files/en-us/web/api/performanceresourcetiming/domainlookupstart/index.md
{{APIRef("Performance API")}}{{AvailableInWorkers}}
The domainLookupStart read-only property returns the {{domxref("DOMHighResTimeStamp","timestamp")}} immediately before the browser starts the domain name lookup for the resource.
The domainLookupStart property can have the following values:
0 if the resource was instantaneously retrieved from a cache.0 if the resource is a cross-origin request and no {{HTTPHeader("Timing-Allow-Origin")}} HTTP response header is used.The domainLookupStart and {{domxref("PerformanceResourceTiming.domainLookupEnd", "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 {{domxref("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: ${dns}ms`);
}
});
});
observer.observe({ type: "resource", buffered: true });
Example using {{domxref("Performance.getEntriesByType()")}}, which only shows resource performance entries present in the browser's performance timeline at the time you call this method:
const resources = performance.getEntriesByType("resource");
resources.forEach((entry) => {
const dns = entry.domainLookupEnd - entry.domainLookupStart;
if (dns > 0) {
console.log(`${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 {{HTTPHeader("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
{{Specifications}}
{{Compat}}