Internet DRAFT - draft-ietf-dprive-dtls-and-tls-profiles
draft-ietf-dprive-dtls-and-tls-profiles
dprive S. Dickinson
Internet-Draft Sinodun
Updates: 7858 (if approved) D. Gillmor
Intended status: Standards Track ACLU
Expires: March 15, 2018 T. Reddy
McAfee
September 11, 2017
Usage and (D)TLS Profiles for DNS-over-(D)TLS
draft-ietf-dprive-dtls-and-tls-profiles-11
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). These profiles can
increase the privacy of DNS transactions compared to using only clear
text DNS. This document also specifies new authentication mechanisms
- it describes several ways a DNS client can use an authentication
domain name to authenticate a (D)TLS connection to a DNS server.
Additionally, it defines (D)TLS protocol profiles for DNS clients and
servers implementing DNS-over-(D)TLS. This document updates RFC
7858.
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
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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 March 15, 2018.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Usage Profiles . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. DNS Resolution . . . . . . . . . . . . . . . . . . . . . 10
6. Authentication in DNS-over(D)TLS . . . . . . . . . . . . . . 10
6.1. DNS-over-(D)TLS Startup Configuration Problems . . . . . 10
6.2. Credential Verification . . . . . . . . . . . . . . . . . 11
6.3. Summary of Authentication Mechanisms . . . . . . . . . . 11
6.4. Combining Authentication Mechanisms . . . . . . . . . . . 14
6.5. Authentication in Opportunistic Privacy . . . . . . . . . 14
6.6. Authentication in Strict Privacy . . . . . . . . . . . . 15
6.7. Implementation guidance . . . . . . . . . . . . . . . . . 15
7. Sources of Authentication Domain Names . . . . . . . . . . . 15
7.1. Full direct configuration . . . . . . . . . . . . . . . . 15
7.2. Direct configuration of ADN only . . . . . . . . . . . . 16
7.3. Dynamic discovery of ADN . . . . . . . . . . . . . . . . 16
7.3.1. DHCP . . . . . . . . . . . . . . . . . . . . . . . . 16
8. Authentication Domain Name based Credential Verification . . 17
8.1. PKIX Certificate Based Authentication . . . . . . . . . . 17
8.2. DANE . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.2.1. Direct DNS Lookup . . . . . . . . . . . . . . . . . . 18
8.2.2. TLS DNSSEC Chain extension . . . . . . . . . . . . . 18
9. (D)TLS Protocol Profile . . . . . . . . . . . . . . . . . . . 19
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
11. Security Considerations . . . . . . . . . . . . . . . . . . . 20
11.1. Counter-measures to DNS Traffic Analysis . . . . . . . . 20
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
13.1. Normative References . . . . . . . . . . . . . . . . . . 21
13.2. Informative References . . . . . . . . . . . . . . . . . 23
Appendix A. Server capability probing and caching by DNS clients 24
Appendix B. Changes between revisions . . . . . . . . . . . . . 24
B.1. -11 version . . . . . . . . . . . . . . . . . . . . . . . 25
B.2. -10 version . . . . . . . . . . . . . . . . . . . . . . . 25
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B.3. -09 version . . . . . . . . . . . . . . . . . . . . . . . 26
B.4. -08 version . . . . . . . . . . . . . . . . . . . . . . . 26
B.5. -07 version . . . . . . . . . . . . . . . . . . . . . . . 26
B.6. -06 version . . . . . . . . . . . . . . . . . . . . . . . 26
B.7. -05 version . . . . . . . . . . . . . . . . . . . . . . . 27
B.8. -04 version . . . . . . . . . . . . . . . . . . . . . . . 27
B.9. -03 version . . . . . . . . . . . . . . . . . . . . . . . 27
B.10. -02 version . . . . . . . . . . . . . . . . . . . . . . . 27
B.11. -01 version . . . . . . . . . . . . . . . . . . . . . . . 28
B.12. draft-ietf-dprive-dtls-and-tls-profiles-00 . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
1. Introduction
DNS Privacy issues are discussed in [RFC7626]. The specific issues
described there that are most relevant to this document are
o Passive attacks which eavesdrop on clear text DNS transactions on
the wire (Section 2.4) and
o Active attacks which redirect clients to rogue servers to monitor
DNS traffic (Section 2.5.3).
