DNSEXT Working Group Olafur Gudmundsson
INTERNET-DRAFT July 2001
<draft-ietf-dnsext-delegation-signer-01.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.
Internet-Drafts are working documents of the Internet Engineering Task
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This draft expires on January 15, 2002.
Copyright Notice
Copyright (C) The Internet Society (2001). All rights reserved.
Abstract
One of the biggest problems in DNS occur when records of the same type
can appear on both sides of an delegation. If the contents of these
sets differs clients can get confused. RFC2535 KEY records follows
the same model as for NS records, parent is responsible for the
records but the child is responsible for the contents. This document
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proposes to store a different record in the parent that specifies
which one of the child's keys are authorized to sign the child's KEY
set. This change is not backwards compatible with RFC2535 but
simplifies DNSSEC operation.
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 sources it is common that the data gets out of sync. NS record
in a zone indicates that there is a delegation at this name 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 communication is needed between the two parties, to transmit
key sets up to parent and then the signed set or signatures down to
child. If the KEY set resides at the parent[Parent] the communication
is reduced having only child send updated key sets to parent.
DNSSEC[RFC2535] requires that the parent store NULL key set for
unsecure children, this complicates resolution process as in many
cases as servers for both parent and child need to be queried for KEY
set the [Parent] proposal simplifies this.
Further complication of the DNSSEC KEY model is that KEY record is
used to store DNS zone keys and public keys for other protocols. This
can lead to large key sets at delegation points. 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. Example of protocols are IPSEC, HTTP,
SMTP, SSH etc.
2. Key set may require frequent updates.
3. Probability of compromised/lost keys increases and triggers
emergency key rollover.
4. Parent may refuse sign key sets with NON DNS zone keys.
5. Parent may not have QoS on key changes that meets child's
expectations.
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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, 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.
1.1 - 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 will use 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 then the KEY set at the child. This is
cryptographically equivalent to just using KEY records.
Communication between the parent and child is reduced as the parent
only needs to know of changes in DNS zone KEY(s) used to sign the apex
KEY set. If other KEY records are stored at the zone apex, the parent
does not need to be aware of them.
This approach has the advantage that it minimizes the communication
between the parent and child and the child is the authority for the
KEY set with full control over the contents. This enables each to
operate and maintain each zone independent of each other. 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. The child can just as well use the same key to sign all records
in its zone.
Another advantage is that this model fits well with slow rollout of
DNSSEC and islands of security model. In the islands of security model
someone that trusts "good.example." preconfigures 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. If DS records can
also be used to identify trusted keys instead of KEY records.
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The main disadvantage of this approach is double the number of
signatures that need to be verified for the each delegation KEY set.
There is no impact on verifying other record sets.
1.2 - Reserved words
The key words "MUST", "MUST NOT", "SHOULD", and "MAY" in this document
are to be interpreted as described in RFC2119.
2 - DS (Delegation KEY signer) record:
2.1 Protocol change
DS record MUST only appear at secure delegations in the parent zone.
The record lists the child's keys that SHOULD sign the child's key
set. Insecure delegation MUST NOT have a DS record, the presence of
DS record SHOULD be considered a hint that the child might be secure.
Resolver MUST only trust KEY records that match a DS record.
NOTE: It has been suggested that NULL DS record for insecure children
is better than no record. The advantage is to have authenticated
denial of child's security status, the drawback is for large
delegating zones there will be many NULL DS records. If parent uses
NXT records adding NXT record to the authority section in the cases
when no DS record exists at delegation will give the same result as
NULL DS record.
WG please comment on which approach is better.
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 CAN 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 instructed to trust the key
used, via preconfiguration.
Authorative server answering a query, that has the OK bit set[OKbit],
MUST include the DS set in the additional section if the answer is a
referral and there is space. Caching servers SHOULD return the DS
record in the additional section under the same condition.
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2.1.1 - 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 to be only stored at 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 should not have to be changed if it
supports unknown types. Different TTL values on the childs 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 the
parent 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 think 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 that KEY records in parents
or children make prevent DNSSEC to ever get deployed.
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2.2 Wire format of DS record
The DS record consists of algorithm, size, 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 | size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| algorithm | SHA-1 digest |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (20 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| |
+-+-+-+-+-+-+-+-+
The key tag is calculated as specified in RFC2535, the size is the
size of the public key in bits as specified in the document specifying
the algorithm. 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 25 bytes, regardless of the key size.
NOTE: if 160 bits is to large the SHA-1 digest can be shortened but
that weakens the overall security of the system.
2.2.1 Justifications for fields
The algorithm and size fields are here to allow resolvers to quickly
identify the candidate KEY records to examine. Key Tag is to allow
quick check if this is a good candidate. The key tag is redundant but
provides some greater assurance than SHA-1 digest on its own. SHA-1 is
a strong cryptographic checksum, it is hard for attacker to generate a
KEY record that has the same SHA-1 digest. Making sure that the KEY
record is a valid public key is much harder. Combining the name of the
key and the key data as input to the digest provides stronger
assurance of the binding. Combining the SHA-1 with the other fields
makes the task of the attacker is as hard breaking the public key.
Even if someone creates a database of all SHA-1 key hashes seen so
far, the addition of the name renders that database useless for
attacks against random zones.
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2.3 Presentation format of DS record
The presentation format of DS record consists of 2 numbers, followed
by either the name of the signature algorithm or the algorithm number.
The digest is to be presented in hex.
2.4 Justifications for compact format
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.
Each DS record has RDATA size of 25, regardless of the size of the
keys, keeping the answers from the parent smaller than if public key
was used. The smallest currently defined KEY record RDATA is 70 bytes.
Compact DS format is also better suited to list trusted keys for
islands of security in configuration files.
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
cumbersome communications that DS is supposed to solve.
Resolvers will not get confused as they will select signatures with
the KEY they trust and ignore the other one.
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=12345 size=1024 alg=dsa <foofoo>
secure.example. NS ns1.secure.example.
NS ns1.secure.example. s
secure.example. NXT NS SIG NXT DS tail.example.
secure.example. SIG(NXT)
secure.example. SIG(DS)
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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=dsa>
KEY <tag=54321 size=512 alg=rsa/sha1>
KEY <tag=32145 size=1024 alg=dsa>
secure.example. SIG(KEY) <key-tag=12345 size=1024 alg=dsa>
secure.example. SIG(SOA) <key-tag=54321 size=512 alg=rsa/sha1>
secure.example. SIG(NS) <key-tag=54321 size=512 alg=rsa/sha1>
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 supposed to sign the KEY record at secure.example. In
this example "secure.example." has three different KEY records and the
one corresponding to 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.
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. For 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 holds ratios hold true.
Resolver may require an extra query to get the DS record but 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. DS requires much less storage in large delegation
zones 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 that has only the role of signing a key set, comes from
discussions with Bill Manning and Perry Metzger on how to put in a
single root key in all resolver that lives for a long time. Brian
Wellington, Dan Massey, Edward Lewis, Havard Eidnes, Jakob Schlyter,
Mark Kosters, Miek Gieben, Roy Arens, Scott Rosen have provided
usefull 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.
In the representation of DS record, there is a possibility that an
attacker can generate an valid KEY that matches all the checks 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 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.
Major editing of arguments and some reordering of text for clarity.
Added section on transition issues.
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