Roben Kleene

On Cloudflare Warp & Privacy

Cloudflare recently announced Warp, their performance and security focused VPN. Warp will be added to the app, which, as of now, acts as an easy way to setup their “privacy-first consumer DNS service”, also called, on your mobile device:

Truth be told, the App was really just a lead up to today. We had a plan on how we could radically improve the performance, security, and privacy of the mobile Internet well beyond just DNS. To pull it off, we needed to understand the failure conditions when a VPN app switched between cellular and WiFi, when it suffered signal degradation, tried to register with a captive portal, or otherwise ran into the different conditions that mobile phones experience in the field.

If you’re interested in trying it out, you can claim your place in line in the app:

So, beginning today, what you can do is claim your place in line to be among the first to get Warp. If you already have the App on your phone, you can update it through the Apple App Store or the Google Play Store. If you don’t yet have the App you can download it free from Apple or Google. Once you’ve done that you’ll see an option to claim your place in line for Warp.

Until Warp arrives, the app just configures your device to use their DNS provider, which Cloudflare touts as having a couple of advantages: The first is speed, Cloudflare is a CDN, and therefore has the infrastructure to make a fast DNS service (DNSPerf currently ranks them as the fastest DNS provider). The second advantage, one that’s much more subtle and interesting, is privacy.

HTTPS, DNS, SNI & Privacy

The name “” refers to two things: A DNS service that can be configured manually1, and an app in the App Store, that automatically configures your device to use their DNS provider via a VPN.

If you haven’t done anything special to configure your DNS, then you’re probably using your ISP’s DNS provider. This means your DNS requests go straight to your ISP and they can see (and potentially store) the domain of every website that you visit (if you’re using HTTPS, then this applies only to the domain, not the path, so is visible but /2019/06/16/on-cloudflare-warp-privacy/ is not). In fact, even if you’ve manually setup as your DNS provider, without using the app, then your ISP can still see all of your DNS requests, because DNS is an unencrypted protocol.

This is where another feature of the app comes in: It also supports DNS over HTTPS and DNS over TLS. Neither of these protocols are supported by iOS (or macOS) by default, so they’re implemented using a VPN. Cloudflare CEO Matthew Prince shares the reasoning behind the VPN in a comment on the app announcement:

iOS, unfortunately, only allows you to set DNS settings on a per-WiFi-network basis. That means, you need to set your DNS settings for each WiFi network. And, even if you do that, it won’t cover you when you’re using your cellular provider. Moreover, while is fast and more privacy respecting, iOS, by default, does not support encrypted DNS (either DNS over TLS or DNS over HTTPS). The only way to support across all networks and to add encrypted DNS support was to setup a VPN profile. We’re hopeful that both iOS and Android will provide more flexibility in the future but, for now, that was the only technical way to make it work. Note: we are only proxying DNS traffic via the VPN. Non-DNS traffic is not routed through the VPN.

Once the VPN is turned on, your DNS queries go to Cloudflare’s DNS servers, not your ISP’s, and they are encrypted.

So if you’re using the Cloudflare app, does that mean your ISP can no longer see which domains you’re visiting? Unfortunately, the answer is still no. First of all, the domain is also sent unencrypted as part of the initial TLS handshake when establishing an HTTPS connection as part of Server Name Indication (SNI). This is done so multiple domains can use the same IP address, and, since the domains might have different SSL certificates, it is done before the encrypted connection is established.

In addition to the domain being sent unencrypted for SNI, the IP address of the server you are visiting is also sent unencrypted (as part of the lower-level TCP connection that HTTPS sits on top of). The IP address isn’t as revealing as the actual domain, and with the proliferation of Cloud platforms, it’s often impossible to map an IP address back to a single domain, but it’s still more information than you’d be ideally sharing.

So while the app without Warp does block one way your ISP can see which domains you’re visiting, it leaves another open with SNI. And the IP address of the server you’re visiting is also always visible. So it’s more of a steps towards a more private internet rather than a complete solution.

Cloudflare Warp

Enter Cloudflare Warp. With Warp, all traffic is routed through the VPN and all traffic is encrypted until it exits the VPN. So the domains you are visiting are no longer visible to your ISP. The only thing they can see is that you are visiting the IP address of one of Cloudflare’s VPN servers. Here’s Prince again, in a blog post introducing Warp, summarizing its benefits:

This doesn’t just apply to your web browser but to all apps running on your phone. Any unencrypted connections are encrypted automatically and by default. Warp respects end-to-end encryption and doesn’t require you to install a root certificate or give Cloudflare any way to see any encrypted Internet traffic we wouldn’t have otherwise.

Unfortunately, a lot of the Internet is still unencrypted. For that, Warp automatically adds encryption from your device to the edge of Cloudflare’s network — which isn’t perfect, but is all other VPNs do and it does address the largest threats typical Internet users face. One silver lining is that if you browse the unencrypted Internet through Warp, when it’s safe to do so, Cloudflare’s network can cache and compress content to improve performance and potentially decrease your data usage and mobile carrier bill.

The privacy benefit of a VPN is that two kinds of information that were once visible to the same entity are now divided between two separate entities: Who you are, and which websites you are visiting2. With a VPN, your ISP knows who you are (because presumably they are billing you), but they don’t know which websites you are visiting. whether this is an improvement has a lot to do with how much you trust your VPN provider, in this case Cloudflare. This means in order to find out which websites that you personally visited means having access to the data stored by two separate companies. And that even assumes Cloudflare is actually storing that data, which they explicitly state they are not, and have hired an external firm to verify it:

We will never log your IP address (the way other companies identify you). And we’re not just saying that. We’ve retained KPMG to audit our systems annually to ensure that we’re doing what we say.

Given all the privacy benefits of using a VPN, why aren’t they already more widespread? Traditionally, the problem with a VPN is performance. Since all your traffic has to be routed through the VPN, it means that your traffic cannot take the most direct route to its destination. This introduces lag, here’s how Prince describes it:

VPNs suck, especially on mobile. They add latency, drain your battery, and, in many cases, are run by companies with motivations that are opposite to actually keeping your data private and secure.

He goes into some more technical details of how they’re planning to improve performance, but suffice to say Cloudflare is in a great position to offer a performant VPN, because they can leverage the infrastructure of their CDN service.

I wrote this piece to try and figure out whether and Warp are worth using for myself, and I’ve concluded that they are3. Hiding the domains you’re visiting from your ISP (not to mention the rest of your unencrypted traffic) is a privacy improvement—even if it means that now Cloudflare can see them. Cloudflare doesn’t know who you are4, like your ISP does, and Cloudflare has at least made a promise not to store your information. Unlike the ISPs, who not only have made no such promise, but they’ve lobbied for the right share your browsing history for profit. The less they know the better.

  1. can also be configured manually as a DNS server on macOS, iOS, or your router, but if configured that way, DNS traffic, including the domains you visit, will not be encrypted↩︎

  2. In reality, a VPN provider could probably uniquely identify you by analyzing your traffic, so trusting your VPN provider is important (but no more so than trusting your ISP already is). ↩︎

  3. There is one other reason not to use Cloudflare Warp, one that’s a bit more subtle. In general the internet is becoming more centralized, and, at the infrastructure level, one of the companies that it’s centralizing around is Cloudflare. Since their products are less consumer orientated, and more business-to-business, many people aren’t aware just how massive Cloudflare actually is. According to W3Techs, 9.2% of all websites use Cloudflare. In some ways, using a product like Cloudflare Warp is contributing to this centralization. ↩︎

  4. Unless you have a Cloudflare account… privacy and security online is like trying to float a Swiss cheese boat. ↩︎