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.
Wellington Standards Track [Page 9]
