| Internet Engineering Task Force | D. Wessels |
| Internet-Draft | P. Barber |
| Intended status: Experimental | M. Weinberg |
| Expires: April 12, 2019 | Verisign |
| W. Kumari | |
| W. Hardaker | |
| USC/ISI | |
| October 9, 2018 |
Message Digest for DNS Zones
draft-wessels-dns-zone-digest-03
This document describes an experimental protocol and new DNS Resource Record that can be used to provide an message digest over DNS zone data. The ZONEMD Resource Record conveys the message digest data in the zone itself. When a zone publisher includes an ZONEMD record, recipients can verify the zone contents for accuracy and completeness. This provides assurance that received zone data matches published data, regardless of how the zone data has been transmitted and received.
ZONEMD is not designed to replace DNSSEC. Whereas DNSSEC is designed to protect recursive name servers and their caches, ZONEMD protects applications that consume zone files, whether they be authoritative name servers, recursive name servers, or uses of zone file data.
As specified at this time, ZONEMD is not designed for use in large, dynamic zones due to the time and resources required for digest calculation. The ZONEMD record described in this document includes fields reserved for future work to support large, dynamic zones.
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 April 12, 2019.
Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.
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In the DNS, a zone is the collection of authoritative resource records (RRs) sharing a common origin ([RFC7719]). Zones are often stored as files on disk in the so-called master file format [RFC1034]. Zones are generally distributed between name servers using the AXFR [RFC5936], and IXFR [RFC1995] protocols. Zone files can also be distributed outside of the DNS, with such protocols as FTP, HTTP, rsync, and even via email. Currently there is no standard 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 the zone file's authenticity, especially when used in combination with DNSSEC. This technique makes the message digest a part of the zone file itself, allowing verification the zone file as a whole, no matter how it is transmitted. Furthermore, the digest is based on the wire format of zone data. Thus, it independent of presentation format, such as changes in whitespace, capitalization, and comments.
DNSSEC provides three strong security guarantees relevant to this protocol:
This specification is OPTIONAL to implement by both publishers and consumers of zone file data.
The motivation for this protocol enhancement is the desire for the ability to verify the authenticity of a stand-alone zone file, regardless of how it is transmitted. A consumer of zone file data should be able to verify that the data is as-published by the zone operator.
One approach to preventing data tampering and corruption is to secure the distribution channel. The DNS has a number of features that can already be used for channel security. Perhaps the most widely used is DNS transaction signatures (TSIG [RFC2845]). TSIG uses shared secret keys and a message digest to protect individual query and response messages. It is generally used to authenticate and validate UPDATE [RFC2136], AXFR [RFC5936], and IXFR [RFC1995] messages.
DNS Request and Transaction Signatures (SIG(0) [RFC2931]) is another protocol extension designed to authenticate individual DNS transactions. Whereas SIG records were originally designed to cover specific RR types, SIG(0) is used to sign an entire DNS message. Unlike TSIG, SIG(0) uses public key cryptography rather than shared secrets.
The Transport Layer Security protocol suite is also designed to provide channel security. It is entirely possible, for example, to perform zone transfers using DNS-over-TLS ([RFC7858]). Furthermore, one can easily imagine the distribution of zone files over HTTPS-enabled web servers, as well as DNS-over-HTTPS [dns-over-https].
Unfortunately, the protections provided by these channel security techniques are ephemeral and are not retained after the data transfer is complete. They can ensure that the client receives the data from the expected server, and that the data sent by the server is not modified during transmission. However, they do not guarantee that the server transmits the data as originally published, and do not provide any methods to verify data that is read after transmission is complete. For example, a name server loading saved zone data upon restart cannot guarantee that the on-disk data has not been modified. For these reasons, it is preferable to secure the data itself.
Why not simply rely on DNSSEC, which provides certain data security guarantees? Certainly for zones that are signed, a recipient could validate all of the signed RRsets. Additionally, denial-of-existence records can prove that RRsets have not been added or removed. However, not all RRsets in a zone are signed. The design of DNSSEC stipulates that delegations (non-apex NS records) are not signed, and neither are any glue records. Thus, changes to delegation and glue records cannot be detected by DNSSEC alone. Furthermore, zones that employ NSEC3 with opt-out are susceptible to the removal or addition of names between the signed nodes. Whereas DNSSEC is primarily designed to protect consumers of DNS response messages, this protocol is designed to protect consumers of zone files.
