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dprive                                                      A. Edmundson
Internet-Draft                                                P. Schmitt
Intended status: Experimental                                N. Feamster
Expires: January 3, 2019                            Princeton University
A. Mankin
Salesforce
July 2, 2018

Oblivious DNS - Strong Privacy for DNS Queries
draft-annee-dprive-oblivious-dns-00

Abstract

Recognizing the privacy vulnerabilities associated with DNS queries,
a number of standards have been developed and services deployed that
that encrypt a user's DNS queries to the recursive resolver and thus
obscure them from some network observers and from the user's Internet
service provider.  However, these systems merely transfer trust to a
third party.  We argue that no single party should be able to
associate DNS queries with a client IP address that issues those
queries.  To this end, this document specifies Oblivious DNS (ODNS),
which introduces an additional layer of obfuscation between clients
and their queries.  To accomplish this, ODNS uses its own
authoritative namespace; the authoritative servers for the ODNS
namespace act as recursive resolvers for the DNS queries that they
receive, but they never see the IP addresses for the clients that
initiated these queries.  The ODNS experimental protocol is
compatible with existing DNS infrastructure.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF).  Note that other groups may also distribute
working documents as Internet-Drafts.  The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on January 3, 2019.

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Copyright (c) 2018 IETF Trust and the persons identified as the

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document.  Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.  Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

1.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
3.  ODNS Overview . . . . . . . . . . . . . . . . . . . . . . . .   4
4.  Sending and Receiving ODNS Queries  . . . . . . . . . . . . .   5
5.  Replication and Privacy-Preserving Key Distribution . . . . .   6
5.1.  Scalability and Performance Using Anycast . . . . . . . .   6
5.2.  Key Distribution  . . . . . . . . . . . . . . . . . . . .   6
5.3.  QNAME Length  . . . . . . . . . . . . . . . . . . . . . .   7
6.  Backward Compatibility  . . . . . . . . . . . . . . . . . . .   7
7.  IANA considerations . . . . . . . . . . . . . . . . . . . . .   8
8.  Security considerations . . . . . . . . . . . . . . . . . . .   8
9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   8
11. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .   8
12. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
12.1.  Normative References . . . . . . . . . . . . . . . . . .   8
12.2.  Informative References . . . . . . . . . . . . . . . . .   9
Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

Privacy terminology is as described in Section 3 of [RFC6973].

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DNS terminology is as described in [I-D.ietf-dnsop-terminology-bis]
with one modification: we use the definition of Privacy-enabling DNS
server taken from [RFC8310]:

2.  Introduction

Recognizing the privacy vulnerabilities associated with DNS queries,
a number of specifications and services have been developed that
encrypt a user's DNS queries to the recursive resolver and thus
obscure them from some network observers and from the user's Internet
service provider.  However, these systems merely transfer trust to a
third party.  We argue that no single party should be able to
associate DNS queries with a client IP address that issues those
queries, that there should be obfuscation between the client and its
queries.

DNS queries can reveal significant information about the Internet
destinations that a user or device is communicating with.  For
example, the domain names themselves may reveal the websites that a
user is visiting.  In the case of smart-home Internet of Things (IoT)
devices, the DNS queries may reveal the types of devices in user
homes.  Previous work has also demonstrated that DNS lookups can
identify the websites that a user is visiting, even when they are
using an anonymizing service such as Tor [Tor-DNS].  The operator of
a DNS resolver may also retain information about DNS queries and
responses---including the IP addresses that query the domains and the
DNS names that are queries.

Other approaches have layered encryption on top of the DNS query
stream.  For example, DNS-over-TLS [RFC7858], DNS-over-DTLS
[RFC8094], and DNS-over-HTTPS [I-D.ietf-doh-dns-over-https] all send
DNS queries over an encrypted channel, which prevents an eavesdropper
from learning the contents of a DNS lookup but does not prevent the
operator of the recursive resolver from linking queries and IP
cryptography to encrypt DNS requests and responses; it also
authenticates all DNS responses and eliminates any forged responses.
DNSCrypt (ref to be added) encrypts and authenticates DNS traffic
between a client and a recursive resolver.  None of the approaches
prevent the recursive resolver from observing DNS queries and
responses.  Note: a new draft is under development, targetted to for
BCP, that n would offers a policy and best-practices approach to the
problem of recursive resolvers's observation of this data.

