Signaling That an Authoritative DNS server offers DoT
draft-levine-dprive-signal-01
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Network Working Group J. Levine
Internet-Draft Taughannock Networks
Intended status: Informational November 4, 2019
Expires: May 7, 2020
Signaling That an Authoritative DNS server offers DoT
draft-levine-dprive-signal-01
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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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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 May 7, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. General observations . . . . . . . . . . . . . . . . . . . . 2
3. Signaling methods . . . . . . . . . . . . . . . . . . . . . . 3
3.1. EDNS0 option . . . . . . . . . . . . . . . . . . . . . . 3
3.2. DNSKEY flags . . . . . . . . . . . . . . . . . . . . . . 3
3.3. Other DNS records . . . . . . . . . . . . . . . . . . . . 3
3.4. SRV records . . . . . . . . . . . . . . . . . . . . . . . 3
3.5. DANE TLSA . . . . . . . . . . . . . . . . . . . . . . . . 4
3.6. Special server names . . . . . . . . . . . . . . . . . . 4
4. References - Normative . . . . . . . . . . . . . . . . . . . 4
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
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
Depending on your threat model it may be a problem if an intermediate
party can intercept the signal and force a DoT client to use
unencrypted DNS.
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.
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.
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3. Signaling methods
This is a working list of possible signaling 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 supports ADoT. The option could be served by
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 flags
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
served over ADoT. This is resistant to downgrade, since the DNSKEY
is signed, but there's no per-server indication. DNSSEC clients have
to fetch the DNSKEY records anyway so there's no extra round trip.
Since nobody has ever used DNSKEY records with flag values other than
0, 256, and 257, some software may fail if it sees other flag values.
3.3. Other DNS records
A variation of the previous approach is to put some other kind of
DNSSEC signed record at the zone apex lists nameservers expected to
support ADoT, either yet another overloaded TXT record or a new
RRTYPE. The list of names would presumably have to be names already
listed in NS records (but see the next section.) This provides per-
server indication, and is backward compatible, but it makes the DNS
Camel sad.
Another variation puts the signal record at the rDNS name for a
nameserver's IP address.
3.4. SRV records
Servers could publish SRV records for ADoT service discovery. ADoT
cleints would use the servers identified by SRV instead of the NS
servers.
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This is downgrade resistant, backward compatible, and allows per-
server signalling, even allowing non-standard port numbers. There is
potentially an extra round trip for the SRV lookup and more if the
name of the servers aren't the same as the NS servers. The number of
round trips could be limited if servers provide the SRV and related
A/AAAA records as additional data in responses to DNSKEY lookups. It
might lead to unpleasant resolution loops if if SRV records use out
of bailiwick nameservers.
_domain-s._tcp.blah1.example. IN SRV 10 0 853 ns.blah2.example.
. . .
_domain-s._tcp.blah2.example. IN SRV 10 0 4242 ns.blah1.example.
3.5. DANE TLSA
If ADoT servers all have DANE secured TLS certificates, the TLSA
record can be the ADoT signal.
Publishing a TLSA record is straightforward if a zone is already
DNSSEC signed. It's downgrade-resistant, allows per-server signals,
and there's no extra round trip beyond what's needed to do the DANE
validation.
_853._tcp.ns1.blah.example IN TLSA . . ."
3.6. Special server names
Any server that supports ADoT has a name starting with the four
characters "XS--". All names starting with two letters other than
"XN" and two dashes were reserved when IDNs were invented, so these
names are unlikely to collide with any existing names. These are not
IDNs, they're just funny looking ASCII names, and you can't do "XN--
XS--blah" or anything like that.
This is backward compatible, downgrade resistant, needs no extra
round trip, and allows per-server signals. It doesn't allow server
names to be IDNs which should not be a big problem since DNS server
names are not generally shown to users, although it may confuse
people who believe that anything with two dashes must be an IDN.
The Camel is also not crazy about it.
4. References - Normative
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
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[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, DOI 10.17487/RFC4034, March 2005,
<https://www.rfc-editor.org/info/rfc4034>.
Author's Address
John Levine
Taughannock Networks
PO Box 727
Trumansburg, NY 14886
Email: standards@taugh.com
URI: http://jl.ly
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