Network Working Group B. Wellington

Request for Comments: 3007 Nominum

Updates: 2535, 2136 November 2000

Obsoletes: 2137

Category: Standards Track

Secure Domain Name System (DNS) Dynamic Update

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.

Copyright Notice

Copyright (C) The Internet Society (2000). All Rights Reserved.

Abstract

This document proposes a method for performing secure Domain Name

System (DNS) dynamic updates. The method described here is intended

to be flexible and useful while requiring as few changes to the

protocol as possible. The authentication of the dynamic update

message is separate from later DNSSEC validation of the data. Secure

communication based on authenticated requests and transactions is

used to provide authorization.

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 RFC 2119 [RFC2119].

1 - Introduction

This document defines a means to secure dynamic updates of the Domain

Name System (DNS), allowing only authorized sources to make changes

to a zone's contents. The existing unsecured dynamic update

operations form the basis for this work.

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

[RFC2136] is helpful and is assumed by this document. In addition,

knowledge of DNS security extensions [RFC2535], SIG(0) transaction

security [RFC2535, RFC2931], and TSIG transaction security [RFC2845]

is recommended.

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This document updates portions of RFC 2535, in particular section

3.1.2, and RFC 2136. This document obsoletes RFC 2137, an alternate

proposal for secure dynamic update, due to implementation experience.

1.1 - Overview of DNS Dynamic Update

DNS dynamic update defines a new DNS opcode and a new interpretation

of the DNS message if that opcode is used. An update can specify

insertions or deletions of data, along with prerequisites necessary

for the updates to occur. All tests and changes for a DNS update

request are restricted to a single zone, and are performed at the

primary server for the zone. The primary server for a dynamic zone

must increment the zone SOA serial number when an update occurs or

before the next retrieval of the SOA.

1.2 - Overview of DNS Transaction Security

Exchanges of DNS messages which include TSIG [RFC2845] or SIG(0)

[RFC2535, RFC2931] records allow two DNS entities to authenticate DNS

requests and responses sent between them. A TSIG MAC (message

authentication code) is derived from a shared secret, and a SIG(0) is

generated from a private key whose public counterpart is stored in

DNS. In both cases, a record containing the message signature/MAC is

included as the final resource record in a DNS message. Keyed

hashes, used in TSIG, are inexpensive to calculate and verify.

Public key encryption, as used in SIG(0), is more scalable as the

public keys are stored in DNS.

1.3 - Comparison of data authentication and message authentication

Message based authentication, using TSIG or SIG(0), provides

protection for the entire message with a single signing and single

verification which, in the case of TSIG, is a relatively inexpensive

MAC creation and check. For update requests, this signature can

establish, based on policy or key negotiation, the authority to make

the request.

DNSSEC SIG records can be used to protect the integrity of individual

RRs or RRsets in a DNS message with the authority of the zone owner.

However, this cannot sufficiently protect the dynamic update request.

Using SIG records to secure RRsets in an update request is

incompatible with the design of update, as described below, and would

in any case require multiple expensive public key signatures and

verifications.

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SIG records do not cover the message header, which includes record

counts. Therefore, it is possible to maliciously insert or remove

RRsets in an update request without causing a verification failure.

If SIG records were used to protect the prerequisite section, it

would be impossible to determine whether the SIGs themselves were a

prerequisite or simply used for validation.

In the update section of an update request, signing requests to add

an RRset is straightforward, and this signature could be permanently

used to protect the data, as specified in [RFC2535]. However, if an

RRset is deleted, there is no data for a SIG to cover.

1.4 - Data and message signatures

As specified in [RFC3008], the DNSSEC validation process performed by

a resolver MUST NOT process any non-zone keys unless local policy

dictates otherwise. When performing secure dynamic update, all zone

data modified in a signed zone MUST be signed by a relevant zone key.

This completely disassociates authentication of an update request

from authentication of the data itself.

The primary usefulness of host and user keys, with respect to DNSSEC,

is to authenticate messages, including dynamic updates. Thus, host

and user keys MAY be used to generate SIG(0) records to authenticate

updates and MAY be used in the TKEY [RFC2930] process to generate

TSIG shared secrets. In both cases, no SIG records generated by

non-zone keys will be used in a DNSSEC validation process unless

local policy dictates.

Authentication of data, once it is present in DNS, only involves

DNSSEC zone keys and signatures generated by them.

1.5 - Signatory strength

[RFC2535, section 3.1.2] defines the signatory field of a key as the

final 4 bits of the flags field, but does not define its value. This

proposal leaves this field undefined. Updating [RFC2535], this field

SHOULD be set to 0 in KEY records, and MUST be ignored.

2 - Authentication

TSIG or SIG(0) records MUST be included in all secure dynamic update

messages. This allows the server to verifiably determine the

originator of a message. If the message contains authentication in

the form of a SIG(0), the identity of the sender (that is, the

principal) is the owner of the KEY RR that generated the SIG(0). If

the message contains a TSIG generated by a statically configured

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shared secret, the principal is the same as or derived from the

shared secret name. If the message contains a TSIG generated by a

dynamically configured shared secret, the principal is the same as

the one that authenticated the TKEY process; if the TKEY process was

unauthenticated, no information is known about the principal, and the

associated TSIG shared secret MUST NOT be used for secure dynamic

update.

SIG(0) signatures SHOULD NOT be generated by zone keys, since

transactions are initiated by a host or user, not a zone.

DNSSEC SIG records (other than SIG(0)) MAY be included in an update

message, but MUST NOT be used to authenticate the update request.

If an update fails because it is signed with an unauthorized key, the

server MUST indicate failure by returning a message with RCODE

REFUSED. Other TSIG, SIG(0), or dynamic update errors are returned

as specified in the appropriate protocol description.

