Network Error Logging (NEL)

Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

Network Error Logging is a mechanism that can be configured via the NEL HTTP response header. This experimental header allows websites and applications to opt-in to receive reports about failed (and, if desired, successful) network fetches from supporting browsers.

Reports are sent to a reporting group defined within a Report-To header.

Usage

Web applications opt in to this behavior with the NEL header, which is a JSON-encoded object:

http

NEL: { "report_to": "nel",
       "max_age": 31556952 }

An origin considered secure by the browser is required.

The following object keys can be specified in the NEL header:

report_to: The reporting API group to send network error reports to (see below).
max_age: Specifies the lifetime of the policy, in seconds (in a similar way to e.g., HSTS policies are time-restricted). The referenced reporting group should have a lifetime at least as long as the NEL policy.
include_subdomains: If true, this NEL policy is also enabled for all subdomains of the origin, but only for network errors that occur during DNS resolution. The NEL policy will not be enabled for subdomains if include_subdomains is not present, is false, or for other (non-DNS-related) network errors. The reporting group must also be set to include subdomains for this option to have effect.
success_fraction: Floating point value between 0 and 1 which specifies the proportion of successful network requests to report. Defaults to 0, so that no successful network requests will be reported if the key is not present in the JSON payload.
failure_fraction: Floating point value between 0 and 1 which specifies the proportion of failed network requests to report. Defaults to 1, so that all failed network requests will be reported if the key is not present in the JSON payload.

The reporting group referenced above is defined in the usual manner within the Report-To header, for example:

http

Report-To: { "group": "nel",
             "max_age": 31556952,
             "endpoints": [
              { "url": "https://example.com/csp-reports" }
             ]
           }

Error reports

In these examples, the reporting API response content is shown. The top-level "body" key contains the network error report.

HTTP 400 (Bad Request) response

json

{
  "age": 20,
  "type": "network-error",
  "url": "https://example.com/previous-page",
  "body": {
    "elapsed_time": 338,
    "method": "POST",
    "phase": "application",
    "protocol": "http/1.1",
    "referrer": "https://example.com/previous-page",
    "sampling_fraction": 1,
    "server_ip": "192.0.2.172",
    "status_code": 400,
    "type": "http.error",
    "url": "https://example.com/bad-request"
  }
}

DNS name not resolved

Note that the phase is set to dns in this report and no server_ip is available to include.