Message Digest for DNS Zones

In the DNS, a zone is the collection of authoritative resource records (RRs) sharing a common origin (), 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 . Sometimes zones are distributed outside of the DNS, with such protocols as FTP, HTTP, rsync, and so on. While zone files are self-contained, currently there is no way to verify the authenticity of a stand-alone zone file without relying on an external checksum or signature. 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. DNS transaction signatures (TSIG ) uses a message digest to protect individual query and response messages. TSIG is generally used to authenticate and validate UPDATE AXFR , and IXFR 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, allowing anything to verify the zone file as a whole, no matter how it is transmitted. DNSSEC provides three strong security guarantees relevant to this protocol: whether or not to expect DNSSEC records in the zone, whether or not to expect a &RRNAME; record in a signed zone, and whether or not the &RRNAME; 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. 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 completeness 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 with any DNS zone. This specification is OPTIONAL to implement by both publishers and consumers of zone file data.

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 .

This section describes the &RRNAME; Resource Record, including its fields, wire format, and presentation format. The Type value for the &RRNAME; RR is TBD. The &RRNAME; 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 &RRNAME; RR. Having exactly one &RRNAME; record per zone simplifies this protocol and eliminates confusion around downgrade attacks, at the expense of algorithm agility.

The &RRNAME; RDATA wire format is encoded as follows:

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 ( 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 &RRNAME; meaningful. Without the serial number, a stand-alone &RRNAME; digest has no association to any particular zone file. If there is agreement that &RRNAME; responses are not useful, this field could be removed. See also the end of Security Considerations.

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 . The status of &RRNAME; 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 . At the time of this writing the following digest types are defined: Value Description Status Reference 1 SHA1 Mandatory 2 SHA256 Mandatory 3 GOST R 34.11-94 Optional 4 SHA384 Optional

The Digest field is a variable-length sequence of octets containing the message digest. describes how to calculate the digest for a zone. 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 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 &RRNAME; RR.

example.com. 86400 IN &RRNAME; ( 2018031500 4 FEBE3D4CE2EC2FFA4BA9 9D46CD69D6D29711E552 17057BEE7EB1A7B641A4 7BA7FED2DD5B97AE499F AFA4F22C6BD647DE )

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 ), and canonical ordering for RRs within an RRset (Section 6.3 of ). It also adopts DNSSEC's canonical RR form (Section 6.2 of ). 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 ( 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.

In preparation for calculating the zone digest, any existing &RRNAME; records MUST first be deleted from the zone. Prior to calculation of the digest, and prior to signing with DNSSEC, a placeholder &RRNAME; 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.

Following addition of the placeholder record, the zone MAY be signed with DNSSEC. Note that when the digest calculation is complete, and the &RRNAME; 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 of RRs in the zone, in the order described above, subject to the inclusion/exclusion rules described below, and then applying the digest algorithm:

When calculating the digest, the following inclusion/exclusion rules apply: More than one SOA MUST NOT be included. The placeholder &RRNAME; RR MUST be included. If the zone is signed, DNSSEC RRs MUST be included, except: The RRSIG covering &RRNAME; MUST NOT be included.

Once the zone digest has been calculated, its value is then copied to the Digest field of the &RRNAME; record. If the zone is signed with DNSSEC, the appropriate RRSIG records covering the &RRNAME; record MUST then be added. Because the &RRNAME; placeholder was added prior to signing, the zone will already have the appropriate denial-of-existence (NSEC, NSEC3) records.

The recipient of a zone that has a message digest record can verify the zone by calculating the digest as follows: 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 below. For zones that are provably signed, the existence of the &RRNAME; record MUST be verified. If the &RRNAME; record provably does not exist, digest verification cannot be done. If the &RRNAME; record does provably exist, but is not found in the zone, digest verification MUST NOT be considered successful. For zones that are provably signed, the SOA RR and &RRNAME; RR(set) MUST have valid signatures, chaining up to a trust anchor. If DNSSEC validation of the SOA or &RRNAME; records fails, digest verification MUST NOT be considered successful. The SOA Serial field MUST exactly match the &RRNAME; Serial field. If the fields to not match, digest verification MUST NOT be considered successful. The &RRNAME; 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. The zone digest is calculated using the algorithm described in . Note in particular that digested &RRNAME; RRs MUST be placeholders and their RRSIGs are not included in the digest. 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. If the zone is to be served and transferred, the original (not placeholder) &RRNAME; RRs MUST be sent to recipients so that downstream clients can verify the zone.

This document uses a new DNS RR type, &RRNAME;, whose value TBD has been allocated by IANA from the "Resource Record (RR) TYPEs" subregistry of the "Domain Name System (DNS) Parameters" registry.

The &RRNAME; Digest Type field has the same semantics as the DS RR Digest Type field. Thus, it does not add new IANA protocol registry requirements.

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 RECOMMENDS that the verifier determine whether or not to expect DNSSEC signatures for the zone in step . The attacker might perform a downgrade attack by removing the &RRNAME; record. This is why REQUIRES that the verifier checks DNSSEC denial-of-existence proofs in step . The attacker might alter the Digest Type or Digest fields of the &RRNAME; record. Such modifications are detectable only with DNSSEC validation.

Nothing in this specification prevents clients from making, and servers from responding to, &RRNAME; queries. One might consider how well &RRNAME; responses could be used in a distributed denial-of-service amplification attack. The &RRNAME; RR is moderately sized, much like the DS RR. A single &RRNAME; 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 &RRNAME; 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 &RRNAME; queries with the REFUSED RCODE without loss of functionality. Note that refusal would prevent ensuring that a zone-walk is complete.

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.

The authors are currently working on an implementation in C, using the ldns library . This implementation is able to perform the following functions: Read input zone file, output zone file with &RRNAME; placeholder. Compute zone digest over signed zone file and update &RRNAME; record. Re-compute DNSSEC signature over &RRNAME; record. 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.

RFC Editor: Please remove this section. This section lists substantial changes to the document as it is being worked on. From -00 to -01: Removed requirement to sort by RR CLASS. Added Kumari and Hardaker as coauthors. Added Change Log section. Minor clarifications and grammatical edits.