Authentication and (D)TLS Profile for DNS-over-(D)TLS
draft-ietf-dprive-dtls-and-tls-profiles-08
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (dprive WG) | |
|---|---|---|---|
| Authors | Sara Dickinson , Daniel Kahn Gillmor , Tirumaleswar Reddy.K | ||
| Last updated | 2017-03-02 (Latest revision 2017-01-18) | ||
| Replaces | draft-dgr-dprive-dtls-and-tls-profiles | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text htmlized pdfized bibtex | ||
| Reviews |
SECDIR Last Call review
(of
-09)
Not Ready
OPSDIR Last Call review
(of
-09)
Has Issues
TSVART Last Call review
Ready with Nits
|
||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Tim Wicinski | ||
| Shepherd write-up | Show Last changed 2017-01-24 | ||
| IESG | IESG state | Waiting for Writeup | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | Terry Manderson | ||
| Send notices to | "Tim Wicinski" <tjw.ietf@gmail.com> | ||
| IANA | IANA review state | IANA OK - No Actions Needed |
draft-ietf-dprive-dtls-and-tls-profiles-08
dprive S. Dickinson
Internet-Draft Sinodun
Intended status: Standards Track D. Gillmor
Expires: July 22, 2017 ACLU
T. Reddy
Cisco
January 18, 2017
Authentication and (D)TLS Profile for DNS-over-(D)TLS
draft-ietf-dprive-dtls-and-tls-profiles-08
Abstract
This document discusses Usage Profiles, based on one or more
authentication mechanisms, which can be used for DNS over Transport
Layer Security (TLS) or Datagram TLS (DTLS). This document also
specifies new authentication mechanisms - it describes several ways a
DNS client can use an authentication domain name to authenticate a
DNS server. Additionally, it defines (D)TLS profiles for DNS clients
and servers implementing DNS-over-(D)TLS.
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 July 22, 2017.
Copyright Notice
Copyright (c) 2017 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
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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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Usage Profiles . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. DNS Resolution . . . . . . . . . . . . . . . . . . . . . 9
6. Authentication in DNS-over(D)TLS . . . . . . . . . . . . . . 9
6.1. DNS-over-(D)TLS Startup Configuration Problems . . . . . 9
6.2. Credential Verification . . . . . . . . . . . . . . . . . 10
6.3. Summary of Authentication Mechanisms . . . . . . . . . . 10
6.4. Combining Authentication Mechanisms . . . . . . . . . . . 13
6.5. Authentication in Opportunistic Privacy . . . . . . . . . 13
6.6. Authentication in Strict Privacy . . . . . . . . . . . . 13
6.7. Implementation guidance . . . . . . . . . . . . . . . . . 14
7. Sources of Authentication Domain Names . . . . . . . . . . . 14
7.1. In Band Sources: SRV Service Label . . . . . . . . . . . 14
7.2. Out of Band Sources . . . . . . . . . . . . . . . . . . . 15
7.2.1. Full direct configuration . . . . . . . . . . . . . . 15
7.2.2. Direct configuration of name only . . . . . . . . . . 15
7.2.3. DHCP . . . . . . . . . . . . . . . . . . . . . . . . 16
8. Authentication Domain Name based Credential Verification . . 16
8.1. PKIX Certificate Based Authentication . . . . . . . . . . 16
8.2. DANE . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.2.1. Direct DNS Lookup . . . . . . . . . . . . . . . . . . 18
8.2.2. TLS DNSSEC Chain extension . . . . . . . . . . . . . 18
9. (D)TLS Protocol Profile . . . . . . . . . . . . . . . . . . . 18
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
11. Security Considerations . . . . . . . . . . . . . . . . . . . 19
11.1. Counter-measures to DNS Traffic Analysis . . . . . . . . 19
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
13.1. Normative References . . . . . . . . . . . . . . . . . . 20
13.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Server capability probing and caching by DNS clients 23
Appendix B. Changes between revisions . . . . . . . . . . . . . 23
B.1. -08 version . . . . . . . . . . . . . . . . . . . . . . . 23
B.2. -07 version . . . . . . . . . . . . . . . . . . . . . . . 23
B.3. -06 version . . . . . . . . . . . . . . . . . . . . . . . 24
B.4. -05 version . . . . . . . . . . . . . . . . . . . . . . . 24
B.5. -04 version . . . . . . . . . . . . . . . . . . . . . . . 24
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B.6. -03 version . . . . . . . . . . . . . . . . . . . . . . . 25
B.7. -02 version . . . . . . . . . . . . . . . . . . . . . . . 25
B.8. -01 version . . . . . . . . . . . . . . . . . . . . . . . 25
B.9. draft-ietf-dprive-dtls-and-tls-profiles-00 . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
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'. Note that this document has the
Intended status of Experimental.
