RFC3008 - Domain Name System Security (DNSSEC) Signing Authority

Network Working Group B. Wellington

Request for Comments: 3008 Nominum

Updates: 2535 November 2000

Category: Standards Track

Domain Name System Security (DNSSEC) Signing Authority

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Abstract

This document proposes a revised model of Domain Name System Security

(DNSSEC) Signing Authority. The revised model is designed to clarify

earlier documents and add additional restrictions to simplify the

secure resolution process. Specifically, this affects the

authorization of keys to sign sets of records.

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 [RFC2119].

1 - IntrodUCtion

This document defines additional restrictions on DNSSEC signatures

(SIG) records relating to their authority to sign associated data.

The intent is to establish a standard policy followed by a secure

resolver; this policy can be augmented by local rules. This builds

upon [RFC2535], updating section 2.3.6 of that document.

The most significant change is that in a secure zone, zone data is

required to be signed by the zone key.

Familiarity with the DNS system [RFC1034, RFC1035] and the DNS

security extensions [RFC2535] is assumed.

2 - The SIG Record

A SIG record is normally associated with an RRset, and "covers" (that

is, demonstrates the authenticity and integrity of) the RRset. This

is referred to as a "data SIG". Note that there can be multiple SIG

records covering an RRset, and the same validation process should be

repeated for each of them. Some data SIGs are considered "material",

that is, relevant to a DNSSEC capable resolver, and some are

"immaterial" or "extra-DNSSEC", as they are not relevant to DNSSEC

validation. Immaterial SIGs may have application defined roles. SIG

records may exist which are not bound to any RRset; these are also

considered immaterial. The validation process determines which SIGs

are material; once a SIG is shown to be immaterial, no other

validation is necessary.

SIGs may also be used for transaction security. In this case, a SIG

record with a type covered field of 0 is attached to a message, and

is used to protect message integrity. This is referred to as a

SIG(0) [RFC2535, RFC2931].

The following sections define requirements for all of the fields of a

SIG record. These requirements MUST be met in order for a DNSSEC

capable resolver to process this signature. If any of these

requirements are not met, the SIG cannot be further processed.

Additionally, once a KEY has been identified as having generated this

SIG, there are requirements that it MUST meet.

2.1 - Type Covered

For a data SIG, the type covered MUST be the same as the type of data

in the associated RRset. For a SIG(0), the type covered MUST be 0.

2.2 - Algorithm Number

The algorithm specified in a SIG MUST be recognized by the client,

and it MUST be an algorithm that has a defined SIG rdata format.

2.3 - Labels

The labels count MUST be less than or equal to the number of labels

in the SIG owner name, as specified in [RFC2535, section 4.1.3].

2.4 - Original TTL

The original TTL MUST be greater than or equal to the TTL of the SIG

record itself, since the TTL cannot be increased by intermediate

servers. This field can be ignored for SIG(0) records.

2.5 - Signature EXPiration and Inception

The current time at the time of validation MUST lie within the

validity period bounded by the inception and expiration times.

2.6 - Key Tag

There are no restrictions on the Key Tag field, although it is

possible that future algorithms will impose constraints.

2.7 - Signer's Name

The signer's name field of a data SIG MUST contain the name of the

zone to which the data and signature belong. The combination of

signer's name, key tag, and algorithm MUST identify a zone key if the

SIG is to be considered material. The only exception that the

signer's name field in a SIG KEY at a zone apex SHOULD contain the

parent zone's name, unless the KEY set is self-signed. This document

defines a standard policy for DNSSEC validation; local policy may

override the standard policy.

There are no restrictions on the signer field of a SIG(0) record.

The combination of signer's name, key tag, and algorithm MUST

identify a key if this SIG(0) is to be processed.

2.8 - Signature

There are no restrictions on the signature field. The signature will

be verified at some point, but does not need to be examined prior to

verification unless a future algorithm imposes constraints.

3 - The Signing KEY Record

Once a signature has been examined and its fields validated (but

before the signature has been verified), the resolver attempts to

locate a KEY that matches the signer name, key tag, and algorithm

fields in the SIG. If one is not found, the SIG cannot be verified

and is considered immaterial. If KEYs are found, several fields of

the KEY record MUST have specific values if the SIG is to be

considered material and authorized. If there are multiple KEYs, the

following checks are performed on all of them, as there is no way to

determine which one generated the signature until the verification is

performed.