Mitigating against these attacks increases the privacy of DNS
transactions, however many of the other issues raised in [RFC7626]
still apply.
Two documents that provide ways to increase 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 Datagram Transport Layer Security (DTLS) [RFC8094],
referred to here simply as 'DNS-over-DTLS'. Note that this
document has the Category of Experimental.
Both documents are limited in scope to communications 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 provide improved levels of
mitigation against the attacks described above compared to using only
clear text DNS.
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Section 5 presents a generalized 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 DNS server authentication. The Profiles described are:
o A Strict Profile that requires an encrypted connection and
successful authentication of the DNS server which mitigates both
passive eavesdropping and client re-direction (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
limited or no mitigation against such attacks 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
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 a Subject Public Key Info (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.
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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:
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. For emphasis, in this document the term
does not apply to authoritative servers.
o Privacy-enabling DNS server: A DNS server that implements DNS-
over-TLS [RFC7858] and may optionally implement DNS-over-DTLS
[RFC8094]. The server should also offer at least one of the
credentials described in Section 8 and implement 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 [RFC8094].
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 privacy-enabling DNS server. Sources of
authentication domain names are discussed in Section 7.
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o SPKI Pinsets: [RFC7858] describes the use of cryptographic digests
to "pin" public key information in a manner similar to HTTP Public
Key Pinning [RFC7469] (HPKP). 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.
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].
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o Any server identifier other than domain names, including IP
addresses, organizational names, country of origin, etc.
4. Discussion
One way to mitigate against passive attackers eavesdropping on clear
text DNS transactions is to encrypt 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. To also mitigate against active attackers pretending to
be the server, the client must authenticate the (D)TLS connection to
the server.
This document discusses Usage Profiles, which provide differing
levels of attack mitigation 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 future work.
The term 'privacy-enabling DNS server' is used throughout this
document. This is a DNS server that:
o MUST implement DNS-over-TLS [RFC7858].
o MAY implement DNS-over-DTLS [RFC8094].
o SHOULD offer at least one of the credentials described in
Section 8.
o Implements the (D)TLS profile described in Section 9.
5. Usage Profiles
A DNS client has a choice of Usage Profiles available to increase the
privacy of DNS transactions. This choice is briefly discussed in
both [RFC7858] and [RFC8094]. These Usage Profiles are:
o Strict profile: 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 mitigates against both
passive and active attacks providing the client with the best
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available privacy for DNS. 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."
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.
In all these cases the DNS client is willing to continue with a
connection to the DNS Server and perform resolution of queries.
The use of Opportunistic Privacy is intended to support
incremental deployment of increased privacy with a view to
widespread adoption of the Strict profile. It should be employed
when the DNS client might otherwise settle for cleartext; it
provides the maximum protection available depending on the
combination of factors described above. If all the configured DNS
Servers are DNS Privacy servers then it provides protection
against passive attacks but not active ones.
Both profiles can include an initial meta query (performed using an
Opportunistic lookup) to obtain the IP address for the privacy-
enabling DNS server to which the DNS client will subsequently
connect. The rationale for permitting this for the Strict profile is
that requiring such meta queries to also be performed using the
Strict profile would introduce significant deployment obstacles.
However, it should be noted that in this scenario an active attack is
possible on the meta query. Such an attack could result in a Strict
profile client connecting to a server it cannot authenticate and so
not obtaining DNS service, or an Opportunistic Privacy client
connecting to a server controlled by the attacker. DNSSEC validation
can detect the attack on the meta query and results in the client not
obtaining DNS service (for both Usage profiles) because it will not
proceed to connect to the server in question (see Section 7.2)
To compare the two Usage profiles the table below shows a successful
Strict profile along side the 3 possible outcomes of an Opportunistic
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profile. In the best case scenario for the Opportunistic profile (an
authenticated and encrypted connection) it is equivalent to the
Strict profile. In the worst case scenario it is equivalent to clear
text. Clients using the Opportunistic profile 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. One reason to
fallback without trying every available privacy-enabling DNS server
is if latency is more important than attack mitigation, see
Appendix A. The Opportunistic profile therefore provides varying
protection depending on what kind of connection is actually used
including no attack mitigation at all.
