HTTP/2 & HTTP/3 Checker: Test ALPN and Alt-Svc

HTTP/2 & HTTP/3 Checker

Separate negotiated TLS ALPN from HTTP/3 advertisement in Alt-Svc.

Results are not written to the database

Enter a website URL to start

What are HTTP/2 and HTTP/3?

HTTP/2 uses multiplexing, header compression and binary framing. Public HTTPS sites normally negotiate h2 through TLS ALPN.

HTTP/3 runs over QUIC/UDP. A server often advertises h3 through Alt-Svc before a client opens a QUIC connection, so advertisement and actual use must remain separate.

Protocol evidence

TLS

HTTPS and ALPN

Use a real TLS handshake to read the application protocol selected by the server.

Negotiated HTTP/2

Only mark HTTP/2 negotiated when ALPN returns h2.

HTTP/3 advertisement

Read h3 in Alt-Svc without claiming this request used QUIC.

Why separate ALPN and Alt-Svc?

Claiming HTTP/3 from a CDN brand or an Alt-Svc header alone creates a false result. Reliable checks explain the evidence boundary.

Verify HTTP/2

Confirm that the TLS endpoint and reverse proxy negotiate h2.

Confirm h3 advertisement

Check whether browsers receive an HTTP/3 upgrade signal.

Avoid false claims

Show negotiation and advertisement as separate states.

How does the protocol checker work?

The tool safely fetches the page for Alt-Svc and performs a lightweight TLS ALPN handshake to the final HTTPS host.

01
Follow safe redirects
Reach the final public HTTP or HTTPS URL.
02
Negotiate ALPN
Offer h2 and http/1.1 to the final TLS endpoint.
03
Read Alt-Svc
Check whether the final response advertises h3.
04
Report separate states
Show HTTPS, ALPN, HTTP/2 negotiation and HTTP/3 advertisement.

HTTP/2 and HTTP/3 FAQ

Does Alt-Svc h3 mean this request used HTTP/3?

No. It advertises an alternative service that the client may use later.

Is HTTP/3 required for SEO?

No. Stable, fast and crawlable delivery matters more; HTTP/2 is sufficient for most sites.

Why is HTTP/2 associated with HTTPS?

The standard allows h2c, but mainstream browsers use HTTP/2 on public sites primarily through HTTPS ALPN.

Are results stored?

No business record is stored; identical targets use short caching.

Continue with compression, caching, CDN and TLS certificates.

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