Web applications opt in to this behavior 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:

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, 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.

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:

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

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

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:

dns.unreachable

The user's DNS server is unreachable

dns.name_not_resolved

The user's DNS server responded but was unable to resolve an IP address for the requested URI.

dns.failed

Request to the DNS server failed due to reasons not covered by previous errors (e.g. SERVFAIL)

dns.address_changed

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.timed_out

TCP connection to the server timed out

tcp.closed

The TCP connection was closed by the server

tcp.reset

The TCP connection was reset

tcp.refused

The TCP connection was refused by the server

tcp.aborted

The TCP connection was aborted

tcp.address_invalid

The IP address is invalid

tcp.address_unreachable

The IP address is unreachable

tcp.failed

The TCP connection failed due to reasons not covered by previous errors

http.error

The user agent successfully received a response, but it had a 4xx or 5xx status code

http.protocol.error

The connection was aborted due to an HTTP protocol error

http.response.invalid

Response is empty, has a content-length mismatch, has improper encoding, and/or other conditions that prevent user agent from processing the response

http.response.redirect_loop

The request was aborted due to a detected redirect loop

http.failed

The connection failed due to errors in HTTP protocol not covered by previous errors