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 was
out of scope for the Working Group charter at the time this document
was finished.
This document specifies 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. Section 5 presents a generalised
discussion of Usage Profiles by separating the Usage Profile, which
is based purely on the security properties it offers the user, from
the specific mechanism(s) that are used for authentication. The
Profiles described are:
o A Strict Profile that 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 An Opportunistic Profile that will attempt, but does not require,
encryption and successful authentication; it therefore provides no
privacy guarantees but offers maximum chance of DNS service.
The above Usage Profiles attempt authentication of the server using
at least one authentication mechanism. Section 6.4 discusses how to
combine authentication mechanisms to determine the overall
authentication result. Depending on that overall authentication
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result (and whether encryption is available) the Usage Profile will
determine if the connection should proceed, fallback or fail.
One authentication mechanism is already described in [RFC7858]. That
document specifies an SPKI based authentication mechanism for DNS-
over-TLS in the context of a specific case of a Strict Usage Profile
using that single authentication mechanism. Therefore the "Out-of-
band Key-pinned Privacy Profile" described in [RFC7858] would qualify
as a "Strict Usage Profile" that used SPKI pinning for
authentication.
This document extends the use of SPKI pinset based authentication so
that it is considered a general authentication mechanism that can be
used with either DNS-over-(D)TLS Usage Profile. That is, the SPKI
pinset mechanism described in [RFC7858] MAY be used with DNS-
over-(D)TLS.
This document also describes a number of additional authentication
mechanisms all of which specify how a DNS client should authenticate
a DNS server based on an 'authentication domain name'. In
particular, the following is described:
o How a DNS client can obtain the combination of an authentication
domain name and IP address for a DNS server. See Section 7.
o What are the acceptable credentials a DNS server can present to
prove its identity for (D)TLS authentication based on a given
authentication domain name. See Section 8.
o How a DNS client can verify that any given credential matches the
authentication domain name obtained for a DNS server. See
Section 8.
In Section 9 this document 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 all currently specified authentication mechanisms.
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:
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o DNS client: a DNS stub resolver or forwarder. 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. 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 8.
* Implements the (D)TLS profile described in Section 9.
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
[I-D.ietf-dprive-dnsodtls]. Specific terms will be used for any
statement that applies to either protocol alone.
o Authentication domain name: A domain name that can be used to
authenticate a DNS Privacy enabling server. Sources of
authentication domain names are discussed in Section 7.
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 Authentication information: Information a DNS client may use as
the basis of an authentication mechanism. In this context that
can be either a:
* a SPKI pinset or
* an authentication domain name
o Reference Identifier: a Reference Identifier as described in
[RFC6125], constructed by the DNS client when performing TLS
authentication of a DNS server.
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o Credential: Information available for a DNS server which proves
its identity for authentication purposes. Credentials discussed
here include:
* PKIX certificate
* DNSSEC validated chain to a TLSA record
but may also include SPKI pinsets.