3 - Policy

All policy is configured by the zone administrator and enforced by

the zone's primary name server. Policy dictates the authorized

actions that an authenticated principal can take. Policy checks are

based on the principal and the desired action, where the principal is

derived from the message signing key and applied to dynamic update

messages signed with that key.

The server's policy defines criteria which determine if the key used

to sign the update is permitted to perform the requested updates. By

default, a principal MUST NOT be permitted to make any changes to

zone data; any permissions MUST be enabled though configuration.

The policy is fully implemented in the primary zone server's

configuration for several reasons. This removes limitations imposed

by encoding policy into a fixed number of bits (such as the KEY RR's

signatory field). Policy is only relevant in the server applying it,

so there is no reason to expose it. Finally, a change in policy or a

new type of policy should not affect the DNS protocol or data format,

and should not cause interoperability failures.

3.1 - Standard policies

Implementations SHOULD allow access control policies to use the

principal as an authorization token, and MAY also allow policies to

grant permission to a signed message regardless of principal.

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A common practice would be to restrict the permissions of a principal

by domain name. That is, a principal could be permitted to add,

delete, or modify entries corresponding to one or more domain names.

Implementations SHOULD allow per-name access control, and SHOULD

provide a concise representation of the principal's own name, its

subdomains, and all names in the zone.

Additionally, a server SHOULD allow restricting updates by RR type,

so that a principal could add, delete, or modify specific record

types at certain names. Implementations SHOULD allow per-type access

control, and SHOULD provide concise representations of all types and

all "user" types, where a user type is defined as one that does not

affect the operation of DNS itself.

3.1.1 - User types

User types include all data types except SOA, NS, SIG, and NXT. SOA

and NS records SHOULD NOT be modified by normal users, since these

types create or modify delegation points. The addition of SIG

records can lead to attacks resulting in additional workload for

resolvers, and the deletion of SIG records could lead to extra work

for the server if the zone SIG was deleted. Note that these records

are not forbidden, but not recommended for normal users.

NXT records MUST NOT be created, modified, or deleted by dynamic

update, as their update may cause instability in the protocol. This

is an update to RFC 2136.

Issues concerning updates of KEY records are discussed in the

Security Considerations section.

3.2 - Additional policies

Users are free to implement any policies. Policies may be as

specific or general as desired, and as complex as desired. They may

depend on the principal or any other characteristics of the signed

message.

4 - Interaction with DNSSEC

Although this protocol does not change the way updates to secure

zones are processed, there are a number of issues that should be

clarified.

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4.1 - Adding SIGs

An authorized update request MAY include SIG records with each RRset.

Since SIG records (except SIG(0) records) MUST NOT be used for

authentication of the update message, they are not required.

If a principal is authorized to update SIG records and there are SIG

records in the update, the SIG records are added without

verification. The server MAY examine SIG records and drop SIGs with

a temporal validity period in the past.

4.2 - Deleting SIGs

If a principal is authorized to update SIG records and the update

specifies the deletion of SIG records, the server MAY choose to

override the authority and refuse the update. For example, the

server may allow all SIG records not generated by a zone key to be

deleted.

4.3 - Non-explicit updates to SIGs

If the updated zone is secured, the RRset affected by an update

operation MUST, at the completion of the update, be signed in

accordance with the zone's signing policy. This will usually require

one or more SIG records to be generated by one or more zone keys

whose private components MUST be online [RFC3008].

When the contents of an RRset are updated, the server MAY delete all

associated SIG records, since they will no longer be valid.

4.4 - Effects on the zone

If any changes are made, the server MUST, if necessary, generate a

new SOA record and new NXT records, and sign these with the

appropriate zone keys. Changes to NXT records by secure dynamic

update are explicitly forbidden. SOA updates are allowed, since the

maintenance of SOA parameters is outside of the scope of the DNS

protocol.

5 - Security Considerations

This document requires that a zone key and possibly other

cryptographic secret material be held in an on-line, network-

connected host, most likely a name server. This material is at the

mercy of host security to remain a secret. Exposing this secret puts

DNS data at risk of masquerade attacks. The data at risk is that in

both zones served by the machine and delegated from this machine.

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Allowing updates of KEY records may lead to undesirable results,

since a principal may be allowed to insert a public key without

holding the private key, and possibly masquerade as the key owner.

6 - Acknowledgements

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

informative comments (in alphabetical order):

Harald Alvestrand

Donald Eastlake

Olafur Gudmundsson

Andreas Gustafsson

Bob Halley

Stuart Kwan

Ed Lewis

7 - References

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

STD 13, RFC 1034, November 1987.

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

Specification", STD 13, RFC 1035, November 1987.

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

"Dynamic Updates in the Domain Name System", RFC 2136,

April 1997.

[RFC2137] Eastlake, D., "Secure Domain Name System Dynamic Update",

RFC 2137, April 1997.

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

RFC 2535, March 1999.

[RFC2845] Vixie, P., Gudmundsson, O., Eastlake, D. and B.

Wellington, "Secret Key Transaction Signatures for DNS

(TSIG)", RFC 2845, May 2000.

[RFC2930] Eastlake, D., "Secret Key Establishment for DNS (TKEY

RR)", RFC 2930, September 2000.

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

(SIG(0)s)", RFC 2931, September 2000.

[RFC3008] Wellington, B., "Domain Name System Security (DNSSEC)

Signing Authority", RFC 3008, November 2000.

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8 - 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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9. Full Copyright Statement

Copyright (C) The Internet Society (2000). All Rights Reserved.

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Acknowledgement

Funding for the RFC Editor function is currently provided by the

Internet Society.

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