Network Working Group                                        R. Van Rein
Intended status: Standards Track                       November 13, 2014
Expires: May 17, 2015

               Kerberos Realm Descriptors in DNS (KREALM)


   This specification defines methods to determine Kerberos realm
   descriptive information for services that are known by their DNS
   name.  Currently, finding such information is done through static
   mappings or educated guessing.  DNS can make this process more
   dynamic, provided that DNSSEC is used to ensure authenticity of
   resource records.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 17, 2015.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  The KREALM Resource Record  . . . . . . . . . . . . . . . . .   3
   3.  Defining Home Realms and Home Records . . . . . . . . . . . .   4
   4.  Querying Realm Descriptors  . . . . . . . . . . . . . . . . .   5
     4.1.  Querying a Domain's Realm Descriptors . . . . . . . . . .   6
     4.2.  Querying a Host's Realm Descriptors . . . . . . . . . . .   6
   5.  Publishing Realm Descriptors  . . . . . . . . . . . . . . . .   7
   6.  Tags in Realm Descriptors . . . . . . . . . . . . . . . . . .   7
     6.1.  Realm Descriptor Tag "realm"  . . . . . . . . . . . . . .   8
     6.2.  Realm Descriptor Tag "service"  . . . . . . . . . . . . .   9
   7.  Efficiency Considerations . . . . . . . . . . . . . . . . . .  10
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  11
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     11.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   When a Kerberos client contacts a service, it needs to obtain 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, such mappings could be dynamically expanded,
   permitting more flexibility than under the current practice.

   Two mappings are needed for the given scenario.  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 protected by the Kerberos protocol 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
   validate it.

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

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   This specification defines how to publish and process Realm
   Descriptors using a newly defined resource record type KREALM.  Each
   of these records holds of a series of tagged string values.  A few of
   these are defined below, others may be added by future

2.  The KREALM Resource Record

   This specification introduces a new DNS resource record that serves
   as a realm descriptor for Kerberos.  The name for this DNS resource
   record type is KREALM, and its numeric value is TBD1.  The
   corresponding RDATA format is as follows:

   /                  KREALM-DATA                  /

   The syntax of KREALM-DATA is the DER encoding of the following ASN.1

           versionNumber INTEGER (0..) DEFAULT 0,
           SET OF SEQUENCE {
               tag IA5String,
               value UTF8String

   This specification makes the assumption that all values are
   represented as strings.

   DISCUSS: RFC 4120 defines realm names with the KerberosString type
   which is a GeneralString, but it then advises to constrain it to
   IA5String or else risk interoperability problems.  It is worth noting
   that the ESC "%" "G" prefix defined in ISO 2022 can be used to
   introduce an UTF8String in the KerberosString, and that
   implementations even exist that insert UTF8String in KerberosString
   fields without even that escape.  It's a jungle out there!  Defining
   UTF-8 for this new field type does not seem to be such a stretch; it
   includes the IA5String subset and it also keeps the door ajar for
   future attempts at I18N of realm names and other Kerberos parameters.
   OTOH, OCTETSTRING would probably be too general, and place too much
   faith in wire representations to be suitable for comparison.

   The advised format for the resource data in master zone files is the
   base64-encoded KREALM-DATA content in DER-encoding.  Although most
   data will be printable string data, it need not adhere to an ASCII
   character set, may include NUL characters and have a somewhat complex

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   grammar; all these aspects would complicate master zone files and
   thus make them more error-prone.

   Tags are registered with IANA, and this document defines the first
   few.  A special range of tags starting with "x-" is available for
   local or experimental use.  Implementations MAY safely ignore tags
   (and the corresponding value) that are not known to them.  An
   application that does not recognise a tag name MUST silently discard

   The versionNumber 0 is used as a version number for this
   specification; it is also the default versionNumber when absent from
   the encoding.  Future updates to this specification MUST use another
   versionNumber IF they are invalidate any assumptions made in this
   specification.  Such new applications SHOULD advise how to setup DNS
   in a backward-compatible manner; they might for instance publish both
   old and new styles of KREALM records.

   Clients requesting KREALM records MUST ensure that the record uses
   proper syntax, including the string formats specified for tag and
   value fields and a versionNumber that the client understands.

   Multiple KREALM records may be supplied under a queried name, and
   there may be multiple that adhere to this syntax; these present
   alternatives that can be tried.  Since DNS does not supply them in
   any order, the DNS client can choose freely in what order to process
   these records.

   In this general form, there are no constraints on the number of
   permissible occurrences of a tag in one or more KREALM records, but
   tags MUST define whether multiple occurrences are permitted, and if
   so, what their interpretation is.

