dprive S. Dickinson
Internet-Draft Sinodun
Intended status: Standards Track D. Gillmor
Expires: April 10, 2017 ACLU
T. Reddy
October 7, 2016

Authentication and (D)TLS Profile for DNS-over-(D)TLS


This document describes how a DNS client can use a domain name to authenticate a DNS server that uses Transport Layer Security (TLS) and Datagram TLS (DTLS). Additionally, it defines (D)TLS profiles for DNS clients and servers implementing DNS-over-TLS and DNS-over-DTLS.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on April 10, 2017.

Copyright Notice

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Table of Contents

1. Introduction

DNS Privacy issues are discussed in [RFC7626]. Two documents that provide DNS privacy between DNS clients and DNS servers are: Section 2 and Section 3). The proposals here might be adapted or extended in future to be used for recursive clients and authoritative servers, but this application is out of scope for the DNS PRIVate Exchange (DPRIVE) Working Group per its current charter.

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

This document defines two Usage Profiles (Strict and Opportunistic) for DTLS [RFC6347] and TLS [RFC5246] which define the security properties a user should expect when using that profile to connect to the available DNS servers. In essence:

Additionally, a number of authentication mechanisms are defined that specify how a DNS client should authenticate a DNS server based on a domain name. In particular, the following is described:

It should be noted that [RFC7858] includes a description of a specific case of a Strict Usage Profile using a single authentication mechanism (SPKI pinning). This draft generalises the picture by separating the Usage Profile, which is based purely on the security properties it offers the user, from the specific mechanism that is used for authentication. Therefore the "Out-of-band Key-pinned Privacy Profile" described in the DNS-over-TLS draft would qualify as a "Strict Usage Profile" that used SPKI pinning for authentication.

This document also defines a (D)TLS protocol profile for use with DNS. This profile defines the configuration options and protocol extensions required of both parties to optimize connection establishment and session resumption for transporting DNS, and to support the authentication mechanisms defined here.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

Several terms are used specifically in the context of this draft:

3. Scope

This document is limited to domain-name-based authentication of DNS servers by DNS clients (as defined in the terminology section), and the (D)TLS profiles needed to support this. As such, the following things are out of scope:

4. Discussion

4.1. Background

To protect against passive attacks DNS privacy requires encrypting the query (and response). Such encryption typically provides integrity protection as a side-effect, which means on-path attackers cannot simply inject bogus DNS responses. For DNS privacy to also provide protection against active attackers pretending to be the server, the client must authenticate the server.

This draft discusses Usage Profiles, which provide differing levels of privacy guarantees to DNS clients, based on the requirements for authentication and encryption, regardless of the context (for example, which network the client is connected to). A Usage Profile is a distinct concept to a usage policy or usage model, which might dictate which Profile should be used in a particular context (enterprise vs coffee shop), with a particular set of DNS Servers or with reference to other external factors. A description of the variety of usage policies is out of scope of this document, but may be the subject of a future I-D.

4.2. Usage Profiles

A DNS client has a choice of privacy Usage Profiles available. This choice is briefly discussed in both [RFC7858] and [I-D.ietf-dprive-dnsodtls]. In summary, the usage profiles are:

To compare the two Usage profiles the table below shows successful Strict Privacy along side the 3 possible successful outcomes of Opportunistic Privacy. In the best case scenario for Opportunistic 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 intermediate cases and eventually the worst case scenario in order to obtain a response. It therefore provides no privacy guarantee to the user and varying protection depending on what kind of connection is actually used. Note that there is no requirement in Opportunistic to notify the user what type of connection is actually used, the 'detection' described below is only possible if such connection information is available. This is discussed in Section 5.

DNS Privacy Protection by Usage Profile and type of attacker
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

Since Strict Privacy provides the strongest privacy guarantees it is preferable to Opportunistic Privacy.

However since the two profiles require varying levels of configuration (or a trusted relationship with a provider) and DNS server capabilities, DNS clients will need to carefully select which profile to use based on their communication privacy needs. For the case where a client has a trusted relationship with a provider it is expected that the provider will provide either a domain name or SPKI pinset via a secure out-of-band mechanism and therefore Strict Privacy should be used.

4.2.1. DNS Resolution

A DNS client SHOULD select a particular usage profile when resolving a query. A DNS client MUST NOT fallback from Strict Privacy to Opportunistic Privacy during the resolution process as this could invalidate the protection offered against active attackers.

