INTERNET-DRAFT DNSIND Key Rollover
UPDATES RFC 1996 April 1999
Expires October 1999
Domain Name System (DNS) Security Key Rollover
------ ---- ------ ----- -------- --- --------
Donald E. Eastlake 3rd, Mark Andrews
Status of This Document
This draft, file name draft-ietf-dnsind-rollover-00.txt, is intended
to be become a Proposed Standard RFC. Distribution of this document
is unlimited. Comments should be sent to the DNS working group
mailing list <namedroppers@internic.net> or to the authors.
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months. Internet-Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet-
Drafts as reference material or to cite them other than as a
``working draft'' or ``work in progress.''
The list of current Internet-Drafts can be accessed at
The list of Internet-Draft Shadow Directories can be accessed at
Abstract
Deployment of Domain Name System (DNS) security with good cryptologic
practice will involve large volumes of key rollover traffic. A
standard format and protocol for such messages will be necessary for
this to be practical and is specified herein.
[Note: this draft has been moved to dnsind from dnssec as part of the
ongoing combination of these working groups. It would have been
draft-ietf-dnssec-rollover-01.txt otherwise.]
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Table of Contents
Status of This Document....................................1
Abstract...................................................1
Table of Contents..........................................2
1. Introduction............................................3
2. Key Rollover Scenario...................................3
3. Rollover Operation......................................5
3.1 Rollover to Parent.....................................5
3.2 Rollover to Children...................................6
4. Secure Zone Cuts and Joinders...........................7
5. Security Considerations.................................8
6. IANA Considerations.....................................9
References................................................10
Authors Address...........................................10
Expiration and File Name..................................11
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1. Introduction
The Domain Name System (DNS) [RFC 1034, 1035] is the global
hierarchical replicated distributed database system for Internet
addressing, mail proxy, and other information. The DNS has been
extended to include digital signatures and cryptographic keys as
described in [RFC 2535].
The principle security service provided for DNS data is data origin
authentication. The owner of each zone signs the data in that zone
with a private key known only to the zone owner. Anyone that knows
the corresponding public key can then authenticate that zone data is
from the zone owner. To avoid having to preconfigure resolvers with
all zone's public keys, keys are stored in the DNS with each zone's
key signed by its parent (if the parent is secure).
To obtain high levels of security, keys must be periodically changed,
or "rolled over". The longer a private key is used, the more likely
it is to be compromised due to cryptanalysis, accident, or treachery
[RFC 2541].
In a widely deployed DNS security system, the volume of update
traffic will be large. Just consider the .com zone. If only 10% of
its children are secure and change their keys only once a year, you
are talking about hundreds of thousands of new child public keys that
must be securely sent to the .com manager to sign and return with
their new parent signature. And when .com rolls over its private
key, it will needs to send hundred of thousands of new signatures on
the existing child public keys to the child zones.
It will be impractical to handle such update volumes manually on a
case by case basis. The bulk of such key rollover updates must be
automated.
The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", and "MAY"
in this document are to be interpreted as described in [RFC 2119].
2. Key Rollover Scenario
Although DNSSEC provides for the storage of other keys in the DNS for
other purposes, DNSSEC zone keys are included solely for the purpose
of being retrieved to authenticate DNSSEC signatures. Thus, when a
zone key is being rolled over, the old public key should be left in
the zone, along with the addition of the new public key, for as long
as it will reasonably be needed to authenticate old signatures that
have been cached or are held by applications. Similarly, old parent
SIGs should be retained for a short time after a parent KEY(s) roll
over and new parent SIGs have been installed.
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If DNSSEC were universally deployed and all DNS server's clocks were
synchronized and zone transfers were instantaneous etc., it might be
simultaneous expiration of SIGs everywhere in the DNS. But these
assumptions do not hold. Security aware DNS servers decrease the TTL
of secure RRs served as the expiration of their authenticating SIG(s)
approaches but some dithered fudge must generally be left due to
clock skew, RR retention by applications, and the like. Retaining
old KEYs for a while after rolling over to new KEYs will be necessary
in practical cases.
Assume a middle zone with a secure parent and a secure child wishes
to role over its KEY RRset. This RRset would probably be one KEY RR
per crypto algorithm used to secure the zone, but for this scenario,
we will simply assume it is one KEY RR. The old KEY RR and two SIG
RRs will exist at the apex of the middle zone. (These RRs may also
exist at the leaf node for this zone in its parent if the parent
chooses to store them there.) The contents of the middle zone and the
zone KEY RRs of its secure child will have SIGs under the old key.
The middle zone owner needs to communicate with its parent to obtain
a new parental signature covering both the old and new KEY RRs and
covering just the new KEY RR. The signature on both is needed so the
old KEY can be retain for the period it might be needed to
authenticate old SIGs. The middle zone would probably want to obtain
these in advance so that it can install them at the right time along
with its new SIG RRs covering the content of its zone. Finally, it
needs to give new SIG RRs to its child that cover its KEY RRs so it
must signal its children to ask for such SIG RRs.