There are existing tools and protocols that provide data security, such as OpenPGP [RFC4880] and S/MIME [RFC3851]. In fact, the internic.net site publishes PGP signatures along side the root zone and other files available there. However, this is a detached signature with no strong association to the corresponding zone file other than its timestamp. Non-detached signatures are, of course, possible, but these necessarily change the format of the file being distributed. That is, a zone file signed with OpenPGP or S/MIME no longer looks like a zone file and could not directly be loaded into a name server. Once loaded the signature data is lost, so it does not survive further propagation.
It seems the desire for data security in DNS zones was envisioned as far back as 1997. [RFC2065] is an obsoleted specification of the first generation DNSSEC Security Extensions. It describes a zone transfer signature, aka AXFR SIG, which is similar to the technique proposed by this document. That is, it proposes ordering all (signed) RRsets in a zone, hashing their contents, and then signing the zone hash. The AXFR SIG is described only for use during zone transfers. It did not postulate the need to validate zone data distributed outside of the DNS. Furthermore, its successor, [RFC2535], omits the AXFR SIG, while at the same time introducing an IXFR SIG.
This document introduces a new Resource Record type designed to convey a message digest of the content of a zone file. The digest is calculated at the time of zone publication. Ideally the zone is signed with DNSSEC to guarantee that any modifications of the digest can be detected. The procedures for digest calculation and DNSSEC signing are similar (i.e., both require the same ordering of RRs) and can be done in parallel.
The zone digest is designed to be used on zones that are relatively stable and have infrequent updates. As currently specified, the digest is re-calculated over the entire zone content each time. This specification does not provide an efficient mechanism for incremental updates of zone data. It does, however, reserve a field in the ZONEMD record for future work to support incremental zone digest algorithms (e.g. using Merkle trees).
It is expected that verification of a zone digest would be implemented in name server software. That is, a name server can verify the zone data it was given and refuse to serve a zone which fails verification. For signed zones, the name server needs a trust anchor to perform DNSSEC validation. For signed non-root zones, the name server may need to send queries to validate a chain-of-trust. Digest verification could also be performed externally.
The root zone [InterNIC] is perhaps the most widely distributed DNS zone on the Internet, served by 930 separate instances [RootServers] at the time of this writing. Additionally, many organizations configure their own name servers to serve the root zone locally. Reasons for doing so include privacy and reduced access time. [RFC7706] describes one, but not the only, way to do this. As the root zone spreads beyond its traditional deployment boundaries, the need for verification of the completeness of the zone contents becomes increasingly important.
Since its very early days, the developers of the DNS recognized the importance of secondary name servers and service diversity. However, they may not have anticipated the complexity of modern DNS service provisioning which can include multiple third-party providers and hundreds of anycast instances. Instead of a simple primary-to-secondary zone distribution system, today it is possible to have multiple levels, multiple parties, and multiple protocols involved in the distribution of zone data. This complexity introduces new places for problems to arise. The zone digest protects the integrity of data that flows through such systems.
DNS Response Policy Zones is "a method of expressing DNS response policy information inside specially constructed DNS zones..." [RPZ]. A number of companies provide RPZ feeds, which can be consumed by name server and firewall products. Since these are zone files, AXFR is often, but not necessarily used for transmission. While RPZ zones can certainly be signed with DNSSEC, the data is not queried directly, and would not be subject to DNSSEC validation.
ICANN operates the Centralized Zone Data Service [CZDS], which is a repository of top-level domain zone files. Users request access to the system, and to individual zones, and are then able to download zone data for certain uses. Adding a zone digest to these would provide CZDS users with assurances that the data has not been modified. Note that ZONEMD could be added to CZDS zone data independently of the zone served by production name servers.
Since the zone digest does not depend on presentation format, it could be used to compare multiple copies of a zone received from different sources, or copies generated by different processes.
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] [RFC8174] when, and only when, they appear in all capitals, as shown here.
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.
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 | Reserved | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Digest |
/ /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
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] and values found in the IANA protocol registry for DS digest types [iana-ds-digest-types].
The status of ZONEMD digest types (e.g., mandatory, optional, deprecated), however, are independent of those for DS digest types.
