Signalling That an Authoritative DNS server offers DoT
draft-levine-dprive-signal-00

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Network Working Group                                          J. Levine
Internet-Draft                                      Taughannock Networks
Intended status: Informational                          November 2, 2019
Expires: May 5, 2020

         Signalling That an Authoritative DNS server offers DoT
                     draft-levine-dprive-signal-00

Abstract

   DNS resolvers that wish to use DNS over TLS to authoritative servers
   (ADoT) need some way to tell whether server offers DoT.  This
   document describes some ways that a server might signal that it uses
   DoT.

Status of This Memo

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

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   This Internet-Draft will expire on May 5, 2020.

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   Copyright (c) 2019 IETF Trust and the persons identified as the
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  General observations  . . . . . . . . . . . . . . . . . . . .   2
   3.  Signaling methods . . . . . . . . . . . . . . . . . . . . . .   2
     3.1.  EDNS0 option  . . . . . . . . . . . . . . . . . . . . . .   2
     3.2.  DNSKEY  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.3.  Special server names  . . . . . . . . . . . . . . . . . .   3
   4.  References - Normative  . . . . . . . . . . . . . . . . . . .   3
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   4

1.  Introduction

   The Domain Name System[RFC1034] [RFC1035] uses a directed presumably
   acyclic graph of servers to provide authoritative answers to queries.
   The link from one server to the next is provided by an NS record in
   the zone on the upper server that points to the lower server.  For
   zones signed with DNSSEC, the upper server zone contains DS records
   that contain hashes of signing keys in DNSKEY records in the zone on
   the lower server.

2.  General observations

   In several of the following schemes, the client probes the server to
   see whether it offers ADoT.  In those cases, the client presumably
   remembers what servers it's probed so there's only one probe per
   server.

   The probe query would generally use query minimization to limit
   leakage of the requested name.  Even so, if a server handles many
   zones, this leaks the name of the zone being probed.

   Some zones have servers run by multiple operators.  (The DNS root is
   a well known example.)  It is possible that some of the servers will
   offer ADoT and some will not.  Some of the schemes below handle per-
   server signals, some don't.

3.  Signaling methods

   This is a working list of possible signalling methods.  Just because
   they're in the list doesn't mean that anyone thinks they're a good
   idea.

3.1.  EDNS0 option

   We define a new EDNS0 option edns-adot.  The client sends an edns-
   adot option in its request, and the server responds with a value of 0
   or 1 to say whether it suports ADoT.  The option could be served by

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   the upper level server along with the NS records, which avoids the
   extra probe, or by the lower level server.

   This is easy to implement, but since the OPT isn't signed, it's
   subject to downgrade attacks.  If served by the upper level server,
   there's no per-server indication, but also no extra round trip.

3.2.  DNSKEY

   A DNSKEY [RFC4034] at the apex of the zone signals that ADoT is
   available.  The simplest approach would be to use one of the
   unassigned DNSKEY flags to indicate that the zone is expected to be
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