3.  Defining Home Realms and Home Records

   One of the tags defined by this specification is the "realm" tag
   [Section 6.1] that specifies a realm name.  Realm names can be mapped
   [Section 7.2.3 of [RFC4120]] to DNS names.  Those realm names in
   "realm" tags of KREALM records that that map to the FQDN at which
   that KREALM record was found, are hereby defined as "Home Realms".

   KREALM records of which all realm name tags are Home Realms, are
   hereby defined as "Home Records".  Note how KREALM records are not
   defined to be Home Realms when they contain a mixture of Home Tags
   and "realm" tags that are not Home Tags.

   In general, the DNS name to which a realm name maps can hold
   information about the location of a KDC [Section 7.2.3 of [RFC4120]]

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   and other realm-specific services.  This does not change; but for
   Home Realms, the same FQDN is the basis for finding the KREALM record
   and the KDC and other services; for other KREALM records that Home
   Records, the KREALM tag is a reference to a realm that translates to
   another DNS name, which in turn serves as the basis for finding the
   KDC and other realm services.

   The following specifications are strict about their reliance on
   DNSSEC for KREALM records.  The reason for this is to ensure that
   both the pointer to the actual service and its containing realm are
   inserted into DNS by the same responsble party.  When combined with
   cryptographic ensurance of reaching the proper KDC for a realm, this
   provides a secure mechanism for access to realms can be created,
   whether or not they are the realm into which a user logged on.

   Mechanisms for cryptographic ensurance of reaching realms is standard
   in Kerberos implementations; based on DNSSEC this may even be
   extended with more dynamic mechanisms, but that is not defined in
   this specification.  A result that this specification may have is
   that the user knows the realms to ask for, even if it is a realm that
   was hitherto unknown.  In that sense, the KREALM record may be a
   stepping stone in loosely connected links between Kerberos realms.

4.  Querying Realm Descriptors

   The following subsections define two procedures for finding Kerberos
   realm descriptors for the DNS name of a service.  One procedure
   starts from a domain name, the other starts from the host name of a

   When dealing with services found through DNS SRV [RFC2782], a choice
   between the use of a domain name or host name is possible.  In these
   situations, the FQDN of the SRV queries, without the _Service._Proto
   prefix, MUST be used in the procedure for domain name queries, and
   the procedure for querying a domain should be followed rather than
   the procedure for a host name.

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

   To give one possible implementation, a Kerberos client may send DNS
   queries with the DNSSEC OK bit [RFC3225] set to enable DNSSEC, and
   require that the Authenticated Data bit [RFC3655] is set in the

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   response to indicate 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 that resolver
   is reached over a loopback interface, then the Kerberos client has
   implemented the requirements for Secure use of the answer and
   authority sections in DNS responses.

   The result may contain KREALM records that do not adhere to the
   syntax of this specification; such KREALM records MUST be removed
   from the result.  In the sequence of tag-and-value pairs, there may
   be tags that are unknown; such tags and their value MUST be ignored
   when further processing the results.  Finally, some tags are
   specifically registered with IANA for use in Home Tags, and those
   MUST be ignored if the KREALM record is not a Home Record.

   When no Secure DNS responses are received, this procedure MUST be
   terminated without extracting realm descriptive information from DNS.
   Such termination need not be fatal; non-DNS procedures may exist to
   find a realm name.

4.1.  Querying a Domain's Realm Descriptors

   To find Kerberos realm descriptors for a domain name, a DNS client
   conducts a KREALM query targeted directly at the domain name.

   Where this specification speaks of querying a domain, its
   interpretation of a domain is that of a name space, which may or may
   not have a host attached, but which is likely to have services
   attached, for instance through MX or SRV records.  Domain names also
   occur in many naming schemes after an optional username and @ symbol,
   such as the domain name (that also happens to be termed "realm", but
   without connection to Kerberos realms) in a Network Access Identifier

4.2.  Querying a Host's Realm Descriptors

   To find a realm descriptor for a host name, a KREALM query is
   performed to the same FQDN as that of the host name.  If this fails
   with a secure denial, a fallback KREALM query is done on the FQDN of
   the zone apex.  The reason to use the zone apex in this role is that
   it signifies the start of administrative control over a zone,
   generally making it cover a larger DNS name range than a single host
   name, while still residing under the same operational control as the
   host name itself -- which is valuable from a security viewpoint.

   Without a signed denial, no fallback query is performed; this
   mitigates denial of service attacks on that host name's FQDN.  With a
   signed denial, evidence of non-existence of the KREALM record is

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   returned, and part of that is a field named the Signer's Name.  This
   field must contain the zone name [Section 3.1.7 of [RFC4034]], and
   this value is used as the FQDN of the zone apex for the fallback
   query.  The querying client MUST ensure that this property holds;
   that is, it MUST NOT proceed with the fallback query if the Signer's
   Name does not have the original host name as a subordinate name.