ODNS (1) obfuscates the queries that a recursive resolver sees from
the clients that issue DNS queries; and (2) obfuscates the client's
IP address from upper levels of the DNS hierarchy that ultimately
resolve the query (that is, the authoritative servers).  ODNS

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operates in the context of the existing DNS protocol, allowing the
existing deployed infrastructure to remain unchanged.  A client sends
an encrypted query to a recursive resolver, which then forwards the
query to an authoritative DNS server that can resolve ODNS queries.
The recursive resolver never sees the DNS domain that the client
queries, and the ODNS server never sees the IP address of the client.

ODNS requires a modified client stub resolver, and a modified
authoritative DNS server.  The stub resolver must take an existing
DNS name, encrypt it, and append the ODNS domain to ensure that the
query is forwarded to the ODNS authoritative DNS server.  The
authoritative DNS server for ODNS must also act as a recursive DNS
resolver; it must not only reply for the ODNS namespace but also
ultimately retrieve the DNS record that corresponds to the client's
initial query.

3.  ODNS Overview

ODNS operates similarly to conventional DNS, but adds two components:
(1)each client runs a modified stub resolver; and (2) ODNS runs an
authoritative name server that also acts as a recursive DNS resolver
for the original DNS query:

o  The client's stub resolver obfuscates the domain that the client
is requesting (via symmetric encryption), resulting in the
client's configured recursive resolver being unaware of the
requested domain.

o  The authoritative name server for ODNS separates the clients'
identities from their corresponding DNS requests, such that the
name servers cannot learn who is requesting specific domains.

As detailed in [RFC7626], operators of recursive DNS resolvers see
individual IP addresses along with the fully qualified domain name
those IPs request.  Operators of authoritative resolvers may also be
able to learn information about the client by using one of the
extensions to DNS, notably EDNS0 Client Subnet (ECS) [RFC7871].  ECS
authoritative DNS servers higher in the DNS hierarchy (not only
recursive DNS resolvers).  ODNS hides a client's IP address from the
authoritative name servers at different levels of the DNS hierarchy.

The configured (non-ODNS) recursive DNS resolver knows the client IP
address but never sees the domain that it queries.  ODNS requires the
client to use a custom local stub resolver, which hides the requested
domain from the recursive resolver.  The ODNS stub resolver, which
runs at the client, encrypts the original DNS query for the ODNS
authoritative DNS server before it appends the domain for the ODNS

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namespace to the query, which causes the recursive resolver to
forward the encrypted domain name on to the ODNS authoritative
server.  NOTE: for simplicity, we sometimes say that this
authoritative server is for .odns, but any authoritative DNS domain
can run an ODNS server.  Even if there was a TLD, there would be
leakage of information, because the IP addresses of clients and
recursive resolvers would be seen at the root.  Experiments can be
of [RFC7706].

When an ODNS authoritative DNS server receives a DNS query, it
removes any client information from the request (e.g., the client IP
address, EDNS0 client subnet information) before performing
additional DNS lookups.  The ODNS name server then switches to acting
as a recursive resolver.  The authoritative server forwards any
response to the original recursive DNS resolver, which in turn sends
the response to the client.

The recursive DNS resolver receives the request from the client, but
cannot identify the genuine domain.  It parses the TLD (.odns) and
forwards the request onto the .odns authoritative server.  Because
the session key was originally encrypted with the authoritative
server's public key, the authoritative server can decrypt the session
key with its private key, and subsequently decrypt the domain with
the session key.  The authoritative server then acts as a recursive
resolver and contacts the necessary name servers to resolve the
domain.  Once an answer is obtained, the authoritative server
encrypts the domain with the session key, appends the .odns TLD and
forwards the response to the recursive DNS resolver.  As explained by
the use of session keys, the recursive resolver cannot learn the
domains a client requests, despite being able to learn who the client
is.

