Internet Engineering Task Force D. Wessels
Internet-Draft P. Barber
Intended status: Standards Track M. Weinberg
Expires: October 2, 2018 Verisign
March 31, 2018
Message Digest for DNS Zones
draft-wessels-dns-zone-digest-00
Abstract
This document describes a protocol and DNS Resource Record used to
provide a message digest over DNS zone data. In particular, it
describes how to compute, sign, represent, and use the message digest
to verify the contents of a zone for accuracy and completeness. The
ZONEMD Resource Record type is introduced for conveying the message
digest data.
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
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This Internet-Draft will expire on October 2, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. The ZONEMD Resource Record . . . . . . . . . . . . . . . . . 4
3.1. ZONEMD RDATA Wire Format . . . . . . . . . . . . . . . . 4
3.1.1. The Serial Field . . . . . . . . . . . . . . . . . . 4
3.1.2. The Digest Type Field . . . . . . . . . . . . . . . . 5
3.1.3. The Digest Field . . . . . . . . . . . . . . . . . . 5
3.2. ZONEMD Presentation Format . . . . . . . . . . . . . . . 5
3.3. ZONEMD Example . . . . . . . . . . . . . . . . . . . . . 6
4. Calculating the Digest . . . . . . . . . . . . . . . . . . . 6
4.1. Canonical Format and Ordering . . . . . . . . . . . . . . 6
4.1.1. Order of RRsets Having the Same Owner Name . . . . . 6
4.1.2. Special Considerations for SOA RRs . . . . . . . . . 6
4.2. Add ZONEMD Placeholder . . . . . . . . . . . . . . . . . 7
4.3. Optionally Sign the Zone . . . . . . . . . . . . . . . . 7
4.4. Calculate the Digest . . . . . . . . . . . . . . . . . . 7
4.4.1. Inclusion/Exclusion Rules . . . . . . . . . . . . . . 7
4.5. Update ZONEMD RR . . . . . . . . . . . . . . . . . . . . 8
5. Verifying Zone Message Digest . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6.1. ZONEMD RRtype . . . . . . . . . . . . . . . . . . . . . . 9
6.2. ZONEMD Digest Type . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7.1. Attacks Against the Zone Digest . . . . . . . . . . . . . 9
7.2. Attacks Utilizing the Zone Digest . . . . . . . . . . . . 9
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
10. Implementation Status . . . . . . . . . . . . . . . . . . . . 10
10.1. Authors' Implementation . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
In the DNS, a zone is the collection of authoritative resource
records (RRs) sharing a common origin ([RFC7719]), which can be
distributed from primary to secondary name servers. Zones are often
stored as files on disk in the so-called master file format
[RFC1034]. Sometimes zones are distributed outside of the DNS, with
such protocols as FTP, HTTP, rsync, and so on. While zone files are
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self-contained, currently there is no way to verify the authenticity
of a stand-alone zone file.
This document introduces a new RR type that serves as a cryptographic
message digest of the data in a zone file. It allows a receiver of
the zone file to verify its authenticity, especially when used in
combination with DNSSEC.
Message digests are also found in the TSIG protocol. DNS transaction
signatures (TSIG [RFC2845]) uses a message digest to protect
individual query and response messages. TSIG is generally used to
authenticate and validate UPDATE [RFC2136] AXFR [RFC5936], and IXFR
[RFC1995] messages. However, TSIG's protections are ephemeral,
existing only "on the wire," and are not retained after the
transaction is complete. Additionally, TSIG utilizes shared secret
keys, which are not available to third parties.
The technique described in this document makes the message digest a
part of the zone file itself, and allows anyone to verify the zone
file as a whole, no matter how it is transmitted.
DNSSEC provides three strong security guarantees relevant to this
protocol:
1. whether or not to expect DNSSEC records in the zone,
2. whether or not to expect a ZONEMD record in a signed zone, and
3. whether or not the ZONEMD record has been altered since it was
signed.
FOR DISCUSSION: currently this document does not require DNSSEC.
Should it be a prerequisite?
The motivation for including a message digest in a zone file comes
largely from the DNS root zone. At the time of this writing, there
is increased attention to the idea of widely distributing the root
zone, beyond the root server system. [RFC7706] describes how a
recursive resolver can serve the root zone via a loopback address.
As the root zone spreads beyond its traditional deployment
boundaries, the need for verification of the zone contents becomes
increasingly important.
Nothing in this specification, however, is specific to the root zone.
The zone digest is designed to work for any DNS zone.
