Today we heard an announcement that Cloudflare, Apple, and Fastly are collaborating on a new technology for improving privacy of DNS queries using a technology they call Oblivious DoH (ODoH).
This is an exciting development. This posting examines the technology in more detail and looks at some of the challenges this will need to overcome before it can be deployed more widely.
How ODoH Provides Privacy for DNS Queries
Oblivious DoH is a simple mixnet protocol for making DNS queries. It uses a proxy server to provide added privacy for query streams.
This looks something like:
A common criticism of DNS over HTTPS (DoH) is that it provides DoH resolvers with lots of privacy-sensitive information. Currently all DNS resolvers, including DoH resolvers, see the contents of queries and can link that to who is making those queries. DoH includes connection reuse, so resolvers can link requests from the same client using the connection.
In Oblivious DoH, a proxy aggregates queries from multiple clients so that the resolver is unable to link queries to individual clients. ODoH protects the IP address of the client, but it also prevents the resolver from linking queries from the same client together. Unlike an ordinary HTTP proxy, which handle TLS connections to servers, ODoH proxies handle queries that are individually encrypted.
ODoH prevents resolvers from assembling profiles on clients by collecting the queries they make, because resolvers see queries from a large number of clients all mixed together.
An ODoH proxy learns almost nothing from this process as ODoH uses HPKE to encrypt the both query and answer with keys chosen by the client and resolver.
The privacy benefits of ODoH can only be undone if both the proxy and resolver cooperate. ODoH therefore recommends that the two services be run independently, with the operator of each making a commitment to respecting privacy.
The privacy advantages provided by the ODoH design come at a higher cost than DoH, where a client just queries the resolver directly:
- The proxy adds a little latency as it needs to forward queries and responses.
- HPKE encryption adds up to about 100 bytes to each query.
- The client and resolver need to spend a little CPU time to add and remove the encryption.
Cloudflare’s tests show that the overall effect of ODoH on performance is quite modest. These early tests even suggest some improvement for the slowest queries. If those performance gains can be kept as they scale up their deployment, that would be strong justification for deployment.
Why This Design
A similar outcome might be achieved using a proxy that supports HTTP CONNECT. However, in order to avoid the resolver from learning which queries come from the same client, each query would have to use a new connection.
That gets pretty expensive. While you might be able to tricks to drive down latency like sending the TLS handshake with the HTTP CONNECT, it means that every request uses a separate TCP connection and a round trip to establish the connection.
It is also possible to use something like Tor, which provides superior privacy protection. Tor is a lot more expensive.
Using HPKE and a multiplexed protocol like HTTP/2 or HTTP/3 avoids per-query connection setup costs. However, the most important thing is that it involves only minimal additional latency to get the privacy benefits.
Key Management in DNS
The proposal puts HPKE keys for the resolver in the DNS. The idea is that clients can talk to the resolver directly to get these, then use that information to protect its queries. As the keys are DNS records, they can be retrieved from any DNS resolver, which is a potential advantage.
This also means that this ODoH design depends on DNSSEC. Many clients rely on their resolver to perform DNSSEC validation, which doesn’t help here. So this makes it difficult to deploy something like this incrementally in clients.
A better option might be to offer the HPKE public key information in response to a direct HTTP request to the resolver. That would ensure that the key could be authenticated by the client using HTTPS and the Web PKI.
Trustworthiness of Proxies
Both client and resolver will want to authenticate the proxy and only allow a trustworthy proxy. The protocol design means that the need for trust in the proxy is limited, but it isn’t zero.
Clients need to trust that the proxy is hiding their IP address. A bad proxy could attach the client IP address to every query they forward. Clients will want some way of knowing that the proxy won’t do this.
Resolvers will likely want to limit the number of proxies that they will accept requests from, because the aggregated queries from a proxy of any reasonable size will look a lot like a denial of service attack. Mixing all the queries together denies resolvers the ability to do per-client rate limiting, which is a valuable denial of service protection measure. Resolvers will need to apply much more generous rate limits for these proxies and trust that the proxies will take reasonable steps to ensure that individual clients are not able to generate abusive numbers of queries.
This means that proxies will need to be acceptable to both client and resolver. Early deployments will be able to rely on contracts and similar arrangements to guarantee this. However, if use of ODoH is to scale out to support large numbers of providers of both proxies and resolvers, it could be necessary to build systems for managing these relationships.
Proxying For Other Applications
One obvious thing with this design is that it isn’t unique to DNS queries. In fact, there are a large number of request-response exchanges that would benefit from the same privacy benefits that ODoH provides. For example, Google this week announced a trial of a similar technology for preloading content.
A generic design that enabled protection for HTTP queries of any sort would be ideal. My hope is that we can design that protocol.
Once you look to designing a more generic solution, there are a few extra things that might improve the design. Automatic discovery of HTTP endpoints that allow oblivious proxying is one potential enhancement. Servers could advertise both keys and the proxies they support so that clients can choose to use those proxies to mask their address. This might involve automated proxy selection or discovery and even systems for encoding agreements. There are lots of possibilities here.
One criticism regarding DoH deployments is that they encourage consolidation of DNS resolver services. For ODoH - at least in the short term - options for ODoH resolvers will be limited, which could push usage toward a small number of server operators in exchange for the privacy gains ODoH provides.
During initial roll-out, the number of proxy operators will be limited. Also, using a larger proxy means that your queries are mixed in with more queries from other people, providing marginally better privacy. That might provide some impetus to consolidate.
Deploying automated discovery systems for acceptable proxies might help mitigate the worst centralization effects, but it seems likely that this will not be a feature of early deployments.
In the end, it would be a mistake to cry “centralization” in response to early trial deployments of a technology, which are naturally limited in scope. Furthermore, it’s hard to know what the long term impact on the ecosystem will be. We might never be able to separate the effect of existing trends toward consolidation from the effect of new technology.
I like the model adopted here. The use of a proxy neatly addresses one of the biggest concerns with the rollout of DoH: the privacy risk of having a large provider being able to gather information about streams of queries that can be linked to your IP address.
ODoH breaks streams of queries into discrete transactions that are hard to assemble into activity profiles. At the same time, ODoH makes it hard to attribute queries to individuals as it hides the origin of queries.
My sense is that the benefits very much outweigh the performance costs, the protocol complexity, and the operational risks. ODoH is a pretty big privacy win for name resolution. The state of name resolution is pretty poor, with much of it still unprotected from snooping, interception, and poisoning. The deployment of DoH went some way to address that, but came with some drawbacks. Oblivious DoH takes the next logical step.
This is something all current DNS resolvers get, but the complaint is about the scale at which this information is gathered. Some people are unhappy that network operators are unable to access this information, but I regard that as a feature. ↩︎
OK, proxies do handle individual, unencrypted HTTP requests, but that capability is hardly ever used any more now that 90% of the web is HTTPS. ↩︎
Using 0-RTT doesn’t work here without some fiddly changes to TLS because the session ticket used for TLS allows the server to link connections together, which isn’t what we need. ↩︎
This also makes ODoH far more susceptible to traffic analysis, but it relies on volume and the relative similarity of DNS queries to help manage that risk. ↩︎
The recursion here means that the designers of ODoH probably deserve a prize of some sort. ↩︎
The willful IP blindness proposal goes into more detail on what might be required for this. ↩︎