INTERNET-DRAFT Donald E. Eastlake 3rd
CyberCash, Inc.
Expires: 27 August 1996 28 February 1996
Secure Domain Name System Dynamic Update
------ ------ ---- ------ ------- ------
Status of This Document
This draft, file name draft-ietf-dnssec-update-00.txt, is intended to
be become a Proposed Standard RFC. Distribution of this document is
unlimited. Comments should be sent to the DNS security mailing list
<dns-security@tis.com> or the author.
This document is an Internet-Draft. 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
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``working draft'' or ``work in progress.''
To learn the current status of any Internet-Draft, please check the
1id-abstracts.txt listing contained in the Internet-Drafts Shadow
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nic.nordu.net (North Europe), ftp.nis.garr.it (South Europe),
munnari.oz.au (Pacific Rim), or ftp.is.co.za (Africa).
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Abstract
Domain Name System (DNS) protocol extensions have been defined to
authenticate the data in DNS and provide key distribution services
(draft-ietf-dnssec-secext-*.txt). DNS Dynamic Update operations have
also been defined (draft-ietf-dnsind-dynDNS-*.txt>, but without a
detailed description of strong security for the update operation.
This draft describes how to use DNS digital signatures covering
requests and data to secure updates and restrict them to those
authorized to perform them as indicated by the updater's possession
of cryptographic keys.
Acknowledgements
The contributions of the following person to this draft are
gratefully acknowledged:
Charlie Kaufman
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Table of Contents
Status of This Document....................................1
Abstract...................................................2
Acknowledgements...........................................2
Table of Contents..........................................3
1. Introduction............................................4
1.1 Overview of DNS Dynamic Update.........................4
1.2 Overview of DNS Security...............................4
2. Two Basic Strategies....................................6
3. Keys....................................................8
3.1 Update Keys............................................8
3.1.1 Update Key Name Scope................................8
3.1.2 Update Key Class Scope...............................8
3.1.3 Update Key Signatory Field...........................8
3.2 Zone Keys and Update Modes............................10
3.3 Wildcard Key Punch Through............................11
4. Update Signatures......................................12
4.1 Update Request Signatures.............................12
4.2 Update Data Signatures................................12
5. The in-key.int. Domain.................................13
6. Security Considerations................................15
References................................................15
Author's Address..........................................15
Expiration and File Name..................................15
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1. Introduction
Dynamic update operations have been defined for the Domain Name
System (DNS) in draft-ietf-dsnind-dynDNS-*.txt but without a detailed
description of strong security for those updates. Means of securing
the DNS and using it for key distribution have been defined in
draft-ietf-dnssec-sexect-*.txt.
This draft proposes techniques based on the defined DNS security
mechanisms to authenticate DNS updates. In addition, a secure in-
key.int. domain is defined with special security policies. This in-
key domain permits access to entities by their key if the entity has
been registered in that domain.
Familiarity with the DNS system [RFC 1034, 1035] is assumed.
Familiarity with the DNS security and dynamic update proposals will
be helpful.
1.1 Overview of DNS Dynamic Update
DNS dynamic update defines a new DNS opcode, new DNS request and
response structure if that opcode is used, and new error codes. An
update can specify complex combinations of deletion and insertion
(with or without pre-existence testing) of resource records (RRs)
with one or more owner names; however, all testing and changes for
any particular DNS update request are restricted to a single zone.
The server for a secure dynamic zone must increment the zone SOA
serial number when an update occurs or the next time the SOA is
retrieved if one or more updates have occurred since the previous SOA
retrieval and the updates themselves did not update the SOA.
1.2 Overview of DNS Security
DNS security authenticates data in the DNS by also storing digital
signatures in the DNS as resource records (RRs). A SIG RR provides a
digital signature on the set of all RRs with the same owner name and
class as the SIG and whose type is the type covered by the SIG. The
SIG RR cryptographically binds the covered RR set to the signer, time
signed, signature expiration date, etc. There is a key associated
with every secure zone and all data in the secure zone is signed
either by this zone key or by a dynamic update key tracing its
authority to the zone key.
DNS security also defines transaction SIGs and request SIGs.
Transaction SIGs appear at the end of a response. Transaction SIGs
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authenticate the response and bind it to the corresponding request
with the key of the host that the responding DNS server is running
on. Request SIGs appear at the end of a request and authenticate the
request. Request SIGs are the primary means of authenticating update
requests.
