Internet-Draft Unilateral Encrypted Authoritative DNS November 2021
Gillmor & Salazar Expires 22 May 2022 [Page]
Workgroup:
dprive
Internet-Draft:
draft-dkgjsal-dprive-unilateral-probing-00
Published:
Intended Status:
Informational
Expires:
Authors:
D.K. Gillmor
ACLU
J. Salazar
A19

Unilateral Opportunistic Deployment of Encrypted Recursive-to-Authoritative DNS

Abstract

This draft sets out steps that DNS servers (recursive resolvers and authoritative servers) can take unilaterally (without any coordination with other peers) to defend DNS query privacy against a passive network monitor. The steps in this draft can be defeated by an active attacker, but should be simpler and less risky to deploy than more powerful defenses. The draft also introduces (but does not try to specify) the semantics of signalling that would permit defense against an active attacker.

The goal of this draft is to simplify and speed deployment of opportunistic encrypted transport in the recursive-to-authoritative hop of the DNS ecosystem. With wider easy deployment of the underlying transport on an opportunistic basis, we hope to facilitate the future specification of stronger cryptographic protections against more powerful attacks.

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 https://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 22 May 2022.

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Table of Contents

1. Introduction

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 ([RFC2119] and [RFC8174]) when, and only when, they appear in all capitals, as shown here.

1.2. Terminology

  • "unilateral" means capable of opportunistic probing deployment without external coordination with any of the other parties
  • Do53 refers to traditional cleartext DNS over port 53 ([RFC1035])
  • DoQ refers to DNS-over-QUIC ([I-D.ietf-dprive-dnsoquic])
  • DoT refers to DNS-over-TLS ([RFC7858])
  • DoH refers to DNS-over-HTTPS ([RFC8484])
  • Encrypted transports refers to DoQ, DoT, and DoH collectively

2. Priorities

This document aims to provide guidance to implementers who want to simply enable protection against passive network observers.

In particular, it focuses on mechanisms that can be adopted unilaterally by recursive resolvers and authoritative servers, without any explicit coordination with the other parties. This guidance provides opportunistic security (see [RFC7435]) -- encrypting things that would otherwise be in the clear, without interfering with or weakening stronger forms of security.

2.1. Minimizing Negative Impacts

It also aims to minimize potentially negative impacts caused by the probing of encrypted transports -- for the systems that adopt these guidelines, for the parties that they communicate with in the "second hump" of the DNS camel, and for uninvolved third parties. The negative impacts that we specifically try to minimize are:

  • excessive bandwidth use
  • excessive computational resources (CPU and memory in particular)
  • amplification attacks (where DNS resolution infrastructure is wielded as part of a DoS attack)

2.2. Protocol Choices

While this document focuses specifically on strategies used by DNS servers, it does not go into detail on the specific protocols used, as those protocols --- in particular, DoT and DoQ --- are described in other documents.

This document does not pursue the use of DoH in this context, because a DoH client needs to know the path part of a DoH endpoint URL, and there are currently no mechanisms for a DNS resolver to predict the path on its own, in an opportunistic or unilateral fashion, without incurring in excessive use of resources. For instance, a recursive resolver in theory could guess the full path to a queried IP address by trying all the URL paths that the client has in records and see if one of those works, but even though it can be expected that this would work 99% of the time with fewer than 100 probes, this technique would likely incur in excessive resource consumption potentially leading to vulnerabilities and amplification attacks. The authors of this draft particularly welcome ideas and contributions from the community that lead to a suitable mechanism for unilaterally probing for DoH-capable authoritative servers, for later consideration in this or other drafts.

3. Guidance for Authoritative Servers

An authoritative server SHOULD implement and deploy DNS-over-TLS (DoT) on TCP port 853.

An authoritative server MAY implement and deploy DNS-over-QUIC (DoQ) on UDP port 853.

3.1. Authentication

For unilateral deployment, an authoritative server does not need to offer any particular form of authentication.

The simplest deployment would simply provide a self-issued, regularly-updated X.509 certificate. This mechanism is supported by many TLS and QUIC clients, and will be acceptable for any opportunistic connection.

Possible alternate forms of server authentication include:

  • an X.509 Certificate issued by a widely-known certification authority associated with the common NS names used for this authoritative server
  • DANE authentication (potentially including the TLS handshake)

An authoritative DNS server that wants to handle unilateral queries MUST NOT rely on SNI to multiplex distinct authoritative DNS services on a single IP address.

3.2. Resource Exhaustion

A well-behaved recursive resolver may keep an encrypted connection open to an authoritative server, to amortize the costs of connection setup for both parties.