3. Scope
This document is limited to describing
o Usage Profiles based on general authentication mechanisms
o The details of domain-name-based authentication of DNS servers by
DNS clients (as defined in the terminology section)
o The (D)TLS profiles needed to support authentication in DNS-
over-(D)TLS.
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 The details of how to perform SPKI-pinset-based authentication.
This is defined in [RFC7858].
o Any server identifier other than domain names, including IP
address, organizational name, country of origin, etc.
4. Discussion
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.
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
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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 future work.
5. 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]. These 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 authentication 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 Profile is discussed in detail in
Section 6.6.
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
* an encrypted connection
* a clear text connection
depending on the fallback logic of the client, the available
authentication information and the capabilities of the DNS Server.
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In all these 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 outcomes of Opportunistic
Privacy. In the best case scenario for Opportunistic Privacy (an
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 the intermediate case 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 Security to notify
the user what type of connection is actually used, the 'detection'
described below is only possible if such connection information is
available. However, if it is available and the user is informed that
an unencrypted connection was used to connect to a server then the
user should assume (detect) that the connection is subject to both
active and passive attack since the DNS queries are sent in clear
text. This might be particularly useful if a new connection to a
certain server is unencrypted when all previous connections were
encrypted. Similarly if the user is informed that an encrypted but
unauthenticated connection was used then user can detect that the
connection may be subject to active attack. This is discussed in
Section 6.5.
+---------------+------------+------------------+-----------------+
| 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
Strict Privacy provides the strongest privacy guarantees and
therefore SHOULD always be implemented in DNS clients along with
Opportunistic Privacy.
A DNS client that implements DNS-over-(D)TLS SHOULD NOT default to
the use of clear text (no privacy).
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The choice between the two profiles depends on a number of factors
including which is more important to the particular client:
o DNS service at the cost of no privacy guarantee (Opportunistic) or
o guaranteed privacy at the potential cost of no DNS service
(Strict).
Additionally the two profiles require varying levels of configuration
(or a trusted relationship with a provider) and DNS server
capabilities therefore DNS clients will need to carefully select
which profile to use based on their communication privacy needs.
A DNS server that implements DNS-over-TLS SHOULD provide at least one
credential in order that those DNS clients that wish to do so are
able to use Strict Privacy (see Section 2).
5.1. 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.
6. Authentication in DNS-over(D)TLS
This section describes authentication mechanisms and how they can be
used in either Strict or Opportunistic Privacy for DNS-over-(D)TLS.
6.1. DNS-over-(D)TLS Startup Configuration Problems
Many (D)TLS clients use PKIX authentication [RFC6125] based on an
authentication 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. Such a DNS client therefore has a
bootstrap problem. DNS clients typically know only the IP address of
a DNS server.
In this case, before connecting to a DNS server, a DNS client needs
to learn the authentication domain name it should associate with the
IP address of a DNS server for authentication purposes. Sources of
authentication domains names are discussed in Section 7.
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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6.2. Credential Verification
The use of SPKI pinset verification is discussed in [RFC7858].
In terms of domain name based verification, once an authentication
domain name is known for a DNS server a choice of authentication
mechanisms can be used for credential verification. Section 8
discusses these mechanisms in detail, namely PKIX 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 8.2.
6.3. Summary of Authentication Mechanisms
This section provides an overview of the various authentication
mechanisms. The table below indicates how the DNS client obtains
information to use for authentication for each option; either
statically via direct configuration or dynamically. Of course, the
Opportunistic Usage Profile does not require authentication and so a
client using that profile may choose to connect to a Privacy Enabling
DNS server on the basis of just an IP address.