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 the user can detect that the
connection may be subject to active attack. In other words for the
cases where no protection is provided against an attacker (N) it is
possible to detect that an attack might be happening (D). 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: Attack protection by Usage Profile and type of attacker
The Strict profile provides the best attack mitigation and therefore
SHOULD always be implemented in DNS clients that implement
Opportunistic Privacy.
A DNS client that implements DNS-over-(D)TLS SHOULD NOT be configured
by default to use only clear text.
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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 attack mitigation (Opportunistic) or
o best available attack mitigation 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 needs.
A DNS server that implements DNS-over-(D)TLS SHOULD provide at least
one credential so that those DNS clients that wish to do so are able
to use the Strict profile (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 of a given query as this
could invalidate the protection offered against attackers. It is
anticipated that DNS clients will use a particular Usage Profile for
all queries to all configured servers until an operational issue or
policy update dictates a change in the profile used.
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, as it will typically only know the IP address of
its 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 domain names are discussed in Section 7.
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One advantage of this domain name based approach is that it
encourages association of stable, human recognizable identifiers with
secure DNS service providers.
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 7.1) |
| | | | |
| 3 | ADN | IP | ADN only: Opportunistic lookups to |
| | | | a NP DNS server for A/AAAA (see |
| | | | Section 7.2) |
| | | | |
| 4 | | ADN + IP | DHCP: DHCP configuration only (see |
| | | | Section 7.3.1) |
| | | | |
| 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
Server Name Indication (SNI) field in the Client Hello in TLS
when communicating with a privacy-enabling DNS server)
- Overhead of on-going key management required
2. ADN
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+ Minimal leakage
+ One-off direct configuration only
3. ADN only
+ Minimal one-off direct configuration, only a human
recognizable domain name needed
- A/AAAA meta queries leaked to network provided DNS server
that may be subject to active attack (attack can be mitigated
by DNSSEC validation).
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 if
used with a Strict Usage profile
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 that may be subject to active attack
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. In this
case the client SHOULD treat the SPKI pin as specified in Section 2.6
of [RFC7469] with regard to user defined trust anchors. 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 and which
know authentication information for a given privacy-enabling DNS
server SHOULD 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 and encrypted connection. In this case, whilst a
client cannot know the reason for an authentication failure, from a
security standpoint the client should consider an active attack in
progress and proceed under that assumption. For example, a client
that implements a nameserver selection algorithm that preferentially
uses nameservers which successfully authenticated (see Section 5)
might not continue to use the failing server if there were
alternative servers available.
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 e.g, where all the configured servers
support DNS-over-(D)TLS and successfully authenticate.
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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 which
attempt to re-direct clients to rogue DNS servers.
A DNS client requiring Strict Privacy MUST either use one of the
sources listed in Section 7 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. If using a Strict Usage profile
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. 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.
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In this case, direct configuration for a DNS client would consist of
both an IP address and an authentication domain name for each DNS
server.
7.2. Direct configuration of ADN only
A DNS client may be configured directly and securely with only the
authentication domain name of each of its privacy-enabling DNS
servers. 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 an untrusted recursive
DNS resolver to establish the IP address of the intended privacy-
enabling DNS resolver by doing a lookup of A/AAAA records. Such
records SHOULD be DNSSEC validated when using a Strict Usage profile
and MUST be validated when using Opportunistic Privacy. Private DNS
resolution can now be done by the DNS client against the pre-
configured privacy-enabling DNS resolver, using the IP address
gathered from the untrusted DNS resolver.
A DNS client so configured that successfully connects to a privacy-
enabling DNS server MAY choose to locally cache the server host IP
addresses in order to not have to repeat the opportunistic lookup.
7.3. Dynamic discovery of ADN
This section discusses the general case of a DNS client discovering
both the authentication domain name and IP address dynamically. This
is not possible at the time of writing by any standard means.
