 is built on QUIC and normally uses UDP port 443. It can cut latency, handle connection drops more gracefully and often loads faster – especially on mobile networks. That is why so many people look for an HTTP3 check, QUIC test or HTTP/3 test.

We show two things. First, whether your server advertises HTTP/3 at all (via the Alt-Svc header). Second, whether a real HTTP/3 connection can actually be established. If your result is "NO", the cause is usually the server configuration, the CDN, blocked UDP/443, or simply that HTTP/3 has not been enabled yet.

Want HTTP/3 for WordPress without the tinkering? At turbopress hosting HTTP/3 is on by default (LiteSpeed Enterprise) – tuned for performance and security.

Keywords: HTTP/3 check, HTTP3 check, HTTP/3 test, HTTP3 test, QUIC check, QUIC test, h3 check.

HTTP/3 check (QUIC) for your domain

HTTP/3 status

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HTTP/3 check & HTTP3 test – what does it mean?