dprive S. Dickinson
Internet-Draft Sinodun
Intended status: Standards Track D. Gillmor
Expires: January 9, 2017 ACLU
T. Reddy
Cisco
July 8, 2016
Authentication and (D)TLS Profile for DNS-over-(D)TLS
draft-ietf-dprive-dtls-and-tls-profiles-03
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-
Drafts is at http://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 9, 2017.
Copyright Notice
Copyright (c) 2016 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
Provisions Relating to IETF Documents
(http://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
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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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Background . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Usage Profiles . . . . . . . . . . . . . . . . . . . . . 6
4.2.1. DNS Resolution . . . . . . . . . . . . . . . . . . . 8
4.3. Authentication . . . . . . . . . . . . . . . . . . . . . 8
4.3.1. DNS-over-(D)TLS Bootstrapping Problems . . . . . . . 8
4.3.2. Credential Verification . . . . . . . . . . . . . . . 8
4.3.3. Implementation guidance . . . . . . . . . . . . . . . 9
5. Authentication in Opportunistic DNS-over(D)TLS Privacy . . . 9
6. Authentication in Strict DNS-over(D)TLS Privacy . . . . . . . 9
7. In Band Source of Domain Name: SRV Service Label . . . . . . 10
8. Out of Band Sources of Domain Name . . . . . . . . . . . . . 10
8.1. Full direct configuration . . . . . . . . . . . . . . . . 10
8.2. Direct configuration of name only . . . . . . . . . . . . 10
8.3. DHCP . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9. Credential Verification . . . . . . . . . . . . . . . . . . . 12
9.1. X.509 Certificate Based Authentication . . . . . . . . . 12
9.2. DANE . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.2.1. Direct DNS Lookup . . . . . . . . . . . . . . . . . . 13
9.2.2. TLS DNSSEC Chain extension . . . . . . . . . . . . . 13
10. Combined Credentials with SPKI Pinsets . . . . . . . . . . . 14
11. (D)TLS Protocol Profile . . . . . . . . . . . . . . . . . . . 14
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
13. Security Considerations . . . . . . . . . . . . . . . . . . . 15
13.1. Counter-measures to DNS Traffic Analysis . . . . . . . . 15
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
15.1. Normative References . . . . . . . . . . . . . . . . . . 16
15.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. Server capability probing and caching by DNS clients 19
Appendix B. Changes between revisions . . . . . . . . . . . . . 19
B.1. -03 version . . . . . . . . . . . . . . . . . . . . . . . 19
B.2. -02 version . . . . . . . . . . . . . . . . . . . . . . . 19
B.3. -01 version . . . . . . . . . . . . . . . . . . . . . . . 20
B.4. draft-ietf-dprive-dtls-and-tls-profiles-00 . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
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1. Introduction
DNS Privacy issues are discussed in [RFC7626]. Two documents that
provide DNS privacy between DNS clients and DNS servers are:
o Specification for DNS over Transport Layer Security (TLS)
[RFC7858], referred to here as simply 'DNS-over-TLS'
o DNS-over-DTLS (DNSoD) [I-D.ietf-dprive-dnsodtls], referred to here
simply as 'DNS-over-DTLS'
Both documents are limited in scope to encrypting DNS messages
between stub clients and recursive resolvers and the same scope is
applied to this document (see Section 2 and Section 3). The
proposals here might be adapted or extended in future to be used for
recursive clients and authoritative servers, but this application is
out of scope for the DNS PRIVate Exchange (DPRIVE) Working Group per
its current charter.
This document defines two Usage Profiles (Strict and Opportunistic)
for DTLS [RFC6347] and TLS [RFC5246] which define the security
properties a user should expect when using that profile to connect to
the available DNS servers. In essence:
o the Strict Profile requires an encrypted connection and successful
authentication of the DNS server which provides strong privacy
guarantees (at the expense of providing no DNS service if this is
not available).
o the Opportunistic Profile will attempt, but does not require,
encryption and successful authentication; it therefore provides no
privacy guarantees but offers maximum chance of DNS service.