However since, for example, a future DHCP extension could (in
principle) provide this mechanism the required security properties of
such mechanisms are outlined here.
When using a Strict profile the dynamic discovery technique used as a
source of authentication domain names MUST be considered secure and
trustworthy. This requirement does not apply when using an
Opportunistic profile given the security expectation of that profile.
7.3.1. DHCP
In the typical case today, a DHCP server [RFC2131] [RFC3315] provides
a list of IP addresses for DNS resolvers (see Section 3.8 of
[RFC2132]), but does not provide an authentication domain name for
the DNS resolver, thus preventing the use of most of the
authentication methods described here (all those that are based on a
mechanism with ADN in Table 2).
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This document does not specify or request any DHCP extension to
provide authentication domain names. However, if one is developed in
future work the issues outlined in Section 8 of [RFC7227] should be
taken into account as should the Security Considerations in
Section 23 of [RFC3315]).
This document does not attempt to describe secured and trusted
relationships to DHCP servers, which is a purely DHCP issue (still
open, at the time of writing.) 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 a 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 one Reference Identifier for the server based
on the authentication domain name: A DNS-ID which is simply the
authentication domain name itself.
If the Reference Identifier is found in the PKIX 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 the Reference Identifier. In
particular, it MUST NOT inspect the Subject field itself.
8.2. DANE
DANE [RFC6698] provides mechanisms to anchor 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
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"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
Trust Anchor and End Entity based schemes (PKIX-TA(0) and PKIX-
EE(1) respectively) for Opportunistic Security.
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.[authentication-domain-name] for TLS
_853._udp.[authentication-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
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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.
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 a green-field deployment DNS
clients and servers MUST implement only (D)TLS 1.2 or later. For
example, implementing TLS 1.3 [I-D.ietf-tls-tls13] is also an option.
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 (This statement updates [RFC7858]).
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). A client
MUST only indicate support for raw public keys if it has an SPKI
pinset pre-configured (for interoperability reasons).
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 [RFC8094].
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10. IANA Considerations
This memo includes no request to IANA.
11. Security Considerations
Security considerations discussed in [RFC7525], [RFC8094] and
[RFC7858] apply to this document.
DNS Clients SHOULD implement support for the mechanisms described in
Section 8.2 and offering a configuration option which limits
authentication to using only those mechanisms (i.e., with no fallback
to pure PKIX based authentication) such that authenticating solely
via the PKIX infrastructure can be avoided.
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 implement the following
relevant DNS extensions
o EDNS(0) padding [RFC7830], which allows encrypted queries and
responses to hide their size making analysis of encrypted traffic
harder.
Guidance on padding policies for EDNS(0) is provided in
[I-D.ietf-dprive-padding-policy]
DNS-over-(D)TLS clients SHOULD implement 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
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.
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12. Acknowledgments
Thanks to the authors of both [RFC8094] 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, <https://www.rfc-
editor.org/info/rfc2119>.
[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, <https://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, <https://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,
<https://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, <https://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, <https://www.rfc-editor.org/info/rfc6347>.
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[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, <https://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, <https://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, <https://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, <https://www.rfc-editor.org/info/rfc7671>.
[RFC7830] Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830,
DOI 10.17487/RFC7830, May 2016, <https://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, <https://www.rfc-editor.org/info/rfc7858>.
[RFC7918] Langley, A., Modadugu, N., and B. Moeller, "Transport
Layer Security (TLS) False Start", RFC 7918,
DOI 10.17487/RFC7918, August 2016, <https://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, <https://www.rfc-
editor.org/info/rfc7924>.
[RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
Transport Layer Security (DTLS)", RFC 8094,
DOI 10.17487/RFC8094, February 2017, <https://www.rfc-
editor.org/info/rfc8094>.
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13.2. Informative References
[CRIME] Rizzo, J. and T. Duong, "The CRIME Attack", 2012.
[dnssec-trigger]
NLnetLabs, "Dnssec-Trigger", May 2014,
<https://www.nlnetlabs.nl/projects/dnssec-trigger/>.
