Recursive to Authoritative DNS with Unauthenticated Encryption

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Table of Contents

1. Introduction

A recursive resolver using traditional DNS over port 53 may wish instead to use encrypted communication with authoritative servers in order to limit snooping of its DNS traffic by passive or on-path attackers. The recursive resolver can use unauthenticated encryption (defined in [OPPORTUN]) to achieve this goal.¶

This document describes the use case for unauthenticated encryption in recursive resolvers in Section 1.1. The encryption method with authoritative servers can be DNS-over-TLS [DNSOTLS] (DoT), DNS-over-HTTPS [DNSOHTTPS] (DoH), and/or DNS-over-QUIC [DNSOQUIC] (DoQ).¶

The document also describes a discovery method that shows if an authoritative server supports encryption in Section 2.¶

See [FULL-AUTH] for a description of the use case and a proposed mechanism for fully-authenticated encryption.¶

NOTE: The draft uses the SVCB record as a discovery mechanism for encryption by a particular authoritative server. Any record type that can show multiple types of encryption (currently DoT, DoH, and DoQ) can be used for discovery. Thus, this record type might change in the future, depending on the discussion in the DPRIVE WG.¶

2. Discovery of Authoritative Server Encryption

An authoritative server that supports DNS with encryption makes itself discoverable by publishing one or more DNS SVCB records that contain "alpn" parameter keys. SVCB records are defined in [SVCB], and the DNS extension to those records is defined in [DNS-SVCB].¶

A recursive resolver discovers whether an authoritative server supports DNS with encryption by looking for cached SVCB records for the name of the authoritative server with a positive answer. A cached DNS SVCB record with a negative answer indicates that the authoritative server does not support any encrypted transport.¶

A resolver MAY also use port probing, although the mechanism for that is not described here.¶

If the cache has no positive or negative answers for any SVCB record for any of a zone's authoritative servers, the resolver MAY send queries for the SVCB records (and for the A/AAAA records of names mentioned in those SVCB records) for some or all of the zone's authoritative servers and wait for a positive response so that the resolver can use DNS with encryption for the original query. In this situation, the resolver MAY instead just use classic DNS for the original query but simultaneously queue queries for the SVCB (and subsequent A/AAAA) records for some or all of the zone's authoritative servers so that future queries might be able to use DNS with encryption.¶

DNSSEC validation of SVCB RRsets used strictly for this discovery mechanism is not mandated.¶

3. Processing Discovery Responses

After a resolver has DNS SCVB records in its cache (possibly due to having just queried for them), it needs to use those records to try to find an authoritative server that uses DNS with encryption. This section describes how the resolver can make that selection.¶

A resolver MUST NOT attempt encryption for a server that has a negative response in its cache for the associated DNS SVCB record.¶

After sending out all requests for SVCB records for the authoritative servers in the NS RRset for a name, if all of the SVCB records for those authoritative servers in the cache are negative responses, the resolver MUST use classic (unencrypted) DNS instead of encryption. Similarly, if none of the DNS SVCB records for the authoritative servers in the cache have supported "alpn" parameters, the resolver MUST use classic (unencrypted) DNS instead of encryption.¶

If there are any DNS SVCB records in the cache for the authoritative servers for a zone with supported "alpn" parameters, the resolver MUST try each indicated authoritative server using DNS with encryption until it successfully sets up a connection. The resolver only attempts to use the encrypted transports that are in the associated SVCB record for the authoritative server. (( Note that this completely prohibits "simple port 853 probing" even though that is what some operators are currently doing. Does the WG want to be this strict? ))¶

A resolver SHOULD keep a DNS with encryption session to a particular server open if it expects to send additional queries to that server in a short period of time. [DNS-OVER-TCP] says "both clients and servers SHOULD support connection reuse" for TCP connections, and that advice could apply as well for DNS with encryption, especially as DNS with encryption has far greater overhead for re-establishing a connection. If the server closes the DNS with encryption session, the resolver can possibly re-establish a DNS with encryption session using encrypted session resumption.¶

For any DNS with encryption protocols, TLS version 1.3 [TLS-13] or later MUST be used.¶

A resolver following this protocol does not need to authenticate TLS servers. Thus, when setting up a TLS connection, if the server's authentication credentials do not match those expected by the resolver, the resolver continues with the TLS connection. Privacy-oriented resolvers (defined in [PRIVACY-REC]) following this protocol MUST NOT indicate that they are using encryption because this protocol is susceptible to on-path attacks.¶

# Inputs
ns_names = List of NS Rdatas from the NS RRset for the queried name
can_do_secure = List of secure transports supported by resolver
secure_names_and_transports = Empty list, filled in below

# Fill secure_names_and_transports with (name, transport) tuples
for this_name in ns_names:
  if signal_rrset(this_name) is in the resolver cache:
    if signal_rrset(this_name) positively does not exist:
      continue
    for this_transport in signal_rrset(this_name):
      if this_transport in can_do_secure:
        add (this_name, this_transport) to secure_names_and_transports
  else: # signal_rrset(this_name) is not in the resolver cache
    queue a query for signal_rrset(this_name) for later caching

# Query over secure transport until successful
for (this_name, this_transport) tuple in secure_names_and_transports:
  query using this_transport on this_name
  if successful:
    finished

# Got here if no this_name/this_transport query was successful
#   or if secure_names_and_transports was empty
query using classic DNS on any/all ns_names; finished

4. Serving with Encryption

An operator of an authoritative server following this protocol SHOULD publish SVCB records as described in Section 2. If they cannot publish such records, the security properties of their authoritative servers will not be found. If an operator wants to test serving using encryption, they can publish SVCB records with short TTLs and then stop serving with encryption after removing the SVCB records and waiting for the TTLs to expire.¶

It is acceptable for an operator of authoritative servers to only offer encryption on some of the named authoritative servers, such as when the operator is determining how far to roll out encrypted service.¶

A server MAY close an encrypted connection at any time. For example, it can close the session if it has not received a DNS query in a defined length of time. The server MAY close an encrypted session after it sends a DNS response; however, it might also want to keep the session open waiting for another DNS query from the resolver. [DNS-OVER-TCP] says "both clients and servers SHOULD support connection reuse" for TCP connections, and that advice could apply as well for DNS with encryption, especially as DNS with encryption has far greater overhead for re-establishing a connection. If the server closes the DNS with encryption session, the resolver can possibly re-establish a DNS with encryption session using encrypted session resumption.¶

For any DNS with encryption protocols, TLS version 1.3 [TLS-13] or later MUST be used.¶

5. IANA Considerations

(( Update registration for TCP/853 to also include ADoT ))¶

(( Maybe other updates for DoH and DoQ ))¶

6. Security Considerations

The method described in this document explicitly allows a resolver to perform DNS communications over traditional unencrypted, unauthenticated DNS on port 53, if it cannot find an authoritative server that advertises that it supports encryption. The method described in this document explicitly allows a resolver using encryption to choose to allow unauthenticated encryption. In either of these cases, the resulting communication will be susceptible to obvious and well-understood attacks from an attacker in the path of the communications.¶

7. Acknowledgements

Puneet Sood contributed many ideas to early drafts of this document.¶

The DPRIVE Working Group has contributed many ideas that keep shifting the focus and content of this document.¶

Authors' Addresses