Additionally, a number of authentication mechanisms are defined that
specify how a DNS client should authenticate a DNS server based on a
domain name. In particular, the following is described:
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.
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It should be noted that [RFC7858] includes a description of a
specific case of a Strict Usage Profile using a single authentication
mechanism (SPKI pinning). This draft generalises the picture by
separating the Usage Profile, which is based purely on the security
properties it offers the user, from the specific mechanism that is
used for authentication. Therefore the "Out-of-band Key-pinned
Privacy Profile" described in the DNS-over-TLS draft would qualify as
a "Strict Usage Profile" that used SPKI pinning for authentication.
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 mechanisms 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 [RFC7858] 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.
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 [RFC7858] and DNS-over-DTLS
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[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: [RFC7858] 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 [RFC7858].
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.
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
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provide protection against active attackers pretending to be the
server, the client must authenticate the server.
This draft discusses Usage Profiles, which provide differing levels
of privacy guarantees to DNS clients, based on the requirements for
authentication and encryption, regardless of the context (for
example, which network the client is connected to). A Usage Profile
is a distinct concept to a usage policy or usage model, which might
dictate which Profile should be used in a particular context
(enterprise vs coffee shop), with a particular set of DNS Servers or
with reference to other external factors. A description of the
variety of usage policies is out of scope of this document, but may
be the subject of a future I-D.
4.2. Usage Profiles
A DNS client has a choice of privacy usage profiles available. This
choice is briefly discussed in both [RFC7858] 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 privacy-enabling 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 profile can include some initial meta queries
(performed using Opportunistic Privacy) to securely obtain the IP
address and authentication information for the privacy-enabling
DNS server to which the DNS client will subsequently connect. The
rationale for this is that requiring Strict Privacy for such meta
queries would introduce significant deployment obstacles. This
profile 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."
The use of Opportunistic Privacy is intended to support
incremental deployment of security capabilities with a view to
widespread adoption of Strict Privacy. It should be employed when
the DNS client might otherwise settle for cleartext; it provides
the maximum protection available. As described in [RFC7435] it
might result in
* an encrypted and authenticated connection
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* an encrypted connection
* a clear text connection
* hard failure
depending on the fallback logic of the client, the available
authentication information and the capabilities of the DNS Server.
In the first three cases the DNS client is willing to continue
with a connection to the DNS Server and perform resolution of
queries.
To compare the two Usage profiles the table below shows successful
Strict Privacy along side the 3 possible successful outcomes of
Opportunistic Privacy. In the best case scenario for Opportunistic
(authenticated and encrypted connection) it is equivalent to Strict
Privacy. In the worst case scenario it is equivalent to clear text.
Clients using Opportunistic Privacy SHOULD try for the best case but
MAY fallback to intermediate cases and eventually the worst case
scenario in order to obtain a response. It therefore provides no
privacy guarantee to the user and varying protection depending on
what kind of connection is actually used. Note that there is no
requirement in Opportunistic to notify the user what type of
connection is actually used, the detection described below is only
possible if such connection information is available. This is
discussed in Section 5.
+---------------+------------+------------------+-----------------+
| Usage Profile | Connection | Passive Attacker | Active Attacker |
+---------------+------------+------------------+-----------------+
| Strict | A, E | P | P |
| Opportunistic | A, E | P | P |
| Opportunistic | E | P | N (D) |
| Opportunistic | | N (D) | N (D) |
+---------------+------------+------------------+-----------------+
P == protection; N == no protection; D == detection is possible; A ==
Authenticated Connection; E == Encrypted Connection
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.
However since the two profiles require varying levels of
configuration (or a trusted relationship with a provider) and DNS
server capabilities, DNS clients will need to carefully select which
profile to use based on their communication privacy needs. For the
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case where a client has a trusted relationship with a provider it is
expected that the provider will provide either a domain name or SPKI
pinset via a secure out-of-band mechanism and therefore Strict
Privacy should be used.