However, some authoritative servers may have insufficient resources available to keep many connections open concurrently.

An authoritative server facing resource exhaustion SHOULD cleanly close open connections from recursive resolvers based on the authoritative's preferred prioritization.

A reasonable prioritization scheme would be to close connections in this order, until resources are back in control:

  • connections with no outstanding queries, ordered by idle time (longest idle time gets closed first)
  • connections with outstanding queries, ordered by age of outstanding query (oldest outstanding query gets closed first)

4. Guidance for recursive resolvers

This section outlines a probing policy suitable for unilateral adoption by any recursive resolver. Following this policy should not result in failed resolutions or significant delay.

4.1. Overall recursive resolver Settings

A recursive resolver implementing this draft must set system-wide values for some default parameters. These parameters may be set independently for each supported encrypted transport, though a simple implementation may keep the parameters constant across encrypted transports.

Table 1: recursive resolver system parameters per encrypted transport
Name Description Suggested Default
persistence How long should the recursive resolver remember successful encrypted transport connections? 3 days (259200 seconds)
damping How long should the recursive resolver remember unsuccessful encrypted transport connections? 1 day (86400 seconds)
timeout How long should the recursive resolver wait for an initiated encrypted connection to complete? 30 seconds

This document uses the notation E-foo to refer to the foo parameter for the encrypted transport E.

For example DoT-persistence would indicate the length of time that the recursive resolver will remember that an authoritative server had a successful connection over DoT.

This document also assumes that the resolver maintains a list of outstanding cleartext queries destined for the authoritative resolver's IP address X. This list is referred to as Do53-queries[X]. This document does not attempt to describe the specific operation of sending and receiving cleartext DNS queries (Do53) for a recursive resolver. Instead it describes a "bolt-on" mechanism that extends the recursive resolver's operation on a few simple hooks into the recursive resolver's existing handling of Do53.

Implementers or deployers of DNS recursive resolvers that follow the strategies in this document are encouraged to report their preferred values of these parameters.

4.2. Recursive Resolver Requirements

To follow this guidance, a recursive resolver MUST implement at least one of either DoT or DoQ in its capacity as a client of authoritative nameservers.

A recursive resolver SHOULD implement the client side of DNS-over-TLS (DoT). A recursive resolver MAY implement the client side of DNS-over-QUIC (DoQ).

While this document focuses on the recursive-to-authoritative hop, a recursive resolver implementing these strategies SHOULD also accept queries from its clients over some encrypted transport (current common transports are DoH or DoT).

4.3. Authoritative Server Encrypted Transport Connection State

The recursive resolver SHOULD keep a record of the state for each authoritative server it contacts, indexed by the IP address of the authoritative server and the encrypted transports supported by the recursive resolver.

Each record should contain the following fields for each supported encrypted transport, each of which would initially be null:

Table 2: recursive resolver state per authoritative IP, per encrypted transport
Name Description Retain Across Reset
session The associated state of any existing, established session (the structure of this value is dependent on the encrypted transport implementation). If session is not null, it may be in one of two states: pending or established N
initiated Timestamp of most recent connection attempt Y
completed Timestamp of most recent completed handshake Y
status Enumerated value of success or fail or timeout, associated with the completed handshake Y
resumptions A stack of resumption tickets (and associated parameters) that could be used to resume a prior successful connection Y
queries A queue of queries intended for this authoritative server, each of which has additional status early, unsent, or sent N
last-activity A timestamp of the most recent activity on the connection N

Note that the session fields in aggregate constitute a pool of open connections to different servers.

With the exception of the session, queries, and last-activity fields, this cache information should be kept across restart of the server unless explicitly cleared by administrative action.

This document uses the notation E-foo[X] to indicate the value of field foo for encrypted transport E to IP address X.

For example, DoT-initiated[192.0.2.4] represents the timestamp when the most recent DoT connection packet was sent to IP address 192.0.2.4.

4.4. Probing Policy

When a recursive resolver discovers the need for an authoritative lookup to an authoritative DNS server using IP address X, it retrieves the records associated with X from its cache.

The following sections presume that the time of the discovery of the need for lookup is time T0.

If any of the records discussed here are absent, they are treated as null.

The recursive resolver must know to decide whether to initially send a query over Do53, or over any of the supported encrypted transports (DoT or DoQ).

Note that a resolver might initiate this query via any or all of the known transports. When multiple queries are sent, the initial packets for each connection can be sent concurrently, similar to "Happy Eyeballs" ([RFC8305]). However, unlike Happy Eyeballs, when one transport succeeds, the other connections do not need to be terminated, but can instead be continued to establish whether the IP address X is capable of corresponding on the relevant transport.

