Network Working Group                                        R. Van Rein
Intended status: Informational                            March 15, 2016
Expires: September 16, 2016

         Declaring Kerberos Realm Names in DNS (_kerberos TXT)


   This specification defines a method to determine Kerberos realm names
   for services that are known by their DNS name.  Currently, such
   information can only be found in static mappings or through educated
   guesses.  DNS can make this process more flexible, provided that
   DNSSEC is used to assure authenticity of resource records.

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   This Internet-Draft will expire on September 16, 2016.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Defining _kerberos TXT Resource Records . . . . . . . . . . .   3
   3.  Querying Kerberos Realm Names . . . . . . . . . . . . . . . .   5
   4.  Publishing Kerberos Realm Names . . . . . . . . . . . . . . .   6
   5.  Efficiency Considerations . . . . . . . . . . . . . . . . . .   7
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   When a Kerberos client contacts a service, it needs to acquire a
   service ticket, and for that it needs to contact the KDC for a realm
   under which the service is run.  To map a service name into a realm
   name and then into a KDC, clients tend to use static mappings or
   educated guesses; the client's KDC may or may not be involved in this
   process.  Through DNS, the static mappings could be replaced by
   dynamic lookups, and migrate from local client configuration into the
   hands of the party administrating a server's presence in DNS.  This
   brings improved flexibility and centralisation, which is
   operationally desirable.

   Two mappings are needed for a client to contact a service.  One is a
   mapping from the FQDN of a service to its realm name; the other is a
   mapping from the realm name to the Kerberos-specific services such as
   the KDC.  The latter mapping is published in SRV records [RFC4120]
   and such traffic is usually protected by Kerberos itself.  The first
   mapping however, has hitherto not been standardised and is ill-
   advised over unsecured DNS because the published information is then
   neither validated by DNS nor does it lead to a protocol that could
   provide end-to-end validation for it.

   With the recent uprise of DNSSEC, it is now possible to make a
   reliable judgement on the authenticity of data in DNS, which enables
   the standardisation of the first mapping in the form of resource
   records under DNSSEC.

   This specification defines a method to publish and process Kerberos
   realm names in TXT resource records.  These records hold a case-
   sensitive string with the realm name.  Multiple TXT records may be

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   defined as alternatives in contexts that welcome this; they suggest
   iterating over the values until a ticket can be procured.

   It is suggested to use the name "_kerberos TXT" to informally refer
   to the style of using DNS that is introduced in this specification.

2.  Defining _kerberos TXT Resource Records

   This specification uses the TXT resource record type in DNS to
   represent a Kerberos realm name.  The corresponding RDATA format is
   as follows:

   /                   REALMNAME                   /

   The REALMNAME is represented as a <character-string> [RFC1035] which
   starts with a single-byte length, followed by as many bytes of realm
   name as the length byte's value.  The RDATA field therefore has a
   length of 1 up to 256 bytes, to hold a realm name of 0 up to 255
   bytes.  For instance, a realm EXAMPLE.ORG would be represented with
   the following RDATA, written in the notation for unkonwn resource
   record types [RFC3597]:

   \# 12 ( 0b 45 58 41 4d 50 4c 45 2e 4f 52 47 )

   The REALMNAME represents a Kerberos realm name, not a DNS name;
   invalid names SHOULD be ignored.  The empty string is considered an
   invalid REALMNAME, and it should be noted that a REALMNAME may exceed
   the size constraints of a DNS name.  The Kerberos realm name MUST NOT
   have an additional trailing dot unless it is actually part of the
   Kerberos realm name.  The realm name is never relative to the DNS
   name at which the TXT record was found.  Finally, there is no
   required relationship (such as partial overlap) between the realm
   name and the DNS name at which the TXT record was found.

   The TXT record can hold one or more <character-string> values in an
   ordered sequence, and implementations of this specification MUST NOT
   reject TXT records with multiple <character-string>s.  This
   specification only describes the meaning of the first <character-
   string> as a REALMNAME, and leaves the interpretation of further
   <character-string>s to future specifications.  Until these
   specifications are adopted, master zone files SHOULD NOT introduce
   these extra <character-string>s.  If such future specifications
   intend to specify Kerberos aspects that do not include a realm name,
   then they can mention an invalid realm name such as an empty

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   Realm names in Kerberos are often domain-styled [Section 6.1 of
   [RFC4120]], in which case they look like DNS names but are case
   sensitive; unlike the DNS names used as lookup keys in the DNS
   hierarchy, the REALMNAME format follows the <character-string> format
   in being case-sensitive.

