Last-Mile Browser Network Visibility. Zero JavaScript.
Direct telemetry into DNS lookup timeouts, TCP connection drops, TLS handshake failures, and origin 5xx spikes—reported natively by modern browser user agents without client-side JavaScript SDKs.
Interactive Network Error Logging Playground
Experience browser-level network telemetry without installing anything. Simulate edge DNS, TLS, and gateway failures, inspect live W3C reports+json payloads, and copy copy-paste deployment configs.
Live W3C NEL Payload
application/reports+jsonStandardized report dispatched by browser user agent
[
{
"age": 1420,
"type": "network-error",
"url": "https://example.com/api/v1/checkout",
"user_agent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/130.0.0.0 Safari/537.36",
"body": {
"uri": "https://example.com/api/v1/checkout",
"referrer": "",
"sampling_fraction": 1,
"server_ip": null,
"protocol": null,
"method": "GET",
"status_code": 0,
"elapsed_time": 42,
"phase": "dns",
"type": "dns.name_not_resolved"
}
}
]Diagnostic Analysis: DNS Resolution Failure
Authoritative DNS server timed out or zone record does not exist. TCP socket connection was never attempted.
Native Browser Out-of-Band Delivery
Browser buffers failure report in isolated network stack memory queue. Out-of-band report is dispatched upon network availability.
Server & Edge Deployment Snippet
Production-ready response headers configured for your network stack
Token Architecture & Verification Guidance
Template ModeNELSink requires a valid ingestion token in report endpoints (/r/{token}) to authenticate incoming browser reports. In production, tokens are minted permanently once domain ownership is verified via an Apex DNS TXT record (nelsink=<account_token>).
# NELSink Network Error Logging (NEL) Policy
add_header NEL '{"report_to":"nelsink","max_age":2592000,"include_subdomains":true,"failure_fraction":1,"success_fraction":0}' always;
add_header Report-To '{"group":"nelsink","max_age":2592000,"endpoints":[{"url":"https://in.nelsink.com/r/{token}"}],"include_subdomains":true}' always;
add_header Reporting-Endpoints 'nelsink="https://in.nelsink.com/r/{token}"' always;Instructs browsers to log 100% of network failures (failure_fraction: 1.0) with 30-day cache TTL.
Modern RFC 8941 Structured Fields header routing reports to NELSink edge ingress.
What is Network Error Logging (W3C NEL)?
Network Error Logging (W3C NEL) is a standardized browser mechanism that empowers web applications to receive automatic, out-of-band telemetry whenever real users fail to reach your servers.
Traditional Monitoring: The "Pre-JS" Blind Spot
JavaScript APMs (Datadog, Sentry, New Relic) and analytics scripts require a successful page render before they can execute. When authoritative DNS drops, edge certificates expire, or firewalls block TCP handshakes, no JavaScript ever runs. Synthetic monitors only test from cloud datacenters, leaving real-world end-user failures completely invisible.
W3C NEL: Native Browser Network Telemetry
W3C NEL runs directly inside the browser user agent's network stack. Once a browser visits your site and receives your policy header, it caches the configuration. Any subsequent failure—across DNS, TCP, TLS, or HTTP—is recorded in browser memory and dispatched asynchronously in the background once connectivity is restored.
How Network Error Logging Operates
Policy Advertisement
Your server or CDN advertises standard NEL and Reporting-Endpoints HTTP response headers on secure connections.
Client Storage
The user agent locally caches the policy for your domain and subdomains for the specified lifetime (max_age).
Native Failure Capture
If any future request encounters a connection reset, DNS timeout, or TLS abort, the browser creates an application/reports+json report.
Asynchronous Dispatch
The browser buffers reports and uploads them out-of-band to NELSink with zero page latency or JavaScript footprint.
Core Operational Use Cases
When traditional synthetics and application metrics fail, W3C NEL provides definitive ground truth.
DNS & Anycast Failures
Diagnose authoritative nameserver outages, NXDOMAIN poisoning, and anycast routing blackholes before customer support queues escalate.
TLS & Certificate Issues
Detect expired SSL certificates, SNI mismatch errors, and cipher negotiation failures on specific client cohorts and mobile operating systems.
ISP Eyeball Corroboration
Group errors by client Autonomous System Numbers (ASNs) to distinguish global infrastructure outages from local transit carrier disruptions.
Zero-Overhead Monitoring
Zero JavaScript bundle weight, zero client CPU consumption, and zero impact on Interaction to Next Paint (INP) or Core Web Vitals.
Why Engineering Teams Choose NELSink
Comprehensive last-mile visibility into browser connectivity, edge routing, and global ISP health.
Global Edge Telemetry
Capture real-world connection drops, DNS resolution errors, and SSL/TLS handshake failures from browser clients across every geography and ISP network without installing heavy client-side SDKs.
Actionable Last-Mile Intelligence
Transform raw browser error reports into clear root-cause diagnostics. Correlate regional transit flaps, identify top failing endpoints, and evaluate domain reliability scores in real time.
Frictionless Security & Governance
Enterprise domain verification via standard DNS records, automated token rotation with zero report drop, and universal deployment across Cloudflare, Nginx, Apache, Caddy, and modern clouds.