4.4.1. Sending a query over Do53

For any of the supported encrypted transports E, if either of the following holds true, the resolver SHOULD NOT send a query to X over Do53:

  • E-session[X] is in the established state, or
  • E-status[X] is success, and (T - E-completed[X]) < persistence

Otherwise, if there is no outstanding session for any encrypted transport, and the last successful encrypted transport connection was long ago, the resolver sends a query to X over Do53. When it does so, it inserts a handle for the query in Do53-queries[X].

4.4.2. Receiving a response over Do53

When a successful response R is received in cleartext from authoritative server X for a query Q that was sent over Do53, the recursive resolver should:

  • If Q is in Do53-queries[X]:

    • Return R to the requesting client
  • Remove Q from Do53-queries[X]

  • For each supported encrypted transport E:

    • If Q is in E-queries[X]:

      • Remove Q from E-queries[X]

But if R is unsuccessful (e.g. SERVFAIL):

  • if Q is not in any of *-queries[X]:

    • Return SERVFAIL to the client

4.4.3. Initiating a connection over encrypted transport

If any E-session[X] is in the established, the recursive resolver SHOULD NOT initiate a new connection to X over any other transport, but should instead send a query through the existing session (see Section 4.4.8). FIXME: What if there's a preferred transport, but the established session does not correspond to that preferred transport?

Otherwise, the timer should examine and possibly refresh its state for encrypted transport E to authoritative IP address X:

  • if E-session[X] is in state pending, and

  • T - E-initiated[X] > E-timeout, then

    • set E-session[X] to null and
    • set E-status[X] to timeout

When resources are available to attempt a new encrypted transport, the resolver should only initiate a new connection to X over E as long as one of the following holds true:

  • E-status[X] is success, or
  • E-status[X] is fail or timeout and (T - E-completed[X]) > damping, or
  • E-status[X] is null and E-initiated[X] is null

When initiating a session to X over encrypted transport E, if E-resumptions[X] is not empty, one ticket should be popped off the stack and used to try to resume a previous session. Otherwise, the initial Client Hello handshake should not try to resume any session.

When initiating a connection, the resolver should take the following steps:

  • set E-initiated[X] to T0
  • store a handle for the new session (which should have pending state) in E-session[X]
  • insert a handle for the query that prompted this connection in E-queries[X], with status unsent or early, as appropriate (see below).
4.4.3.1. Early Data

Modern encrypted transports like TLS 1.3 offer the chance to store "early data" from the client into the initial Client Hello in some contexts. A resolver that initiates a connection over a encrypted transport according to this guidance in a context where early data is possible SHOULD send the DNS query that prompted the connection in the early data, according to the sending guidance in Section 4.4.8.

If it does so, the status of Q in E-queries[X] should be set to early instead of unsent.

4.4.3.2. Resumption Tickets

When initiating a new connection (whether by resuming an old session or not), the recursive resolver SHOULD request a session resumption ticket from the authoritative server. If the authoritative server supplies a resumption ticket, the recursive resolver pushes it into the stack at E-resumptions[X].

4.4.3.3. Server Name Indication

For modern encrypted transports like TLS 1.3, most client implementations expect to send a Server Name Indication (SNI) in the Client Hello.

There are two complications with selecting or sending SNI in this unilateral probing:

  • Some authoritative servers are known by more than one name; selecting a single name to use for a given connection may be difficult or impossible.
  • In most configurations, the contents of the SNI field is exposed on the wire to a passive adversary. This potentially reveals additional information about which query is being made, based on the NS of the query itself.

To avoid additional leakage and complexity, a recursive resolver using only this guidance SHOULD NOT send any SNI to the authoritative when attempting encrypted transport.

Alternately, if the recursive resolver implements Encrypted Client Hello ([I-D.ietf-tls-esni] and the appropriate records are available for the NS in question, the recursive resolver MAY send an SNI under encryption.

4.4.3.4. Authoritative Server Authentication

A recursive resolver following this guidance MAY attempt to verify the server's identity by X.509 certificate or DANE. When doing so, the identity would presumably be based on the NS name used for a given query.

However, since this probing policy is unilateral and opportunistic, the client SHOULD NOT consider it a failure if an encrypted transport handshake that does not authenticate to any particular expected name.

To avoid the complexity of authoritative servers with multiple simultaneous names, or multiple names over time, this draft does not attempt to describe what name a recursive resolver should use when validating an authoritative server, or what the recursive resolver should do with an authentication success.

