The following is the technical specification of NO Resource Records [50] as discussed in chapter 6. It was submitted as a independent submission to the IETF DNS Extensions Work Group [18]. The Work Group adopted it is an official work item. This is the second version (not a final version) of the document.
Network Working Group S.A. Josefsson
Internet-Draft RSA Security
Expires: February 22, 2001 August 24, 2000
Authenticating denial of existence in DNS with minimum disclosure
(or; An alternative to DNSSEC NXT records)
draft-ietf-dnsext-not-existing-rr-00
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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Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This draft present an alternative to NXT records, used to achieve
authenticated denial of existence of a domain name, class and type.
Problems with NXT records, as specified in RFC 2535, are identified.
One solution, the NO record, is presented.
The NO record differ from the NXT record by using a cryptographic
hash value instead of the domain name. This prevent an adversery
from collecting information by "chaining" through a zone. It also
remove delegation point concerns in NXT records. The document also
describe hash truncation and record merging that reduces
storage/network load.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 3
2. Context . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The NO Resource Record . . . . . . . . . . . . . . . . . . 4
3.1 Idea . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2 NO RDATA Format . . . . . . . . . . . . . . . . . . . . . 4
3.3 NO RDATA on-the-wire format example . . . . . . . . . . . 6
3.4 Owner Names . . . . . . . . . . . . . . . . . . . . . . . 6
3.5 Additional Complexity Due To Wildcards . . . . . . . . . . 7
3.6 No Considerations at Delegation Points . . . . . . . . . . 7
3.7 Hash Truncation and Dynamic Updates . . . . . . . . . . . 8
3.8 Reducing Number of Records . . . . . . . . . . . . . . . . 9
3.9 Hash Collisions . . . . . . . . . . . . . . . . . . . . . 9
3.10 Authenticating Denial of NO Records . . . . . . . . . . . 9
3.11 Case Considerations . . . . . . . . . . . . . . . . . . . 10
3.12 Presentation Format . . . . . . . . . . . . . . . . . . . 10
3.13 Examples . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.13.1 Adding NO Records to a Zone . . . . . . . . . . . . . . . 10
3.13.2 Simple NO creating entity . . . . . . . . . . . . . . . . 11
3.13.3 Advanced NO creating entity . . . . . . . . . . . . . . . 11
3.13.4 Resolver Query for Non-existing Domain . . . . . . . . . . 11
3.13.5 Resolver Query for Non-existing Type At Existing Domain . 12
4. Security Considerations . . . . . . . . . . . . . . . . . 13
5. IANA Considerations . . . . . . . . . . . . . . . . . . . 13
References . . . . . . . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . 14
Full Copyright Statement . . . . . . . . . . . . . . . . . 15
1. Introduction
"Domain Name Security Extensions", RFC 2535 [1], specifies several
extensions to DNS that provides data integrity and authentication.
Among them is the NXT record, used to achieve authenticated denial
of existence of domains, and authenticated denial of existence on
certain class/types on existing domains.
As a consequence of NXT records it is possible to "chain" through a
zone secured by DNS security extensions, collecting all names and/or
records in the zone. This is the main problem that motivated this
draft.
2. Context
There have been arguments that the "chain" problem of NXT records is
a non-issue. Often used is the argument that information in DNS is
public, and if you wish to keep information private, you shouldn't
publish it in DNS. This might be true, but nonetheless major
service providers and companies are restricting access to zone
transfers. Also, people have debated whether NXT records should be
part of DNSSEC at all because of this problem [5].
Another aspect exist. When DNS is used to store certificates, as
described in RFC 2538 [2], certificates can identify individuals
and/or carry authentication information for special purposes. This
context has been the motivation for developing this draft.
The delegation considerations for NXT records (different RRsets in
the parent and child for the same domain) has also been regarded as
a flaw of the NXT records.
3. The NO Resource Record
This section describe the NO Resource Record.
3.1 Idea
A straight-forward extension to the NXT record that minimize
disclosure of information is to store a one-way function value
instead of the actual domain name. This is similar to NXT records;
where NXT records secure a interval where no existing domain names
are to be found, NO records secure a interval where no one-way value
of existing domain names are to be found.