DNS security also permits the storage of public keys in the DNS via
KEY RRs. These KEY RRs are also, of course, authenticated by SIG
RRs. KEY RRs for zones are stored in their superzone and subzone
servers, if any, so that the secure DNS tree of zones can be
traversed by a security aware resolver.
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2. Two Basic Strategies
A dynamic secure zone is any secure DNS zone containing one or more
KEY RRs that can authorize dynamic updates, i.e., entity or user KEY
RRs with the signatory field non-zero and whose zone KEY RR signatory
field indicates that updates are implemented. There are two basic
modes of dynamic secure zone which relate to the update strategy,
mode A and mode B. A summary comparison table is given below and
then each mode is described.
SUMMARY OF DYNAMIC SECURE ZONE MODES
CRITERIA: | MODE A | MODE B
-------------------------+--------------------+-------------------
Zone Key | Off line | On line
-------------------------+--------------------+-------------------
Server Workload | Low | High
-------------------------+--------------------+-------------------
Static Data Security | Very High | Medium-High
-------------------------+--------------------+-------------------
Dynamic Data Security | Medium | Medium-High
-------------------------+--------------------+-------------------
Dynamic Key Rollover | No | Yes
-------------------------+--------------------+-------------------
Key Restrictions | Fine grain | Coarse grain
-------------------------+--------------------+-------------------
For mode A, the zone owner key and static zone master file are always
kept off-line for maximum security of the static zone contents. Any
dynamicly added or changed RRs are signed in the secure zone by their
authorizing dynamic update key and they are backed up, along with
this SIG RR, in a separate online dynamic master file. In this type
of zone, server computation is minimized since the server need only
check signatures on the update data, which has already been signed by
the updater, generally a much faster operation than signing data.
However, the AXFR SIG and NXT RRs which covers the zone under the
zone key will not cover dynamically added data. Thus, for type A
dynamic secure zones, zone transfer security is not provided for
dynamically added RRs, where they could be omitted, and
authentication is not provided for the server denial of the existence
of a dynamically added type. Key rollover for an entity that can
authorize dynamic updates is more cumbersome since the authority of
their key must be traceable to a zone key and so, in general, they
must securely communicate a new key to the zone authority for manual
transfer to the off line static master file. Because the dynamicly
added RRs retain their update KEY signed SIG, finer grained control
control of updates can be implemented via bits in the KEY RR
signatory field. NOTE: for this mode the zone SOA must be signed by a
dynamic update key and that private key must be kept on line so that
the SOA can be changed for updates.
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For mode B, the zone owner key and master file are kept on-line at
the zone primary server. When authenticated updates succeed, SIGs
under the zone key for the resulting data (including the possible NXT
type bit map changes) are calculated and these SIG (and possible NXT)
changes are entered into the zone and the unified on-line master
file. (The zone transfer AXFR SIG may be recalculated for each
update or on demand when a zone transfer is requested and it is out
of date.) This requires considerably more computational effort on
the part of the server as the public/private keys are generally
arranged so that signing (calculating a SIG) is more effort than
verifying a signature. The security on static data in the zone is
decreased because the ultimate state of the static date being served
and the ultimate zone authority private key are all on-line on the
net. This means that if the primary server is subverted, false data
could be authenticated to secondaries and other servers/resolvers.
On the other hand, this mode of operation means that data added
dynamically is more secure than in mode A. Dynamic data will be
covered by the AXFR SIG and thus fully protected during zone
transfers and will be included in NXT RRs so that it can be falsely
denied by a server only to the same extent that static data can
(i.e., if it is within a wild card scope). Maintaining the zone key
on-line also means that dynamic update keys which are signed by the
zone key can be dynamically updated since the zone key is available
to dynamically sign new values. Finally, because the zone key is used
to sign all the zone data, the information as to who originated the
current state of dynamic RR sets is lost making unavailable the
effects of some of the update control bits in the KEY RR signatory
field.
NOTE: The Mode A / Mode B distinction only effects the validation
and performance of update requests. It has no effect on retrievals.
[It might be possible to dream up additional modes but I think they
would be more complicated. A mode where things are temporarily
signed by the entity key but lated changed to being signed off line
by the zone key doesn't work well. You could also have a mode where
the zone key SIGs were added to and also covered the entity
signature. But the problem with any delayed addition of zone
signatures tends to be that you have to delay deletes of such
material until you can zone sign new NXT RRs, etc.]