[I-D.ietf-dprive-padding-policy]
Mayrhofer, A., "Padding Policy for EDNS(0)", draft-ietf-
dprive-padding-policy-01 (work in progress), July 2017.
[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-04 (work in
progress), June 2017.
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-21 (work in progress),
July 2017.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://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,
<https://www.rfc-editor.org/info/rfc2132>.
[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
2003, <https://www.rfc-editor.org/info/rfc3315>.
[RFC3646] Droms, R., Ed., "DNS Configuration options for Dynamic
Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
DOI 10.17487/RFC3646, December 2003, <https://www.rfc-
editor.org/info/rfc3646>.
[RFC7227] Hankins, D., Mrugalski, T., Siodelski, M., Jiang, S., and
S. Krishnan, "Guidelines for Creating New DHCPv6 Options",
BCP 187, RFC 7227, DOI 10.17487/RFC7227, May 2014,
<https://www.rfc-editor.org/info/rfc7227>.
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[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
December 2014, <https://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, <https://www.rfc-editor.org/info/rfc7469>.
[RFC7626] Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
DOI 10.17487/RFC7626, August 2015, <https://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, <https://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 capabilities of privacy-enabling 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
unauthenticated or clear text connections. DNS clients using an
Opportunistic Usage profile should try all available servers
(possibly in parallel) in order to obtain an authenticated and
encrypted connection before falling back. (RATIONALE: This approach
can increase latency while discovering server capabilities but
maximizes the chance of sending the query over an authenticated and
encrypted connection.)
Appendix B. Changes between revisions
[Note to RFC Editor: please remove this section prior to
publication.]
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B.1. -11 version
Section 5: Re-ordered and re-worded the text in section on
Opportunistic profile to make the protection offered by Opportunistic
clearer.
Section 5: Provide a more detailed analysis of attacks on the meta
queries
Section 7.2: Re-introduce a requirement to DNSSEC validate the meta-
queries making it as SHOULD for Strict and a MUST for Opportunistic.
B.2. -10 version
Clarified the specific attacks the Usage profiles mitigate against.
Revised wording in the draft relating 'security/privacy guarantees'
and generally improved consistency of wording throughout the
document.
Corrected and added a number of references:
o RFC7924 is now Normative
o RFC7918 and RFC8094 are now Normative (and therefore Downrefs)
o draft-ietf-tls-tls13, draft-ietf-dprive-padding-policy, RFC3315
and RFC7227 added
Terminology: Update definition of Privacy-enabling DNS server and
moved normative definition to section 4.
Section 5 and 6.3: Included discussion of the additional attacks
possible when using meta-queries to bootstrap the DNS service
Section 5: Added sentence on why Opportunistic Profile may fallback
for latency reasons.
Section 5.1: Added discussion of when clients might change Usage
Profiles.
Section 6.4: Added caveat on use of combined authentication re
RFC7469.
Section 6.5: Added more detail on how authentication results might be
used in Opportunistic. Opportunistic clients now SHOULD try for the
best case.
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Section 7.3: Re-worked this section and the discussion of DHCP.
Section 9: Removed unnecessary text, added condition on use of
RFC7250 (Raw public keys).
Section 11.: More detail on padding policies.
Numerous editorial corrections.
B.3. -09 version
Remove the SRV record to simplify the draft.
Add suggestion that clients offer option to avoid using only PKIX
authentication.
Clarify that the MUST on implementing TLS session resumption updates
RFC7858.
Update page header to be '(D)TLS Authentication for TLS'.
B.4. -08 version
Removed hard failure as an option for Opportunistic Usage profile.
Added a new section comparing the Authentication Mechanisms
B.5. -07 version
Re-work of the Abstract and Introduction to better describe the
contents in this version.
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.6. -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.
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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.7. -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.8. -04 version
Introduction: Add comment that DNS-over-DTLS draft is Experiments
Update 2 I-D references to RFCs.
B.9. -03 version
Section 9: Update DANE section with better references to RFC7671 and
RFC7250
B.10. -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.
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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.11. -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.12. 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
McAfee, Inc.
Embassy Golf Link Business Park
Bangalore, Karnataka 560071
India
Email: TirumaleswarReddy_Konda@McAfee.com
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