The benefit, of course, is that an adversary does not yield any
useful information from the record. Specifically, "chaining"
through a zone to collect all records is no longer possible.
This idea has been extended in this document into allowing (but not
requiring) one record to deny existence of several records, and
truncating the hash value to the shortest unique prefix to preserve
space.
3.2 NO RDATA Format
The RDATA for a NO RR consists of one or more fields with the
following structure. The structure have the following elements: a
zero-terminated list of RR types, a hash length specifier, and a
hash value, as shown below. Both the "RR type" list and the "next
hash value" fields are of variable width.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| /
/ owner RR type list /
/ |
+---------------+-----------------------------------------------+
| # hash octets | SHA-1 hash value /
+---------------+ /
/ /
+---------------------------------------------------------------+
Replacing the NXT RR "type bit map" field is a variable length list
of RR types. Each RR type is 16 bits. As the list is of variable
length, a end-of-list indicator is require. End of the list is
signalled by a all-zero RR type. Each element is a 2 byte RR type.
The list MUST be sorted in RR type order. The change from NXT's
bitmap field removes the limit of authenticating only the first 127
RR types.
The RR type list indicates what types exist at the previous hash
value -- i.e. the first RR type list in the RRdata of a NO record
indicate what RR types exist at the domain name that hashes to the
owner name of that NO record. The second RR type list, if any, in
the RRdata of a NO record indicate what RR types exist at the domain
name that hashes the first SHA-1 value in the RRdata. And so on.
See below for a complete example of the on-the-wire-format of a NO
record with hash truncation and record merging and its
interpretation.
Length of the hash value field is denoted by the # hash octets
fields, it is a unsigned integer ranging from 0 to 255. The meaning
of a zero length integer is reserved.
The SHA-1 hash value field is a variable length field containing the
actual hash value.
The NO RRs for a zone SHOULD be automatically calculated and added
to the zone when SIGs are added. The NO RR's TTL SHOULD NOT exceed
the zone minimum TTL.
The type number for the NO RR is TBD.
NO RR's MUST only be signed by zone level keys [7].
3.3 NO RDATA on-the-wire format example
The following is a example of the on-the-wire-format of a complete
NO resource record were hash truncation and record merging is used.
It is the on-the-wire format of the NO record in section 3.13.1.2.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RR type 1 (A) | RR type 0 (end-of-list) |
+---------------+-----------------------------------------------+
| 1 hash octet | Hash 0x22 | RR type 2 (NS) |
+---------------+---------------+-------------------------------+
| RR type 6 (SOA) | RR type 15 (MX) |
+-------------------------------+---------------+---------------+
| RR type 0 (end-of-list) | 1 hash octet | Hash 0x83 |
+-------------------------------+---------------+---------------+
| RR type 1 (A) | RR type 0 (end-of-list) |
+---------------+-----------------------------------------------+
| 1 hash octet | Hash 0x90 | RR type 1 (A) |
+---------------+---------------+-------------------------------+
| RR type 16 (TXT) | RR type 0 (end-of-list) |
+---------------+---------------+-------------------------------+
| 1 hash octet | Hash 0x1b |
+-------------------------------+
The intepretation here is that the domain that corresponds with the
NO owner name ("1b.example.org.", not shown above) have a A record,
that the domain that hash to 0x22 (truncated to one octet) have a
NS, SOA and MX record, that the domain that hash to 0x83 (truncated
to 1 octet) have a A record etc. Note that the last hash value of NO
RRdata does not have a RR type list in the same record.
3.4 Owner Names
As the previous NO RR format describe, the "next domain name" of NXT
records is replaced by its hash value. This removes the possibility
of chaining both backwards and forwards through a zone.
But without also modifying the owner names of NO records it is still
not difficult to chain through a zone. Consider querying a server
for (say) "m.example.org", the reply could contain a NO record for
"g.example.org", now an adversary can lookup records for
"g.example.org", and then issue a query for a domain that would sort
(in "the canonical order" described in RFC 2535) just before
"g.example.org". Applying the technique over and over again, all
records in the zone can still be collected.
