Internet DRAFT - draft-hoffman-dprive-dns-tls-alpn
draft-hoffman-dprive-dns-tls-alpn
Network Working Group P. Hoffman
Internet-Draft VPN Consortium
Intended status: Standards Track October 22, 2014
Expires: April 25, 2015
Using TLS and ALPN for Privacy Between DNS Stub and Recursive Resolvers
draft-hoffman-dprive-dns-tls-alpn-00
Abstract
DNS queries and responses can contain information that reveals
important information about the person who caused the queries, and it
would be better if eavesdroppers were unable to see DNS traffic.
This document describes how to use TLS for encrypting DNS traffic
between a system acting as a DNS stub resolver and a system acting as
a DNS recursive resolver. It defines how to transport DNS queries
and responses under TLS on port 443 using ALPN.
Discussion of this draft should take place in the DPRIVE WG.
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
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on April 25, 2015.
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Copyright (c) 2014 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Other Designs . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Specification of Using HTTPS Between a DNS Stub Resolver and
a Recursive Resolver . . . . . . . . . . . . . . . . . . . . 4
2.1. Design Rationale . . . . . . . . . . . . . . . . . . . . 4
2.2. Stub Resolver Policy . . . . . . . . . . . . . . . . . . 5
2.3. Privacy Through DNS Forwarders . . . . . . . . . . . . . 5
2.4. Use by Authoritative Servers . . . . . . . . . . . . . . 5
3. Privacy Considerations . . . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
4.1. ALPN Identification Sequence . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
As described in [I-D.bortzmeyer-dnsop-dns-privacy], there are many
reasons why a user or system making a DNS query would like the query
and the response to not be seen by others. The best way to make a
query and response private is to use encryption, and TLS is a
commonly-deployed protocol that provides encryption to clients and
servers. This document describes how to use TLS for encrypting DNS
traffic between a system acting as a stub resolver and a system
acting as a recursive resolver.
This document defines how to transport DNS [RFC1035] queries and
responses under TLS on port 443 using ALPN [RFC7301]. Using ALPN for
this allows the use of the commonly-allowed port 443 while not
confusing any HTTP servers that might be running under TLS.
Because there is currently no expectation of privacy for DNS queries,
this document defines the use of opportunistic security as described
in [I-D.dukhovni-opportunistic-security] for adding privacy for DNS
traffic between a stub resolver and a recursive resolver.
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The protocol described in this document cannot be used by a stub
resolver to trust the DNSSEC validation status of responses from a
recursive server. Such trust might be described in a different
protocol that always uses authenticated TLS, but not the one here.
1.1. Other Designs
There have been many designs proposed for using TLS to protect DNS
traffic between a stub resolver and a recursive resolver. Among them
are:
o [draft-hzhwm-dprive-start-tls-for-dns] describes DNS over TCP
begun on port 53 as normal, but there is an in-band signal to
change the transport to TLS.
o [draft-hoffman-dprive-dns-tls-https] describes how to easily wrap
DNS queries in HTTP requests and interpret DNS responses in the
HTTP respones; the HTTP here is always run under TLS on port 443.
o [draft-hoffman-dprive-dns-tls-newport] describes DNS over TCP is
begun on a port specific to the protocol.
(Yet a different design, call DNSCrypt, has a fair amount of
deployment. A pointer will be added here for the technical
specification of that design if it becomes available.)
1.2. 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 RFC
2119, BCP 14 [RFC2119].
The roles of agents that make DNS requests, and those that give DNS
responses have been loosely named over time. Because this protocol
is meant to be used between specific types of agents, they need to be
defined here. [[ Note: if these are adequately defined in existing
RFCs in ways that the community agrees on, it would be better to
simply repeat those definitions. ]]
Stub resolver: A system that sends DNS queries with the intention of
using the answers locally.
Authoritative server: A system that responds to DNS queries with
information about zones for which it is authoritative.
Recursive resolver: A system that receives DNS queries and either
responds to those queries from a local cache or sends queries to
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authoritative servers in order to get the answers to the original
queries. These systems are also commonly called "recursive
servers".
DNS forwarder: A system receives a DNS query from a stub resolver,
possibly changes the query, sends the resulting query to a
recursive resolver, receives the response from the recursive
resolver, possibly changes the response, and sends the resulting
response to the stub resolver. [RFC5625] does not give a specific
definition for DNS forwarder, but describes in detail what
features they need to support. The protocol interfaces for DNS
forwarders are exactly the same as those for recursive resolvers
(for interactions with DNS stubs) and as those for stub resolvers
(for interactions with recursive resolvers).
2. Specification of Using HTTPS Between a DNS Stub Resolver and a
Recursive Resolver
A stub resolver MAY attempt to communicate with a recursive resolver
using TLS [RFC5246] over port 443.
The protocol in this document runs the DNS protocol directly under
TLS on port 443. The way that a server knows that the client is
going to run DNS instead of other protocols that run on port 443 is
by using ALPN [RFC7301].
If the recursive resolver responds on port 443, both the client and
the server MUST use the ALPN [RFC7301] extension to TLS, and MUST use
"dns" as the identification sequence in ALPN. After the TLS
connection is established, the client and server communicate using
the normal DNS protocol defined in [RFC1035] and all the relevant
updates.