BEFORE ROLLOVER SHORTLY AFTER AFTER ROLLOVER
m.p.x SIG(KEY) M2
c.m.p.x SIG(KEY) C1
p = parent, m = middle, c = child, GP = grandparent key
P* = parent key, M* = middle zone key, C* = child key
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3. Rollover Operation
Rollover operations use a DNS request syntactically identical to the
UPDATE request [RFC 2136] (except that the operation code is ROLLOVER
which is equal to (TBD)) and use a new form of NOTIFY [RFC 1996].
Considerations for such requests to the parent and children of a zone
are givens below.
All rollover operations involve significant amounts of cryptographic
calculations. Appropriate rate limiting SHOULD be applied to avoid
denial of service attacks.
[This draft does not consider cross-certification key rollover.]
3.1 Rollover to Parent
A zone rolling over its KEY RRset sends an upward ROLLOVER request to
its parent. Actually, it will normally do two upward ROLLOVERs, one
for a combined KEY RRset of its old and new KEYs and one for just its
new KEY RRset, as discussed above.
The server selection algorithm in [RFC 2136] section 4 should be
used. A child needs to be configured with or determine the name of
its parent but SHOULD NOT remember the location of its parent other
than via normal DNS caching of address RRs so that rollover will
continue to work if its parent servers are moved.
The ROLLOVER request Zone should be specified as the parent zone.
The request Update section has the new KEY RRset on which the parent
signature is requested along with the requesting zone's SIG(s) under
its old KEY(s) as RRs to be "added" to the parent zone. The
inception and expiration times in this child SIG or SIGs are the
requested inception and expiration times for the new parent SIG(s).
The "prerequisites" section has the old child KEY RRset with the
parent SIG (see next paragraph).
An upward ROLLOVER request MUST be signed and if not signed a BADAUTH
response generated. The signature MUST be under the previous zone
KEY, so the parent can validate it, or under a valid TSIG key
[draft-ietf-dnsind-tsig-*.txt] arranged with the parent. Including
the "prerequisite" section as specified above enables a parent that
keeps no record of its children's KEYs to still authenticate a
child's ROLLOVER request based on the old child KEY because the
parent is presented with its own SIG on the old KEY.
If the ROLLOVER command is erroneous or violates parental policy, an
Error response is returned. If a parent retains copies of its
children's KEYs, it may use that knowledge to validate ROLLOVER
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request SIGs and ignore the "prerequisites" section.
If the ROLLOVER command is OK and the parent can sign online, its
response MAY include the new parent SIG(s) in the response Update
section. This response MUST be sent to the originator of the
request.
If the parent can not sign online, it should return a response with
an empty Update section and queue the SIG(s) calculation request.
This response MUST be sent to the originator of the request.
ROLLOVER response messages MUST always include the actual parent's
SOA signed with a key the child should recognize in the Additional
Information section (see section 4 below).
Regardless of whether the server has sent the new signatures above,
it MUST, once it has calculated the new SIG(s), send a ROLLOVER to
the child zone using the DNS port (53) and the server selection
algorithm defined in RFC 2136, Section 4. This ROLLOVER reqeust
contains the KEY RRset that triggered it and the new SIG(s). There
are several reasons for sending the ROLLOVER response regardless of
whether the new SIG RR(s) were sent in the original response. One is
to provide an indication to the operators of the zone in the event
someone is trying to hijack the zone. Another is that this maximizes
the probability of the response getting through.
Although the parent zone need not hold or serve the child's key, if
it does the ROLLOVER command REQUEST SHOULD NOT automatically update
the parent zone. A later UPDATE command can be used to actually put
the new KEY into the parent zone if desired and supported by parent
policy.
This document does not cover the question of parental policy on key
rollovers. Parents may have restrictions on how far into the future
they will sign KEY RRsets, what algorithms or key lengths they will
support, might require payment for the service, etc. The signing of
a future KEY by a parent is, to some extent, a granting of future
authoritative existence to the controller of the child private key
even if the child zone ownership should change. The only effective
way of invalidating such future signed child public keys would be for
the parent to roll over its key(s), which might be an expensive
operation.
3.2 Rollover to Children
When a secure zone is going to rollover its key(s), it needs to re-
sign the zone keys of any secure children under its new key(s). The
parent simply notifIES the child via a rollover NOTIFY [RFC 1996]
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that the parent KEY(s) have changed. The child then proceeds to do
an upward ROLLOVER request, as described in 3.1 above to obtain the
new parental SIG(s).
A rollover NOTIFY is a NOTIFY request [RFC 1996] that has a QTYPE of
SIG and the owner name of the child zone. The answer section has the
current parent SOA signed by a key the child will know (see section
4).
A rollover NOTIFY MUST be signed and if not signed a BADAUTH response
generated. The signature MUST be under the previous parental zone
KEY, so the child can validate it, or under a valid TSIG key [draft-
ietf-dnsind-tsig-*.txt] negotiated between parent and child.
The rollover NOTIFY can be sent to any of the nameservers for the
child using the nameserver selection algorithm defined in RFC 2136,
Section 4. Nameservers for the child zone receiving a rollover
NOTIFY query will forward the rollover NOTIFY in the same manner as
an UPDATE is forwarded.