4.2.1. DNS Resolution
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.
4.3.2. Credential Verification
The use of SPKI pinset verification is discussed in [RFC7858].
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.
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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 [RFC7858]
which MAY be used for DNS-over-(D)TLS.
DNS clients issuing queries under an opportunistic profile which know
of a domain name or SPKI pinset for a given privacy-enabling 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, since the fact that authentication
information is available indicates that the server in question is a
privacy-enabling DNS server to which it should be possible to
establish an authenticated, encrypted connection. In this case,
whilst a client cannot know the reason for an authentication failure,
from a privacy standpoint the client should consider an active attack
in progress and proceed under that assumption. Attempting
authentication is also useful for debugging or diagnostic purposes if
there are means to report the result. 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 [RFC7858].
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
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.
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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.
A DNS client might learn of a default recursive DNS resolver from an
untrusted source (such as DHCP's DNS server option [RFC3646]). It
can then use opportunistic DNS connections to untrusted recursive DNS
resolver to establish the IP address of the intended privacy-enabling
DNS server by doing a lookup of SRV records. Such records MUST be
validated using DNSSEC. Private DNS resolution can now be done by
the DNS client against the configured privacy-enabling DNS server.
Example:
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o A DNSSEC validating DNS client is configured with the domain name
dns.example.net for a privacy-enabling DNS server
o Using Opportunistic Privacy to a default DNS resolver (acquired,
for example, using DHCP) the client performs look ups for
* SRV record for _domain-s._tcp.dns.example.net to obtain the
server host name
* A and/or AAAA lookups to obtain IP address for the server host
name
o Client validates all the records obtained in the previous step
using DNSSEC.
o If the records successfully validate the client proceeds to
connect to the privacy-enabling DNS server using Strict Privacy.
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.
In the future, 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.
Use of such a mechanism with any DHCP server when using an
Opportunistic profile is reasonable, given the security expectation
of that profile. However when using a Strict profile the DHCP
servers used as sources of domain names MUST be considered secure and
trustworthy. 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.]
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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
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
key trust with DNSSEC. However this requires the DNS client to have
a domain name for the DNS Privacy Server which must be obtained via a
trusted source.
This section assumes a solid understanding of both DANE [RFC6698] and
DANE Operations [RFC7671]. A few pertinent issues covered in these
documents are outlined here as useful pointers, but familiarity with
both these documents in their entirety is expected.
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."
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It is noted that [RFC7671] covers the following topics:
o Section 4.1: Opportunistic Security and PKIX Usages and
Section 14: Security Considerations, which both discuss the use of
PKIX-TA(0) and PKIX-EE(1) for OS.
o Section 5: Certificate-Usage-Specific DANE Updates and Guidelines.
Specifically Section 5.1 which outlines the combination of
Certificate Usage DANE-EE(3) and Selector Usage SPKI(1) with Raw
Public Keys [RFC7250]. Section 5.1 also discusses the security
implications of this mode, for example, it discusses key lifetimes
and specifies that validity period enforcement is based solely on
the TLSA RRset properties for this case. [QUESTION: Should an
appendix be added with an example of how to use DANE without X.509
certificates?]
o Section 13: Operational Considerations, which discusses TLSA TTLs
and signature validity periods.
The specific DANE record for a DNS Privacy Server 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
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.ietf-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.
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10. Combined Credentials with SPKI Pinsets
The SPKI pinset profile described in [RFC7858] 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.
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).
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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 is provided in [RFC7858] and that specific
to DTLS it is provided in[I-D.ietf-dprive-dnsodtls].
12. IANA Considerations
This memo includes no request to IANA.
