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: koehler-mechtersen.de – HTTP/3 not active

Stored result from Nov 6, 2026, 19:42 Check again now →

koehler-mechtersen.de HTTP/3 inactive The server neither advertises HTTP/3 nor answers over QUIC.

HTTP/3 status

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

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

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

TLS 1.3

IPv6 Whether an AAAA record is published in DNS.

YES

koehler-mechtersen.de at netcup runs without HTTP/3.

At turbopress, HTTP/3 is on out of the box – servers in Germany. We'll move your site over for free, emails included.

See the free migration →

Prefer to do it yourself? How to enable HTTP/3 on nginx, LiteSpeed, Cloudflare and Plesk.

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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/koehler-mechtersen.de/"><img src="https://h3check.de/badge/koehler-mechtersen.de.svg?lang=en" alt="HTTP/3 status of koehler-mechtersen.de" width="170" height="30"></a>

API: /api/v1/check?domain=koehler-mechtersen.de

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?