DNSEXT Working Group Olafur Gudmundsson
INTERNET-DRAFT September 2001
<draft-ietf-dnsext-delegation-signer-02.txt>
Updates: RFC 1035, RFC 2535, RFC 3008.
Delegation Signer record in parent.
Status of this Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
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This draft expires on February 20, 2002.
Copyright Notice
Copyright (C) The Internet Society (2001). All rights reserved.
Abstract
The Delegation Signer (DS) RR set is stored in a delegating (parent)
zone at each delegation point, and indicates the keys used in the
delegated (child) zone. The main design goal of the DS RR simplify the
operation of secure delegations by eliminating the need to store the
same RR in parent and child, as is done with the NS RR set and the KEY
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set in RFC2535.
Secure resolvers need to take an additional step with DS to verify a
child's KEY RR set. Operationally this schema is much simpler as
operation of the two zones at delegation is now decoupled to great
extent.
1 - Introduction
Familiarity with the DNS system [RFC1035], DNS security extensions
[RFC2535] and DNSSEC terminology [RFC3090] is important.
When the same data can reside in two administratively different DNS
zones, the data frequently gets out of sync. NS record in a zone
indicates that this name is a delegation and the NS record lists the
authorative servers for the real zone. Based on actual measurements
10-30% of all delegations in the Internet have differing NS sets at
parent and child. There are number of reasons for this, including lack
of communication between parent and child and bogus name-servers being
listed to meet registrar requirements.
DNSSEC [RFC2535,RFC3008,RFC3090] specifies that child must have its
KEY set signed by the parent to create a verifiable chain of KEYs.
There is some debate, where the signed KEY set should reside,
parent[Parent] or child[RFC2535]. If the KEY set resides at the child,
frequent two way communication is needed between the two parties.
First the child needs to transmit the key set to parent and then the
parent must send the signed set or signatures to child. If the KEY set
resides at the parent the communication is reduced as the child only
sends changed key sets to parent.
DNSSEC[RFC2535] requires that the parent store NULL key set for
unsecure children, this complicates resolution process in many cases
as servers for both parent and child need to be queried for KEY set if
the child server does not return a KEY set. Storing the KEY record
only in the parent zone simplifies this and allows the elimination of
the NULL key set.
Another complication of the DNSSEC KEY model is that KEY record is
used to store DNS zone keys and public keys for other protocols.
There are number of potential problems with this including:
1. KEY set may become quite large if many applications/protocols
store their keys at the zone apex. Possible protocols are IPSEC,
HTTP, SMTP, SSH and others that use public key cryptography.
2. Key set may require frequent updates.
3. Probability of compromised/lost keys increases and triggers
emergency key rollover procedures.
4. Parent may refuse sign key sets with NON DNS zone keys.
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5. Parent may not meet the child's expectations in turnaround time
in resigning the key set.
Given these and other reasons there is good reason to explore
alternatives to using only KEY records to create chain of trust.
Some of these problems can be reduced or eliminated by operational
rules or protocol changes. To reduce the number of keys at apex, a
rule to require applications to store their KEY records at the SRV
name for that application is one possibility. Another is to restrict
KEY record to DNS keys only and create a new type for all non DNS
keys. Third possible solution is to ban the storage of non DNS related
keys at zone apex. There are other possible solutions but they are
outside the scope of this document.
1.2 - Reserved words
The key words "MAY","MAY NOT", "MUST", "MUST NOT", "REQUIRED",
"RECOMMENDED", "SHOULD", and "SHOULD NOT" in this document are to be
interpreted as described in RFC2119.
2 - DS (Delegation KEY Signer)
2.1 - Delegation Signer Record model
This document proposes an alternative to the KEY record chain of
trust, that uses a special record that can only reside at the parent.
This record will identify the key(s) that child are allowed to self
sign its own KEY set.
The chain of trust is now established by verifying the parent KEY set,
the DS set from the parent and the KEY set at the child. This is
cryptographically equivalent to just using KEY records.