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3. Keys
Dynamic update requests depend on update keys as described in section
3.1 below. In addition, the zone secure dynamic update mode and
availability of some options is indicated in the zone key. Finally,
a special rule is used in searching for KEYs to validate updates as
described in section 3.3.
3.1 Update Keys
All update requests to a secure zone must include signatures by one
or more key(s) that together can authorize that update. In order for
the Domain Name System (DNS) server receiving the request to confirm
this, the key or keys must be available to and authenticated by that
server as a specially flagged KEY Resource Record. The one exception
is in the in-key.int. domain as described in section 5.
The scope of authority of such keys is indicated by their KEY RR
owner name, class, and signatory field flags as described below. In
addition, such KEY RRs must be entity or user keys and not have the
authentication use prohibited bit on. All parts of the actual update
must be within the scope of at least one of the keys used for a
request SIG on the update request as described in section 4.
3.1.1 Update Key Name Scope
The owner name of any update authorizing KEY RR must (1) be the same
as the owner name of any RRs being added or deleted or (2) if the
owner name of the KEY is a wildcard, it must include within its
extended scope (see section 3.3) the name of any RRs being added or
deleted and those RRs must be in the same zone.
3.1.2 Update Key Class Scope
The class of any update authorizing KEY RR must be the same as the
class of any RR's being added or deleted.
3.1.3 Update Key Signatory Field
The four bit "signatory field" (see draft-ietf-dnssec-secext-*.txt)
of any update authorizing KEY RR must be non-zero. The bits have the
meanings described below for non-zone keys (see section 3.2 for zone
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type keys).
UPDATE KEY RR SIGNATORY FIELD BITS
0 1 2 3
+-----------+-----------+-----------+-----------+
| zone | strong | unique | general |
+-----------+-----------+-----------+-----------+
Bit 0, zone control - If nonzero, this key is authorized to attach,
detach, and move zones by creating and deleting NS, glue, and
zone KEY RR(s). If zero, the key can not authorize any update
that would effect such RRs. This bit is meaningful for both
type A and type B dynamic secure zones.
NOTE: do not confuse the "zone" signatory field bit with the
"zone" key type bit.
Bit 1, strong update - If nonzero, this key is authorized to add and
delete RRs even if there are other RRs with the same owner name
and class that are authenticated by a SIG signed with a
different dynamic update KEY. If zero, the key can only
authorize updates where any existing dynamic RRs of the same
owner and class are authenticated by a SIG using the same key.
This bit is meaningful only for type A dynamic zones and is
ignored in type B dynamic zones.
Keeping this bit zero on multiple KEY RRs with the same or
nested wild card owner names permits multiple entities to exist
that can create and delete names but can not effect RRs with
different owner names from any they created. In effect, this
creates two levels of dynamic update key, strong and weak, where
weak keys are limited in interfering with each other but a
strong key can interfere with any weak keys or other strong
keys.
Bit 2, unique name update - If nonzero, this key is authorized to add
and update RRs for only a single owner name. If there already
exist RRs with multiple names signed by this key, they may be
updated but no new name created until the number of existing
names is reduced to zero. This bit is meaningful only for mode
A dynamic zones and is ignored in mode B dynamic zones. This bit
is meaningful only if the owner name is a wildcard. (Any
dynamic update KEY with a non-wildcard name is, in effect, a
unique name update key.)
This bit can be used to restrict a KEY from flooding a zone with
new names. In conjunction with a local administratively imposed
limit on the number of dynamic RRs with a particular name, it
can completely restrict a KEY from flooding a zone with RRs.
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Bit 3, general update - The general update signatory field bit has no
special meaning. If the other three bits are all zero, it must
be one so that the field is non-zero in the update key. The
meaning of all values of the signatory field with the general
bit and one or more other signatory field bits on is reserved.
All the signatory bit update authorizations described above only
apply if the update is within the name and class scope as per
sections 3.1.1 and 3.1.2.
3.2 Zone Keys and Update Modes
Zone type keys are automatically authorized to sign anything in their
zone, of course, regardless of the value of their signatory field.
