Internet DRAFT - draft-dgr-dprive-dtls-and-tls-profiles
draft-dgr-dprive-dtls-and-tls-profiles
dprive S. Dickinson
Internet-Draft Sinodun
Intended status: Standards Track D. Gillmor
Expires: June 25, 2016 ACLU
T. Reddy
Cisco
December 23, 2015
Authentication and (D)TLS Profile for DNS-over-TLS and DNS-over-DTLS
draft-dgr-dprive-dtls-and-tls-profiles-00
Abstract
This document describes how a DNS client can use a domain name to
authenticate a DNS server that uses Transport Layer Security (TLS)
and Datagram TLS (DTLS). Additionally, it defines (D)TLS profiles
for DNS clients and servers implementing DNS-over-TLS and DNS-over-
DTLS.
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-
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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 June 25, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Background . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Usage Profiles . . . . . . . . . . . . . . . . . . . . . 5
4.3. Authentication . . . . . . . . . . . . . . . . . . . . . 6
4.3.1. DNS-over-(D)TLS Bootstrapping Problems . . . . . . . 6
4.3.2. Credential Verification . . . . . . . . . . . . . . . 7
4.3.3. Implementation guidance . . . . . . . . . . . . . . . 7
5. Authentication in Opportunistic DNS-over(D)TLS Privacy . . . 7
6. Authentication in Strict DNS-over(D)TLS Privacy . . . . . . . 7
7. In Band Source of Domain Name: SRV Service Label . . . . . . 8
8. Out of Band Sources of Domain Name . . . . . . . . . . . . . 8
8.1. Full direct configuration . . . . . . . . . . . . . . . . 8
8.2. Direct configuration of name only . . . . . . . . . . . . 8
8.3. DHCP . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9. Credential Verification . . . . . . . . . . . . . . . . . . . 9
9.1. X.509 Certificate Based Authentication . . . . . . . . . 9
9.2. DANE . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.2.1. Direct DNS Lookup . . . . . . . . . . . . . . . . . . 10
9.2.2. TLS DNSSEC Chain extension . . . . . . . . . . . . . 11
10. Combined Credentials with SPKI Pinsets . . . . . . . . . . . 11
11. (D)TLS Protocol Profile . . . . . . . . . . . . . . . . . . . 12
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
13. Security Considerations . . . . . . . . . . . . . . . . . . . 13
13.1. Counter-measures to DNS Traffic Analysis . . . . . . . . 13
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
15.1. Normative References . . . . . . . . . . . . . . . . . . 14
15.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. Server capability probing and caching by DNS clients 16
Appendix B. Changes between revisions . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
The DPRIVE working group has two active documents that provide DNS
privacy between DNS clients and DNS servers (to address the concerns
in [RFC7626]):
o DNS-over-TLS [I-D.ietf-dprive-dns-over-tls]
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o DNS-over-DTLS [I-D.ietf-dprive-dnsodtls]
This document defines usage profiles and authentication mechanisms
for DTLS [RFC6347] and TLS [RFC5246] that specify how a DNS client
should authenticate a DNS server based on a domain name. In
particular, it describes:
o How a DNS client can obtain a domain name for a DNS server to use
for (D)TLS authentication.
o What are the acceptable credentials a DNS server can present to
prove its identity for (D)TLS authentication based on a given
domain name.
o How a DNS client can verify that any given credential matches the
domain name obtained for a DNS server.
This document also defines a (D)TLS protocol profile for use with
DNS. This profile defines the configuration options and protocol
extensions required of both parties to optimize connection
establishment and session resumption for transporting DNS, and to
support the authentication profiles defined here.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Several terms are used specifically in the context of this draft:
o DNS client: a DNS stub resolver or forwarder/proxy. In the case
of a forwarder, the term "DNS client" is used to discuss the side
that sends queries.
o DNS server: a DNS recursive resolver or forwarder/proxy. In the
case of a forwarder, the term "DNS server" is used to discuss the
side that responds to queries.
o Privacy-enabling DNS server: A DNS server that:
* MUST implement DNS-over-TLS [I-D.ietf-dprive-dns-over-tls] and
MAY implement DNS-over-DTLS [I-D.ietf-dprive-dnsodtls].
* Can offer at least one of the credentials described in
Section 9.
* Implements the (D)TLS profile described in Section 11.