To prevent this, the owner names of NO records is replaced by its
hash value. As DNS places limits on what characters can be used in
owner names, the owner name uses a base 16 "hex" encoding [6].
In order to remove the (very small) chance of generated NO record
names colliding with existing, "real", domains, all NO records MUST
be stored directly in the "_no" domain of the zone. I.e., a zone
"example.org." store its NO records as, say,
"35a4d1._no.example.org.".
3.5 Additional Complexity Due To Wildcards
Proving that a non-existent name response is correct, or that a
wildcard expansion response is correct, makes things a little more
complex.
In particular, when a non-existent name response is returned, an NO
must be returned showing that the exact name did not exist and, in
general, one or more additional NO need to be returned to also prove
that there wasn't a wildcard whose expansion should have been
returned. (There is no need to return multiple copies of the same
NO.) These NOs, if any, are returned in the authority section of the
response.
Furthermore, if a wildcard expansion is returned in a response, in
general one or more NOs needs to also be returned in the authority
section to prove that no more specific name (including possibly more
specific wildcards in the zone) existed on which the response should
have been based.
3.6 No Considerations at Delegation Points
When NXT records are used to deny existance, there exists a special
case at delegation points. Namely, that two distinct RRsets exist
for the same owner name, one in the parent zone and one in the child
zone.
$ORIGIN parent.example.org.
@ SOA
NS
NXT child SOA NS SIG NXT
child NS foo
NXT next NS SIG NXT
next A 127.0.0.2
$ORIGIN child.parent.example.org.
@ SOA
NS
NXT name SOA NS SIG NXT
name A 127.0.2.1
NXT child.parent.example.org.
With NO records, the parent would deny existance of domains in
"_no.parent.example" and the child in
"_no.child.parent.example.org". Thus no NO RRset collision occur.
3.7 Hash Truncation and Dynamic Updates
Entities that create NO records MAY truncate the hash field. It
MUST NOT truncate hash fields so they are no longer unique
throughout a zone. Hash truncations MUST only be done to octet
offsets. Truncation MUST be such that less significant bits are
truncated, i.e. higher-order bits are preserved (see the examples).
Zones that are dynamically updated will have to calculate and add NO
records on-the-fly. If hash truncation is used, adding a new name
to the zone will require updating (and signing) NO records for the
conflicting name(s).
As this recalculation might be quite inefficient, the use of
"shortest unique prefix" truncation in dynamically updated zones is
not recomended. However, a truncation to, say, 64 bits might be
possible if the administrators are willing to have their software
perform costly operations once every ~2^32 update (on average).
Since truncation (and also "compression" described in the next
section) make it impossible to predict the corresponding NO record
given a domain name, resolvers should not ask for a hashed NO record
(aaaa._no.example.org. IN NO) for a known domain name if they want
to find out what types exist at the domain. Instead, a resolver
might ask for NO records on the original name (www.example.org. IN
NO). Such records will never exist, and the correct NO record will
be returned by the server.
To summarize, the behaviour of hash truncation should be
configurable in the entity that creates NO records, to accomodate
different usage-patterns. If the zone is not intended to be
dynamicly updated, the use of hash truncation reduces size and is
recomended.
3.8 Reducing Number of Records
Entitities that create NO records MAY deny existence for several
records per NO record. Entities that create NO records should take
care so that each resulting NO record is not "too large". [Comments
on this? Should there be a specific limit? It might be left as a
DNS Operational consideration]
Merging several NO records into one record also place more work on
the resolver. Instead of parsing two hash values for each NO record
to determine if it's applicaple, a resolver will have to parse
several hash values and compare each.
The NO RR record consist of one or more RR type list / hash values,
described above, and a resolver need to parse the entire record to
collect each individual field. I.e., a NO parse algorithm could
looke like: read RRs, stop when you read a zero RR type, read hash
length indicator, read hash size, if the entire record is read stop,
otherwise read RRs, stop when you read a zero RR type, etc..
3.9 Hash Collisions
Hash value collisions are expected not to occur. For SHA-1, the
probability that this should happen is 1 out of 2^80 on average.