This specification is OPTIONAL to implement by both publishers and
consumers of zone file data.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. The ZONEMD Resource Record
This section describes the ZONEMD Resource Record, including its
fields, wire format, and presentation format. The Type value for the
ZONEMD RR is TBD. The ZONEMD RR is class independent. The RDATA of
the resource record consists of three fields: Serial, Digest Type,
and Digest.
FOR DISCUSSION: This document is currently written as though a zone
MUST NOT contain more than one ZONEMD RR. Having exactly one ZONEMD
record per zone simplifies this protocol and eliminates confusion
around downgrade attacks, at the expense of algorithm agility.
3.1. ZONEMD RDATA Wire Format
The ZONEMD RDATA wire format is encoded as follows:
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Serial |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Digest Type | |
+-+-+-+-+-+-+-+-+ Digest +
/ /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.1.1. The Serial Field
The Serial field is a 32-bit unsigned integer in network order. It
is equal to the serial number from the zone's SOA record ([RFC1035]
section 3.3.13) for which the message digest was generated.
FOR DISCUSSION: the serial number is included in order to make DNS
response messages of type ZONEMD meaningful. Without the serial
number, a stand-alone ZONEMD digest has no association to any
particular zone file. If there is agreement that ZONEMD responses
are not useful, this field could be removed. See also the end of
Security Considerations.
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3.1.2. The Digest Type Field
The Digest Type field is an 8-bit unsigned integer, with meaning
equivalent to the Digest Type of the DS resource record, as defined
in section 5.1.3 of [RFC4034].
The status of ZONEMD digest types (e.g., mandatory, optional,
deprecated) SHALL always match the status for DS records. This
information can be found in the IANA protocol registry for DS digest
types [iana-ds-digest-types].
At the time of this writing the following digest types are defined:
+-------+-----------------+-----------+-----------+
| Value | Description | Status | Reference |
+-------+-----------------+-----------+-----------+
| 1 | SHA1 | Mandatory | [RFC3658] |
| 2 | SHA256 | Mandatory | [RFC4509] |
| 3 | GOST R 34.11-94 | Optional | [RFC5933] |
| 4 | SHA384 | Optional | [RFC6605] |
+-------+-----------------+-----------+-----------+
Table 1: Digest Types
3.1.3. The Digest Field
The Digest field is a variable-length sequence of octets containing
the message digest. Section 4 describes how to calculate the digest
for a zone. Section 5 describes how to use the digest to verify the
contents of a zone.
3.2. ZONEMD Presentation Format
The presentation format of the RDATA portion is as follows:
The Serial field MUST be represented as an unsigned decimal integer.
The Digest Type field MUST be represented as an unsigned decimal
integer.
The Digest MUST be represented as a sequence of case-insensitive
hexadecimal digits. Whitespace is allowed within the hexadecimal
text.
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3.3. ZONEMD Example
The following example shows a ZONEMD RR.
example.com. 86400 IN ZONEMD ( 2018031500 4 FEBE3D4CE2EC2FFA4BA9
9D46CD69D6D29711E552
17057BEE7EB1A7B641A4
7BA7FED2DD5B97AE499F
AFA4F22C6BD647DE )
4. Calculating the Digest
4.1. Canonical Format and Ordering
Calculation of the zone digest REQUIRES the RRs in a zone to be in a
consistent format and ordering. Correct ordering of the zone depends
on (1) ordering of owner names in the zone, (2) ordering of RRsets
with the same owner name, and (3) ordering of RRs within an RRset.
This specification adopts DNSSEC's canonical ordering for names
(Section 6.1 of [RFC4034]), and canonical ordering for RRs within an
RRset (Section 6.3 of [RFC4034]). It also adopts DNSSEC's canonical
RR form (Section 6.2 of [RFC4034]). However, since DNSSEC does not
define a canonical ordering for RRsets having the same owner name,
that ordering is defined here.
4.1.1. Order of RRsets Having the Same Owner Name
For the purposes of calculating the zone digest, RRsets having the
same owner name MUST first be ordered by their numeric RR CLASS, and
second by their numeric RR TYPE.
4.1.2. Special Considerations for SOA RRs
When AXFR is used to transfer zone data, the first and last records
are always the SOA RR ([RFC5936] Section 2.2). Because of this, zone
files on disk often contain two SOA RRs. When calculating the zone
digest, the first SOA RR MUST be included and any subsequent SOA RRs
MUST NOT be included.
Additionally, per established practices, the SOA record is generally
the first record in a zone file. However, according to the
requirement to sort RRsets with the same owner name by type, the SOA
RR (type value 2) might not be first in the digest calculation. If
the zone has an A RR (type value 1) at the apex, it MUST be processed
before the SOA RR.