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o (D)TLS: For brevity this term is used for statements that apply to
both Transport Layer Security [RFC5246] and Datagram Transport
Layer Security [RFC6347]. Specific terms will be used for any
statement that applies to either protocol alone.
o DNS-over-(D)TLS: For brevity this term is used for statements that
apply to both DNS-over-TLS [I-D.ietf-dprive-dns-over-tls] and DNS-
over-DTLS [I-D.ietf-dprive-dnsodtls]. Specific terms will be used
for any statement that applies to either protocol alone.
o Credential: Information available for a DNS server which proves
its identity for authentication purposes. Credentials discussed
here include:
* X.509 certificate
* DNSSEC validated chain to a TLSA record
but may also include SPKI pinsets.
o SPKI Pinsets: [I-D.ietf-dprive-dns-over-tls] describes the use of
cryptographic digests to "pin" public key information in a manner
similar to HPKP [RFC7469]. An SPKI pinset is a collection of
these pins that constrains a DNS server.
o Reference Identifier: a Reference Identifier as described in
[RFC6125], constructed by the DNS client when performing TLS
authentication of a DNS server.
3. Scope
This document is limited to domain-name-based authentication of DNS
servers by DNS clients (as defined in the terminology section), and
the (D)TLS profiles needed to support this. As such, the following
things are out of scope:
o Authentication of authoritative servers by recursive resolvers.
o Authentication of DNS clients by DNS servers.
o SPKI-pinset-based authentication. This is defined in
[I-D.ietf-dprive-dns-over-tls]. However, Section 10 does describe
how to combine that approach with the domain name based mechanism
described here.
o Any server identifier other than domain names, including IP
address, organizational name, country of origin, etc.
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4. Discussion
4.1. Background
To protect against passive attacks DNS privacy requires encrypting
the query (and response). Such encryption typically provides
integrity protection as a side-effect, which means on-path attackers
cannot simply inject bogus DNS responses. For DNS privacy to also
provide protection against active attackers pretending to be the
server, the client must authenticate the server.
4.2. Usage Profiles
A DNS client has a choice of privacy usage profiles available. This
choice is briefly discussed in both [I-D.ietf-dprive-dns-over-tls]
and [I-D.ietf-dprive-dnsodtls]. In summary, the usage profiles are:
o Strict Privacy: the DNS client requires both an encrypted and
authenticated connection to a DNS Server. A hard failure occurs
if this is not available. This requires the client to securely
obtain information it can use to authenticate the server. This
provides strong privacy guarantees to the client. This is
discussed in detail in Section 6.
o Opportunistic Privacy: the DNS client uses Opportunistic Security
as described in [RFC7435]
"... the use of cleartext as the baseline communication
security policy, with encryption and authentication negotiated
and applied to the communication when available."
In the best case scenario (authenticated and encrypted connection)
this is equivalent to Strict Privacy, in the worst case (clear
text connection) this is equivalent to No Privacy. Clients will
try for the best case but are willing to fallback to intermediate
cases and eventually the worst case scenario in order to obtain a
response. This provides an undetermined privacy guarantee to the
user depending on what kind of connection is actually used. This
is discussed in section Section 5
o No Privacy: the DNS client does not require or attempt to use
either encryption or authentication. Queries are always sent in
clear text. This provides no privacy guarantees to the client.
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+-----------------------+------------------+-----------------+
| Usage Profile | Passive Attacker | Active Attacker |
+-----------------------+------------------+-----------------+
| No Privacy | N | N |
| Opportunistic Privacy | P | N (D) |
| Strict Privacy | P | P |
+-----------------------+------------------+-----------------+
P == protection; N == no protection; D == detection is possible
Table 1: DNS Privacy Protection by Usage Profile and type of attacker
Since Strict Privacy provides the strongest privacy guarantees it is
preferable to Opportunistic Privacy which is preferable to No
Privacy. However since the different profiles require varying levels
of configuration (or a trusted relationship with a provider) DNS
clients will need to carefully select which profile to use based on
their communication privacy needs.
A DNS client SHOULD select a particular usage profile when resolving
a query. A DNS client MUST NOT fallback from Strict Privacy to
Opportunistic Privacy during the resolution process as this could
invalidate the protection offered against active attackers.
4.3. Authentication
This document describes authentication mechanisms that can be used in
either Strict or Opportunistic Privacy for DNS-over-(D)TLS.
