DNS Privacy Protection by Usage Profile and type of attacker

Usage Profile	Passive Attacker	Active Attacker
No Privacy	N	N
Opportunistic Privacy	N (D)	N (D)
Strict Privacy	P	P

P == protection; N == no protection; D == detection is possible

Since Strict Privacy provides the strongest privacy guarantees it is preferable to Opportunistic Privacy which is preferable to No Privacy. However since the different profiles require varying levels of configuration (or a trusted relationship with a provider) DNS clients will need to carefully select which profile to use based on their communication privacy needs. 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.

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 [I-D.ietf-dprive-dns-over-tls].

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

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

4.3.3. Implementation guidance

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

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

An Opportunistic Security [RFC7435] profile is described in [I-D.ietf-dprive-dns-over-tls] which MAY be used for DNS-over-(D)TLS.

DNS clients issuing queries under an opportunistic profile which know of a domain name 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 [I-D.ietf-dprive-dns-over-tls].

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

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

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

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

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

Example service records (for TLS and DTLS respectively):

• _domain-s._tcp.dns.example.com. SRV 0 1 853 dns1.example.com. _domain-s._tcp.dns.example.com. SRV 0 1 853 dns2.example.com.
• _domain-s._udp.dns.example.com. SRV 0 1 853 dns3.example.com.

8. Out of Band Sources of Domain Name

8.1. Full direct configuration

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

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

8.2. Direct configuration of name only

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

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:

• A DNSSEC validating DNS client is configured with the domain name dns.example.net for a privacy-enabling DNS server
• 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
• Client validates all the records obtained in the previous step using DNSSEC.
• If the records successfully validate the client proceeds to connect to the privacy-enabling DNS server using Strict Privacy.

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

8.3. DHCP

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

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

Note that this requires the client to trust the DHCP server, and to have a secured/authenticated connection to it. Therefore this mechanism may be limited to only certain environments. This document does not attempt to describe secured and trusted relationships to DHCP servers.

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

[QUESTION: The authors would like to solicit feedback on the use of DHCP to determine whether to pursue a new DHCP option in a later version of this draft, or defer it.]

9. Credential Verification

9.1. X.509 Certificate Based Authentication

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

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

• DNS-ID: dns.example.com
• SRV-ID: _domain-s.dns.example.com

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

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

9.2. DANE

DANE [RFC6698] provides mechanisms to root certificate and raw public keys trust with DNSSEC. However this requires a domain name which must be obtained via a trusted source.

It is noted that [RFC6698] says

• "Clients that validate the DNSSEC signatures themselves MUST use standard DNSSEC validation procedures. Clients that rely on another entity to perform the DNSSEC signature validation MUST use a secure mechanism between themselves and the validator."

The specific DANE record would take the form:

• _853._tcp.[server-domain-name] for TLS
• _853._udp.[server-domain-name] for DTLS

9.2.1. Direct DNS Lookup

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

9.2.2. TLS DNSSEC Chain extension

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

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

[ TODO: specify guidance for DANE parameters to be used here. For example, a suggestion to use Certificate Usage of 3 (EE-DANE) (section 2.1.1 of [RFC6698]) and a Selector of 1 (SPKI) (section 2.1.2) would completely remove all X.509 and certificate authorities from the verification path and allows for private certification ]

[ TODO: discuss combination of DNSSEC Chain Extension with cert validation. Note that the combination depends on the Certificate Usage value of the TLSA response. ]

10. Combined Credentials with SPKI Pinsets

The SPKI pinset profile described in [I-D.ietf-dprive-dns-over-tls] MAY be used with DNS-over-(D)TLS.

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

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

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:

• TLS session resumption without server-side state [RFC5077] which eliminates the need for the server to retain cryptographic state for longer than necessary.
• 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:

• TLS False Start [I-D.ietf-tls-falsestart] which reduces round-trips by allowing the TLS second flight of messages (ChangeCipherSpec) to also contain the (encrypted) DNS query
• Cached Information Extension [I-D.ietf-tls-cached-info] which avoids transmitting the server's certificate and certificate chain if the client has cached that information from a previous TLS handshake

[NOTE: The references to (works in progress) should be upgraded to MUST's if those references become RFC's prior to publication of this document.]

Guidance specific to TLS or DTLS is provided in either [I-D.ietf-dprive-dnsodtls] or [I-D.ietf-dprive-dns-over-tls].

12. IANA Considerations

This memo includes no request to IANA.

13. Security Considerations

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

13.1. Counter-measures to DNS Traffic Analysis

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

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

• EDNS(0) padding [I-D.ietf-dprive-edns0-padding], which allows encrypted queries and responses to hide their size.

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

• Privacy Election using Client Subnet in DNS Queries [I-D.ietf-dnsop-edns-client-subnet]. If a DNS client does not include an EDNS0 Client Subnet Option with a SOURCE PREFIX-LENGTH set to 0 in a query, the DNS server may potentially leak client address information to the upstream authoritative DNS servers. A DNS client ought to be able to inform the DNS Resolver that it does not want any address information leaked, and the DNS Resolver should honor that request.

14. Acknowledgements

Thanks to the authors of both [I-D.ietf-dprive-dnsodtls] and [I-D.ietf-dprive-dns-over-tls] for laying the ground work that this draft builds on and for reviewing the contents. The authors would also like to thank John Dickinson, Shumon Huque, Melinda Shore, Gowri Visweswaran, Ray Bellis, Stephane Bortzmeyer and Jinmei Tatuya 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.

15.2. Informative References

[CRIME]	Rizzo, J. and T. Duong, "The CRIME Attack", 2012.
[dnssec-trigger]	NLnetLabs, "Dnssec-Trigger", May 2014.
[I-D.ietf-dnsop-edns-client-subnet]	Contavalli, C., Gaast, W., tale, t. and W. Kumari, "Client Subnet in DNS Queries", Internet-Draft draft-ietf-dnsop-edns-client-subnet-06, December 2015.
[I-D.ietf-dprive-dns-over-tls]	Zi, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D. and P. Hoffman, "Specification for DNS over TLS", Internet-Draft draft-ietf-dprive-dns-over-tls-09, March 2016.
[I-D.ietf-dprive-dnsodtls]	Reddy, T., Wing, D. and P. Patil, "DNS over DTLS (DNSoD)", Internet-Draft draft-ietf-dprive-dnsodtls-05, March 2016.
[I-D.ietf-dprive-edns0-padding]	Mayrhofer, A., "The EDNS(0) Padding Option", Internet-Draft draft-ietf-dprive-edns0-padding-03, March 2016.
[I-D.ietf-tls-cached-info]	Santesson, S. and H. Tschofenig, "Transport Layer Security (TLS) Cached Information Extension", Internet-Draft draft-ietf-tls-cached-info-22, January 2016.
[I-D.ietf-tls-falsestart]	Langley, A., Modadugu, N. and B. Moeller, "Transport Layer Security (TLS) False Start", Internet-Draft draft-ietf-tls-falsestart-01, November 2015.
[I-D.shore-tls-dnssec-chain-extension]	Shore, M., Barnes, R., Huque, S. and W. Toorop, "A DANE Record and DNSSEC Authentication Chain Extension for TLS", Internet-Draft draft-shore-tls-dnssec-chain-extension-02, October 2015.
[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.

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. -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.2. 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