13. Security Considerations
Security considerations discussed in [RFC7525],
[I-D.ietf-dprive-dnsodtls] and [RFC7858] 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 [RFC7830], 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 [RFC7871]. 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
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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
[RFC7858] 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, Ray
Bellis, Stephane Bortzmeyer, Jinmei Tatuya, Paul Hoffman and
Christian Huitema for review and discussion of the ideas presented
here.
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>.
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[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>.
[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>.
[RFC7671] Dukhovni, V. and W. Hardaker, "The DNS-Based
Authentication of Named Entities (DANE) Protocol: Updates
and Operational Guidance", RFC 7671, DOI 10.17487/RFC7671,
October 2015, <http://www.rfc-editor.org/info/rfc7671>.
[RFC7830] Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830,
DOI 10.17487/RFC7830, May 2016,
<http://www.rfc-editor.org/info/rfc7830>.
[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, <http://www.rfc-editor.org/info/rfc7858>.
15.2. Informative References
[CRIME] Rizzo, J. and T. Duong, "The CRIME Attack", 2012.
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[dnssec-trigger]
NLnetLabs, "Dnssec-Trigger", May 2014,
<https://www.nlnetlabs.nl/projects/dnssec-trigger/>.
[I-D.ietf-dprive-dnsodtls]
Reddy, T., Wing, D., and P. Patil, "DNS over DTLS
(DNSoD)", draft-ietf-dprive-dnsodtls-06 (work in
progress), April 2016.
[I-D.ietf-tls-cached-info]
Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension", draft-ietf-tls-
cached-info-23 (work in progress), May 2016.
[I-D.ietf-tls-dnssec-chain-extension]
Shore, M., Barnes, R., Huque, S., and W. Toorop, "A DANE
Record and DNSSEC Authentication Chain Extension for TLS",
draft-ietf-tls-dnssec-chain-extension-00 (work in
progress), June 2016.
[I-D.ietf-tls-falsestart]
Langley, A., Modadugu, N., and B. Moeller, "Transport
Layer Security (TLS) False Start", draft-ietf-tls-
falsestart-02 (work in progress), May 2016.
[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>.
[RFC3646] Droms, R., Ed., "DNS Configuration options for Dynamic
Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
DOI 10.17487/RFC3646, December 2003,
<http://www.rfc-editor.org/info/rfc3646>.
[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>.
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[RFC7626] Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
DOI 10.17487/RFC7626, August 2015,
<http://www.rfc-editor.org/info/rfc7626>.
[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,
<http://www.rfc-editor.org/info/rfc7871>.
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.)
Appendix B. Changes between revisions
[Note to RFC Editor: please remove this section prior to
publication.]
B.1. -03 version
Section 9: Update DANE section with better references to RFC7671 and
RFC7250
B.2. -02 version
Introduction: Added paragraph on the background and scope of the
document.
Introduction and Discussion: Added more information on what a Usage
profiles is (and is not) the the two presented here.
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Introduction: Added paragraph to make a comparison with the Strict
profile in RFC7858 clearer.
Section 4.2: Re-worked the description of Opportunistic and the
table.
Section 8.3: Clarified statement about use of DHCP in Opportunistic
profile
Title abbreviated.
B.3. -01 version
Section 4.2: Make clear that the Strict Privacy Profile can include
meta queries performed using Opportunistic Privacy.
Section 4.2, Table 1: Update to clarify that Opportunistic Privacy
does not guarantee protection against passive attack.
Section 4.2: Add sentence discussing client/provider trusted
relationships.
Section 5: Add more discussion of detection of active attacks when
using Opportunistic Privacy.
Section 8.2: Clarify description and example.
B.4. draft-ietf-dprive-dtls-and-tls-profiles-00
Re-submission of draft-dgr-dprive-dtls-and-tls-profiles with name
change to draft-ietf-dprive-dtls-and-tls-profiles. Also minor nits
fixed.
Authors' Addresses
Sara Dickinson
Sinodun Internet Technologies
Magdalen Centre
Oxford Science Park
Oxford OX4 4GA
UK
Email: sara@sinodun.com
URI: http://sinodun.com
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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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