4.3.1. DNS-over-(D)TLS Bootstrapping Problems
Many (D)TLS clients use PKIX authentication [RFC6125] based on a
domain name for the server they are contacting. These clients
typically first look up the server's network address in the DNS
before making this connection. A DNS client therefore has a
bootstrap problem. DNS clients typically know only the IP address of
a DNS server.
As such, before connecting to a DNS server, a DNS client needs to
learn the domain name it should associate with the IP address of a
DNS server for authentication purposes. Sources of domains names are
discussed in Section 7 and Section 8.
One advantage of this domain name based approach is that it
encourages association of stable, human recognisable identifiers with
secure DNS service providers.
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4.3.2. Credential Verification
The use of SPKI pinset verification is discussed in
[I-D.ietf-dprive-dns-over-tls].
In terms of domain name based verification, once a domain name is
known for a DNS server a choice of mechanisms can be used for
authentication. Section 9 discusses these mechanisms in detail,
namely X.509 certificate based authentication and DANE.
Note that the use of DANE adds requirements on the ability of the
client to get validated DNSSEC results. This is discussed in more
detail in Section 9.2.
4.3.3. Implementation guidance
Section 11 describes the (D)TLS profile for DNS-over(D)TLS.
Additional considerations relating to general implementation
guidelines are discussed in both Section 13 and in Appendix A.
5. Authentication in Opportunistic DNS-over(D)TLS Privacy
An Opportunistic Security [RFC7435] profile is described in
[I-D.ietf-dprive-dns-over-tls] which MAY be used for DNS-over-(D)TLS.
DNS clients issuing queries under an opportunistic profile which know
of a domain name for a DNS server MAY choose to try to authenticate
the server using the mechanisms described here. This is useful for
detecting (but not preventing) active attack, and for debugging or
diagnostic purposes if there are means to report the result of the
authentication attempt. This information can provide a basis for a
DNS client to switch to (preferred) Strict Privacy where it is
viable.
6. Authentication in Strict DNS-over(D)TLS Privacy
To authenticate a privacy-enabling DNS server, a DNS client needs to
know the domain name for each server it is willing to contact. This
is necessary to protect against active attacks on DNS privacy.
A DNS client requiring Strict Privacy MUST either use one of the
sources listed in Section 8 to obtain a domain name for the server it
contacts, or use an SPKI pinset as described in
[I-D.ietf-dprive-dns-over-tls].
A DNS client requiring Strict Privacy MUST only attempt to connect to
DNS servers for which either a domain name or a SPKI pinset is known
(or both). The client MUST use the available verification mechanisms
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described in Section 9 to authenticate the server, and MUST abort
connections to a server when no verification mechanism succeeds.
With Strict Privacy, the DNS client MUST NOT commence sending DNS
queries until at least one of the privacy-enabling DNS servers
becomes available.
A privacy-enabling DNS server may be temporarily unavailable when
configuring a network. For example, for clients on networks that
require registration through web-based login (a.k.a. "captive
portals"), such registration may rely on DNS interception and
spoofing. Techniques such as those used by DNSSEC-trigger [dnssec-
trigger] MAY be used during network configuration, with the intent to
transition to the designated privacy-enabling DNS servers after
captive portal registration. The system MUST alert by some means
that the DNS is not private during such bootstrap.
7. In Band Source of Domain Name: SRV Service Label
This specification adds a SRV service label "domain-s" for privacy-
enabling DNS servers.
Example service records (for TLS and DTLS respectively):
_domain-s._tcp.dns.example.com. SRV 0 1 853 dns1.example.com.
_domain-s._tcp.dns.example.com. SRV 0 1 853 dns2.example.com.
_domain-s._udp.dns.example.com. SRV 0 1 853 dns3.example.com.
8. Out of Band Sources of Domain Name
8.1. Full direct configuration
DNS clients may be directly and securely provisioned with the domain
name of each privacy-enabling DNS server. For example, using a
client specific configuration file or API.
In this case, direct configuration for a DNS client would consist of
both an IP address and a domain name for each DNS server.
8.2. Direct configuration of name only
A DNS client may be configured directly and securely with only the
domain name of its privacy-enabling DNS server. For example, using a
client specific configuration file or API.
It can then use opportunistic DNS connections to untrusted DNS
servers (e.g. provided by the local DHCP service) to establish the IP
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address of the intended privacy-enabling DNS server by doing a lookup
of SRV records. Such records MUST be validated using DNSSEC.
