[Doh] New Privacy Considerations Section Proposal

[Patrick McManus <[e-mail]>]

Hi All,

The authors have a proposed privacy considerations section available as a
PR at: https://github.com/dohwg/draft-ietf-doh-dns-over-https/pull/200 and
a snapshot is also included at the end of this email. (though obviously the
PR will be a living document).

First, thanks for the all the comments and especially to Sara for some of
the significant text that made its way into the draft text.

The text does not, cannot really, describe the whole universe of
http/tls/tcp/ip fingerprinting - but it does raise it as an issue and try
to give enough illustrative examples to motivate consideration of these
things in your implementation. And in the end, that's its recommendation -
consider these things and don't use them blindly. The construction that
"DoH inherits the HTTPS profile so understand that please" seems most apt
and in scope.

It would be really great if we had something published in the HTTP space
that deal with this. We don't AFAICT, and that's feedback Mark and I have
already started noodling over for HTTPbis. (If we had it, the DoH text
would be shorter :))

Comments and discussion are of course welcome - its just a pull request.

-Patrick

---

# Privacy Considerations {#PrivacyConsiderations}

{{RFC7626}} discusses DNS Privacy Considerations in both "On the
wire" (Section 2.4), and "On the server" (Section 2.5) contexts. This
is also a useful framing for DoH's privacy considerations.

## On The Wire {#OnTheWire}

DoH encrypts DNS traffic and requires authentication of the
server. This mitigates both passive surveillance {{RFC7258}} and
active attacks attempting to divert DNS traffic to rogue servers
({{RFC7626}} Section 2.5.1). DNS over TLS {{RFC7858}} provides
similar protections, while direct UDP and TCP based transports are
vulnerable to this class of information leak.

Additionally, the use of the HTTPS default port 443 and the ability to
mix DoH traffic with other HTTPS traffic on the same connection can
deter on-path devices from interfering with DNS operations and make
DNS traffic analysis more difficult.

## On The Server {#OnTheServer}

A DoH application is built on IP, TCP, TLS, and HTTP. Each layer
contains one or more common features that can be used to correlate
different queries to the same identity. DoH inherits the privacy
properties of the HTTPS stack and is not known to introduce new
concerns beyond that of HTTPS.

At the IP level, the client address provides obvious correlation
information. This can be mitigated by use of a NAT, proxy, VPN, or
simple address rotation over time. It may be aggravated by use of a
DNS server that can correlate real-time addressing information with
other personal identifiers, such as when a DNS server and DHCP server
are operated by the same entity.

TCP-based solutions may seek performance through the use of TCP Fast
Open {{RFC7413}}. The cookies used in TCP Fast Open allow servers to
correlate different TCP sessions together.

TLS based implementations often achieve better handshake performance
through the use of some form of session resumption mechanism such as
session tickets {{RFC5077}}. Session resumption creates trivial
mechanisms for a server to correlate different TLS connections
together.

HTTP's feature set can also be used for identification and tracking in
a number of different ways. For example, authentication request header
fields explicitly identify profiles in use, and HTTP Cookies are
designed as an explicit state tracking mechanism between the client
and serving site and often are used as an authentication mechanism.

Additionally, the User-Agent and Accept-Language request header fields
often convey specific information about the client version or locale.
This facilitates content negotiation and operational work-arounds for
implementation bugs. Request header fields that control caching
can expose state information about a subset of the client's
history. Mixing DoH requests with other HTTP requests on the same
connection also provides an opportunity for richer data correlation.

Implementations of DoH clients and servers need to consider the benefit
and privacy impact of all these features, and their deployment context,
when deciding whether or not to enable them.