4.4.4. Establishing an encrypted transport connection

When an encrypted transport connection actually completes (e.g., the TLS handshake completes) at time T1, the resolver sets E-completed[X] to T1 and does the following:

If the handshake completed successfully:

  • update E-session[X] so that it is in state established
  • set E-status[X] to success

  • for each query Q in E-queries[X]:

    • if early data was accepted and Q is early,

      • set the status of Q to sent

    • otherwise:

      • send Q through the session (see Section 4.4.8), and set the status of Q to sent
  • set E-last-activity[X] to T1

4.4.5. Failing to establish an encrypted transport connection

If, at time T2 an encrypted transport handshake completes with a failure (e.g. a TLS alert),

  • set E-session[X] to null
  • set E-status[X] to fail
  • set E-completed[X] to T2

  • for each query Q in E-queries[X]:

    • if Q is not present in any other *-queries[X] or in Do53-queries[X], report SERVFAIL to the requesting client. FIXME: should there be retries in this scenario? Or should the recursive then try a different encrypted transport?

4.4.6. Encrypted transport failure

Once established, an encrypted transport might fail for a number of reasons (e.g., decryption failure, or improper protocol sequence).

If this happens:

  • set E-session[X] to null
  • set E-status[X] to fail

  • for each query Q in E-queries[X]:

    • if Q is not present in any other *-queries[X] or in Do53-queries[X], report SERVFAIL to the requesting client. FIXME: should a resumption ticket be used here for this previously successful connection? Or a retry?

FIXME: are there specific forms of failure that we might handle differently? For example, What if a TCP timeout closes an idle DoT connection? What if a QUIC stream ends up timing out but other streams on the same QUIC connection are going through? Do the described scenarios cover the case when an encrypted transport's port is made unavailable/closed?

4.4.7. Handling clean shutdown of encrypted transport connection

At time T3, the recursive resolver may find that authoritative server X cleanly closes an existing outstanding connection (most likely due to resource exhaustion, see Section 3.2).

When this happens:

  • set E-session[X] to null

  • for each query Q in E-queries[X]:

    • if Q is not present in any other *-queries[X] or in Do53-queries[X], report SERVFAIL to the requesting client.

4.4.8. Sending a query over encrypted transport

When sending a query to an authoritative server over encrypted transport at time T4, the recursive resolver should take a few reasonable steps to ensure privacy and efficiency.

When sending query Q, the recursive resolver should ensure that its state in E-queries[X] is set to sent.

The recursive resolver also sets E-last-activity[X] to T4.

In addition, the recursive resolver should consider the following guidance:

4.4.8.1. Avoid EDNS client subnet

To protect the privacy of the client, the recursive resolver SHOULD NOT send EDNS(0) Client Subnet information to the authoritative server ([RFC7871]) unless explicitly authorized to do so by the client.

4.4.8.2. Pad to standard policy

To increase the anonymity set for each query, the recursive resolver SHOULD use EDNS(0) padding according to policies described in [RFC8467].

4.4.8.3. Send queries in separate channels

When multiple queries are multiplexed on a single encrypted transport to a single authoritative server, the recursive resolver MUST offer distinct query ID fields for every outstanding query on a connection, and MUST be capable of receiving responses out of order.

To the extent that the encrypted transport can avoid head-of-line blocking (e.g. QUIC can use a separate stream per query) the recursive resolver SHOULD avoid head-of-line blocking.

4.4.9. Receiving a response over encrypted transport

When a response R for query Q arrives at the recursive resolver over encrypted transport E from authoritative server with IP address X at time T5, if Q is in E-queries[X], the recursive resolver takes the following steps:

  • Remove R from E-queries[X]
  • Set E-last-activity[X] to T5

  • If R is successful:

    • send R to the requesting client

    • For each supported encrypted transport N other than E:

      • If Q is in N-queries[X]:

        • Remove Q from N-queries[X]

    • If Q is in Do53-queries[X]:

      • Remove Q from Do53-queries[X]

  • Otherwise (R is unsuccessful, e.g., SERVFAIL):

    • If Q is not in Do53-queries[X] or any other *-queries[X]:

      • Return SERVFAIL to the requesting client FIXME: What response should be sent to the clients in the case that extended DNS errors are used in an authoritative's response?

4.4.10. Resource Exhaustion

Over time, a recursive resolver following this policy may find that it is limited in resources, and may prefer to close some outstanding connections.

This could be done by checking available/consumed resources on a fixed schedule, by having a policy of only keeping a fixed number of connections open, by checking resources when activity occurs, or by some other cadence.

When existing connections should be closed, the recursive resolver should use a reasonable prioritization scheme to close outstanding connections.