Example: For a DNSSEC validating DNS client configured in this way to
do strict DNS privacy to dns.example.net, it would opportunistically
look up the SRV for _domain-s._tcp.dns.example.net and determine
addresses (via opportunistic A and/or AAAA lookups) for the resulting
SRV response(s). The records obtained during this process would only
be used if they were validated by the client using DNSSEC.
A DNS client so configured that successfully connects to a privacy-
enabling DNS server MAY choose to locally cache the looked up
addresses in order to not have to repeat the opportunistic lookup.
8.3. DHCP
Some clients may have an established trust relationship with a known
DHCP [RFC2131] server for discovering their network configuration.
In the typical case, such a DHCP server provides a list of IP
addresses for DNS servers (see section 3.8 of [RFC2132]), but does
not provide a domain name for the DNS server itself.
A DHCP server might use a DHCP extension to provide a list of domain
names for the offered DNS servers, which correspond to IP addresses
listed.
Note that this requires the client to trust the DHCP server, and to
have a secured/authenticated connection to it. Therefore this
mechanism may be limited to only certain environments. This document
does not attempt to describe secured and trusted relationships to
DHCP servers.
[NOTE: It is noted (at the time of writing) that whilst some
implementation work is in progress to secure IPv6 connections for
DHCP, IPv4 connections have received little to no implementation
attention in this area.]
[QUESTION: The authors would like to solicit feedback on the use of
DHCP to determine whether to purse a new DHCP option in a later
version of this draft, or defer it.]
9. Credential Verification
9.1. X.509 Certificate Based Authentication
When a DNS client configured with a domain name connects to its
configured DNS server over (D)TLS, the server may present it with an
X.509 certificate. In order to ensure proper authentication, DNS
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clients MUST verify the entire certification path per [RFC5280]. The
DNS client additionally uses [RFC6125] validation techniques to
compare the domain name to the certificate provided.
A DNS client constructs two Reference Identifiers for the server
based on the domain name: A DNS-ID and an SRV-ID [RFC4985]. The DNS-
ID is simply the domain name itself. The SRV-ID uses a "_domain-s."
prefix. So if the configured domain name is "dns.example.com", then
the two Reference Identifiers are:
DNS-ID: dns.example.com
SRV-ID: _domain-s.dns.example.com
If either of the Reference Identifiers are found in the X.509
certificate's subjectAltName extension as described in section 6 of
[RFC6125], the DNS client should accept the certificate for the
server.
A compliant DNS client MUST only inspect the certificate's
subjectAltName extension for these Reference Identifiers. In
particular, it MUST NOT inspect the Subject field itself.
9.2. DANE
DANE [RFC6698] provides mechanisms to root certificate and raw public
keys trust with DNSSEC. However this requires a domain name which
must be obtained via a trusted source.
It is noted that [RFC6698] says
"Clients that validate the DNSSEC signatures themselves MUST use
standard DNSSEC validation procedures. Clients that rely on
another entity to perform the DNSSEC signature validation MUST use
a secure mechanism between themselves and the validator."
The specific DANE record would take the form:
_853._tcp.[server-domain-name] for TLS
_853._udp.[server-domain-name] for DTLS
9.2.1. Direct DNS Lookup
The DNS client MAY choose to perform the DNS lookups to retrieve the
required DANE records itself. The DNS queries for such DANE records
MAY use opportunistic encryption or be in the clear to avoid trust
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recursion. The records MUST be validated using DNSSEC as described
above in [RFC6698].
9.2.2. TLS DNSSEC Chain extension
The DNS client MAY offer the TLS extension described in
[I-D.shore-tls-dnssec-chain-extension]. If the DNS server supports
this extension, it can provide the full chain to the client in the
handshake.
If the DNS client offers the TLS DNSSEC Chain extension, it MUST be
capable of validating the full DNSSEC authentication chain down to
the leaf. If the supplied DNSSEC chain does not validate, the client
MUST ignore the DNSSEC chain and validate only via other supplied
credentials.