   DISCUSS: Note that most name servers will also return a SOA record
   with a negative response; this addition however, is not guaranteed
   and it may be removed from the response due to frame size
   constraints.  This is why the SOA record is not preferred for finding
   the secondary description.

5.  Publishing Realm Descriptors

   The default position for KREALM records that describe a realm is in
   the apex of a zone.  Such KREALM records may be Home Records, but
   that is not a requirement, because on Kerberos realm may cover any
   number of DNS zones.

   When a KREALM record is published in the zone apex, it will cover all
   SRV records for that domain name, as well as all host names defined
   in the same zone.  SRV records for subordinate names to the zone apex
   need a separate KREALM record.  If a host name requires overridden
   KREALM record, then this may be specified in a KREALM record with the
   same FQDN as the host name.

   One form of overriding KREALM definitions worth noting is one that
   does not define a Kerberos realm at all; such a record can be used to
   undefine any realm names that are defined in the zone apex.

   Note that KREALM 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

6.  Tags in Realm Descriptors

   The names of tags are partitioned into two types:

   o  General Tags can be used in any KREALM records;

   o  Home Tags can only be used in Home Records.

   Any Home Tags that occur in other KREALM records than Home Records
   MUST be ignored.

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6.1.  Realm Descriptor Tag "realm"

   The tag "realm" MAY be present in all KREALM records, and it MUST be
   recognised and processed by implementations of this specification; in
   other words, the tag is not optional.

   The value of a "realm" tag provides a realm name for the queried
   FQDN.  The permissible values of this tag conform to the permissible
   names of realm names [Section 6.1 of [RFC4120]], which a conforming
   application MUST validate before processing the value.  This
   includes, but is not limited to, domain-style realm names.  Since the
   value field is a general UTF8String, it is to be treated as a case-
   sensitive string [RFC4343], just like realm names.

   It is possible to define zero "realm" tags in a KREALM record.  This
   indicates that no realm is defined by that record.  This is not an
   invalid condition; other KREALM records, if any, or other service-to-
   realm mappings may still be used.

   When multiple "realm" tags occur in one KREALM record, then they
   present alternative suggestions to combine with all other tags in the
   same KREALM record.

   An example use of the "realm" tag in a TXT record is  IN KREALM "MBgxFjAUFgVyZWFsbQwLRVhBTVBMRS5DT00="

   Written out, the RDATA holds the following DER representation:

            IA5STRING    realm

   Since the value of all "realm" tags map to the FQDN of this KREALM
   record, this KREALM record is a Home Record.  As a result, tags that
   are registered as Home Tags may be added to this realm descriptor.

   In addition to the previous example, the following tag indicates that
   there is no realm, and so it is useless to request a service ticket
   for, as far as this domain descriptor in DNS is
   concerned:  IN KREALM "MAIxAA=="

   The RDATA for this KREALM record encodes no tags and no values at

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6.2.  Realm Descriptor Tag "service"

   The tag "service" MAY be present in any KREALM record, and it SHOULD
   be recognised by implementations of this specification.  The tag is

   The value of a "service" tag is the name of a service, as used in
   principal names.  This can be used as a hint to clients that need to
   match "service" tags.  The occurrence of a "service" tag and a
   "realm" tag in the same KREALM record is a hint that a service ticket
   for the combination probably exists.  Note that the value of this tag
   is a general UTF8String, and that it is case-sensitive [RFC4343].

   The purpose of this tag is to enable clients to locally select
   alternatives that it may wish to pursue; adding a "service" tag may
   improve the speed of resolution when multiple alternatives are listed
   in DNS, which is especially fruitful when future initiatives would
   require public key cryptography for realm crossover.

   When no "service" tag is defined in a KREALM record, then no hint for
   selection is available; processing must then continue under the
   assumption that any desired service name may be available for the
   realm description.  In contrast, when one or more "service" tags are
   defined in a KREALM record, then this set may be considered a
   complete specification of available services.  Note that multiple
   KREALM records may exist, each of which may or may not define
   "service" tags.

   When multiple "service" tags occur in one KREALM record, then they
   present alternative suggestions to combine with all other tags in the
   same KREALM record.

   An example use of this tag in a KREALM record is  IN KREALM "ME8xTTAOFgdzZXJ2aWNlDANmdHAwDxYHc2Vydmlj

   This RDATA contains the following data structure:

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            IA5STRING    service
            UTF8STRING   ftp
            IA5STRING    service
            UTF8STRING   HTTP
            IA5STRING    realm
            IA5STRING    realm

   This matches the following principal names, found by iterating over
   all combinations of "server" and "realm" values:

   o  HTTP/

   o  HTTP/

   o  ftp/

   o  ftp/

   Apart from the case-insensitivity of the DNS name "",
   this list is a complete list of principal names matched by the KREALM
   record.  For instance, a service named "krbtgt" is not described by
   this KREALM record.