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4.2. Add ZONEMD Placeholder
In preparation for calculating the zone digest, any existing ZONEMD
records MUST first be deleted from the zone.
Prior to calculation of the digest, and prior to signing with DNSSEC,
a placeholder ZONEMD record MUST be added to the zone. This serves
two purposes: (1) it allows the digest to cover the Serial and Digest
Type field values, and (2) ensures that appropriate denial-of-
existence (NSEC, NSEC3) records are created if the zone is signed
with DNSSEC.
In the placeholder record, the Serial field MUST be set to the
current SOA Serial. The Digest Type field MUST be set to the value
for the chosen digest algorithm. The Digest field MUST be set to all
zeroes and of length appropriate for the chosen digest algorithm.
4.3. Optionally Sign the Zone
Following addition of the placeholder record, the zone MAY be signed
with DNSSEC. Note that when the digest calculation is complete, and
the ZONEMD record is updated, the signature(s) for that record MUST
be recalculated and updated as well.
4.4. Calculate the Digest
The zone digest is calculated by concatenating the canonical form of
RRs in the zone, in the order described above, subject to the
inclusion/exclusion rules described below, and then applying the
digest algorithm:
digest = digest_algorithm( RR(1) | RR(2) | RR(3) | ... )
where "|" denotes concatenation, and
RR(i) = owner | type | class | TTL | RDATA length | RDATA
4.4.1. Inclusion/Exclusion Rules
When calculating the digest, the following inclusion/exclusion rules
apply:
o More than one SOA MUST NOT be included.
o The placeholder ZONEMD RR MUST be included.
o If the zone is signed, DNSSEC RRs MUST be included, except:
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o The RRSIG covering ZONEMD MUST NOT be included.
4.5. Update ZONEMD RR
Once the zone digest has been calculated, its value is then copied to
the Digest field of the ZONEMD record.
If the zone is signed with DNSSEC, the appropriate RRSIG records
covering the ZONEMD record MUST then be added. Because the ZONEMD
placeholder was added prior to signing, the zone will already have
the appropriate denial-of-existence (NSEC, NSEC3) records.
5. Verifying Zone Message Digest
The recipient of a zone that has a message digest record can verify
the zone by calculating the digest as follows:
1. The verifier SHOULD first determine whether or not to expect
DNSSEC records in the zone. This can be done by examining
locally configured trust anchors, or querying for (and
validating) DS RRs in the parent zone. For zones that are
provably unsigned, digest validation continues at step 4 below.
2. For zones that are provably signed, the existence of the ZONEMD
record MUST be verified. If the ZONEMD record provably does not
exist, digest verification cannot be done. If the ZONEMD record
does provably exist, but is not found in the zone, digest
verification MUST NOT be considered successful.
3. For zones that are provably signed, the SOA RR and ZONEMD RR(set)
MUST have valid signatures, chaining up to a trust anchor. If
DNSSEC validation of the SOA or ZONEMD records fails, digest
verification MUST NOT be considered successful.
4. The SOA Serial field MUST exactly match the ZONEMD Serial field.
If the fields to not match, digest verification MUST NOT be
considered successful.
5. The ZONEMD Digest Type field MUST be checked. If the verifier
does not support the given digest type, it SHOULD report that the
zone digest could not be verified due to an unsupported
algorithm.
6. The zone digest is calculated using the algorithm described in
Section 4.4. Note in particular that the Digest field of any
ZONEMD RRs MUST be zeroed prior to calculation of the digest.
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7. The calculated digest is compared to the received digest. If the
two digest values match, verification is considered successful.
Otherwise, verification MUST NOT be considered successful.
6. IANA Considerations
6.1. ZONEMD RRtype
This document uses a new DNS RR type, ZONEMD, whose value TBD has
been allocated by IANA from the "Resource Record (RR) TYPEs"
subregistry of the "Domain Name System (DNS) Parameters" registry.
6.2. ZONEMD Digest Type
The ZONEMD Digest Type field has the same semantics as the DS RR
Digest Type field. Thus, it does not add new IANA protocol registry
requirements.
7. Security Considerations
7.1. Attacks Against the Zone Digest
The zone digest allows the receiver to verify that the zone contents
haven't been modified since the zone was generated/published.
Verification is strongest when the zone is also signed with DNSSEC.
An attacker, whose goal is to modify zone content before it is used
by the victim, may consider a number of different approaches.
The attacker might perform a downgrade attack to an unsigned zone.
This is why Section 5 RECOMMENDS that the verifier determine whether
or not to expect DNSSEC signatures for the zone in step 1.
The attacker might perform a downgrade attack by removing the ZONEMD
record. This is why Section 5 REQUIRES that the verifier checks
DNSSEC denial-of-existence proofs in step 2.