[ TODO: specify guidance for DANE parameters to be used here. For
example, a suggestion to use Certificate Usage of 3 (EE-DANE)
(section 2.1.1 of [RFC6698]) and a Selector of 1 (SPKI) (section
2.1.2) would completely remove all X.509 and certificate authorities
from the verification path and allows for private certification ]
[ TODO: discuss combination of DNSSEC Chain Extension with cert
validation. Note that the combination depends on the Certificate
Usage value of the TLSA response. ]
10. Combined Credentials with SPKI Pinsets
The SPKI pinset profile described in [I-D.ietf-dprive-dns-over-tls]
MAY be used with DNS-over-(D)TLS.
This draft does not make explicit recommendations about how a SPKI
pinset based authentication mechanism should be combined with a
domain based mechanism from an operator perspective. However it can
be envisaged that a DNS server operator may wish to make both an SPKI
pinset and a domain name available to allow clients to choose which
mechanism to use. Therefore, the following is guidance on how
clients ought to behave if they choose to configure both, as is
possible in HPKP [RFC7469].
A DNS client that is configured with both a domain name and a SPKI
pinset for a DNS sever SHOULD match on both a valid credential for
the domain name and a valid SPKI pinset when connecting to that DNS
server.
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11. (D)TLS Protocol Profile
This section defines the (D)TLS protocol profile of DNS-over-(D)TLS.
There are known attacks on (D)TLS, such as machine-in-the-middle and
protocol downgrade. These are general attacks on (D)TLS and not
specific to DNS-over-TLS; please refer to the (D)TLS RFCs for
discussion of these security issues. Clients and servers MUST adhere
to the (D)TLS implementation recommendations and security
considerations of [RFC7525] except with respect to (D)TLS version.
Since encryption of DNS using (D)TLS is virtually a green-field
deployment DNS clients and server MUST implement only (D)TLS 1.2 or
later.
Implementations MUST NOT offer or provide TLS compression, since
compression can leak significant amounts of information, especially
to a network observer capable of forcing the user to do an arbitrary
DNS lookup in the style of the CRIME attacks [CRIME].
Implementations compliant with this profile MUST implement all of the
following items:
o TLS session resumption without server-side state [RFC5077] which
eliminates the need for the server to retain cryptographic state
for longer than necessary.
o Raw public keys [RFC7250] which reduce the size of the
ServerHello, and can be used by servers that cannot obtain
certificates (e.g., DNS servers on private networks).
Implementations compliant with this profile SHOULD implement all of
the following items:
o TLS False Start [I-D.ietf-tls-falsestart] which reduces round-
trips by allowing the TLS second flight of messages
(ChangeCipherSpec) to also contain the (encrypted) DNS query
o Cached Information Extension [I-D.ietf-tls-cached-info] which
avoids transmitting the server's certificate and certificate chain
if the client has cached that information from a previous TLS
handshake
[NOTE: The references to (works in progress) should be upgraded to
MUST's if those references become RFC's prior to publication of this
document.]
Guidance specific to TLS or DTLS is provided in either
[I-D.ietf-dprive-dnsodtls] or [I-D.ietf-dprive-dns-over-tls].
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12. IANA Considerations
This memo includes no request to IANA.
13. Security Considerations
Security considerations discussed in [RFC7525],
[I-D.ietf-dprive-dnsodtls] and [I-D.ietf-dprive-dns-over-tls] apply
to this document.
13.1. Counter-measures to DNS Traffic Analysis
This section makes suggestions for measures that can reduce the
ability of attackers to infer information pertaining to encrypted
client queries by other means (e.g. via an analysis of encrypted
traffic size, or via monitoring of resolver to authoritative
traffic).
DNS-over-(D)TLS clients and servers SHOULD consider implementing the
following relevant DNS extensions
o EDNS(0) padding [I-D.ietf-dprive-edns0-padding], which allows
encrypted queries and responses to hide their size.
DNS-over-(D)TLS clients SHOULD consider implementing the following
relevant DNS extensions
o Privacy Election using Client Subnet in DNS Queries
[I-D.ietf-dnsop-edns-client-subnet]. If a DNS client does not
include an EDNS0 Client Subnet Option with a SOURCE PREFIX-LENGTH
set to 0 in a query, the DNS server may potentially leak client
address information to the upstream authoritative DNS servers. A
DNS client ought to be able to inform the DNS Resolver that it
does not want any address information leaked, and the DNS Resolver
should honor that request.