Communication between the parent and child is greatly reduced, since
the child only needs to notify parent about changes in keys that sign
its apex KEY RRset. Parent is ignorant of all other keys in the
child's apex KEY RRset, and the child maintains full control over the
apex KEY set and its content. Child can maintain any policies over
its DNS and other KEY usage with minimal impact on parent. Thus if
child wants to have frequent key rollover for its DNS keys parent
does not need to be aware of it as the child can use one key to only
sign its apex KEY set and other keys to sign the other record sets in
the zone.
This model fits well with slow roll out of DNSSEC and islands of
security model. In the islands of security model someone that trusts
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"good.example." can preconfigure a key from "good.example." as a
trusted keys and from then on trusts any data that is signed by that
key or has a chain of trust to that key. If "example." starts
advertising DS records, "good.example." does not have to change
operations, by suspending self-signing. DS records can also be used to
identify trusted keys instead of KEY records. One further advantage
is the information stored in the parent is minimized, as only records
for secure delegations are needed.
The main disadvantage of this approach that verifying delegations KEY
set requires twice as many signature verification operations. There
is no impact on the number of signatures verified for other RR sets.
2.2 Protocol change
A DS RR set MUST appear at each secure delegation from a secure zone.
If a DS RR set accompanies the NS RR set, the intent is to state that
the child zone is secured. If an NS RR set exists without a DS RR set
the intent is to state that the child zone is unsecure.
The public keys indicated in the DS RR set are the keys the child has
informed the parent, the child allows to sign the child zone apex KEY
RR set. Barring emergency, the intent of the DS RR set it to indicate
to state the child's zone keyset signing keys. If the child's APEX is
not signed by any KEY indicated in the DS RR set than any of number of
problems may have occurred, and are described later.
Updates RFC2535 sections 2.3.4 and 3.4, as well as RFC3008 section
2.7: Delegating zones MUST NOT store KEY records for delegations. The
only records that can appear at delegation in parent are NS, SIG, NXT
and DS.
Zone MUST self sign its apex KEY set, it SHOULD sign it with a key
that corresponds to a DS record in the parent. The KEY used to sign
the apex KEY RRset MAY sign other RRsets in the zone.
If child apex KEY RRset is not signed with one of the keys specified
in the DS record the child is locally secure[RFC3090] and SHOULD only
be considered secure the resolver has been configured to trust the key
used.
Authorative server for a zone with DS records MUST include the DS
records in answers for a delegation, when the OKbit[okbit] is set in
the query and if space is available in answer. DS records SHOULD have
lower priority than address records but higher priority than KEY
records.
Caching servers SHOULD return the DS record in the additional section
under the same condition.
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2.2.1 - Comments on protocol change
Over the years there has been various discussions on that the
delegation model in DNS is broken as there is no real good way to
assert if delegation exists. In RFC2535 version of DNSSEC the
authentication of a delegation is the NS bit in the NXT bitmap at the
delegation point. Something more explicit is needed and the DS record
addresses this for secure delegations.
DS record is the first DNS record that can only appear on the upper
side of a delegation. NS records appear at both sides as do SIG and
NXT. All other records can only appear at the lower side. This will
cause some problems as servers authorative for parent may reject DS
record even if the server understands unknown types, or not hand them
out unless explicitly asked. Similarly a nameserver acting as a
authorative for child and as a caching recursive server may never
return the DS record. A caching server does not care from which side
DS record comes from and thus does not have to be changed if it
supports unknown types. Different TTL values on the child's NS set
and parents DS set may cause the DS set to expire before the NS set.
In this case an non-DS aware server would ask the child server for the
DS set and get NXDOMAIN answer. DS aware server will know to ask a
parent DNS server for the DS record.
Secure resolvers need to know about the DS record and how to interpret
it. In the worst case, introducing the DS record, doubles the
signatures that need to be checked to validate a KEY set.