The MUST-level requirement for ALPN is because a server might host
both DNS and secure web services on the same IP address.
2.1. Design Rationale
A recursive resolver SHOULD offer authentication using one or more of
the many methods allowed by TLS, and the stub resolver SHOULD
authenticate the recursive resolver if it can. However, if the stub
resolver cannot authenticate the recursive resolver during TLS setup,
the stub resolver SHOULD still complete the handshake in order to
achieve encrypted communication.
A typical form of authentication for a recursive resolver would be a
PKIX [RFC5280] certificate that has a CommonName (CN) that is the IP
address that stub resolvers use to connect to it. Note that there
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are many other standardized types of TLS authentication that can be
used, such as raw public keys keys [RFC7250].
The TLS connection is kept up for as long as each party is willing to
do so.
2.2. Stub Resolver Policy
A stub resolver MAY use policy to allow unauthenticated encryption
(which can possibly be intercepted by an on-path adversary) or
authenticated encryption (which might prevent all DNS resolution if
the server does not have correct authentication credentials) when
contacting a recursive resolver using this protocol.
It is expected that users will want one of the following policies
available to them:
o The stub resolver MUST achieve authenticated TLS with a recursive
server; if that can't be achieved, the stub resolver refuses to
send out DNS queries
o The stub resolver tries to achieve authenticated TLS with a
recursive server; if it cannot achieve authenticated TLS, it tries
to achieve unauthenticated TLS; if that can't be achieved, the
stub resolver refuses to send out DNS queries
o The stub resolver tries to achieve authenticated TLS with a
recursive server; if it cannot achieve authenticated TLS, it tries
to achieve unauthenticated TLS; if that can't be achieved, the
stub resolver uses normal DNS cleartext on port 53
o The stub resolver doesn't want to try TLS at all, and uses normal
DNS cleartext on port 53
2.3. Privacy Through DNS Forwarders
A stub resolver cannot tell whether it is sending queries to a
recursive resolver or to a DNS forwarder. Therefore, a DNS forwarder
that acts as a TLS server for DNS requests SHOULD attempt to use TLS
with its upstream resolver(s) to maximize the confidentiality of its
stub clients.
2.4. Use by Authoritative Servers
There is absolutely no expectation that any authoritative server will
deploy this protocol. Thus, a DNS recursive resolver that tries to
contact an authoritative server on TCP port 443 in hopes of keeping
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its communication private is probably wasting its time and delaying
getting the actual answer over port 53.
3. Privacy Considerations
This entire document is about improving privacy for DNS requests and
responses.
4. IANA Considerations
4.1. ALPN Identification Sequence
IANA is requested add the following value to the "Application-Layer
Protocol Negotiation (ALPN) Protocol IDs" registry. That registry is
populated by expert review, and such a review will be requested if
this document progresses.
Protocol Identification Sequence Reference
DNS 0x64 0x6e 0x73 ("dns") This document
5. Security Considerations
An adversary who can observe encrypted queries from stub resolvers,
and can simultaneously observe the cleartext queries from a recursive
resolver to authoritative servers, might be able to associate those
two sets of queries and thus ascertain that a particular client asked
a particular query. Such observations can be prevented by the
recursive resolver already having the answer in its cache. If a
recursive resolver has ample room in its cache, it can make the
adversary's job harder by refreshing entries in its cache before the
TTL on those entries time out, thereby preventing the adversary's
ability to associate encrypted queries with cleartext ones.
6. Acknowledgements
Many people have thought about protecting DNS queries and responses,
and various discussions with those people resulted in this document.
The following have made significant contributions to this document:
Jacob Appelbaum, Carsten Bormann, Tatuya JINMEI, Warren Kumari, and
Paul Wouters.
7. References
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7.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
7.2. Informative References
[I-D.bortzmeyer-dnsop-dns-privacy]
Bortzmeyer, S., "DNS privacy considerations", draft-
bortzmeyer-dnsop-dns-privacy-02 (work in progress), April
2014.
[I-D.dukhovni-opportunistic-security]
Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", draft-dukhovni-opportunistic-
security-04 (work in progress), August 2014.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines", BCP
152, RFC 5625, August 2009.
[RFC7250] Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S., and
T. Kivinen, "Using Raw Public Keys in Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", RFC 7250, June 2014.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, July 2014.
[draft-hoffman-dprive-dns-tls-https]
Hoffman, P., "Using HTTPS for Privacy Between DNS Stub and
Recursive Resolvers", draft-hoffman-dns-tls-https ,
October 2014.
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[draft-hoffman-dprive-dns-tls-newport]
Hoffman, P., "Using TLS on a New Port for Privacy Between
DNS Stub and Recursive Resolvers", draft-hoffman-dns-tls-
newport , October 2014.
[draft-hzhwm-dprive-start-tls-for-dns]
Hu, Z., "TLS for DNS: Initiation and Performance
Considerations", draft-hzhwm-dprive-start-tls-for-dns ,
October 2014.
Author's Address
Paul Hoffman
VPN Consortium
Email: paul.hoffman@vpnc.org
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