Set up Network Error Logging

Two response headers on your origin make the browser report every failed request to fastmon, including pages that never ran the tracker. Only Chromium browsers send reports. What the reports show is on Network Error Logging.

Set it upCopy Anchor Link

1. Enter the application's collector_hash in the panel below, or open this page with ?collector_hash=. Every snippet on the page then uses it.
2. Send the two headers on every HTTPS response of each host.
3. Make sure every host that sends them is a site of that application, or a subdomain of one.
4. Verify that a report arrives.

A report whose host is no site of the application is dropped, and a report never creates a site. www.shop.example counts for shop.example, notshop.example does not.

The headersCopy Anchor Link

Report-To: {"group":"fastmon","max_age":86400,"endpoints":[{"url":"https://fastmon.site/c/n/{collector_hash}"}]}NEL: {"report_to":"fastmon","max_age":86400,"success_fraction":0,"failure_fraction":1}

An application with a custom collector endpoint uses its own host instead of fastmon.site.

Where to set them:

• On every response, error pages included. On a site behind several addresses, an error page without the headers is lost.
• At the outermost server you control. The ingress or load balancer answers its own 502, which the application never sees.
• Over HTTPS only. The browser ignores a policy from a plain HTTP response.
• On each host. www.shop.example and shop.example are two policies.

ExamplesCopy Anchor Link

server {  listen 443 ssl;  server_name shop.example;  add_header Report-To '{"group":"fastmon","max_age":86400,"endpoints":[{"url":"https://fastmon.site/c/n/{collector_hash}"}]}' always;  add_header NEL '{"report_to":"fastmon","max_age":86400,"success_fraction":0,"failure_fraction":1}' always;}

include_subdomainsCopy Anchor Link

"include_subdomains":true in the NEL header extends the policy to every subdomain of the host. For those subdomains the browser reports DNS failures only. fastmon keeps such a report when the subdomain is a site of the application, or a subdomain of the only site. A subdomain that serves its own responses is better covered by its own headers.

VerifyCopy Anchor Link

1. The headers arrive. In the browser's network tab, check any response of the host for Report-To and NEL.
2. A report goes out. Load a page in Chrome, then open a path that does not exist. The 404 has to carry both headers too. Chrome uploads an http.error report after about a minute.
3. The endpoint answers. Test it without a browser, with a registered site in place of shop.example:

curl -i https://fastmon.site/c/n/{collector_hash} \-H 'Content-Type: application/reports+json' \--data '[{"age":0,"type":"network-error","url":"https://shop.example/missing","body":{"type":"http.error","phase":"application","status_code":404,"elapsed_time":12,"method":"GET","protocol":"http/1.1","referrer":"","sampling_fraction":1}}]'

The endpoint answers 204 whether it kept the report or dropped it. The Sites and hosts card on the dashboard shows each site's last report.

TrapsCopy Anchor Link

• Cloudflare replaces your headers when its own Network Error Logging setting is on. Turn it off for the zone.
• Shopify storefronts allow no custom response headers, so a Shopify shop cannot send a policy.
• Report-To is required. Chromium's NEL does not read Reporting-Endpoints.
• Each application has its own hash. A report for a site of another application drops, even in the same organization.
• A hash rotation stops the reports silently. Put the new collector_hash into Report-To.
• A first-party proxy has to forward /c/n/ and the browser's OPTIONS preflight. A location /c/ block covers both.
• Removing the headers does not remove the policy. Browsers keep reporting for up to max_age seconds. To stop sooner, send NEL with "max_age":0 for a day first.