TODO (in -01 or later): Create an ASCII diagram form of Figure 1 from
odns.cs.princeton.edu

4.  Sending and Receiving ODNS Queries

TODO (in -01 or later): Create an ASCII diagram form of Figure 2 from
odns.cs.princeton.edu

o  When a client generates a DNS request, the local stub resolver
generates a symmetric session key, encrypts the domain name with
the session key, encrypts the session key with the authoritative
server's public key, and appends the .odns TLD to the encrypted
domain. (www.example.com_k.odns.)  The stub also appends the
session key encrypted under the ODNS authoritative server's public
key k_PK)

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o  The client sends the query in the Additional Information portion
of the DNS query to the recursive resolver, which then sends it to
the authoritative name server for ODNS.

o  The authoritative server for ODNS queries decrypts the session
key, which it uses to decrypt the domain in the query.

o  The authoritative server forwards a recursive DNS request to the
appropriate name server for the original domain, which then
returns the answer to the ODNS server.

o  The ODNS server returns the answer to the client's recursive
resolver.

Other authoritative DNS servers see incoming DNS requests, but these
only see the IP address of the ODNS authoritative resolver, which
effectively proxies the DNS request for the original client.  The
client's original recursive resolver can learn the client's IP
address, but cannot learn the domain names in the client's DNS
queries.

5.  Replication and Privacy-Preserving Key Distribution

5.1.  Scalability and Performance Using Anycast

To achieve scalability the authoritative server is replicated in a
variety of geographical locations and all replicas are assigned to
both an anycast IP address as well as a unique unicast IP address.
Using anycast, all servers that share the IP address are able to
answer a query.  When a recursive sends a DNS query to the ODNS
authoritative server, the query will be routed by BGP to the
nearest'' authoritative server.  And because the recursive resolver
(an open resolver) is also anycast, both the recursive and the ODNS
authoritative server should be the optimal choices based on the
client's network connectivity {\it without revealing the client's
location}. This results in maximizing the performance of ODNS by
minimizing the network path that queries must traverse.

5.2.  Key Distribution

Use of anycast and multiple authoritative replicas introduce a key
distribution challenge for ODNS.  The ODNS stub server uses the
public key of the authoritative server to encrypt session keys in
ODNS queries.  Based on best practices, we cannot share public /
private keypairs across all of the replicated authoritative servers.
Likewise, in order to preserve user identity privacy the key
distribution must be done in a way that the authoritative server
never learns the identity (i.e., IP address) of a stub.  This

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disqualifies out-of-band key exchange as in EncDNS.  Instead, we
leverage the DNS infrastructure itself to distribute keys while
maintaining privacy.  We have defined a special'' query (e.g.,
special.odns) that we use to select a specific authoritative server
as well as distribute the appropriate public key.

The client's stub resolver sends a special ODNS query to the
recursive resolver, which will in turn use the anycast address to
locate the nearest authoritative server.  The authoritative that
receives the query responds with an OPT record that includes a self-
certifying name (e.g., ABC.odns), such that the name of the server is
derived from the public key itself and is associated with an instance
of the authoritative nameserver listening on the unique unicast IP
address, and the authoritative server's public key; this response is
returned to the client's stub resolver via the recursive.  Subsequent
ODNS queries at the stub append the unique name of the authoritative
that responded to the special query, which means that the requests
will all reach the same server and the client encrypt using the
appropriate public key.

5.3.  QNAME Length

In principle, a query could include the encrypted query and / or
session key in a special Resource Record (RR) in the Additional
Information'' section of a DNS message (known as an OPT), but we
discovered that, in practice, most open resolvers strip all OPT
records before forwarding the query on to the authoritative
nameserver.  In this case, ODNS cannot simply use an OPT to
communicate the session key.  ODNS overcomes this challenge by
placing the encrypted key in the QNAME field of the DNS message; the
QNAME field consists of 4 sets of 63 bytes, which limits both the key
size and encryption scheme used.  For this reason, ODNS uses 16-byte
AES session keys and encrypts the session keys using the Elliptic
Curve Integrated Encryption Scheme (ECIES)~.  Once the session key is
encrypted, the resulting value takes up 44 bytes of the QNAME field.
In the future, we envision an ODNS-specific OPT code that would cause
recursive resolvers to maintain and forward the ODNS OPT record
intact to the authoritative nameserver.  Such a mechanism allows for
the use of larger encryption keys as OPT records can be much larger
(typically 4096 bytes) that the space alloted for QNAMEs.

