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What's in Your DNS Query?

Russ White

Privacy problems are an area of wide concern for individual users of the Internet — but what about network operators? Geoff Huston wrote an article earlier this year concerning privacy in DNS and the various attempts to make DNS private on the part of the IETF — the result can be summarized with this long, but entertaining, quote:

"The Internet is largely dominated, and indeed driven, by surveillance, and pervasive monitoring is a feature of this network, not a bug. Indeed, perhaps the only debate left today is one over the respective merits and risks of surveillance undertaken by private actors and surveillance by state-sponsored actors. ... We have come a very long way from this lofty moral stance on personal privacy into a somewhat tawdry and corrupted digital world, where "do no evil!" has become 'don't get caught!'"

Before diving into a full-blown look at the many problems with DNS security, it is worth considering what kinds of information can leak through the DNS system. Let's ignore the recent discovery that DNS queries can be used to exfiltrate data; instead, let's look at more mundane data leakage from DNS queries.

For instance, say you work in a marketing department for a company that is just about to release a new product. To build the marketing and competitive materials your sales critters will need to stand in front of customers, you do a lot of research around competitor products. In the process, you examine, in detail, each of the competing product's pages. Or perhaps you work in a company that is determining whether another purchasing or merging with another company might be a good idea. Or you are working on a new externally facing application, or component in an existing application, that relies on a new connection point into your network.

All of these processes can lead to a lot of DNS queries. For someone who knows what they are looking for, the pattern of queries may be enough to examine strings queried from search engines and other information, ultimately leading to someone being able to guess a lot about that new product, what company your company is thinking about buying or merging with, what your new application is going to do, etc. DNS is a treasure trove of information at a personal and organizational level.

Operators and protocol designers have been working for years to resolve these problems, making DNS queries "more private;" Geoff Huston's article provides a good overview of many of these attempts. DNS over HTTPS (DoH), a recent (and ongoing) attempt bears a closer look.

DNS is normally sent "in plain text" over the network; anyone who can capture the packets can read not only the query but also the responses. The simplest way to solve this problem is to encrypt the DNS data in flight using something like TLS — hence DoT, or DNS over TLS. One problem with DoT is it is carried over a unique port number, which means it is probably blocked by default by most packet filters, and can easily be blocked by administrators who either do not know what this traffic is or do not want it on their network. To solve this, DoH carries TLS encrypted traffic in a way that makes it look just like an HTTPS session. If you block DoH traffic, you will also block access to web servers running HTTPS. This is the logical "end" of carrying everything else over HTTPS to avoid the impact of stateful and stateless packet filters and the impact of middle boxes on Internet traffic.

The good result is, in fact, that DNS traffic can no longer be "spied on" by anyone outside servers in the DNS system itself. Whether or not this is "enough" privacy is a matter of conjecture, however. Servers within the DNS system can still collect information about what queries you are making; if the server has access to other information about you or your organization, combining this data into a profile, or using it to determine some deeper investigation is warranted by looking at other sources of data, is pretty simple. Ultimately, DoH is only really useful if you trust your DNS provider.

Do you? Perhaps more importantly — should you?

DNS providers are like any other business; they must buy hardware, connectivity, and the time of smart people who can make the system work, troubleshoot the system when it fails, and think about ways of improving the system. If the service is free…

DoH, however, has another problem Geoff outlines in his article — DNS is moved up the stack, so it no longer runs over TCP and UDP directly, but instead, it runs over HTTPS. This means local applications, like browsers, can run DNS queries independently of the operating system. In fact, because these queries are TLS encrypted, the operating system itself cannot even "see" the contents of these DNS queries. This might be a good thing — or might be a bad thing. If nothing else, it means the browser, or any other application, can choose to use a resolver not configured by the local operating system. A browser maker, for instance, can direct their browser to send all DNS queries made within the browser to their DNS server, exposing another source of information about users (and the organizations they work for).

Remember that time you mistyped an internal hostname in your browser? Thankfully, you had a local DNS server configured, so the query did not go out to a resolver on the Internet. With DoH, the query can go out to an open resolver on the Internet regardless of how your local systems are configured. Something to ponder.

The bottom line is this — the nature of DNS makes it extremely difficult to secure. Somehow you have to have someone operate, and pay for, an open database of names which translate to addresses. Somehow you have to have a protocol that allows this database to be queried. All of these "somehows" expose information, and there is no clear way to hide that information. You can solve parts of the problem, but not the whole problem. Solving one part of the problem seems to make another part of the problem worse.

If you haven't found the tradeoff, you haven't looked hard enough.

In the end, though, the privacy of DNS queries at a personal and organizational level is something you need to think about.

By Russ White, Infrastructure Architect at Juniper Networks
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Related topics: Cybersecurity, DNS, DNS Security, Privacy
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