DNSOP G. Guette
Internet-Draft IRISA/INRIA Rennes
Expires: August 8, 2004 O. Courtay
ENST-Bretagne
February 8, 2004
Requirements for Automated Key Rollover in DNSsec
draft-ietf-dnsop-key-rollover-requirements-00
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Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document describes problems that appear during an automated
rollover and gives the requirements for the design of communication
between parent zone and child zone in an automated rollover process.
This document is essentially about key rollover, the rollover of
one other Resource Record present at delegation point (NS RR) is
also discussed.
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1. Introduction
The DNS security extensions (DNSsec) [1] uses public-key cryptography
and digital signatures. It stores the public keys in KEY Resource
Records (RRs). Because old keys and frequently used keys are
vulnerable, they must be changed periodically. In DNSsec this is the
case for Zone Signing Keys (ZSKs) and Key Signing Keys (KSKs) [2, 4].
Automation of key rollover process is necessary for large zones
because inside a large zone, there are too many changes to handle for
a single administrator.
Let us consider for example a zone with one million child zones among
which only 10% of secured child zones. If the child zones change their
keys once a year on average, that implies 300 changes per day for the
parent zone. All these changes are hard to manage manually.
Automated rollover is optional and resulting from an agreement
between the administrator of the parent zone and the administrator of
the child zone. Of course, key rollover can also be done manually by
administrators.
This document describes the requirements for the design of messages
of automated key rollover process.
2. The Key Rollover Process
Key rollover consists in replacing the DNSsec keys used to sign
resource records in a given DNS zone file. There are two types of
rollover, ZSK rollover and KSK rollover.
In ZSK rollover, all changes are local to the zone that changes its
key: there is no need to contact other zones (e.g. parent zone) to
propagate the performed changes because this type of key have no
associated DS records in the parent zone.
In KSK rollover, new DS RR(s) MUST be created and stored in the
parent zone. In consequence, the child zone MUST contact its parent
zone and notify it about the KSK change(s).
Manual key rollover exists and works [3]. The key rollover is built
from two parts of different nature:
- An algorithm that generates new keys. It could be local to the
zone
- The interaction between parent and child zone
In this document we focus on the interaction between parent and
child zone servers.
One example of manual key rollover is:
Child zone creates a new KSK, waiting for the creation of the DS
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record in its parent zone and then child zone deletes old key.
In manual rollover, communications are managed by the zone
administrators and the security of these communications is out of
scope of DNSsec.
Automated key rollover MUST use a secure communication between parent
and child zone. In this document we concentrate our efforts on
defining interactions between entities present in key rollover
process that are not explicitly defined in manual key rollover
method.
3. Basic Requirements
The main constraint to respect during a key rollover is that the
chain of trust MUST be preserved. Even if a resolver retrieve some RRs
from recursive name server. Every RR MUST be verifiable at any time,
every message exchanged during rollover MUST be authenticated and
data integrity MUST be guaranteed.
Two entities are present during a KSK rollover: the child zone and
its parent zone. These zones are generally managed by different
administrators. These administrators MUST agree on some parameters
like availability of automated rollover, the maximum delay between
notification of changes in the child zone and the resigning of the
parent zone. The child zone needs to know this delay to schedule its
changes.
During an automated rollover process, data are transmitted between
the primary name server of the parent and the the primary name server
of the child zone.
The reason is that the IP address of the primary name server is easy
to obtain.
Other solutions based on machine dedicated to the rollover are not
suitable solutions because of the difficulty to obtain the IP
addresses of the dedicated machine in an automated manner.
4. Messages authentication and information exchanged
Every exchanged message MUST be authenticated and the authentication
tool MUST be a DNSsec tool such as TSIG [5], SIG(0) [6] or DNSsec
request with verifiable SIG records.
Once the changes related to a KSK are made in a child zone, this zone
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MUST notify its parent zone in order to create the new DS RR and
store this DS RR in parent zone file.
The parent zone MUST receive all the child Keys that needs the
creation of an associated DS RRs in the parent zone.
Some errors could occur during transmission between child zone and
parent zone. Key rollover solution MUST be fault tolerant, i.e. at
any time the rollover MUST be in a consistent state and all RRs MUST
be verifiable, even if an error occurs. That is to say that it MUST
remains a valid chain of trust.
5. Emergency Rollover
A key of a zone might be compromised and this key MUST be changed as
soon as possible. Fast changes could break the chain of trust. The
part of DNS tree having this zone as apex can become unverifiable,
but the break of the chain of trust is necessary if we want to no one
can use the compromised key to spoof DNS data.
Parent zone behavior after an emergency rollover in one of its child
zone is an open discussion.
Should we define:
- an EMERGENCY flag. When a child zone does an emergency KSK change,
it uses the EMERGENCY flag to notify its parents that the chain of
trust is broken and will stay broken until right DS creation and a
parent zone resigning.
- a maximum time delay after next parent zone resigning, we ensure
that after this delay the parent zone is resigned and the right DS
is created.
- that no pre-defined behavior for the parent zone is needed
6. Other Resource Record concerned by automatic rollover
NS records are also present at delegation point, so when the child
zone changes some NS records, the corresponding records at
delegation point in parent zone MUST be updated. NS records are
concerned by rollover and this rollover could be automated too. In
this case, when the child zone notifies its parent zone that some NS
records have been changed, the parent zone MUST verify that these NS
records are present in child zone before doing any changes in its own
zone file. This allow to avoid inconsistency between NS records at
delegation point and NS records present in the child zone.
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7. Security consideration
This document describes requirements to design an automated key
rollover in DNSsec based on DNSsec security. In the same way the, as
plain DNSsec, the automatic key rollover contains no mechanism
protecting against denial of service (DoS) resistant. The security
level obtain after an automatic key rollover, is the security level
provided by DNSsec.
8. Acknowledgments
The authors want to acknowledge Mohsen Souissi, Bernard Cousin,
Bertrand Leonard and members of IDsA project for their contribution
to this document.
Normative references
[1] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[2] Gudmundsson, O., "Delegation Signer Resource Record",
draft-ietf-dnsext-delegation-signer-15 (work in progress),
June 2003.
[3] Kolkman, O. and Gieben, R., "DNSSEC key operations",
draft-ietf-dnsext-operational-practices (work in progress),
June 2003.
[4] Kolkman, O. and Schlyter, J., "KEY RR Secure Entry Point Flag"
draft-ietf-dnsext-keyrr-key-signing-flag-10 (work in progress),
September 2003.
[5] Vixie, P., Gudmundsson, O., Eastlake, D., and Wellington, B.,
"Secret Key Transaction Authentication for DNS (TSIG)", RFC
2845, May 2000.
[6] Eastlake, D., "DNS Request and Transaction Signatures (SIG(0)s)",
RFC 2931, September 2000.
[7] Eastlake, D.,"DNS Security Operational Considerations", RFC
2541, March 1999.
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Author's Addresses
Gilles Guette
IRISA/INRIA Rennes
Campus Universitaire de Beaulieu
35042 Rennes France
Phone : (33) 02 99 84 71 32
Fax : (33) 02 99 84 25 29
E-mail : gguette@irisa.fr
Olivier Courtay
ENST-Bretagne
2, rue de la chtaigneraie
35512 Cesson Cvign CEDEX France
Phone : (33) 02 99 84 71 31
Fax : (33) 02 99 84 25 29
olivier.courtay@enst-bretagne.fr
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