Unless the master server is configured to initiate an automatic
ROLLOVER it MUST seek to inform its operators that a rollover NOTIFY
request has been received. This could be done by a number of methods
including generating a log message, generating an email request to
the child zone's SOA RNAME or any other method defined in the
server's configuration for the zone. The default SHOULD be to send
mail to the zone's SOA RNAME. As with all rollover operations, care
should be taken to rate limit these messages so prevent them being
used to facilitate a denial of service attack.
Once the message has been sent (or suppressed if so configured) to
the child zone's administrator the master server for the child zone
is free to respond to the rollover NOTIFY request.
4. Secure Zone Cuts and Joinders
There are two other events that have some similarity to key rollover.
The first is when a secure zone the is more than one level deep has a
zone cut introduced inside it. For example, assume zone example.com
zone cut could be introduced such that b.c.example.com became a
separate child zone of example.com. A real world exampe would be a
company that structures its DNS as host.branch.company.example. It
might start out will all of these names in one zone but later decide
to delegate all or some of the branches to branch zone file
maintainers.
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The second is when a secure zone absorbs a child zone eliminating a
zone cut. This is simply the inverse of the previous paragraph.
From the point of view of the parent zone above the splitting zone or
above the upper of the two combining zones, there is no change.
When a zone is split by introducing a cut, the newly created child
must be properly configured.
However, from the point of view of a child of the splitting zone
which becomes a grandchild or a grandchild that becomes a child due
to joinder, there is a change in parent name. Therefore, in general,
there is a change in parent KEY(s). Unless the entity that handles
rollovers for the zone whose parent name has changed is appropriately
updated, future automated rollover will fail because it will be sent
to the old parent.
For this reason and so that other consistency checks can be made, the
parent SOA and SIG(SOA) are always included in the Answer section of
rollover NOTIFY requests and in ROLLOVER responsess. For automated
rollover to the new cut or joined state to work, these SOAs must be
signed with old KEY(s) of the former parent so the signatures can be
validated by the zone whose parent name is changing. In the case of
a joinder, if the private key of the pinched out middle zone is not
available, then manual update of the former grandchild, now child,
will be necessary. In the case of introducing a cut, operational
coordination with the former parent, now grandparent, signing the
initial updates to the former child, now grandchild, will be needed
to automate the reconfiguration of the zones.
5. Security Considerations
The security of ROLLOVER or UPDATE requests is essential, otherwise
false children could steal parental authorization or a false parent
could cause a child to install an invalid signature on its zone key,
etc.
A ROLLOVER request can be authenticated by request SIG(s)under the
old zone KEY(s) of the requestor [RFC 2535]. The response SHOULD
have transaction SIG(s) under the old zone KEY(s) of the responder.
(This public key security could be used to rollover a zone to the
unsecured state but at that point it would generally not be possible
to roll back without manual intervention.)
Alternatively, if there is a prior arrangement between a child and a
parent, ROLLOVER requests and responses can be secured and
authenticated using TSIG [draft-ietf-dnsind-tsig-*.txt]. (TSIG
security could be used to rollover a zone to unsecured and to
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rollover an unsecured zone to the secured state.)
A server that implements online signing SHOULD have the ability to
black list a zone and force manual processing or demand that a
particular signature be used to generate the ROLLOVER request. This
it to allow ROLLOVER to be used even after a private key has been
compromised.
6. IANA Considerations
The DNS operation code (TBD) is assigned to ROLLOVER. There are no
other IANA considerations in this document.
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References
[RFC 1034] - "Domain names - concepts and facilities", P.
Mockapetris, 11/01/1987.
[RFC 1035] - "Domain names - implementation and specification", P.
Mockapetris, 11/01/1987.
[RFC 1996] - "A Mechanism for Prompt Notification of Zone Changes
(DNS NOTIFY)", P. Vixie, August 1996.
[RFC 2119] - "Key words for use in RFCs to Indicate Requirement
Levels", S. Bradner. March 1997.
[RFC 2136] - "Dynamic Updates in the Domain Name System (DNS
UPDATE)", P. Vixie, Ed., S. Thomson, Y. Rekhter, J. Bound. April
1997.
[draft-ietf-dnsind-tsig-*.txt]
[RFC 2535] - "Domain Name System Security Extensions", D. Eastlake.
March 1999.
[RFC 2541] - "DNS Security Operational Considerations", D. Eastlake.
March 1999.
Authors Address
Donald E. Eastlake 3rd
IBM
65 Sindegan Hill Road, RR #1
Carmel, NY 10512
Telephone: +1 914-276-2668 (h)
+1 914-784-7913 (w)
FAX: +1 914-784-3833 (w)
EMail: dee3@us.ibm.com
Mark Andrews
Internet Software Consortium
1 Seymour Street
Dundas Valley, NSW 2117
AUSTRALIA
Telephone: +61-2-9871-4742
Email: marka@isc.org
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Expiration and File Name
This draft expires in October 1999.
Its file name is draft-ietf-dnsind-rollover-00.txt.
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