NEL
NEL (Network Error Logging) lets a site declare that the browser should report network errors (DNS/TLS/HTTP failures) to a designated endpoint.
Purpose
It lets site operators collect client-side network errors (connection failures, TLS errors, and the like) to monitor reachability and performance.
Mechanism
The site sends an NEL header with report_to and max_age; on later request failures, the browser POSTs error reports (URL, phase, type, server IP) to the Reporting API endpoint.
Use cases
- Client-side network error monitoring
- TLS failure collection
- Reachability reporting
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