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+---+------------+-------------+------------------------------------+
| # | Static | Dynamically | Short name: Description |
| | Config | Obtained | |
+---+------------+-------------+------------------------------------+
| 1 | SPKI + IP | | SPKI: SPKI pinset(s) and IP |
| | | | address obtained out of band |
| | | | [RFC7858] |
| | | | |
| 2 | ADN + IP | | ADN: ADN and IP address obtained |
| | | | out of band (see Section 7.2.1) |
| | | | |
| 3 | ADN | IP | ADN only: DNSSEC validated |
| | | | Opportunistic lookups to a NP DNS |
| | | | server for SRV, A/AAAA (see |
| | | | Section 7.2.2) |
| | | | |
| 4 | | ADN + IP | DHCP: DHCP configuration only (see |
| | | | Section 7.2.3) |
| | | | |
| 5 | [ADN + IP] | [ADN + IP] | DANE: DNSSEC chain obtained via |
| | | TLSA record | Opportunistic lookups to NP DNS |
| | | | server (see Section 8.2.1) |
| | | | |
| 6 | [ADN + IP] | [ADN + IP] | TLS extension: DNSSEC chain |
| | | TLSA record | provided by PE DNS server in TLS |
| | | | DNSSEC chain extension (see |
| | | | Section 8.2.2) |
+---+------------+-------------+------------------------------------+
SPKI == SPKI pinset(s), IP == IP Address, ADN == Authentication
Domain Name, NP == Network provided, PE == Privacy-enabling, [ ] ==
Data may be obtained either statically or dynamically
Table 2: Overview of Authentication Mechanisms
The following summary attempts to present some key attributes of each
of the mechanisms (using the 'Short name' from Table 2), indicating
attractive attributes with a '+' and undesirable attributes with a
'-'.
1. SPKI
+ Minimal leakage (Note that the ADN is always leaked in the
SNI field in the Client Hello in TLS when communicating with a
privacy enabling DNS server)
- Overhead of on-going key management required
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2. ADN
+ Minimal leakage
+ One-off direct configuration only
3. ADN only
+ Minimal one-off direct configuration, only a human
recognisable domain name needed
- SRV, A/AAAA meta queries leaked to network provided DNS
server
4. DHCP
+ No static config
- Requires a non-standard or future DHCP option in order to
provide the ADN
- Requires secure and trustworthy connection to DHCP server
5. DANE
The ADN and/or IP may be obtained statically or dynamically
and the relevant attributes of that method apply
+ DANE options (e.g. matching on entire certificate)
- Requires a DNSSEC validating stub implementation (deployment
of which is limited at the time of writing)
- DNSSEC chain meta queries leaked to network provided DNS
server
6. TLS extension
The ADN and/or IP may be obtained statically or dynamically
and the relevant attributes of that method apply
+ Reduced latency compared with 'DANE'
+ No network provided DNS server required if ADN and IP
statically configured
+ DANE options (e.g. matching on entire certificate)
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- Requires a DNSSEC validating stub implementation
6.4. Combining Authentication Mechanisms
This draft does not make explicit recommendations about how an 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 an authentication 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 an authentication domain
name and a SPKI pinset for a DNS server SHOULD match on both a valid
credential for the authentication domain name and a valid SPKI pinset
(if both are available) when connecting to that DNS server. The
overall authentication result should only be considered successful if
both authentication mechanisms are successful.
6.5. Authentication in Opportunistic 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
authentication information 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.
6.6. Authentication in Strict Privacy
To authenticate a privacy-enabling DNS server, a DNS client needs to
know authentication information for each server it is willing to
contact. This is necessary to protect against active attacks on DNS
privacy.
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A DNS client requiring Strict Privacy MUST either use one of the
sources listed in Section 7.2 to obtain an authentication 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 at least one piece of authentication
information is known. The client MUST use the available verification
mechanisms described in Section 8 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.
6.7. Implementation guidance
Section 9 describes the (D)TLS profile for DNS-over(D)TLS.
Additional considerations relating to general implementation
guidelines are discussed in both Section 11 and in Appendix A.
7. Sources of Authentication Domain Names
7.1. In Band Sources: 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.
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7.2. Out of Band Sources
7.2.1. Full direct configuration
DNS clients may be directly and securely provisioned with the
authentication 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.
7.2.2. Direct configuration of name only
A DNS client may be configured directly and securely with only the
authentication 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:
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.
