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To test HTTP/2, check the protocol actually negotiated on a real connection. In a browser, enable the Network panel’s Protocol column and look for h2. From a terminal, confirm that your curl build includes HTTP/2, then run curl --http2 -v https://example.com and look for ALPN selecting h2 followed by an HTTP/2 response. These checks are more reliable than a server setting or an Alt-Svc header because they observe the client, TLS, proxy and edge that served the request.
What a successful HTTP/2 test proves
HTTP/2 is negotiated per connection. A site can use HTTP/2 on one hostname, CDN edge or client while another connection falls back to HTTP/1.1. Redirects, third-party origins, corporate proxies and separate browser connections can therefore produce different protocol labels on the same page.
For HTTPS, the client and server negotiate the h2 Application-Layer Protocol Negotiation (ALPN) identifier during TLS. A successful connection then uses HTTP/2 frames and sends the protocol connection preface. Seeing an HTTP/2 response on the tested connection is the confirmation you want; an advertised capability alone is not.
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- Open the page using
https://. - Open Developer Tools (usually F12 or Ctrl+Shift+I).
- Select Network.
- Right-click the request-header row, choose the column picker, and enable Protocol (the exact menu wording varies by browser).
- Reload with the Network panel open. Inspect the document request and important same-origin assets.
A value of h2 means that request used HTTP/2. Values such as http/1.1 mean that particular connection did not. Check the document first, then CSS, JavaScript and image requests that matter to your page. A third-party asset can legitimately use a different protocol because it has a different origin or connection.
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Why one row is not the whole site
- A redirect may be served by one host and the final page by another.
- Separate origins open separate connections and can negotiate different protocols.
- Existing browser connections may be reused; close DevTools, clear the page, or use a fresh private window when validating a change.
- A VPN, proxy, antivirus filter or enterprise gateway can terminate TLS and create a different protocol hop.
Reproducible command-line test with curl
1. Confirm that curl can speak HTTP/2
Run:
curl -VRead the Features: line. It must contain HTTP2. If it does not, --http2 cannot work regardless of your server configuration. Install a curl package built with HTTP/2 support or use another current build.
2. Force HTTP/2 negotiation over HTTPS
curl --http2 -v https://example.comReplace the URL with the exact hostname and path you need to test. In verbose output, the confirming sequence looks like:
* ALPN: server accepted to use h2
* using HTTP/2
> GET / HTTP/2
< HTTP/2 200The wording differs slightly by curl version, but you should see ALPN acceptance of h2 and an HTTP/2 request or response. Save the output when comparing a direct origin, CDN hostname and public hostname.
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* ALPN: server did not agree to a protocol
> GET / HTTP/1.1
< HTTP/1.1 200 OKThis proves that this connection used HTTP/1.1. It does not by itself prove that every edge or hostname is misconfigured. Investigate the specific network path, TLS policy, proxy and client build.
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Useful curl variants
curl --http2 -I -v https://example.comtests headers without downloading the body, although some servers treatHEADdifferently fromGET.curl --http2 https://example.com -o page.htmlperforms a normal download while keeping protocol negotiation enabled.curl --http2-prior-knowledge http://example.comis for cleartext HTTP/2 only when you already know the endpoint supports it. Do not use it as a general HTTPS test.
HTTPS versus cleartext HTTP/2
For https://, HTTP/2 discovery normally occurs through TLS ALPN, using the two-octet protocol identifier h2. For http://, there is no TLS ALPN exchange. A client must have prior knowledge or receive out-of-band information. The older h2c Upgrade mechanism is deprecated by the HTTP/2 standard, so an ordinary cleartext request is not evidence that HTTP/2 is available.
Use an HTTPS URL when checking a public website. If you operate an internal cleartext service, document the expected prior-knowledge behavior and test it with a client explicitly designed for that deployment.