However, collisions are actually only a problem if the domain names
producing the same hash value have different sets of existing types.
Consider the following records
; SHA-1(one.example.org) = SHA-1(two.example.org)
one.example.org. IN A 1.2.3.4
two.example.org. IN A 5.6.7.8
Given that no other RR types exist for neither domain, both
"one.example.org" and "two.example.org" would be authenticated not
to exist by the same NO record.
3.10 Authenticating Denial of NO Records
NO records (together with SIG records) authenticate denial for other
types in a zone. Unlike NXT records that re-use the namespace in
the zone, NO records are not intended to authenticate denial of
other NO records.
3.11 Case Considerations
Before calculating SHA-1 hash values, domain names MUST be converted
into canonical format as described in RFC 2535. This is to make hash
comparisons possible.
3.12 Presentation Format
NO RRs do not appear in original unsigned zone master files since
they should be derived from the zone as it is being signed.
If a signed file with NO records is printed or NO records are
printed by debugging code, they appear as a list of unsigned
integers or RR mnemonics, and the hash value in base 16 hex
representation [6] with "0x" prepended (to distinguish it from
integer RR codes). The zero RR that terminate the list of RR types,
and the hash value length indicator, does not appear.
See the next section for examples of printed NO RRs.
3.13 Examples
This section contain examples of NO records, using the reserved
domain exmaple.org [3].
3.13.1 Adding NO Records to a Zone
Consider the following zone file.
$ORIGIN example.org.
@ IN SOA ...
@ IN NS ns
@ IN MX 0 server
ns IN A ...
www IN A ...
sERVEr IN A ...
sERVEr IN TXT "text"
; SHA1(example.org.) = 0x222c7a74bc40e818aa53b3eb0b15cd2350fbb3a1
; SHA1(ns.example.org.) = 0x1b7838c69a66eb50cc215f66ee4554d0c4c940a5
; SHA1(www.example.org.) = 0x839ebd4386c0b26d81f147421b5b7036e61438cf
; SHA1(server.example.org.) = 0x906a0ad5e604b1905828499dc8672ecb8de73e2f
Note that hash values are calculcated on the canonical form.
The following two sections describe two valid ways of adding NO
records to a zone.
3.13.2 Simple NO creating entity
A simple NO creator entity might not implement truncation or provide
the possibility to deny more than one records per NO record. In
this case, the following would be added to the zone file.
$ORIGIN _no.example.org.
1b7838c69a66eb50cc215f66ee4554d0c4c940a5
IN NO A 0x222c7a74bc40e818aa53b3eb0b15cd2350fbb3a1
222c7a74bc40e818aa53b3eb0b15cd2350fbb3a1
IN NO NS SOA MX 0x839ebd4386c0b26d81f147421b5b7036e61438cf
839ebd4386c0b26d81f147421b5b7036e61438cf
IN NO A 0x906a0ad5e604b1905828499dc8672ecb8de73e2f
906a0ad5e604b1905828499dc8672ecb8de73e2f
IN NO A TXT 0x1b7838c69a66eb50cc215f66ee4554d0c4c940a5
3.13.3 Advanced NO creating entity
A more advanced NO creator entity might append the following
instead, using both truncation and "compression".
$ORIGIN _no.example.org
1b IN NO A 0x22 NS SOA MX 0x83 A 0x90 A TXT 0x1b A
Note that this contain 5 hash values while the zone only contain 4
records, the last value in the line above is in fact the first hash
value in the zone, closing the circular NO chain.
3.13.4 Resolver Query for Non-existing Domain
Consider a client looking up the non-existant domain name
"baz.example.org.", using the zone file from the previous section.
First, we note the following calculations.
SHA-1(baz.example.org.) = 0xd5d0f98783eec6e9943750f35904304bd1a4090e
SHA-1(*.example.org.) = 0x7ab3776e3b529eb42467cc5d279c88ec951cf021
A server would reply with an RCODE of NXDOMAIN and the authority
section data including the following:
; backwards compatibility
example.org. IN SOA
; prove no baz.example.org
906a0ad5e604b1905828499dc8672ecb8de73e2f.example.org.
