Question 1

What is HTTP/3 and why does it matter?

Accepted Answer

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.

Question 2

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

Accepted Answer

"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.

Question 3

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

Accepted Answer

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.

Question 4

How does the HTTP/3 check work technically?

Accepted Answer

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.

Question 5

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

Accepted Answer

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.

Knowledge