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7.2.3. 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 authentication 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 authentication domain names MUST be
considered secure and trustworthy. This document does not attempt to
describe secured and trusted relationships to DHCP servers.
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.
8. Authentication Domain Name based Credential Verification
8.1. PKIX Certificate Based Authentication
When a DNS client configured with an authentication domain name
connects to its configured DNS server over (D)TLS, the server may
present it with an PKIX 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 authentication domain name: A DNS-ID and an SRV-ID
[RFC4985]. The DNS-ID is simply the authentication domain name
itself. The SRV-ID uses a "_domain-s." prefix. So if the configured
authentication 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 PKIX
certificate's subjectAltName extension as described in section 6 of
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[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.
8.2. DANE
DANE [RFC6698] provides mechanisms to root certificate and raw public
key trust with DNSSEC. However this requires the DNS client to have
an authentication 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."
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.
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
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_853._udp.[server-domain-name] for DTLS
8.2.1. 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].
8.2.2. 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.
9. (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:
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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 [RFC7918] which reduces round-trips by allowing
the TLS second flight of messages (ChangeCipherSpec) to also
contain the (encrypted) DNS query
o Cached Information Extension [RFC7924] which avoids transmitting
the server's certificate and certificate chain if the client has
cached that information from a previous TLS handshake
Guidance specific to TLS is provided in [RFC7858] and that specific
to DTLS it is provided in[I-D.ietf-dprive-dnsodtls].
10. IANA Considerations
This memo includes no request to IANA.
11. Security Considerations
Security considerations discussed in [RFC7525],
[I-D.ietf-dprive-dnsodtls] and [RFC7858] apply to this document.
11.1. 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
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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
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.
12. 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, Christian
Huitema and John Levine for review and discussion of the ideas
presented here.
13. References
13.1. 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>.
13.2. 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, "Specification for DNS
over Datagram Transport Layer Security (DTLS)", draft-
ietf-dprive-dnsodtls-15 (work in progress), December 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-02 (work in
progress), January 2017.
[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>.
[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>.
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[RFC7918] Langley, A., Modadugu, N., and B. Moeller, "Transport
Layer Security (TLS) False Start", RFC 7918,
DOI 10.17487/RFC7918, August 2016,
<http://www.rfc-editor.org/info/rfc7918>.
[RFC7924] Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension", RFC 7924,
DOI 10.17487/RFC7924, July 2016,
<http://www.rfc-editor.org/info/rfc7924>.
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. -08 version
Removed hard failure as an option for Opportunistic Usage profile.
Added a new section comparing the Authentication Mechanisms
B.2. -07 version
Re-work of the Abstract and Introduction to better describe the
contents in this version.
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Terminology: New definition of 'authentication information'.
Scope: Changes to the Scope section.
Moved discussion of combining authentication mechanism earlier.
Changes to the section headings and groupings to make the
presentation more logical.
B.3. -06 version
Introduction: Re-word discussion of Working group charter.
Introduction: Re-word first and third bullet point about 'obtaining'
a domain name and IP address.
Introduction: Update reference to DNS-over-TLS draft.
Terminology: Change forwarder/proxy to just forwarder
Terminology: Add definition of 'Authentication domain name' and use
this throughout
Section 4.2: Remove parenthesis in the table.
Section 4.2: Change the text after the table as agreed with Paul
Hoffman.
Section 4.3.1: Change title and remove brackets around last
statement.
Section 11: Split second paragraph.
B.4. -05 version
Add more details on detecting passive attacks to section 4.2
Changed X.509 to PKIX throughout
Change comment about future I-D on usage policies.
B.5. -04 version
Introduction: Add comment that DNS-over-DTLS draft is Experiments
Update 2 I-D references to RFCs.
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B.6. -03 version
Section 9: Update DANE section with better references to RFC7671 and
RFC7250
B.7. -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.
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.8. -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.9. 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.
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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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