Note: The working group must determine if the tradeoff between more
work in resolvers is justified by the operational simplification of
this model. The author thinks this is a small price to pay to have a
cleaner delegations structure. One argument, put forward is that DNS
should be optimized for read when ever possible, and on the face of it
adding the DS record makes reading data from DNS more expensive. The
operational complexities and legal hurdles related to KEY records in
either parents or children may prevent DNSSEC deployment.
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2.3 Wire format of DS record
The DS (type=TDB) record consists of algorithm, key tag and SHA-1
digest of the public key KEY record allowed to sign the child's
delegation.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| key tag | algorithm | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SHA-1 digest |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (20 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The key tag is calculated as specified in RFC2535, Algorithm MUST be
an algorithm number assigned in the range 1..251. The SHA-1 digest is
calculated over the canonical name of the delegation followed by the
RDATA of the KEY record.
The size of the DS RDATA is 23 bytes, regardless of key size.
2.3.1 Justifications for fields
The algorithm and key tag fields are here to allow resolvers to
quickly identify the candidate KEY records to examine. The key tag
adds some greater assurance than SHA-1 digest on its own. SHA-1 is a
strong cryptographic checksum, it is real hard for attacker to
generate a KEY record that has the same SHA-1 digest. Combining the
name of the key and the key data as input to the digest provides
stronger assurance of the binding.
This format allows concise representation of the keys that child will
use, thus keeping down the size of the answer for the delegation,
reducing the probability of packet overflow. The SHA-1 hash is strong
enough to uniquely identify the key. This is similar to the PGP
footprint.
DS record is also well suited to lists trusted keys for islands of
security in configuration files.
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2.4 Presentation format of DS record
The presentation format of DS record consists of 2 numbers followed by
digest presented in hex.
2.5 Transition issues for installed base
RFC2535 compliant resolver will assume that all DS secured delegations
are locally secure. This is a bad thing, thus it might be necessary
for a transition period to support both DS and SIG@Child. The cost is
one more signatures in the answer and that early adopters have to use
cumbersome communications that DS solves.
This section needs work, it needs list of all cases and find if there
are any where resolvers get confused or can not determine what the
security status of child is.
3 Resolver Example
To create a chain of trust resolver goes from trusted KEY to DS to
KEY.
Assume the key for domain "example." is trusted. In zone "example."
we have
example. KEY <stuff>
secure.example. DS tag=10243 alg=3 <foofoo>
secure.example. NS ns1.secure.example.
NS ns1.secure.example.
secure.example. NXT NS SIG NXT DS unsecure.example.
secure.example. SIG(NXT)
secure.example. SIG(DS)
unsecure.example NS ns1.unsecure.example.
unsecure.example NS ns2.unsecure.example.
unsecure.example. NXT NS SIG NXT .example.
unsecure.example. SIG(NXT)
In zone "secure.example." we have
secure.example. SOA <soa stuff>
secure.example. NS ns1.secure.example.
NS ns1.secure.example.
secure.example. KEY <tag=12345 size=1024 alg=3>
KEY <tag=54321 size=512 alg=5>
KEY <tag=32145 size=1024 alg=3>
secure.example. SIG(KEY) <key-tag=12345 alg=3>
secure.example. SIG(SOA) <key-tag=54321 alg=5>
secure.example. SIG(NS) <key-tag=54321 alg=5>
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In this example the trusted key for "example." signs the DS record for
"secure.example.", making that a trusted record. The DS record states
what key is expected to sign the KEY RRset at "secure.example". Here
"secure.example." has three different KEY records and the KEY
identified in the DS record signs the KEY set, thus the KEY set is
validated and trusted. Note that one of the other keys in the keyset
actually signs the zone data, and resolvers will trust the signatures
as the key appears in the KEY set.
This example has only one DS record for the child but there no reason
to outlaw multiple DS records. More than one DS record is needed
during signing key rollover. It is strongly recommended that the DS
set be kept small.
Resolver determines the security status of "unsecure.example." by
examining the parent size NXT for this name.