4.3. Authentication

This document describes authentication mechanisms that can be used in either Strict or Opportunistic Privacy for DNS-over-(D)TLS.

4.3.1. DNS-over-(D)TLS Bootstrapping Problems

Many (D)TLS clients use PKIX authentication [RFC6125] based on a domain name for the server they are contacting. These clients typically first look up the server's network address in the DNS before making this connection. A DNS client therefore has a bootstrap problem. DNS clients typically know only the IP address of a DNS server.

As such, before connecting to a DNS server, a DNS client needs to learn the domain name it should associate with the IP address of a DNS server for authentication purposes. Sources of domains names are discussed in Section 7 and Section 8.

One advantage of this domain name based approach is that it encourages association of stable, human recognisable identifiers with secure DNS service providers.

4.3.2. Credential Verification

The use of SPKI pinset verification is discussed in [RFC7858].

In terms of domain name based verification, once a domain name is known for a DNS server a choice of mechanisms can be used for authentication. Section 9 discusses these mechanisms in detail, namely X.509 certificate based authentication and DANE.

Note that the use of DANE adds requirements on the ability of the client to get validated DNSSEC results. This is discussed in more detail in Section 9.2.

4.3.3. Implementation guidance

Section 11 describes the (D)TLS profile for DNS-over(D)TLS. Additional considerations relating to general implementation guidelines are discussed in both Section 13 and in Appendix A.

5. Authentication in Opportunistic DNS-over(D)TLS Privacy

An Opportunistic Security [RFC7435] profile is described in [RFC7858] which MAY be used for DNS-over-(D)TLS.

DNS clients issuing queries under an opportunistic profile which know of a domain name or SPKI pinset for a given privacy-enabling DNS server MAY choose to try to authenticate the server using the mechanisms described here. This is useful for detecting (but not preventing) active attack, since the fact that authentication information is available indicates that the server in question is a privacy-enabling DNS server to which it should be possible to establish an authenticated, encrypted connection. In this case, whilst a client cannot know the reason for an authentication failure, from a privacy standpoint the client should consider an active attack in progress and proceed under that assumption. Attempting authentication is also useful for debugging or diagnostic purposes if there are means to report the result. This information can provide a basis for a DNS client to switch to (preferred) Strict Privacy where it is viable.

6. Authentication in Strict DNS-over(D)TLS Privacy

To authenticate a privacy-enabling DNS server, a DNS client needs to know the domain name for each server it is willing to contact. This is necessary to protect against active attacks on DNS privacy.

A DNS client requiring Strict Privacy MUST either use one of the sources listed in Section 8 to obtain a domain name for the server it contacts, or use an SPKI pinset as described in [RFC7858].

A DNS client requiring Strict Privacy MUST only attempt to connect to DNS servers for which either a domain name or a SPKI pinset is known (or both). The client MUST use the available verification mechanisms described in Section 9 to authenticate the server, and MUST abort connections to a server when no verification mechanism succeeds.

With Strict Privacy, the DNS client MUST NOT commence sending DNS queries until at least one of the privacy-enabling DNS servers becomes available.

A privacy-enabling DNS server may be temporarily unavailable when configuring a network. For example, for clients on networks that require registration through web-based login (a.k.a. "captive portals"), such registration may rely on DNS interception and spoofing. Techniques such as those used by DNSSEC-trigger [dnssec-trigger] MAY be used during network configuration, with the intent to transition to the designated privacy-enabling DNS servers after captive portal registration. The system MUST alert by some means that the DNS is not private during such bootstrap.

7. In Band Source of Domain Name: SRV Service Label

This specification adds a SRV service label "domain-s" for privacy-enabling DNS servers.

Example service records (for TLS and DTLS respectively):

8. Out of Band Sources of Domain Name

8.1. Full direct configuration

DNS clients may be directly and securely provisioned with the domain name of each privacy-enabling DNS server. For example, using a client specific configuration file or API.

In this case, direct configuration for a DNS client would consist of both an IP address and a domain name for each DNS server.

8.2. Direct configuration of name only

A DNS client may be configured directly and securely with only the domain name of its privacy-enabling DNS server. For example, using a client specific configuration file or API.