3.1 - Type Flags

The signing KEY record MUST have a flags value of 00 or 01

(authentication allowed, confidentiality optional) [RFC2535, 3.1.2].

A DNSSEC resolver MUST only trust signatures generated by keys that

are permitted to authenticate data.

3.2 - Name Flags

The interpretation of this field is considerably different for data

SIGs and SIG(0) records.

3.2.1 - Data SIG

If the SIG record covers an RRset, the name type of the associated

KEY MUST be 01 (zone) [RFC2535, 3.1.2]. This updates RFC2535,

section 2.3.6. The DNSSEC validation process performed by a resolver

MUST ignore all keys that are not zone keys unless local policy

dictates otherwise.

The primary reason that RFC2535 allows host and user keys to

generate material DNSSEC signatures is to allow dynamic update

without online zone keys; that is, avoid storing private keys in an

online server. The desire to avoid online signing keys cannot be

achieved, though, because they are necessary to sign NXT and SOA sets

[RFC3007]. These online zone keys can sign any incoming data.

Removing the goal of having no online keys removes the reason to

allow host and user keys to generate material signatures.

Limiting material signatures to zone keys simplifies the validation

process. The length of the verification chain is bounded by the

name's label depth. The authority of a key is clearly defined; a

resolver does not need to make a potentially complicated decision to

determine whether a key has the proper authority to sign data.

Finally, there is no additional flexibility granted by allowing

host/user key generated material signatures. As long as users and

hosts have the ability to authenticate update requests to the primary

zone server, signatures by zone keys are sufficient to protect the

integrity of the data to the world at large.

3.2.2 - SIG(0)

If the SIG record is a SIG(0) protecting a message, the name type of

the associated KEY SHOULD be 00 (user) or 10 (host/entity).

Transactions are initiated by a host or user, not a zone, so zone

keys SHOULD not generate SIG(0) records.

A client is either explicitly executed by a user or on behalf of a

host, therefore the name type of a SIG(0) generated by a client

SHOULD be either user or host. A nameserver is associated with a

host, and its use of SIG(0) is not associated with a particular zone,

so the name type of a SIG(0) generated by a nameserver SHOULD be

host.

3.3 - Signatory Flags

This document does not assign any values to the signatory field, nor

require any values to be present.

3.4 - Protocol

The signing KEY record MUST have a protocol value of 3 (DNSSEC) or

255 (ALL). If a key is not specified for use with DNSSEC, a DNSSEC

resolver MUST NOT trust any signature that it generates.

3.5 - Algorithm Number

The algorithm field MUST be identical to that of the generated SIG

record, and MUST meet all requirements for an algorithm value in a

SIG record.

4 - Security Considerations

This document defines a standard baseline for a DNSSEC capable

resolver. This is necessary for a thorough security analysis of

DNSSEC, if one is to be done.

Specifically, this document places additional restrictions on SIG

records that a resolver must validate before the signature can be

considered worthy of DNSSEC trust. This simplifies the protocol,

making it more robust and able to withstand scrutiny by the security

community.

5 - Acknowledgements

The author would like to thank the following people for review and

informative comments (in alphabetical order):

Olafur Gudmundsson

Ed Lewis

6 - References

[RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities",

STD 13, RFC1034, November 1987.

[RFC1035] Mockapetris, P., "Domain Names - Implementation and

Specification", STD 13, RFC1035, November 1987.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate

Requirement Levels", BCP 14, RFC2119, March 1997.

[RFC2136] Vixie (Ed.), P., Thomson, S., Rekhter, Y. and J. Bound,

"Dynamic Updates in the Domain Name System", RFC2136,

April 1997.

[RFC2535] Eastlake, D., "Domain Name System Security Extensions",

RFC2535, March 1999.

[RFC2931] Eastlake, D., "DNS Request and Transaction Signatures

(SIG(0)s )", RFC2931, September 2000.

[RFC3007] Wellington, B., "Simple Secure Domain Name System

(DNS) Dynamic Update", RFC3007, November 2000.

7 - Author's Address

Brian Wellington

Nominum, Inc.

950 Charter Street

Redwood City, CA 94063

Phone: +1 650 381 6022

EMail:

Brian.Wellington@nominum.com

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