Over the past few months, Mozilla has experimented with DNS-over-HTTPS (DoH). The intention is to fix a part of a DNS ecosystem that simply isn’t up to the modern, secure standards that every Internet user should expect. Today, we want to let you know about our next test of the feature.

Our initial tests of DoH studied the time it takes to get a response from Cloudflare’s DoH resolver. The results were very positive – the slowest users show a huge performance improvement. A recent test in our Beta channel confirmed that DoH is fast and isn’t causing problems for our users. However, those tests only measure the DNS operation itself, which isn’t the whole story.

Content Delivery Networks (CDNs) provide localized DNS responses depending on where you are in the network, with the goal being to send you to a host which is near you on the network and therefore will give you the best performance. However, because of the way that Cloudflare resolves names [technical note: it’s a centralized resolver without EDNS Client Subnet], this process works less well when you are using DoH with Firefox.

The result is that the user might get less well-localized results that could result in a slow user experience even if the resolver itself is accurate and fast.

This is something we can test. We are going to study the total time it takes to get a response from the resolver and fetch a web page. To do that, we’re working with Akamai to help us understand more about the performance impact. Firefox users enrolled in the study will automatically fetch data once a day from four test web pages hosted by Akamai, collect information about how long it took to look up DNS and then send that performance information to Firefox engineers for analysis. These test pages aren’t ones that the user would automatically retrieve and just contain dummy content.

A soft rollout to a small portion of users in our Release channel in the United States will begin this week and end next week. As before, this study will use Cloudflare’s DNS-over-HTTPS service and will continue to provide in-browser notifications about the experiment so that everyone is fully informed and has a chance to decline participation in this particular experiment. Moving forward, we are working to build a larger ecosystem of trusted DoH providers, and we hope to be able to experiment with other providers soon.

We don’t yet have a date for the full release of this feature. We will give you a readout of the result of this test and will let you know our future plans at that time. So stay tuned.