A DNS client might learn of a default recursive DNS resolver from an untrusted source (such as DHCP's DNS server option [RFC3646]). It can then use opportunistic DNS connections to untrusted recursive DNS resolver to establish the IP address of the intended privacy-enabling DNS server by doing a lookup of SRV records. Such records MUST be validated using DNSSEC. Private DNS resolution can now be done by the DNS client against the configured privacy-enabling DNS server.


A DNS client so configured that successfully connects to a privacy-enabling DNS server MAY choose to locally cache the looked up addresses in order to not have to repeat the opportunistic lookup.

8.3. DHCP

Some clients may have an established trust relationship with a known DHCP [RFC2131] server for discovering their network configuration. In the typical case, such a DHCP server provides a list of IP addresses for DNS servers (see section 3.8 of [RFC2132]), but does not provide a domain name for the DNS server itself.

In the future, a DHCP server might use a DHCP extension to provide a list of domain names for the offered DNS servers, which correspond to IP addresses listed.

Use of such a mechanism with any DHCP server when using an Opportunistic profile is reasonable, given the security expectation of that profile. However when using a Strict profile the DHCP servers used as sources of domain names MUST be considered secure and trustworthy. This document does not attempt to describe secured and trusted relationships to DHCP servers.

[NOTE: It is noted (at the time of writing) that whilst some implementation work is in progress to secure IPv6 connections for DHCP, IPv4 connections have received little to no implementation attention in this area.]

9. Credential Verification

9.1. X.509 Certificate Based Authentication

When a DNS client configured with a domain name connects to its configured DNS server over (D)TLS, the server may present it with an X.509 certificate. In order to ensure proper authentication, DNS clients MUST verify the entire certification path per [RFC5280]. The DNS client additionally uses [RFC6125] validation techniques to compare the domain name to the certificate provided.

A DNS client constructs two Reference Identifiers for the server based on the domain name: A DNS-ID and an SRV-ID [RFC4985]. The DNS-ID is simply the domain name itself. The SRV-ID uses a "_domain-s." prefix. So if the configured domain name is "dns.example.com", then the two Reference Identifiers are:

If either of the Reference Identifiers are found in the X.509 certificate's subjectAltName extension as described in section 6 of [RFC6125], the DNS client should accept the certificate for the server.

A compliant DNS client MUST only inspect the certificate's subjectAltName extension for these Reference Identifiers. In particular, it MUST NOT inspect the Subject field itself.

9.2. DANE

DANE [RFC6698] provides mechanisms to root certificate and raw public key trust with DNSSEC. However this requires the DNS client to have a domain name for the DNS Privacy Server which must be obtained via a trusted source.

This section assumes a solid understanding of both DANE [RFC6698] and DANE Operations [RFC7671]. A few pertinent issues covered in these documents are outlined here as useful pointers, but familiarity with both these documents in their entirety is expected.

It is noted that [RFC6698] says

It is noted that [RFC7671] covers the following topics:

The specific DANE record for a DNS Privacy Server would take the form:

9.2.1. Direct DNS Lookup

The DNS client MAY choose to perform the DNS lookups to retrieve the required DANE records itself. The DNS queries for such DANE records MAY use opportunistic encryption or be in the clear to avoid trust recursion. The records MUST be validated using DNSSEC as described above in [RFC6698].

9.2.2. TLS DNSSEC Chain extension

The DNS client MAY offer the TLS extension described in [I-D.ietf-tls-dnssec-chain-extension]. If the DNS server supports this extension, it can provide the full chain to the client in the handshake.

If the DNS client offers the TLS DNSSEC Chain extension, it MUST be capable of validating the full DNSSEC authentication chain down to the leaf. If the supplied DNSSEC chain does not validate, the client MUST ignore the DNSSEC chain and validate only via other supplied credentials.

10. Combined Credentials with SPKI Pinsets

The SPKI pinset profile described in [RFC7858] MAY be used with DNS-over-(D)TLS.

This draft does not make explicit recommendations about how a SPKI pinset based authentication mechanism should be combined with a domain based mechanism from an operator perspective. However it can be envisaged that a DNS server operator may wish to make both an SPKI pinset and a domain name available to allow clients to choose which mechanism to use. Therefore, the following is guidance on how clients ought to behave if they choose to configure both, as is possible in HPKP [RFC7469].

