CheckWebsiteNow

Connection errors

ERR_QUIC_PROTOCOL_ERROR

The browser had a problem with the QUIC (HTTP/3) connection.

Quick answer

QUIC is a newer connection method used by HTTP/3. This error means the QUIC connection failed. Firewalls, VPNs or networks that block this traffic are frequent causes.

When to wait: Rarely; try the network steps first.

If you are visiting the website

  1. Reload once A short network hiccup can break a QUIC connection. Reload the page once.
  2. Try another network Switch from Wi-Fi to mobile data, or the other way round. If the site works there, your network blocks or disturbs QUIC traffic.
  3. Pause your VPN briefly Some VPNs handle QUIC traffic badly. Test once without the VPN, then turn it back on.
  4. Pause HTTPS scanning briefly Firewall or antivirus web filters can interfere with QUIC. Pause only that feature for one test, then switch it back on right away.
  5. Start fresh connections Throw away old connections so the browser builds new ones.
    Windows
    Chrome: chrome://net-internals/#sockets > Flush socket pools. Edge: edge://net-internals/#sockets.
    macOS
    Chrome: chrome://net-internals/#sockets > Flush socket pools. Edge: edge://net-internals/#sockets.
    ChromeOS
    Open chrome://net-internals/#sockets and click Flush socket pools.
    Android
    Close Chrome from the recent apps screen and open it again, or turn Airplane mode on and off once.
  6. Turn off extensions one at a time Privacy and security add-ons can interfere with connections. Turn them off one by one to find the cause, then turn the others back on.
    Windows
    Chrome: More (three dots) > Extensions > Manage extensions. Edge: edge://extensions.
    macOS
    Chrome: More (three dots) > Extensions > Manage extensions. Edge: edge://extensions.
    ChromeOS
    Chrome: More (three dots) > Extensions > Manage extensions.
  7. Update your browser Install the latest browser version, because QUIC support keeps changing. In Chrome on a computer: More > Help > About Google Chrome > Relaunch.

If you run the website

  1. Make sure normal HTTPS works Test with curl --http2 -v https://example.com/ over an ordinary TCP connection. Visitors on networks that block QUIC must still be able to load your site this way.
  2. Check HTTP/3 settings at your CDN On Cloudflare, HTTP/3 is under Speed > Settings > Protocol Optimization. If you suspect HTTP/3 itself, switch it off for a short test and compare error reports.
  3. Check the Alt-Svc header Browsers learn that a site offers HTTP/3 from the Alt-Svc response header. If it advertises a host or port that does not actually answer QUIC, correct or remove it.
  4. Allow UDP if you serve HTTP/3 yourself QUIC runs over UDP, not TCP. If your own server answers HTTP/3, the firewall and load balancer must pass UDP traffic on the advertised port.
  5. Check server error logs Look at your web server and application logs for the time of the error. They usually name the exact cause.

Is it the website or me?

QUIC problems often come from the network. Office firewalls, school filters, some VPNs and hotel Wi-Fi can block or disturb its traffic.

If the site works on mobile data but not on one Wi-Fi network, that network is the likely cause.

What QUIC is, in plain words

QUIC is a newer transport (the delivery layer under the web page) that HTTP/3 uses. It runs over UDP instead of TCP and has encryption built in from the start.

Browsers use it when a website announces that it supports HTTP/3.

For developers: what happens technically

Chromium reports net error -356 when a QUIC connection or stream fails with a protocol error. Typical sources are middleboxes (network devices between browser and server) that alter or drop UDP packets, or an HTTP/3 endpoint that is advertised but misconfigured.

Common questions

Is QUIC unsafe, and should I turn it off?

No. QUIC is an encrypted, standardised protocol (RFC 9000). Testing another network or pausing your VPN tells you more than changing hidden browser settings.

Why does it only happen at work or school?

Those networks often filter traffic and may block or inspect UDP. Ask the IT team; they can see whether QUIC is blocked.

Does this error mean the website is down?

No. The server may work fine over ordinary HTTPS. Our check does not use QUIC, so a green result there together with this error points to the QUIC path.

I own the site. Should I disable HTTP/3?

Only as a short test. If the errors stop, find the misconfiguration instead of leaving HTTP/3 off forever; if they continue, HTTP/3 was not the cause.

Likely causes

  • Common A firewall, VPN or network blocks or disturbs QUIC traffic. (Your side)
  • Sometimes A server or CDN HTTP/3 misconfiguration. (Website side)

What to avoid

  • Do not leave your VPN or security software switched off after testing; turn it back on.

Is the website down? Check it now

One measurement point. Results explain what we saw, not a worldwide verdict.

Sources

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