   In fact, the <character-string> format is a binary format, and DNS
   notation \DDD [Section 5.1 of [RFC1035]] exists to put arbitrary
   bytes in the string notation.  This binary format leaves the door
   ajar for future internationalisation of Kerberos realm names.  Realm
   names are defined with the KerberosString type [Section 5.2.1 of
   [RFC4120]] which is an ASN.1 GeneralString, but its specification
   currently advises to constrain the use of this string type to an
   IA5String (basically using only the first 128 codes of the ASCII
   table) to avoid interoperability problems.  After the <character-
   string>'s length byte, the REALMNAME holds the value of the
   GeneralString, but not its preceding ASN.1 tag and length.

   It is worth noting that the ESC "%" "G" prefix [TODO:xref
   target="ISO2022"/] can be used to introduce an UTF8String in a
   GeneralString, and that implementations exist that insert UTF8String
   values in KerberosString fields without even that escape.  All this
   precedes formal standardisation of internationalisation, but it
   suggests that the RDATA definition for TXT can be supportive of
   future internationalisation of realm names, even if the current
   advised use is limited to the value of an IA5String.

   The format for the resource data in master zone files is standard for
   DNS [RFC1035].  The TXT record is a general record and was not
   especially designed for this purpose.  The reason to use it
   nonetheless is that too many middle boxes suppress unknown DNS
   resource record types; we distinguish the particular use specified
   here by always prefixing a fixed _kerberos label to the DNS name that
   is being investigated.  An example declaration of realm name
   EXAMPLE.ORG for a server named would be:  IN TXT  "EXAMPLE.ORG"

   The RDATA for this TXT record has already been shown above, in the
   generic RDATA section notation.

   As another example, an IMAP server may be specified through SRV
   records [RFC2782], in which case the domain holding those records
   needs to hold the TXT record, rather than the pointed-to server name,
   for example:  IN SRV  10 10 143   IN TXT  "EXAMPLE.ORG"

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   In operational contexts where both the SRV record resolution as the
   direct host name may be applied, it is also possible to define both
   kinds of TXT records; it depends on the application and its
   configuration in which single location it will look for the Kerberos
   realm name.

   Multiple TXT records may be supplied for the same _kerberos-prefixed
   DNS name; these represent alternatives that can be tried by the DNS
   client.  Since DNS delivers the resource records as a set without any
   particular order, the DNS client can choose freely in what order to
   process these records.

   It is possible to create a TXT record for any _kerberos-prefixed DNS
   name, but this specification only provides query procedures for host
   names and domain names.  The use with a domain name has the
   additional use of denoting the precise spelling for a realm name
   under its DNS-mapped name.  DNS-mapped names currently would not
   modify more than the case of a DNS name, and even that is only done
   as the result of DNS compression [RFC4343]; but in a future with
   internationalised realm names there might be more to reconstruct, in
   which case this facility is likely to be helpful.

   It is worth noting that the authority of a _kerberos TXT record is
   determined by the domain name holding it, not the realm contained in
   it.  Since there is no defined relationship between these two pieces
   of information, proper security may require reverse lookups in DNS
   and/or Kerberos.

3.  Querying Kerberos Realm Names

   This sections defines a procedure for determining the Kerberos realm
   names for a server with a given host name or domain name, as well as
   for a DNS-mapped realm name.

   When applications directly know their server host name, perhaps
   because it is mentioned in a URL or in a ticket as a service
   principal name, they will lookup the TXT record in DNS at the server
   host name, prefixed with a _kerberos label.

   When applications locate their servers through a domain name, for
   example via MX [RFC1035] or SRV records [RFC2782], a choice between
   the use of the domain name or the appointed server host names is to
   be made.  In these situations, the TXT query MUST use only the domain
   name, without adding prefixes such as _Service._Proto for SRV
   records, but with an additional label _kerberos prefixed.  The
   appointed servers for a domain service MUST NOT be used to locate TXT
   records, since these may reside under operational control foreign to
   the service domain, which could introduce a security problem; in

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   addition, the possible shared use of a host in the SRV records of
   many domains would mean that they would have to specify alternatives,
   where it is not always certain that clients will be able to process
   more than one such alternative.  The domain name holding the SRV
   record is closer to the client's context and provides a better-
   targeted location for the _kerberos TXT record.