A DNS client that is configured with both a domain name and a SPKI pinset for a DNS server SHOULD match on both a valid credential for the domain name and a valid SPKI pinset if both are available when connecting to that DNS server.

11. (D)TLS Protocol Profile

This section defines the (D)TLS protocol profile of DNS-over-(D)TLS.

There are known attacks on (D)TLS, such as machine-in-the-middle and protocol downgrade. These are general attacks on (D)TLS and not specific to DNS-over-TLS; please refer to the (D)TLS RFCs for discussion of these security issues. Clients and servers MUST adhere to the (D)TLS implementation recommendations and security considerations of [RFC7525] except with respect to (D)TLS version. Since encryption of DNS using (D)TLS is virtually a green-field deployment DNS clients and server MUST implement only (D)TLS 1.2 or later.

Implementations MUST NOT offer or provide TLS compression, since compression can leak significant amounts of information, especially to a network observer capable of forcing the user to do an arbitrary DNS lookup in the style of the CRIME attacks [CRIME].

Implementations compliant with this profile MUST implement all of the following items:

Implementations compliant with this profile SHOULD implement all of the following items:

Guidance specific to TLS is provided in [RFC7858] and that specific to DTLS it is provided in[I-D.ietf-dprive-dnsodtls].

12. IANA Considerations

This memo includes no request to IANA.

13. Security Considerations

Security considerations discussed in [RFC7525], [I-D.ietf-dprive-dnsodtls] and [RFC7858] apply to this document.

13.1. Counter-measures to DNS Traffic Analysis

This section makes suggestions for measures that can reduce the ability of attackers to infer information pertaining to encrypted client queries by other means (e.g. via an analysis of encrypted traffic size, or via monitoring of resolver to authoritative traffic).

DNS-over-(D)TLS clients and servers SHOULD consider implementing the following relevant DNS extensions

DNS-over-(D)TLS clients SHOULD consider implementing the following relevant DNS extensions

14. Acknowledgements

Thanks to the authors of both [I-D.ietf-dprive-dnsodtls] and [RFC7858] for laying the ground work that this draft builds on and for reviewing the contents. The authors would also like to thank John Dickinson, Shumon Huque, Melinda Shore, Gowri Visweswaran, Ray Bellis, Stephane Bortzmeyer, Jinmei Tatuya, Paul Hoffman and Christian Huitema for review and discussion of the ideas presented here.

15. References

15.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[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.
[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.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008.
[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.
[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.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012.
[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.
[RFC7250] Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S. and T. Kivinen, "Using Raw Public Keys in Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250, June 2014.
[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.
[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.
[RFC7830] Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830, DOI 10.17487/RFC7830, May 2016.
[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.

15.2. Informative References

[CRIME] Rizzo, J. and T. Duong, "The CRIME Attack", 2012.
[dnssec-trigger] NLnetLabs, "Dnssec-Trigger", May 2014.
[I-D.ietf-dprive-dnsodtls] Reddy, T., Wing, D. and P. Patil, "Specification for DNS over Datagram Transport Layer Security (DTLS)", Internet-Draft draft-ietf-dprive-dnsodtls-12, September 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", Internet-Draft draft-ietf-tls-dnssec-chain-extension-01, July 2016.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, DOI 10.17487/RFC2131, March 1997.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997.
[RFC3646] Droms, R., "DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646, DOI 10.17487/RFC3646, December 2003.
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection Most of the Time", RFC 7435, DOI 10.17487/RFC7435, December 2014.
[RFC7469] Evans, C., Palmer, C. and R. Sleevi, "Public Key Pinning Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April 2015.
[RFC7626] Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626, DOI 10.17487/RFC7626, August 2015.
[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.
[RFC7918] Langley, A., Modadugu, N. and B. Moeller, "Transport Layer Security (TLS) False Start", RFC 7918, DOI 10.17487/RFC7918, August 2016.
[RFC7924] Santesson, S. and H. Tschofenig, "Transport Layer Security (TLS) Cached Information Extension", RFC 7924, DOI 10.17487/RFC7924, July 2016.

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. -04 version

Introduction: Add comment that DNS-over-DTLS draft is Experiments

Update 2 I-D references to RFCs.

B.2. -03 version

Section 9: Update DANE section with better references to RFC7671 and RFC7250

B.3. -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.4. -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.5. 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
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