PerformanceResourceTiming: domainLookupStart property

The domainLookupStart property can have the following values:

• A DOMHighResTimeStamp immediately before the browser starts the domain name lookup for the resource.
• 0 if the resource was instantaneously retrieved from a cache.
• 0 if the resource is a cross-origin request and no Timing-Allow-Origin HTTP response header is used.

The domainLookupStart and domainLookupEnd properties can be used to measure how long it takes for the DNS lookup to happen.

const dns = entry.domainLookupEnd - entry.domainLookupStart;

Example using a PerformanceObserver, which notifies of new resource performance entries as they are recorded in the browser's performance timeline. Use the buffered option to access entries from before the observer creation.

const observer = new PerformanceObserver((list) => {
  list.getEntries().forEach((entry) => {
    const dns = entry.domainLookupEnd - entry.domainLookupStart;
    if (dns > 0) {
      console.log(`${entry.name}: DNS lookup duration: ${dns}ms`);
    }
  });
});

observer.observe({ type: "resource", buffered: true });

Example using Performance.getEntriesByType(), which only shows resource performance entries present in the browser's performance timeline at the time you call this method:

const resources = performance.getEntriesByType("resource");
resources.forEach((entry) => {
  const dns = entry.domainLookupEnd - entry.domainLookupStart;
  if (dns > 0) {
    console.log(`${entry.name}: DNS lookup duration: ${dns}ms`);
  }
});

If the value of the domainLookupStart property is 0, the resource might be a cross-origin request. To allow seeing cross-origin timing information, the Timing-Allow-Origin HTTP response header needs to be set.

For example, to allow https://developer.mozilla.org to see timing resources, the cross-origin resource should send:

Timing-Allow-Origin: https://developer.mozilla.org

• Timing-Allow-Origin