Network Error Logging

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 web sites 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.


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

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:


The reporting API group to send network error reports to (see below).

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.
If true, the policy applies to all subdomains under the origin that the policy header is set. The reporting group should also be set to include subdomains, if this option is to be enabled.
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.
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:

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

Error reports

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

HTTP 400 (Bad Request) response

  "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": "",
    "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.

  "age": 20,
  "type": "network-error",
  "url": "https://example.com/previous-page",
  "body": {
    "elapsed_time": 18,
    "method": "POST",
    "phase": "dns",
    "protocol": "http/1.1",
    "referrer": "https://example.com/previous-page",
    "sampling_fraction": 1,
    "server_ip": "",
    "status_code": 0,
    "type": "dns.name_not_resolved",
    "url": "https://example-host.com/"

The type of the network error may be one of the following pre-defined values from the specification, but browsers can add and send their own error types:

The user's DNS server is unreachable
The user's DNS server responded but was unable to resolve an IP address for the requested URI.
Request to the DNS server failed due to reasons not covered by previous errors (e.g. SERVFAIL)
For security reasons, if the server IP address that delivered the original report is different to the current server IP address at time of error generation, the report data will be downgraded to only include information about this problem and the type set to dns.address_changed.
TCP connection to the server timed out
The TCP connection was closed by the server
The TCP connection was reset
The TCP connection was refused by the server
The TCP connection was aborted
The IP address is invalid
The IP address is unreachable
The TCP connection failed due to reasons not covered by previous errors
The user agent successfully received a response, but it had a 4xx or 5xx status code
The connection was aborted due to an HTTP protocol error
Response is empty, has a content-length mismatch, has improper encoding, and/or other conditions that prevent user agent from processing the response
The request was aborted due to a detected redirect loop
The connection failed due to errors in HTTP protocol not covered by previous errors


Network Error Logging

Browser compatibilityUpdate compatibility data on GitHub

Chrome Edge Firefox Internet Explorer Opera Safari
NEL 71
Disabled From version 69: this feature is behind the Reporting runtime flag (needs to be set to true).
79 No No 58 No
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
NEL 71 71 No 50 No 10.2

© 2005–2020 Mozilla and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.