At the time of this writing the following digest types are defined:
| Value | Description | Status | Reference |
|---|---|---|---|
| 1 | SHA1 | Deprecated | [RFC3658] |
| 2 | SHA256 | Mandatory | [RFC4509] |
| 3 | GOST R 34.11-94 | Deprecated | [RFC5933] |
| 4 | SHA384 | Optional | [RFC6605] |
The Reserved field is an 8-bit unsigned integer, which is always set to zero. This field is reserved for future work to support efficient incremental updates.
The Digest field is a variable-length sequence of octets containing the message digest. Section 3 describes how to calculate the digest for a zone. Section 4 describes how to use the digest to verify the contents of a zone.
The presentation format of the RDATA portion is as follows:
The Serial field MUST be represented as an unsigned decimal integer.
The Reserved field MUST be represented as an unsigned decimal integer set to zero.
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.
The following example shows a ZONEMD RR.
example.com. 86400 IN ZONEMD ( 2018031500 4 0 FEBE3D4CE2EC2FFA4BA9
9D46CD69D6D29711E552
17057BEE7EB1A7B641A4
7BA7FED2DD5B97AE499F
AFA4F22C6BD647DE )
Calculation of the zone digest REQUIRES the RRs in a zone to be processed 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.
For the purposes of calculating the zone digest, RRsets having the same owner name MUST be numerically ordered by their numeric RR TYPE.
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 6) will not be first in the digest calculation. The zone's NS RRset (type value 2) at the apex MUST be processed before the SOA RR.
In preparation for calculating the zone digest, any existing ZONEMD record 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, Reserved, 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.
It is RECOMMENDED that the TTL of the ZONEMD record match the TTL of the SOA.
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.
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. Therefore, the signer is not required to calculate a signature over the placeholder record at this step in the process, but it is harmless to do so.
The zone digest is calculated by concatenating the canonical on-the-wire form (without name compression) of all 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
When calculating the digest, the following inclusion/exclusion rules apply:
FOR DISCUSSION: Ambiguities about records that are in/out of zone. For example, see Jinmei message to dnsop 2018-06-01 and followups. BIND will load and AXFR data "occluded" by DNAME/NS.
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 or updated. Because the ZONEMD placeholder was added prior to signing, the zone will already have the appropriate denial-of-existence (NSEC, NSEC3) records.
Some implementations of incremental DNSSEC signing might update the zone's serial number for each resigning. However, to preserve the calculated digest, generation of the ZONEMD signature at this time MUST NOT also result in a change of the SOA serial number.
The recipient of a zone that has a message digest record can verify the zone by calculating the digest as follows:
This memo is published as an Experimental RFC. The purpose of the experimental period is to provide the community time to analyze and evaluate to the methods defined in this document, particularly with regard to the wide variety of DNS zones in use on the Internet.
Additionally, the ZONEMD record defined in this document includes a Reserved field. The authors have a particular future use in mind for this field, namely to support efficient digests in large, dynamic zones. We intend to conduct future experiments using Merkle trees of varying depth. The choice of tree depth can be encoded in this reserved field.
The duration of the experiment is expected to be no less than two years from the publication of this document. If the experiment is successful, it is expected that the findings of the experiment will result in an updated document for Standards Track approval.
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.
The ZONEMD Digest Type field has the same values as the DS RR Digest Type field, but with independent implementation status. Therefore, this document expects IANA will create a new "ZONEMD Digest Types" registry.
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 4 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 4 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.
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.
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).
This specification has no impacts on user privacy.
The authors wish to thank David Blacka, Scott Hollenbeck, and Rick Wilhelm for providing feedback on early drafts of this document. Additionally, they thank Joe Abley, Mark Andrews, Olafur Gudmundsson, Paul Hoffman, Evan Hunt, Shumon Huque, Tatuya Jinmei, Burt Kaliski, Shane Kerr, Matt Larson, John Levine, Ed Lewis, Mukund Sivaraman, Petr Spacek, Ondrej Sury, Florian Weimer, Tim Wicinksi, Paul Wouters, and other members of the dnsop working group for their input.
The authors have an open source implementation in C, using the ldns library [ldns-zone-digest]. This implementation is able to perform the following functions:
This implementation does not:
Shane Kerr wrote an implementation of this specification during the IETF 102 hackathon [ZoneDigestHackathon]. This implementation is in Python and is able to perform the following functions:
This implementation does not:
RFC Editor: Please remove this section.
This section lists substantial changes to the document as it is being worked on.
From -00 to -01:
From -01 to -02:
From -02 to -03: