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Versions: 00 01 02 03 04                                                
DNSOP WG                                                Edward Lewis
INTERNET DRAFT                                          NAI Labs
Category: I-D                                           April 13, 2000

                  Handling of DNS Zone Signing Keys

Status of this Memo

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
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at

The list of Internet-Draft Shadow Directories can be accessed at

Comments should be sent to the authors or the DNSOP WG mailing list

This draft expires on October 13, 2000.

Copyright Notice

Copyright (C) The Internet Society (1999,2000).  All rights reserved.


DNS Security Extensions require a greater interaction between zone
administrations sharing a zone cut.  The center of the interaction is
the handling of the zone keys of the child and the signature applied
by the parent.  DNSSEC does not include a protocol for this, but the
means of this interaction need definition to maintain the security of

1 Introduction

DNS Security Extensions require a greater interaction between zone
administrations sharing a zone cut.  The center of the interaction is
the handling of the zone keys of the child and the signature applied
by the parent.  DNSSEC does not include a protocol for this, but the
means of this interaction need definition to maintain the security of

The abstract having been repeated, the following caveats should be
stated.  This document is a work in progress.  This version is a

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complete rewrite because of all of the lessons learned in recent
implementations and workshops held to exercise the first round of
code.  Because of this, the following sections will have to be
considered in light of remarks in this, the introductory section.

Section 2 of the document discusses the issues surrounding keys as
they progress from generation to disposal.  Section 3 discusses the
interactions between a parent and child zone.  Section 4 lists
requirements for a protocol to carry out the steps described in
Section 3.

1.1 On-tree Validation

"On-tree validation" refers to the signing of a child zone's (apex)
KEY RR set by the parent zone's private key.  The child zone would
then publish the KEY RR set and the SIG (KEY) RR as generated by the
parent.  This makes possible building a chain of trust by a resolver
that is verifying an answer it received.  The term "on-tree" refers to
following the DNS domain hierarchy to reach a trusted key, presumably
the root key if no other key is available.  The term "validation"
refers to the digital signature by the parent to prove the integrity,
authentication and authorization of the child' key to sign the child's
zone data.

The term "off-tree validation" refers to the use of some domain name
other than the parent zone to sign a child's KEY RR set.  This ability
has been suggested to be a desirable feature from time to time.
However, securely accomodating off-tree validation is a hard problem.

So, for this document, on-tree validation is assumed to be the only
recognized model.  More liberal validation models require more study.
Starting with a more conservative model makes transitioning to more
liberal models easier, the vice versa is not easy.

2 Life Cycle of a Key Pair

A key pair's lifetime begins with its generation and after a period of
time the pair is disposed of.  The reason for time-limiting keys is
that, given enough time, any secret can become exposed, guessed or
otherwise "broken."  The challenge is to use a key pair for less time
than is needed to "break" or "discover" its private component.

There are three primary factors which determine the span of time a key
pair is useable.  One is the quality of the generation process.  Truly
random number generation and hard to derive numbers increase the
useful life of a  key pair.  Another factor is the safety of the
secret.  The less is it used or placed in a vulnerable position, the
longer it will be useful.  Yet another factor in the so-called
strength of the key pair is length, generally, the longer or bigger a
key, the longer it takes to break or guess it.

2.1 Key Generation

Generating a key is a simple operation with a proper tool, and DNSSEC

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tools are available for this.  At this time, a key pair is requested
that meets the definition of signing zone key pair.  Issues for an
administraitor at this this stage are the cryptographic algorithm and
length of the keys.

DNSSEC has four kinds of algorithms.  One is mandatory-to-implement,
which means that all DNSSEC compliant software (servers, resolvers)
will be able to process the signature.  (DSA is in this category.)
Another category is standard, which means that there is a definition
for the algorithm's use in DNSSEC, but for some reason there may be
software that is unable to implement it.  (RSA is in this category.)
Oddly, the most efficient algorithm for DNSSEC is in the latter

The other two categories are not options at this point.  One is shared
secret (HMAC-MD5) and the other category includes undefined

The meaning of the length of a key varies by algorithm.  In general,
the longer the key, the safer the pair is.  However, as safety
increases, speed of signing and verification drops.  An administratot
should weigh more frequent key generations and deletions against
slower name resolutions when choosing a key length.

2.2 Public Key Validation

Before a zone key pair can be useful, it must be validated by the
parent zone.  To be validated, the public key must be converted into a
KEY RR, complete with a fully qualified owner name, time to live value
and other settings.  The new KEY RR has to be combined with any other
KEY RR's comprising the key set owned by the zone's apex.  Individual
keys are not validated in DNSSEC, entire key sets are validated.

To be validated, all of the keys to be held at the zone's apex are
collected and send to the parent.  The information sent is specificied
in section 4, for now, consider the information to be equivalent to
what would be in the on-the-wire format as send in response to a

Until the response is received from the parent, the private key of the
pair may be put into use, but any signatures resulting from the use
will not be useful in DNSSEC validation.  (They will be considered to
be extra-DNSSEC or immaterial.)

2.3 Signing With Private Key

A private key can be used to sign a zone after generation in parallel
with getting the public key validated.  But the signatures won't be of
use until there is a validating signature over the KEY RR holding the
public key.

There are potentially three ways in which a zone private key can be
put to use.  One is to use the key in a non-server, or off-line,
signer application.  Another is to place the key in a dynamic update

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enabled server.  Yet another, less common use, is in a trusted dynamic
update client that an administrator trusts to properly sign data.

2.3.1 Non-Server Signing

A non-server, signer application is a software process that accepts a
zone master file and signs the data in it with one or more private
keys, and produces a signed master file.  In past DNSSEC documents,
the connectivity of a machine running the process is significant.

If the signer machine is not accessible to the network on which the
data is served, the machine is said to be "off-line."  The purpose for
this is enhanced security of the private key.  A break-in compromising
the server holding the key is less likely if there is no way to
remotely send even just IP traffic to it.

Off-line signing is not a requirement for DNSSEC.  An on-line signer
is permitted, but then host security must be tightly maintained as the
a compromise of crytptographic data is often hard to recover.  As
opposed to the destruction of some files that could be recovered from
backups, exposed cryptographic data could, in the case of DNS, put the
zone's data, and all delegated zone's data, at risk of being hijacked.

Note that if off-line signing is practiced, the generation of keys
should also be done on this machine, or another off-line machine.

2.3.2 Dynamic Update Signing

A private key is needed in dynamic update to sign, at the least, new
SOA records and NXT records.  Updated (changed) data sets must be
signed too.  Dynamic update servers should be careful to keep the
private key secret, including locking it into memory and practice good
host security measures.  The key must not be left in an exposed
configuration file on disk unless the file is properly encrypted.

The same comments apply to any dynamic update client entrusted to
supply signed data.  A wise system administrator will rarely make use
of this feature.

2.4 Loading A New Zone

Assuming the intent of putting a zone key into use is to make the zone
secure to the world, the validated set of KEY RRs holding all the
public keys belonging to the zone's apex must be included in the zone
file.  If a validating SIG (KEY) RR is not available, then the key
set, and all sets signed in the zone will be considered unsecured to
all but zones with preconfigured keys.

2.5 Removing a Key From Use

Removing a key from use is as simple as removing the KEY RR pertaining
to the public key from DNS.  This renders all SIG RR's generated by
the private key meaningless.

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There are a few considerations when it comes to removing the KEY RR
though.  Unless the removal of the KEY RR is in response to a security
emergency (such as a suspected loss of secrecy of the private key),
the private key's use should be terminated first.  Cache's may hold
SIG RR's up to the TTL, so usually, the KEY RR won't be pulled until
the TTL time after the private key is pulled and ideally replaced.

One other consideration is the impact that the removing the KEY RR is
the impact on any delegated zones.  If the KEY RR is used to validate
child zone's keys, then as soon as the KEY RR is removed, the lower
zones are no longer properly secured.

3 Parent-Child Interactions

As stated in the Introduction, on-tree validation is assumed, so all
zone key validation will happen between parent and child.

3.1 New Delegation

At the inception of a new zone, besides the traditional data exchange,
the child should request validation for an initial set of zone keys.
This action would be identical to that described in the next section,
except that there may not be an initial NXT to modify.

Since this is the first set of keys used by the child, the parent
needs to be sure that the child is truly who they claim to be.  There
must be some out of band means used to authenticate the new zone
administrator. Successive key sets can be installed using the first
set's signature, so this authentication is a one-time but crucial

The zone administrations should also make plans to handle "stolen"
child keys.  If a child zone's private key is exposed or stolen, the
zone must be able to install new keys and have the parent sign them.
The parent must authorize the child again to prevent zone jacking.

3.2 Child Initiated Validation

The most efficient way to accomplish the validation of a zone is to
require the child zone to make a request to the parent.  This could be
in response to the child's decision to change keys, to replace a
broken key, or a notification from the parent that the parent has
changed its keys.

The child first has to assemble the entire set of KEY RR's that needs
validation.  The set must be fully specified, meaning that all fields
in the RR must be supplied.  In particular, the TTL's must be set,
this has proven to be a problem in workshops.

The child needs to decide the span of time over which the set is to be
validated.  A child need not wait until a set of keys is needed "now"
to make a request.  Consider the following example.

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A child zone decides to have a different zone key for each month of
the upcoming year.  The child also decides to publish just three of
the keys at any one time - the key for the previous month (to validate
data still being tossed around), the current key, and the key for the
next month (in case the KEY set is cached somewhere beyond the end of
the month).  The child would then assemble 12 KEY RR sets, consisting
of three of the keys at a time, and submit them for validation, one
set per month.

The child should also request the parent sign the keys using a set of
particular algorithms.  If the parent does not recognize any member of
the set, that algorithm cannot be supplied.  The parent should not
return a key of an algorithm that is not requested.

The request is then sent to the parent for validation.  The security
of this transfer is crucial to the safety of the keys and the entire
DNSSEC process.  The child must ensure that the request arrives with
source integrity.  The parent must be able to authenticate the
request, that the request is from an authorized source.
Non-reputability may be a desired feature.

The parent's processing of the validation must not alter the set of
keys submitted by the child.  The parent must not add a NULL key if
there is no zone key present.  If a child submits a key set that has
not zone signing KEY in it, then the child zone will remain unsecured.
It is not the job of the parent to notice this, this is left to the

The parent should not sign any key sets belonging to any member of a
child zone other than the apex.  Other than the apex key set, the
parent should not sign any data  in the child zone.  It is not the job
of the parent to provide security for members of the child zone.

After the SIG (KEY) RR for each KEY RR set to be validated is
generated, the result is returned to the child.  It is important that
the child verify the signatures returned by the parent.  If the
signatures don't verify the appropriate key set, DNSSEC verification
of the zone data will not succeed.

3.3 Parent Initiated Validation

A parent can initiate a validation of a child as a by product of the
parent's changing or revoking of its own keys.  When a parent removes
a key, all children that relied upon the key should be notified that
they need to resubmit keys for validation, as decribed in the previous

The means a parent uses to inform children is not specified here.
Also, whether a parent notifies all children or just children impacted
by the change in keys is not specified.  The latter issue poses an
interesting design decision.

If a parent chooses to notify just the children impacted by the
removal of a key, then the parent must retain the knowledge of which

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key is used to sign which child.  As this sounds like an onerous
burden, consider that the alternative is to notify all children when
any key is removed, and suffer a mass revalidation.

3.4 Removal Of Delegation

When a delegation is ended, data is removed from the parent.  But one
more step is needed, the keys signing the child zone may have to be
removed also.  This stops the child data from being authenticated, if
the child zone servers are not stopped from answering queries.  This
is a consideraton only in a non-cooperative removal of a zone.

If keys validating a delegation are removed, a "Parent Initiated
Validation" will likely ensue.

3.5 Expiration Of A Validation

When a SIG (KEY) RR generated by the parent for a child's key set
expires, there is no requirement that either side act.  The child may
simply wish to revert to an unsecured state.  A parent is under no
obligation to make sure that the child zone's are properly operated.

3.6 Other Adjustments

There is a scenario in which a child may want to have the parent
indicate that the child does not have a set of validated keys.  A
child may discover a problem with the key set, such as the loss of the
private keys (rm -f *) or the exposure of the keys to an untrusted
party.  In this case, the parent should have some means for the child
to request a change in state.

Note that the reverse transition is not necessarily desirable.  A
child should not be given the ability to claim that it has validated
keys without the parent doing the signing.  This draft assumes that
on-tree validation is the only permitted model, and this is what
drives the comment.  Off-tree validation needs much further
development before it can be accomodated in a secure manner.  When
such a model is used, then it might make sense for a child to request
being identified as secured without submitting keys for validation.

4 Requirements on the Validation Process

This section has not been complete yet.

4.1 Child Request For Validation

4.2 Parent Response to Child

4.3 Parent Notification Of Key Removal

4.4 Parent Data Retention

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5 IANA Considerations

This document does not place any requirements on the assigned numbers

6 Security Considerations

This entire document is a note on security considerations.  If the
zone key is mishandled, in a way that compromises its security, then
the security of its zone is compromised.

7 References

8 Author's Address

Edward Lewis
3060 Washington Rd (Rte 97)
Glenwood, MD 21738

9 Acknowledgements

The following individuals and groups have made significant input into
the content of this document: the attendees of the NIC-SE work shop on
DNSSEC, May 18 and 19, 1999, also Olafur Gudmundsson, and Brian

A second workshop held by the CAIRN research network September 29 and
30th also provided input to this document.  Dan Massey has provided
input based upon this workshop and experience with DNSSEC in CAIRN.

10 Full Copyright Statement

Copyright (C) The Internet Society (1999,2000).  All Rights Reserved.

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are
included on all such copies and derivative works.  However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of developing
Internet standards in which case the procedures for copyrights defined
in the Internet Standards process must be followed, or as required to
translate it into languages other than English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

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