14. Acknowledgements
Thanks to the authors of both [I-D.ietf-dprive-dnsodtls] and
[I-D.ietf-dprive-dns-over-tls] for laying the ground work that this
draft builds on and for reviewing the contents. The authors would
also like to thank John Dickinson, Shumon Huque, Melinda Shore, Gowri
Visweswaran and Ray Bellis for review and discussion of the ideas
presented here.
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15. References
15.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4985] Santesson, S., "Internet X.509 Public Key Infrastructure
Subject Alternative Name for Expression of Service Name",
RFC 4985, DOI 10.17487/RFC4985, August 2007,
<http://www.rfc-editor.org/info/rfc4985>.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <http://www.rfc-editor.org/info/rfc5077>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[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, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <http://www.rfc-editor.org/info/rfc6125>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <http://www.rfc-editor.org/info/rfc6698>.
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[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <http://www.rfc-editor.org/info/rfc7250>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <http://www.rfc-editor.org/info/rfc7525>.
15.2. Informative References
[CRIME] Rizzo, J. and T. Duong, "The CRIME Attack", 2012.
[I-D.ietf-dnsop-edns-client-subnet]
Contavalli, C., Gaast, W., tale, t., and W. Kumari,
"Client Subnet in DNS Queries", draft-ietf-dnsop-edns-
client-subnet-06 (work in progress), December 2015.
[I-D.ietf-dprive-dns-over-tls]
Zi, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "DNS over TLS: Initiation and Performance
Considerations", draft-ietf-dprive-dns-over-tls-02 (work
in progress), December 2015.
[I-D.ietf-dprive-dnsodtls]
Reddy, T., Wing, D., and P. Patil, "DNS over DTLS
(DNSoD)", draft-ietf-dprive-dnsodtls-03 (work in
progress), November 2015.
[I-D.ietf-dprive-edns0-padding]
Mayrhofer, A., "The EDNS(0) Padding Option", draft-ietf-
dprive-edns0-padding-01 (work in progress), November 2015.
[I-D.ietf-tls-cached-info]
Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension", draft-ietf-tls-
cached-info-21 (work in progress), December 2015.
[I-D.ietf-tls-falsestart]
Langley, A., Modadugu, N., and B. Moeller, "Transport
Layer Security (TLS) False Start", draft-ietf-tls-
falsestart-01 (work in progress), November 2015.
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[I-D.shore-tls-dnssec-chain-extension]
Shore, M., Barnes, R., Huque, S., and W. Toorop, "A DANE
Record and DNSSEC Authentication Chain Extension for TLS",
draft-shore-tls-dnssec-chain-extension-02 (work in
progress), October 2015.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<http://www.rfc-editor.org/info/rfc2131>.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
<http://www.rfc-editor.org/info/rfc2132>.
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
December 2014, <http://www.rfc-editor.org/info/rfc7435>.
[RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
2015, <http://www.rfc-editor.org/info/rfc7469>.
[RFC7626] Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
DOI 10.17487/RFC7626, August 2015,
<http://www.rfc-editor.org/info/rfc7626>.
Appendix A. Server capability probing and caching by DNS clients
This section presents a non-normative discussion of how DNS clients
might probe for and cache privacy capabilities of DNS servers.
Deployment of both DNS-over-TLS and DNS-over-DTLS will be gradual.
Not all servers will support one or both of these protocols and the
well-known port might be blocked by some middleboxes. Clients will
be expected to keep track of servers that support DNS-over-TLS and/or
DNS-over-DTLS, and those that have been previously authenticated.
If no server capability information is available then (unless
otherwise specified by the configuration of the DNS client) DNS
clients that implement both TLS and DTLS should try to authenticate
using both protocols before failing or falling back to a lower
security. DNS clients using opportunistic security should try all
available servers (possibly in parallel) in order to obtain an
authenticated encrypted connection before falling back to a lower
security. (RATIONALE: This approach can increase latency while
discovering server capabilities but maximizes the chance of sending
the query over an authenticated encrypted connection.)
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Appendix B. Changes between revisions
[Note to RFC Editor: please remove this section prior to
publication.]
Authors' Addresses
Sara Dickinson
Sinodun Internet Technologies
Magdalen Centre
Oxford Science Park
Oxford OX4 4GA
UK
Email: sara@sinodun.com
URI: http://sinodun.com
Daniel Kahn Gillmor
ACLU
125 Broad Street, 18th Floor
New York NY 10004
USA
Email: dkg@fifthhorseman.net
Tirumaleswar Reddy
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: tireddy@cisco.com
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