   Since DNS in general cannot be considered secure, the client MUST
   validate DNSSEC and it MUST dismiss any DNS responses that are
   Insecure, Bogus or Indeterminate [Section 5 of [RFC4033]].  Only the
   remaining Secure responses are to be taken into account.  This
   specification does not require that the DNS client validates the
   responses by itself, but a deployment of _kerberos TXT records SHOULD
   NOT accept DNS responses from a trusted validating DNS resolver over
   untrusted communication channels.

   In addition to the above, the absense of a _kerberos DNS record may
   be meaningful for security decisions.  If such cases, the only denial
   of existence of the _kerberos TXT records MUST be authenticated

   To give one possible implementation, a Kerberos client or its KDC may
   send DNS queries with the Authentic Data (AD) bit set to enable
   DNSSEC [Section 5.7 of [RFC6840]], and thereby request that the
   Authenticated Data bit is set in the response to indicate [RFC3655]
   the Secure state for answer and authority sections of the response.
   When the DNS traffic to and from the validating resolver is
   protected, for instance because the validating resolver is reached
   over a loopback interface, then the Kerberos client or its KDC has
   implemented the requirements for Secure use of the answer and
   authority sections in DNS responses.

   When no Secure DNS responses are received when the DNS query times
   out, then the TXT query MUST be terminated without extracting realm
   names from DNS.  This termination MAY be done immediately upon
   receiving Secure denial for the requested TXT record.  TXT query
   termination need not be fatal; non-DNS procedures may exist to find a
   realm name, including the current practice of static mappings and
   educated guessing.

4.  Publishing Kerberos Realm Names

   To facilitate the dynamic lookup procedures described in this
   specification, _kerberos TXT records should generally be published at
   the domain names and server host names that a Kerberos application
   may want to approach.  Their location is prefixed with an extra
   _kerberos label to distinguish the specific use of the TXT record
   defined herein.

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   As an exception to this general rule, server host names may be
   exempted if they are always looked up through a domain name.  Note
   however, that most query mechanisms that lookup domain names through
   special resource record types have fallbacks to plain server host
   name lookups.  When such direct server host names are supported in a
   deployment of those applications, usually meaning that these server
   host names may occur in Kerberos service principal names, and when
   the realm name cannot be detected through a preferred mechanism for
   such server host names, then an additional _kerberos TXT record
   located at the server name is also going to be helpful.

   There may be situations where a domain represents a Kerberos realm,
   but uses domain-style references such as MX and SRV records to point
   to foreign server host names.  Such foreign server host names are
   often unsuitable to accompany with a _kerberos TXT record, so it is
   advisable to either not refer to the server host names directly in
   service tickets, or to see to it that a realm name will be supplied
   by the KDC.

   These instructions are somewhat flexible, as a result of the pre-
   existing static mechanisms for finding a realm name for a given
   service principal name.  Assuming that the more flexible approach of
   _kerberos TXT records is preferred, a useful functionality test would
   be to access the targeted applications from Kerberos clients without
   any static configuration.

   Zones that intend to provide applications with Kerberos realm names
   through _kerberos TXT records MUST be protected by DNSSEC to make
   them usable to those applications.

   Note that _kerberos TXT records with wildcard names will not work.
   All host names and most domain names define at least one resource
   record (of any type) with the name that the wildcard should cover.
   These defined names cause the wildcards to be suppressed [RFC4592]
   from DNS responses, even when querying a non-existent TXT record.

5.  Efficiency Considerations

   The _kerberos TXT records are intended to define realm names for
   servers whose DNS information is not statically mapped in a Kerberos
   setup.  This may simplify operational control of such static
   mappings.  It may also introduce more dynamicity, which is exptected
   to be useful for such things as realm crossover.

   The lookup of _kerberos TXT records can be done by the Ticket
   Granting Service of a KDC, which can respond with a Server Referral
   [Section 8 of [RFC6806]] to Kerberos clients that enable
   canonicalization.  This can be used for clients that are not setup to

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   query DNS as specified above, and that will assume that a service is
   running under the client's realm.  The caching of DNS records, their
   validation and possibly realm-crossover caching at the KDC can all
   benefit the response time for future lookups by other Kerberos

   The use of _kerberos TXT records helps the Kerberos client and/or its
   KDC to focus on the realm for which a service ought to have a name
   defined.  This generally provides a deterministic path, reducing the
   effort of guessing.

