Network Error Logging Demo

NEL is active on this page — headers sent with every response

Headers sent with this page

These two headers instruct the browser to collect and report network errors

Report-To

{"group":"default","max_age":31536000,"endpoints":[{"url":"https://helios.report-uri.com/a/t/g"}],"include_subdomains":true}

NEL

{"report_to":"default","max_age":31536000,"include_subdomains":true,"failure_fraction":1.0}

What these headers do

Report-To registers a named reporting endpoint with the browser. Once registered, it's remembered for max_age seconds — so it catches errors even on subsequent page loads, without those pages needing to send the header again. NEL activates error collection and points it at the default group above.

Trigger a network error — the browser will queue a report for Report URI

Generate a NEL report

Load a broken resource — the failure will be captured and reported automatically

Note: NEL reports are sent by the browser asynchronously — sometimes on the next page load rather than immediately.

Server error (500)

Fetches a resource that responds with HTTP 500. NEL reports server errors just like network failures.

DNS failure

Attempts to fetch from a non-existent subdomain of this site. The browser logs the DNS error.

What a NEL report looks like

The browser sends a JSON payload to your reporting endpoint. Each report includes the type (e.g., http.error, dns.name_not_resolved), the failed URL, the referring page, the protocol used, and the status code. No JavaScript on the failing page is needed — the browser handles collection and delivery entirely.

Example NEL report payload (sent by the browser)

[{
  "age": 0,
  "type": "network-error",
  "url": "https://report-uri-demo.com/nel/500.php",
  "body": {
    "referrer": "https://report-uri-demo.com/nel/",
    "sampling_fraction": 1.0,
    "server_ip": "203.0.113.42",
    "protocol": "h2",
    "method": "GET",
    "request_headers": {},
    "response_headers": {},
    "status_code": 500,
    "elapsed_time": 120,
    "phase": "application",
    "type": "http.error"
  }
}]