Frame-level diagnosis with nghttp2
When curl shows a fallback or you need proof of SETTINGS, HEADERS and DATA frames, use nghttp2’s nghttp client:
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Do not confuse testing with load generation
nghttp2 also provides h2load, a benchmarking tool. A load test answers how a server behaves under a chosen workload; it does not prove that a normal browser connection negotiated HTTP/2. Keep the questions separate: use DevTools or curl for negotiation, nghttp -nv for frames, and h2load for controlled performance experiments.
How to interpret conflicting results
| Observation | What it establishes | Next check |
|---|---|---|
DevTools shows h2 for the document |
The browser used HTTP/2 on that document connection. | Check redirects and other origins separately. |
curl accepts ALPN h2 |
Your curl connection negotiated HTTP/2. | Repeat from the affected region or network if users report otherwise. |
| curl says ALPN was not agreed and returns HTTP/1.1 | This connection fell back. | Check TLS ALPN, proxy/CDN termination, hostname coverage and curl capability. |
| Origin supports HTTP/2 but public URL does not | The edge or proxy path is not passing through HTTP/2 for this client. | Test the public hostname, not only the origin. |
An Alt-Svc header is present |
The server advertised an alternative service. | Perform an actual negotiated-protocol test; the header alone is not proof. |
Troubleshooting HTTP/1.1 results
curl lacks HTTP2 in Features
Cause: the installed libcurl was built without an HTTP/2 library. Fix: install a distribution package or build that reports HTTP2 with curl -V, then rerun the command.
ALPN is not accepted
Cause: the server, TLS terminator or intermediary did not offer or select h2. Fix: inspect the TLS configuration at the public edge, verify that the certificate and virtual host match the requested hostname, and test without a proxy or VPN where possible.
Only some requests show h2
Cause: redirects, third-party origins, connection reuse or different CDN edges. Fix: inspect the final document request, record each hostname, and test each important origin independently.
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Browser and curl disagree
Cause: different networks, TLS stacks, DNS answers, user agents or proxy policies. Fix: compare the same hostname and URL from the same network, then repeat from the user environment that matters. Do not generalize one curl result to every client or edge.
The page is HTTP/2 but performance did not improve
Protocol selection is only one factor. Connection distance, server work, compression, caching, prioritization, third-party scripts and payload size still affect loading. Use a waterfall and a controlled workload; do not treat an HTTP/2 label as a performance guarantee.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Automate a pass/fail check
A simple shell check can fail a deployment when curl does not report HTTP/2:
set -o pipefail
curl --http2 -sS -D - https://example.com -o /dev/null 2>&1 | tee http2-check.log
if grep -qE 'HTTP/2|using HTTP/2|server accepted to use h2' http2-check.log; then
echo "HTTP/2 negotiated"
else
echo "HTTP/2 not confirmed"
exit 1
fiKeep the raw log as an artifact. For stronger automation, parse curl’s verbose output in your CI language and record hostname, resolved address, timestamp, proxy settings and curl version alongside the result. A pass is valid only for that test path and time.
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Or skip the browser setup
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One request returns PNG, JPEG, WebP or PDF:
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Further reading
For a book-length treatment of frames, streams, upgrading and troubleshooting, see HTTP/2 in Action by Barry Pollard (Manning, 416-page print edition, March 2019, ISBN 9781617295164). Treat its publication date as historical context and verify current implementation details against your server, proxy and client documentation.
Frequently Asked Questions
Does an HTTPS certificate prove HTTP/2 is enabled?
No. A certificate enables TLS, while HTTP/2 still has to be selected through ALPN on the individual connection.
Can I test HTTP/2 with an online checker instead of curl?
You can, but the result describes that checker’s network path. For a reproducible diagnosis, keep a local curl or DevTools result from the client and hostname you care about.
Should I test the origin IP directly?
Only for an origin-specific diagnostic. A direct IP may use a different certificate, virtual host and protocol policy than the public hostname, so it cannot stand in for the user-facing test.
Quick Recap
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