For zone keys, the signatory field bits have different means than
they they do for update keys, as shown below:
ZONE KEY RR SIGNATORY FIELD BITS
0 1 2 3
+-----------+-----------+-----------+-----------+
| mode | strong | unique | general |
+-----------+-----------+-----------+-----------+
Bit 0, mode - This bit indicates the update mode for this zone. Zero
indicates mode A while a one indicates mode B.
Bit 1, strong update - If nonzero, this indicates that the "strong"
key feature described in section 3.1.3 above is implemented and
enabled for this secure zone. If zero, the feature is not
available. Has no effect if the zone is a mode B secure update
zone.
Bit 2, unique name update - If nonzero, this indicates that the
"unique name" feature described in section 3.1.3 above is
implemented and enabled for this secure zone. If zero, this
feature is not available. Has no effect if the zone is a mode B
secure update zone.
Bit 3, general - This bit has no special meeting. If dynamic update
for a zone is supported and the other bits in the zone key
signatory field are zero, it must be a one. The meaning of zone
keys where the signatory field has the general bit and other
bits on is reserved.
If there are multiple KEY RRs for a zone and zone policy is in
transition, they might have different signatory fields. In that
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case, strong and unique name restrictions must be enforced as long as
there is a non-expired zone key being advertised that indicates node
A with the strong or unique name bit on respectively. Mode B updates
must be supported as long as there is a non-expired zone key that
indicates mode B. Mode A updates may be treated as mode B updates at
server option if non-expired zone keys indicate that both are
supported.
A server that will be executing update operations on a zone, that is
the primary master server, MUST not advertize a zone key that will
attract requests for a mode or features that it can not support.
3.3 Wildcard Key Punch Through
Just as a zone key is valid throughout the entire zone, update keys
with wildcard names are valid throughout their extended scope, within
the zone. That is, they remain remain valid for any name that would
match them, even existing specific names within their apparent scope.
If this were not so, then whenever a name within a wildcard scope was
created by dynamic update, it would be necessary to first create a
copy of the KEY RR with this name, because otherwise the existence of
the more specific name would hide the authorizing KEY RR and would
make later updates impossible. An updater could create such a KEY RR
but could not zone sign it. They would have to sign it with the same
key using the wildcard name as signer. Thus in creating, for example,
one hundred type A RRs authorized by a *.1.1.1.in-addr.arpa. KEY RR,
without key punch through 100 As, 100 KEYs, and 200 SIGs would have
to be created as opposed to merely 100 As and 100 SIGs with key punch
through.
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4. Update Signatures
Two kinds of signatures can appear in updates. Request signatures,
which are always required, cover the entire request and authenticate
the DNS header, including opcode, counts, etc., as well as the data.
Data signatures, on the other hand, appear only among the RRs to be
added and are only required for mode A operation.
4.1 Update Request Signatures
An update can effect multiple owner names in a zone. It may be that
these different names are covered by different dynamic update keys.
For every owner name and class effected, the updater must know a
private key valid for that name and class and must prove this by
appending request SIG RRs under each such key.
As specified in draft-ietf-dnssec-secext-*.txt, a request signature
is a SIG RR occurring at the end of a request with a type covered
field of zero. For an update, request signatures occur in the
Additional section. The final "Reserved" section of update requests
in draft-ietf-dnsind-dynDNS-06.txt is hereby redefined as the
Additional section and its corresponding count field is relabeled
ARCOUNT. Each request SIG signs the entire request, including DNS
header, but excluding any other request SIG(s).
4.2 Update Data Signatures
Mode A dynamic secure zones require that the update requester provide
SIG RRs that will authenticate the after update state of all RR sets
that are changed by the update and are non-empty after the update.
These SIG RRs appear in the request as RRs to be added and the
request must delete any previous data SIG RRs that are invalidated by
the request.
In Mode B dynamic secure zones, all zone data is authenticated by
zone key SIG RRs. In this case data signatures need not be included
with the update. A resolver can determine which mode an updatable
secure zone is using by examining the signatory field bits of the
zone KEY RR (see section 3.2).
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5. The in-key.int. Domain
A special domain is defined, the in-key.int. domain, to permit
inverse lookup by key. DNS servers for zones that include any
updatable part of this domain have a special update policy and all
servers and resolvers have a special authentication policy for this
domain. Servers authenticate updates for this domain based on the
requesters knowledge of the private key corresponding to a public key
whose hash is encoded into the RR owner name.