   The definition of _kerberos TXT records under a domain rather than
   under a server host name (for cases where a choice exists, such as
   for applications that use SRV or MX records) should limit the choice
   of realm names, because the domain is not farther away from the
   client's domain/realm than the server host name.

   Constraining the number of paths to a target realm to one is
   especially useful for situations with realm crossover when this
   involves public-key cryptography, because such algorithms are much
   slower than the symmetric algorithms that are normally used for

6.  Privacy Considerations

   The information contained in a _kerberos TXT record for a domain is
   barely more than the domain's DNS name already holds; the _kerberos
   TXT record spells out the case-sensitive realm name, and implies the
   use of Kerberos.  The use of Kerberos is usually also implied by the
   presence of certain SRV records.

   The _kerberos TXT records also marks the names of Kerberised servers.
   Internal use may prohibit such disclosure, but in those use cases it
   is often possible to rely on existing mechanisms for guessing a realm
   name, including simply using the realm name under which a client
   logged on with Kerberos.

7.  Security Considerations

   There is no restriction for _kerberos TXT records to mention realm
   names that map back to DNS names in a disjoint part of the DNS
   hierarcy.  The records could therefore specify realm names for a
   service even if the service is not recognised by the realm.  The KDC
   for the appointed realm would be very clear about that when trying to
   procure a service ticket, so there is no anticipated security issue
   with such misguided use of _kerberos TXT records.

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   The general point is that the use of DNSSEC makes Kerberos accept
   authentic information from the party that publishes the _kerberos TXT
   record, and that party could specify improper realm names or drop
   realm names that are vital to the client.  This is not expected to be
   a security risk either; the party publishing the _kerberos TXT record
   is the same party that publishes the service's records, namely its
   DNS operator.  By publishing the service's record in DNS, this
   operator already has potential control over service denial and other
   man-in-the-middle attacks, so the _kerberos TXT record does not add
   any new powers of abuse.

   When an external attacker would be permitted to spoof a _kerberos TXT
   record in a victim's DNS, then it could be possible for that attacker
   to convince the client that the attacker is the authentic provider
   for the service.  Additional spoofing of host name references could
   then complete the attack.  This has been mitigated by strictly
   requiring Secure validation results from a DNSSEC-aware resolver for
   all _kerberos TXT records.

8.  IANA Considerations


9.  References

9.1.  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,

   [RFC3597]  Gustafsson, A., "Handling of Unknown DNS Resource Record
              (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
              2003, <>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,

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   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              DOI 10.17487/RFC4120, July 2005,

   [RFC4343]  Eastlake 3rd, D., "Domain Name System (DNS) Case
              Insensitivity Clarification", RFC 4343,
              DOI 10.17487/RFC4343, January 2006,

   [RFC6806]  Hartman, S., Ed., Raeburn, K., and L. Zhu, "Kerberos
              Principal Name Canonicalization and Cross-Realm
              Referrals", RFC 6806, DOI 10.17487/RFC6806, November 2012,

9.2.  Informative References

   [RFC3655]  Wellington, B. and O. Gudmundsson, "Redefinition of DNS
              Authenticated Data (AD) bit", RFC 3655,
              DOI 10.17487/RFC3655, November 2003,

   [RFC4592]  Lewis, E., "The Role of Wildcards in the Domain Name
              System", RFC 4592, DOI 10.17487/RFC4592, July 2006,

   [RFC6840]  Weiler, S., Ed. and D. Blacka, Ed., "Clarifications and
              Implementation Notes for DNS Security (DNSSEC)", RFC 6840,
              DOI 10.17487/RFC6840, February 2013,

Appendix A.  Acknowledgements

   Thanks are due to the Kitten Workgroup for discussions during the
   creation of this document.  Especially Greg Hudson, Nico Williams and
   Viktor Dukhovni have added useful input.

   This work was conducted under a grant from the programme "[veilig]
   door innovatie" from the government of the Netherlands.  It has also
   been liberally supported by the NLnet Foundation.

Author's Address

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   Rick van Rein
   Haarlebrink 5
   Enschede, Overijssel  7544 WP
   The Netherlands


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