The attacker might alter the Digest Type or Digest fields of the
ZONEMD record. Such modifications are detectable only with DNSSEC
validation.
7.2. Attacks Utilizing the Zone Digest
Nothing in this specification prevents clients from making, and
servers from responding to, ZONEMD queries. One might consider how
well ZONEMD responses could be used in a distributed denial-of-
service amplification attack.
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The ZONEMD RR is moderately sized, much like the DS RR. A single
ZONEMD RR contributes approximately 40 to 65 octets to a DNS
response, for currently defined digest types. Certainly other query
types result in larger amplification effects (i.e., DNSKEY).
FOR DISCUSSION: The primary purpose of the ZONEMD record is to verify
a zone file prior to being loaded or served by a name server. We
could allow a name server implementation to respond to ZONEMD queries
with the REFUSED RCODE without loss of functionality.
8. Privacy Considerations
This specification has no impacts on user privacy.
9. Acknowledgments
The authors wish to thank David Blacka, Scott Hollenbeck, and Rick
Wilhelm for providing feedback on early drafts of this document.
10. Implementation Status
10.1. Authors' Implementation
The authors are currently working on an implementation in C, using
the ldns library [ldns]. This implementation is able to perform the
following functions:
o Read input zone file, output zone file with ZONEMD placeholder.
o Compute zone digest over signed zone file and update ZONEMD
record.
o Re-compute DNSSEC signature over ZONEMD record.
o Verify zone digest from input zone file.
The authors expect to be able to release this implementation as open
source following submission of this Internet-Draft.
11. References
11.1. Normative References
[iana-ds-digest-types]
IANA, "Delegation Signer (DS) Resource Record (RR) Type
Digest Algorithms", April 2012,
<https://www.iana.org/assignments/ds-rr-types/
ds-rr-types.xhtml>.
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[ldns] NLNet Labs, "The ldns Library", March 2018,
<https://www.nlnetlabs.nl/projects/ldns/>.
[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>.
[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>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3658] Gudmundsson, O., "Delegation Signer (DS) Resource Record
(RR)", RFC 3658, DOI 10.17487/RFC3658, December 2003,
<https://www.rfc-editor.org/info/rfc3658>.
[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>.
[RFC4509] Hardaker, W., "Use of SHA-256 in DNSSEC Delegation Signer
(DS) Resource Records (RRs)", RFC 4509,
DOI 10.17487/RFC4509, May 2006,
<https://www.rfc-editor.org/info/rfc4509>.
[RFC5933] Dolmatov, V., Ed., Chuprina, A., and I. Ustinov, "Use of
GOST Signature Algorithms in DNSKEY and RRSIG Resource
Records for DNSSEC", RFC 5933, DOI 10.17487/RFC5933, July
2010, <https://www.rfc-editor.org/info/rfc5933>.
[RFC6605] Hoffman, P. and W. Wijngaards, "Elliptic Curve Digital
Signature Algorithm (DSA) for DNSSEC", RFC 6605,
DOI 10.17487/RFC6605, April 2012,
<https://www.rfc-editor.org/info/rfc6605>.
11.2. Informative References
[RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
DOI 10.17487/RFC1995, August 1996,
<https://www.rfc-editor.org/info/rfc1995>.
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[RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, DOI 10.17487/RFC2136, April 1997,
<https://www.rfc-editor.org/info/rfc2136>.
[RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
Wellington, "Secret Key Transaction Authentication for DNS
(TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000,
<https://www.rfc-editor.org/info/rfc2845>.
[RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
(AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
<https://www.rfc-editor.org/info/rfc5936>.
[RFC7706] Kumari, W. and P. Hoffman, "Decreasing Access Time to Root
Servers by Running One on Loopback", RFC 7706,
DOI 10.17487/RFC7706, November 2015,
<https://www.rfc-editor.org/info/rfc7706>.
[RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", RFC 7719, DOI 10.17487/RFC7719, December
2015, <https://www.rfc-editor.org/info/rfc7719>.
Authors' Addresses
Duane Wessels
Verisign
12061 Bluemont Way
Reston, VA 20190
Phone: +1 703 948-3200
Email: dwessels@verisign.com
URI: http://verisign.com
Piet Barber
Verisign
12061 Bluemont Way
Reston, VA 20190
Phone: +1 703 948-3200
Email: pbarber@verisign.com
URI: http://verisign.com
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Matt Weinberg
Verisign
12061 Bluemont Way
Reston, VA 20190
Phone: +1 703 948-3200
Email: mweinberg@verisign.com
URI: http://verisign.com
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