IN NO A TXT 0x1b7838c69a66eb50cc215f66ee4554d0c4c940a5
906a0ad5e604b1905828499dc8672ecb8de73e2f.example.org. IN SIG NO
; prove no *.example.org:
222c7a74bc40e818aa53b3eb0b15cd2350fbb3a1.example.org.
IN NO NS SOA MX 0x839ebd4386c0b26d81f147421b5b7036e61438cf
222c7a74bc40e818aa53b3eb0b15cd2350fbb3a1.example.org. IN SIG NO
In order for a client to verify the authenticity of this reply, in
addition of verifying the SIG record, it will also need to calculate
the one-way hash of "baz.example.org." and verify it is contained
inside the interval of any NO record in the authority section.
Also, to prove there are no wildcard records for baz.example.org.,
NO records for possible wildcard expansions are returned. A client
should similarily calculate hash values of possible wildcards and
compare them to the authority section.
Of course, if the zone was generated with the more advanced NO
creating entity, only the NO record from the previous section would
have to be returned.
3.13.5 Resolver Query for Non-existing Type At Existing Domain
Consider a client looking up TXT records for the existing domain
"www.example.org.", again, using the same zone file as previous. A
server would reply with an authority section like the following:
839ebd4386c0b26d81f147421b5b7036e61438cf.example.org.
IN NO A 0x906a0ad5e604b1905828499dc8672ecb8de73e2f
839ebd4386c0b26d81f147421b5b7036e61438cf.example.org. IN SIG NO
The resolver verifies the signature and make sure
SHA-1("bar.example.org.") hashes correctly.
4. Security Considerations
Chaining through all NO records is still technically possible,
altough it cannot be used to collect names and/or records in the
zone (other than NO records themself).
The security of NO record hash values is dependent on the security
of the SHA-1 hash functions used.
It should be pointed out that given this scheme, it is easy to
estimate the number of records within a zone, considering hash
values are supposed to be equally distributed. This can be foiled
by adding any number of bogus records in the zone.
The authentication of NO records is provided by DNS SIG records, as
specified in RFC 2535. The security considerations of RFC 2535 is
not affected by this document, and should also be considered.
5. IANA Considerations
The NO RR type number should be selected by the IANA from the normal
RR type space.
The meaning of a zero hash length value can only be assigned by a
standards action.
Acknowledgements
The idea of encrypting names, that later evolved into just hashing
them, was originally proposed by Jonas Holmerin in private
discussions about DNS Security. Magnus Nyström proposed truncating
the hash values.
Thanks to John Linn and Magnus Nyström for comments on a early
version of this draft.
Olafur Gudmundsson pointed out delegation point issues, suggested
the use of a "_no" subdomain, and suggested replacing the type bit
map field with a sorted list. From the namedroppers mailing list,
I'd like to thank Andrew Draper, Andreas Gustafsson, Peter Koch and
Brian Wellington for comments and suggestions.
References
[1] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[2] Eastlake, D. and O. Gudmundsson, "Storing Certificates in the
Domain Name System (DNS)", RFC 2538, March 1999.
[3] Eastlake, D. and A. Panitz, "Reserved Top Level DNS Names", RFC
2606, June 1999.
[4] NIST, , "Secure Hash Standard", FIPS PUB 180-1, April 1995.
[5] Massey, D., Lehman, T. and E. Lewis, "DNSSEC Implementation in
the CAIRN Testbed.", I.D. draft-ietf-dnsop-dnsseccairn-00.txt,
October 1999.
[6] Josefsson, S.A. (editor), "Base 64, 32 and 16 Encodings", I.D.
draft-josefsson-base-encoding-00.txt, August 2000.
[7] Wellington, B, "Domain Name System Security (DNSSEC) Signing
Authority", I.D. draft-ietf-dnsext-signing-auth-01.txt, May
2000.
Author's Address
Simon Josefsson
RSA Security
Arenavägen 29
121 29 Stockholm
Sweden
Phone: +46 8 7250914
EMail: sjosefsson@rsasecurity.com
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