Q&A What is DNS over TLS (DoT), DNS over Quic (DoQ) and DNS over HTTPS (DoH & DoH3)?

SecurityNightmares

Level 40
Verified
Jan 9, 2020
2,955
(Article is from NextDNS support forum)

DNS is an old protocol lacking all forms of security. Yet, it is one of the most fundamental protocols of the Internet. DoT and DoH are improvements to add transport security to the DNS protocol by reusing the same security layers used by HTTPS: TLS. Both DoT and DoH use TLS. DoH adds HTTP/2 between DNS and TLS for the framing. DoT also has a framing layer inherited from DNS over TCP, but it is ridiculously simple compared to HTTP/2. They both run on top of TCP.

More recently, Quic was added to the mix. Quic is an odd beast that takes TCP, TLS and the stream capability of HTTP/2 and merge them into a natively encrypted protocol implemented on top of UDP. From this new transport protocol, we get two new variants: DoQ which is similar to DoT but is using the stream capability of Quic instead of the DNS over TCP framing, and DoH3 which is DNS over HTTPS/3, HTTP/3 being HTTP over Quic.

All those protocols offer similar advantages but they have some key differences:
  • DoT and DoQ use a custom ports (tcp/853 and udp/8853 respectively) which can be easily blocked by firewalls while DoH uses the same port and protocol as used for all HTTPS web traffic (tcp/443), making it harder to block or even detect. DoH3 uses udp/443, so it is easier to block but still indistinguishable from other web traffic using this protocol, and HTTP/3 capable clients have the capability to fallback to HTTP/2 when this happens.
  • The HTTP/2 protocol used by DoH is significantly more complex than the basic framing employed by DoT. The advantage of DoH is that most HTTP/2 implementations are battle tested and offer good performance, while most DoT implementations get the DoT “spec” wrong, leading to poorer performance. When properly implemented, DoT offers lower complexity, which may theoretically have a small positive impact on battery usage, but it might be a drop in the bucket compared to TLS. The difference in latency should be non-perceivable though.
    DoQ and DoH3 on the other side both use the same framing provided by the Quic protocol, which is greatly inspired by the HTTP/2 protocol. The different in complexity between DoQ and DoH3 is thus even thiner than between DoT and DoH.
  • As DoH uses HTTP, when implemented into a browser, there is the concern of having the same tracking capabilities as used on the web (user-agent, cookies etc.). To date, all popular clients, browsers included, are not sending any fingerprintable headers, run with no cookie jar and don't even send a user-agent.
  • DoQ and DoH3 are more resilient to packet loss. DoT and DoH are running on top of a single TCP connection, meaning that in case of a packet loss, all DNS queries or responses after this packet have to wait for the lost packet to be retransmitted (this is called head of line blocking). Thanks to Quic stream design, a single Quic session can carry multiple individual streams. Each stream is independent, and a loss of a packet only affects the stream it is associated with. With both DoQ and DoH3, each DNS query/response is isolated in its own stream, eliminating the head of line blocking issue described above. Those protocol are thus particularly well suited for mobile or highly congested networks, but won't make any substantial difference on a healthy network. One drawback is that Quic is all implemented in userland, and thus requires more CPU and battery to run than TCP. This can be an issue for bandwidth intensive applications, but DNS being pretty light, the difference should be negligible with most implementations.
Some experts like Paul Vixie recommend DoT over DoH. We don’t share this position and generally recommend DoH as it has less chances of being blocked and implementations are often better and clients supporting HTTP/3 can automatically benefit from it or fallback in case of issue thanks to the Alt-Svc/HTTPSSVC protocol negotiation.
NextDNS supports all 4 protocols. See the setup tab for more information on how to use them.
 
Top