7.  Efficiency Considerations

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

   Since realm names cannot be derived directly from DNS names, clients
   tend to construct various principal names by appending all the realm
   names that they are aware of, and attempting to obtain a service
   ticket for each in turn, until one is found.  The KDC may also
   perform such actions, and return a reference [RFC6806] to a realm for
   consideration.  In general, the list of service principal names that
   may be considered can be relatively long.

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   The use of "realm" tags help the client to focus on those realms for
   which a service has a name defined.  This limit the list of realm
   names to attempt to those realm names that the service suggests.  The
   client does not need to guess as heavily.  Similarly, the combined
   use of "service" tags helps to select only those KREALM records that
   further constrain the scope to search.

   Limiting the length of the list of ticket requests 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 Kerberos.

   The combined publication of multiple "realm" tags with multiple
   "service" tags enables a compact representation of variations that a
   client should iterate over, without the need to store the resulting
   cartesian product in DNS.

8.  Privacy Considerations

   It is common to spread service information in DNS, but for internal
   use this may be considered less desirable.  This is why the "service"
   tag [Section 6.2], is optional.

   Similarly, internal applications may still prefer local definitions
   for realm names that a client should consider; this specification
   does not enforce the KREALM record in those situations.

   For situations which crossover between realms, the choice between
   static configuration in files and KDC configuration versus a dynamic
   configuration in DNS is still a choice; the dynamic option based on
   DNS publishes more information, but dynamic applications are more
   likely to desire such information to be publicly and securely

9.  Security Considerations

   This specification defines a mechanism to redirect from a FQDN to a
   realm name that may not map to that same FQDN, or that define that no
   realm name exists for the originating FQDN.  Publishing such a realm
   descriptor is the prerogative of the DNS administrator who is also in
   charge of publishing the location of the service that is protected by
   Kerberos.  This administrator is generally trusted not to attack the
   security of a zone; DNSSEC makes this assumption even stronger than
   unsecured DNS, and this specification does not reduce or expand on
   that assumption.

   When an external attacker would be permitted to spoof a KREALM record
   in a victim's DNS, then it could be possible for that attacker to

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   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 requiring DNSSEC for
   all such KREALM records.

   Another angle of attack could be due to suppression of KREALM
   records, specifically the ones for a host name which have a fallback
   option at the zone apex.  Such attacks could direct a client to rely
   on information that may form a alternative of lesser security.  Such
   attacks have been mitigated by insisting on signed denials, and by
   stating that a non-responsive DNS server should not lead to the
   assumption that one can move up in the DNS hierarchy.

   The process of finding the zone apex relies on a strict prescription
   in DNSSEC standards.  The field from which the zone apex is taken is
   validated by the signature field of the RRSIG record that holds it.
   This field is necessarily part of a signed denial under DNSSEC.

   The ability to create a KREALM record that references a realm
   operated under another DNS name introduces a potential of setting
   flags for that remote realm that may be counter-productive.  Given
   the open-endedness of the IANA registry for tags, problems that this
   may cause are mitigated by ignoring unknown tags, and treating known
   tags differently when they are registered as Home Tags; such tags are
   not processed for references to realms operated under another DNS

10.  IANA Considerations

   This specification defines a new "Resource Record (RR) Type", to be
   registered in IANA registry of Domain Name System (DNS) Parameters".
   The name of the RRType is KREALM, its value is TBD1 and its meaning
   is "Kerberos realm descriptor".

   This specification establishes a new registry with IANA, whose
   entries are subject to expert review and whose definition must be
   described in a publicly available specification.  The new registry
   will be known as the "DNS KREALM Tag Registry".  Each entry must
   provide a Yes/No flag to indicate if the tag is a Home Tag, meaning
   that it may only be interpreted as part of Home Records.

   The initial entries for this new registry introduced by this
   specification are:

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                | Tag name | Home Tag? | Definition      |
                | realm    | No        | [TBD:THIS-SPEC] |
                | service  | No        | [TBD:THIS-SPEC] |

   Tag names are case-sensitive.  Registration of new tags is subject to
   expert review, and a specification must be created as part of its

   DISCUSS: Suggestions on the submission process for new tags are

   In addition to the foregoing, tag names starting with "x-" are
   reserved for local and experimental use, for which registration is
   neither possible nor required.  These unregistered tags will not be
   protected from name clashes.

11.  References

11.1.  Normative References

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

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

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, March 2005.

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

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

11.2.  Informative References

   [RFC3225]  Conrad, D., "Indicating Resolver Support of DNSSEC", RFC
              3225, December 2001.

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

   [RFC4282]  Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
              Network Access Identifier", RFC 4282, December 2005.

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

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

Author's Address

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


Van Rein                  Expires May 17, 2015                 [Page 14]