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A. NO Resource Records

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)

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 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

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   This Internet-Draft will expire on February 22, 2001.

Copyright Notice

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


   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

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

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

   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

                        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 ("", 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) "", the reply could contain a NO record for
   "", now an adversary can lookup records for
   "", and then issue a query for a domain that would sort
   (in "the canonical order" described in RFC 2535) just before
   "".  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
   "" store its NO records as, say,

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

   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

   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

   @       SOA
           NXT     child   SOA NS SIG NXT
   child   NS      foo
           NXT     next    NS SIG NXT
   next    A

   @       SOA
           NXT     name    SOA NS SIG NXT
   name    A

   With NO records, the parent would deny existance of domains in
   "_no.parent.example" and the child in
   "".  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
   ( 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 ( 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

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( = SHA-1(	IN A	IN A

   Given that no other RR types exist for neither domain, both
   "" and "" 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 [3].

3.13.1 Adding NO Records to a Zone

   Consider the following zone file.

   @	IN SOA ...
   @	IN NS ns
   @	IN MX 0 server
   ns	IN A ...
   www	IN A ...
   sERVEr	IN A ...
   sERVEr	IN TXT "text"

   ; SHA1(          = 0x222c7a74bc40e818aa53b3eb0b15cd2350fbb3a1
   ; SHA1(       = 0x1b7838c69a66eb50cc215f66ee4554d0c4c940a5
   ; SHA1(      = 0x839ebd4386c0b26d81f147421b5b7036e61438cf
   ; SHA1(   = 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.

   		IN NO A 0x222c7a74bc40e818aa53b3eb0b15cd2350fbb3a1
   		IN NO NS SOA MX 0x839ebd4386c0b26d81f147421b5b7036e61438cf
   		IN NO A 0x906a0ad5e604b1905828499dc8672ecb8de73e2f
   		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".

   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
   "", using the zone file from the previous section. 
   First, we note the following calculations.

   SHA-1( = 0xd5d0f98783eec6e9943750f35904304bd1a4090e
   SHA-1(*   = 0x7ab3776e3b529eb42467cc5d279c88ec951cf021

   A server would reply with an RCODE of NXDOMAIN and the authority
   section data including the following:

   ; backwards compatibility		IN SOA

   ; prove no
   		IN NO A TXT 0x1b7838c69a66eb50cc215f66ee4554d0c4c940a5 IN SIG NO

   ; prove no *
   		IN NO NS SOA MX 0x839ebd4386c0b26d81f147421b5b7036e61438cf 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 "" 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,
   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
   "", again, using the same zone file as previous.  A
   server would reply with an authority section like the following:
   		IN NO A 0x906a0ad5e604b1905828499dc8672ecb8de73e2f IN SIG NO

   The resolver verifies the signature and make sure
   SHA-1("") 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.


   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.


   [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

Author's Address

   Simon Josefsson
   RSA Security
   Arenavägen 29
   121 29  Stockholm

   Phone: +46 8 7250914

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