3.1 Resolver cost estimates for DS records
From a RFC2535 resolver point of view for each delegation followed to
chase down an answer one KEY record has to be verified and possibly
some other records based on policy, for example the contents of the NS
set. Once the resolver gets to the appropriate delegation validating
the answer may require verifying one or more signatures. A simple A
record lookup requires at least N delegations to be verified and 1
RRset. For a DS enabled resolver the cost is 2N+1. For MX record the
cost where the target of the MX record is in the same zone as the MX
record the costs are N+2 and 2N+2. In the case of negative answer the
same ratios hold true.
Resolver may require an extra query to get the DS record and this may
add to the overall cost of the query, but this is never worse than
chasing down NULL KEY records from the parent in RFC2535 DNSSEC.
DS adds processing overhead on resolvers, increases the size of
delegation answers but much less than SIG@Parent.
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4 Acknowledgments
Number of people have over the last few years contributed number of
ideas that are captured in this document. The core idea of using one
key to only sign key set, comes from discussions with Bill Manning and
Perry Metzger on how to put in a single root key in all resolvers.
Brian Wellington, Jakob Schlyter, Scott Rosen, Edward Lewis, Dan
Massey, Mark Kosters, Olaf Kolman, Miek Gieben, Havard Eidnes, Donald
Eastlake 3rd., Randy Bush, Rob Austein, Roy Arends, and others have
provided useful comments.
4 - Security Considerations:
This document proposes a change to the validation chain of KEY records
in DNS. The change in is not believed to reduce security in the
overall system, in RFC2535 DNSSEC child must communicate keys to
parent and prudent parents will require some authentication on that
handshake. The modified protocol will require same authentication but
allows the child to exert more local control over its own KEY set.
There is a possibility that an attacker can generate an valid KEY that
matches all the DS fields thus starting to forge data from the child.
This is considered impractical as on average more than 2^80 keys must
be generated before one is found that will match.
DS record is a change to DNSSEC protocol and there is some installed
base of implementations, as well as text books on how to set up
secured delegations. Implementations that do not understand DS record
will not be able to follow the KEY to DS to KEY chain and consider all
zone secured that way insecure.
5 - IANA Considerations:
IANA needs to allocate RR type code for DS from the standard RR type
space.
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References:
[RFC1035] P. Mockapetris, ``Domain Names - Implementation and
Specification'', STD 13, RFC 1035, November 1987.
[RFC2535] D. Eastlake, ``Domain Name System Security Extensions'', RFC
2535, March 1999.
[RFC3008] B. Wellington, ``Domain Name System Security (DNSSEC) Signing
Authority'', RFC 3008, November 2000.
[RFC3090] E. Lewis `` DNS Security Extension Clarification on Zone
Status'', RFC 3090, March 2001.
[OKbit] D. Conrad, ``Indicating Resolver Support of DNSSEC'', work in
progress <draft-ietf-dnsext-dnssec-okbit-02.txt>, April 2001.
[Parent] R. Gieben, T. Lindgreen, ``Parent stores the child's zone
KEYs'', work in progress <draft-ietf-dnsext-parent-stores-
zones-keys-01.txt>, May 2001.
Author Address
Olafur Gudmundsson
3826 Legation Street, NW
Washington, DC, 20015
USA
<ogud@ogud.com>
Appendix A: Changes from Prior versions
Changes from version 01
Deleted KEY size field as it did not contribute anything but
complexity.
Number of wordsmith changes to make document more readable.
The word CAN was used when SHOULD was intended.
Deleted section 2.4 "Justifications for compact format" moved relevant
text to section 2.2.
Reverse alphabetized the acknowledgments section.
Reorganized sections 1 and 2 for readability.
Changes from version 00
Changed name from DK to DS based on working group comments.
Dropped verbose format based on WG comments.
Added text about TTL issue/problem in caching servers.
Added text about islands of security and clarified the cost impact.
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Major editing of arguments and some reordering of text for clarity.
Added section on transition issues.
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