6.  Backward Compatibility

For a new extension to DNS such as ODNS to be widely adopted it must
be backward-compatible with existing infrastructure, as changes to
the DNS system occur over long time scales.  Our design must not rely
upon changes made at recursive resolvers, root nameservers, or TLD
nameservers.  We engineer the ODNS stub and authoritative

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functionality with this in mind as these two locations in the DNS

7.  IANA considerations

For initial experimental deployment of this protocol, the name
obliviousdns.com has been registered.  Its length is a drawback, for
the reasons discussed in Section 5.3 and a shorter privately
registered name may be chosen for future larger-scale
experimentation.  An infrastructure related zone would be more
advantageous choice.  Therefore discussion should resolve the
appropriateness and conditions of a request for a special use domain
name, e.g. odns.arpa.  This falls under the considerations in
[RFC3172].  Notes: because of restrictions on TLD registration,
following the example of .onion [RFC7686] is infeasible.  Traffic for
ODNS traverses normal Internet paths, therefore the IANA special use
registry recently established for Locally-Served DNS Zones, in which
home.arpa has recently been registered [RFC8375], is also not a model
for IANA considerations for the ODNS Namespace.

8.  Security considerations

TODO (some questions to consider): what are residual risks in the
ODNS scheme and additional mitigations?  Is there any increase in
attack surface for the users and operators in ODNS?  Are systems
depending on ODNS vulnerable to DoS in specific ways that should be
mitigated?

9.  Acknowledgements

10.  Contributors

The following contributed significantly to the document:

11.  Changelog

draft-annee-dprive-oblivious-dns-00

o  Initial commit

12.  References

12.1.  Normative References

[I-D.ietf-dnsop-terminology-bis]
Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", draft-ietf-dnsop-terminology-bis-10 (work in
progress), April 2018.

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[RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC3172]  Huston, G., Ed., "Management Guidelines & Operational
Requirements for the Address and Routing Parameter Area
Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172,
September 2001, <https://www.rfc-editor.org/info/rfc3172>.

[RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013,
<https://www.rfc-editor.org/info/rfc6973>.

[RFC7871]  Contavalli, C., van der Gaast, W., Lawrence, D., and W.
Kumari, "Client Subnet in DNS Queries", RFC 7871,
DOI 10.17487/RFC7871, May 2016,
<https://www.rfc-editor.org/info/rfc7871>.

[RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8310]  Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
for DNS over TLS and DNS over DTLS", RFC 8310,
DOI 10.17487/RFC8310, March 2018,
<https://www.rfc-editor.org/info/rfc8310>.

12.2.  Informative References

[I-D.ietf-doh-dns-over-https]
Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", draft-ietf-doh-dns-over-https-12 (work in
progress), June 2018.

[RFC7626]  Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
DOI 10.17487/RFC7626, August 2015,
<https://www.rfc-editor.org/info/rfc7626>.

[RFC7686]  Appelbaum, J. and A. Muffett, "The ".onion" Special-Use
Domain Name", RFC 7686, DOI 10.17487/RFC7686, October
2015, <https://www.rfc-editor.org/info/rfc7686>.

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[RFC7706]  Kumari, W. and P. Hoffman, "Decreasing Access Time to Root
Servers by Running One on Loopback", RFC 7706,
DOI 10.17487/RFC7706, November 2015,
<https://www.rfc-editor.org/info/rfc7706>.

[RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>.

[RFC8094]  Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
Transport Layer Security (DTLS)", RFC 8094,
DOI 10.17487/RFC8094, February 2017,
<https://www.rfc-editor.org/info/rfc8094>.

[RFC8375]  Pfister, P. and T. Lemon, "Special-Use Domain
'home.arpa.'", RFC 8375, DOI 10.17487/RFC8375, May 2018,
<https://www.rfc-editor.org/info/rfc8375>.

[Tor-DNS]  Reschbach, G., Pulls, B., Roberts, L., Winter, P., and N.
Feamster, "The Effect of DNS on Tor's Anonymity", 2016.

Annie Edmundson
Princeton University
Princeton, NJ
United States

Email: annee@cs.princeton.edu

Paul Schmitt
Princeton University
Princeton, NJ
United States

Email: pschmitt@cs.princeton.edu

Nick Feamster
Princeton University
Princeton, NJ
United States

Email: nfeamster@cs.princeton.edu

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Allison Mankin
Salesforce

Email: allison.mankin@gmail.com

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