Normally the only RRs stored in this domain will be a KEY RR and an
authenticating SIG with the SIG signer field pointing to the normal
owner of the KEY. It is expected that an administrative restriction
may be placed on the number of RRs stored under any particular owner
name or charges imposed for additions to this domain.
[It would be possible to just have a PTR and a SIG here but then you
would always have to actually retrieved the KEY(s) at where the PTR
points to to validate anything. Or you could have both a KEY and a
PTR and two SIGs but that would be twice as bulky.]
The owner name associated with a key is
<key-hash>.<key-footprint>.algorithm.in-key.int.
key-hash is the hex representation of the SHA1 [SHA1] hash of the
"public key" portion of the corresponding KEY RR (the portion of
the RDATA after the algorithm octet) with label separating dots
added every fourth hex digit. [should I go for decimal here and
for footprint?] [I'm afraid of hash collisions with MD5 so I
went for the longer SHA...]
key-footprint is the hex representation of the key footprint field of
the KEY RR. [yet more bits to make collisions even less likely]
[could add a key-length label instead of or in addition to the key-
footprint...]
algorithm is the decimal number designating the public key algorithm
from the "algorithm" octet portion of the corresponding key.
Thus, at this time, algorithm will be either 1 or 254. Entries
for algorithm 253 are prohibited.
For example, the RRs in this domain for a purported key with actual
owner name example.tld could be as follows:
$ORIGIN xxxx.xxxx.xxxx.xxxx.xxxx.xxxx.xxxx.xxxx.xxxx.xxxx.xxxx.1.in-key.int.
IN KEY <flags> 0 1 (
45IkskceFGgiWCn/GxHhai6VAuHAoNUz4YoUMxFcby9k/yvedMfQgKzhH5er0Mu/vILz
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80jEeC8aTrO+KKmCaY1tVfSCSqQYn6//11U6Nld= ;key
)
IN SIG KEY 1 3 ( ;type-cov=PTR, alg=1, labels=3
19991202030405 ;signature expiration
19951211100908 ;time signed
2143658709 ;key footprint
example.tld. ;signer
MxFcby9k/yvedMfQgKzhH5er0Mu/vILz45IkskceFGgiWCn/GxHhai6VAuHAoNUz4YoU
1tVfSCSqQYn6//11U6Nld80jEeC8aTrO+KKmCaY= ;signature
)
Retrievals from leaves of this zone are authenticated by validating
the SIG against the KEY with the same owner name and checking that
this owner name correctly reflects the hash and key footprint of the
key. Thus, for this type of validation only, the signer name is
ignored and the SIG is NOT traced back to a known trusted key. In
addition, entries in this domain are "eternal" in that the SIG time
signed and signature expiration are ignored. Note that entries in
this special zone, even when authenticated, give only a hint that the
KEY stored there is or was valid for the signer name. A separate
retrieval from the signer name must be done for confirmation that
they key is currently valid.
Registration in the in-key.int. domain is voluntary. All servers
that include key storage leaves of the in-key.int. domain MUST
operate in mode A.
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6. Security Considerations
Any zone permitting dynamic updates is inherently less secure than a
static secure zone maintained off line as recommended in draft-ietf-
dnssec-secext-*.txt. If nothing else, secure dynamic update requires
on line change to and re-signing of the zone SOA resource record (RR)
to increase the SOA serial number. This means that compromise of the
primary server host could lead to arbitrary serial number changes.
Isolation of dynamic RRs to separate zones from from holding most
static RRs can limit the damage that could occur from breach of a
dynamic zone's security.
References
draft-ietf-dnssec-secext-*.txt
draft-ietf-dnsind-dynDNS-*.txt
[RFC1034] - Domain Names - Concepts and Facilities, P. Mockapetris,
November 1987
[RFC1035] - Domain Names - Implementation and Specifications
[SHA1]
Author's Address
Donald E. Eastlake, 3rd
CyberCash, Inc.
318 Acton Street
Carlisle, MA 01741 USA
Telephone: +1 508-287-4877
+1 508-371-7148 (fax)
+1 703-620-4200 (main office, Reston, Virginia, USA)
email: dee@cybercash.com
Expiration and File Name
This draft expires 27 August 1996.
Its file name is draft-ietf-dnssec-update-00.txt.
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