One reasonable prioritization scheme would be:

  • close outstanding established sessions based on E-last-activity[X] (oldest timestamp gets closed first)

Note that when resources are limited, a recursive resolver following this guidance may also choose not to initiate new connections for encrypted transport.

4.4.11. Maintaining connections

Some recursive resolvers looking to amortize connection costs, and to minimize latency MAY choose to synthesize queries to a particular resolver to keep a encrypted transport session active.

A recursive resolver that adopts this approach should try to align the synthesized queries with other optimizations. For example, a recursive resolver that "pre-fetches" a particular resource record to keep its cache "hot" can send that query over an established encrypted transport session.

5. Signalling for Stronger Defense

This draft does not contemplate the specification of any form of coordinated signalling between authoritative servers and recursive resolvers, as such measures would not be unilateral.

However, the draft highlights the needs of a signaling mechanism for stronger defense.

We highlight the following questions for other specifications to solve:

5.1. Combining Signals with Opportunistic Probing

FIXME: How do the signals get combined with the above opportunistic probing policy? Can we specify that without needing to specify the signalling mechanism itself?

6. IANA Considerations

IANA does not need to do anything for implementers to adopt the guidance found in this draft.

7. Security Considerations

The guidance in this draft provides defense against passive network monitors for most queries. It does not defend against active attackers. It can also leak some queries and their responses due to "happy eyeballs" optimizations when the resolver's cache is cold.

Implementation of the guidance in this draft should increase deployment of opportunistic encrypted DNS transport between recursive resolvers and authoritative servers at little operational risk.

However, implementers should not rely on the guidance in this draft for robust defense against active attackers, but should treat it as a stepping stone en route to stronger defense.

This guidance is only one part of operating a privacy-preserving DNS ecosystem. A privacy-preserving recursive resolver should adopt other practices as well, such as QNAME minimization, local root zone, etc, to reduce the overall leakage of query information that could infringe on the client's privacy.

8. Acknowledgements

9. References

9.1. Normative References

[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.

9.2. Informative References

[RFC1035]
Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, , <https://www.rfc-editor.org/info/rfc1035>.
[I-D.ietf-dprive-dnsoquic]
Huitema, C., Dickinson, S., and A. Mankin, "DNS over Dedicated QUIC Connections", Work in Progress, Internet-Draft, draft-ietf-dprive-dnsoquic-06, , <https://www.ietf.org/archive/id/draft-ietf-dprive-dnsoquic-06.txt>.
[I-D.ietf-tls-esni]
Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS Encrypted Client Hello", Work in Progress, Internet-Draft, draft-ietf-tls-esni-13, , <https://www.ietf.org/archive/id/draft-ietf-tls-esni-13.txt>.
[RFC7435]
Dukhovni, V., "Opportunistic Security: Some Protection Most of the Time", RFC 7435, DOI 10.17487/RFC7435, , <https://www.rfc-editor.org/info/rfc7435>.
[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, , <https://www.rfc-editor.org/info/rfc7858>.
[RFC7871]
Contavalli, C., van der Gaast, W., Lawrence, D., and W. Kumari, "Client Subnet in DNS Queries", RFC 7871, DOI 10.17487/RFC7871, , <https://www.rfc-editor.org/info/rfc7871>.
[RFC8305]
Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2: Better Connectivity Using Concurrency", RFC 8305, DOI 10.17487/RFC8305, , <https://www.rfc-editor.org/info/rfc8305>.
[RFC8467]
Mayrhofer, A., "Padding Policies for Extension Mechanisms for DNS (EDNS(0))", RFC 8467, DOI 10.17487/RFC8467, , <https://www.rfc-editor.org/info/rfc8467>.
[RFC8484]
Hoffman, P. and P. McManus, "DNS Queries over HTTPS (DoH)", RFC 8484, DOI 10.17487/RFC8484, , <https://www.rfc-editor.org/info/rfc8484>.

Appendix A. Document Considerations

[ RFC Editor: please remove this section before publication ]

This document is currently edited as markdown. Minor editorial changes can be suggested via merge requests at https://gitlab.com/dkg/dprive-unilateral-probing or by e-mail to the editor. Please direct all significant commentary to the public IETF DPRIVE mailing list: dprive@ietf.org

The authors' latest draft can be read online in html or pdf or text formats.

Authors' Addresses

Daniel Kahn Gillmor
American Civil Liberties Union
125 Broad St.
New York, NY, 10004
United States of America
Joey Salazar
ARTICLE 19
108-114 Golden Lane
London
EC1Y 0TL
United Kingdom