INTERNET-DRAFT                                             Ken Hornstein
<draft-ietf-cat-krb-dns-locate-00.txt>                               NRL
June 21, 1999                                             Jeffrey Altman
Expires: December 21, 1999                           Columbia University

          Distributing Kerberos KDC and Realm Information with DNS

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.

   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
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   Distribution of this memo is unlimited.  It is filed as <draft-ietf-
   cat-krb-dns-locate-00.txt>, and expires on December 21, 1999.  Please
   send comments to the authors.

Abstract

   Neither the Kerberos V5 protocol [RFC1510] nor the Kerberos V4 proto-
   col [RFC????] describe any mechanism for clients to learn critical
   configuration information necessary for proper operation of the pro-
   tocol.  Such information includes the location of Kerberos key dis-
   tribution centers or a mapping between DNS domains and Kerberos
   realms.

   Current Kerberos implementations generally store such configuration
   information in a file on each client machine.  Experience has shown
   this method of storing configuration information presents problems
   with out-of-date information and scaling problems, especially when

Hornstein, Altman                                               [Page 1]

RFC DRAFT                                                  June 21, 1999

   using cross-realm authentication.

   This memo describes a method for using the Domain Name System
   [RFC1035] for storing such configuration information.  Specifically,
   methods for storing KDC location and hostname/domain name to realm
   mapping information are discussed.

Overview - KDC location information

   KDC location information is to be stored using the DNS SRV RR [RFC
   2052].  The format of this RR is as follows:

   Service.Proto.Realm TTL Class SRV Priority Weight Port Target

   The Service name for Kerberos is always "_kerberos".

   The Proto can be either "_udp" or "_tcp".  If these records are to be
   used, a "_udp" record MUST be included.  If the Kerberos implementa-
   tion supports TCP transport, a "_tcp" record SHOULD be included.

   The Realm is the Kerberos realm that this record corresponds to.

   TTL, Class, SRV, Priority, Weight, Port, and Target have the standard
   meaning as defined in RFC 2052.

Example - KDC location information

   These are DNS records for a Kerberos realm ASDF.COM.  It has two Ker-
   beros servers, kdc1.asdf.com and kdc2.asdf.com.  Queries should be
   directed to kdc1.asdf.com first as per the specified priority.
   Weights are not used in these records.

   _kerberos._udp.ASDF.COM.        IN      SRV     0 0 88 kdc1.asdf.com.
   _kerberos._udp.ASDF.COM.        IN      SRV     1 0 88 kdc2.asdf.com.

Overview - KAdmin location information

   Kadmin location information is to be stored using the DNS SRV RR [RFC
   2052].  The format of this RR is as follows:

   Service.Proto.Realm TTL Class SRV Priority Weight Port Target

   The Service name for Kadmin is always "_kadmin".

   The Proto can be either "_udp" or "_tcp".  If these records are to be
   used, a "_tcp" record MUST be included.  If the Kadmin implementation
   supports UDP transport, a "_udp" record SHOULD be included.

Hornstein, Altman                                               [Page 2]

RFC DRAFT                                                  June 21, 1999

   The Realm is the Kerberos realm that this record corresponds to.

   TTL, Class, SRV, Priority, Weight, Port, and Target have the standard
   meaning as defined in RFC 2052.

Example - Kadmin location information

   These are DNS records for a Kerberos realm ASDF.COM.  It has one Kad-
   min server, kdc1.asdf.com.

   _kadmin._tcp.ASDF.COM.  IN      SRV     0 0 88 kdc1.asdf.com.

Overview - Hostname/domain name to Kerberos realm mapping

   Information on the mapping of DNS hostnames and domain names to Ker-
   beros realms is stored using DNS TXT records [RFC 1035].  These
   records have the following format.

   Service.Name TTL Class TXT Realm

   The Service field is always "_kerberos", and prefixes all entries of
   this type.

   The Name is a DNS hostname or domain name.  This is explained in
   greater detail below.

   TTL, Class, and TXT have the standard DNS meaning as defined in RFC
   1035.

   The Realm is the data for the TXT RR, and consists simply of the Ker-
   beros realm that corresponds to the Name specified.

   When a Kerberos client wishes to utilize a host-specific service, it
   will perform a DNS TXT query, using the hostname in the Name field of
   the DNS query.  If the record is not found, the first label of the
   name is stripped and the query is retried.

   Compliant implementations MUST query the full hostname and the most
   specific domain name (the hostname with the first label removed).
   Compliant implementations SHOULD try stripping all subsequent labels
   until a match is found or the Name field is empty.

Example - Hostname/domain name to Kerberos realm mapping

   For the previously mentioned ASDF.COM realm and domain, some sample
   records might be as follows:

   _kerberos.asdf.com.             IN      TXT     "ASDF.COM"

Hornstein, Altman                                               [Page 3]

RFC DRAFT                                                  June 21, 1999

   _kerberos.mrkserver.asdf.com.   IN      TXT     "MARKETING.ASDF.COM"
   _kerberos.salesserver.asdf.com. IN      TXT     "SALES.ASDF.COM"

   Let us suppose that in this case, a Kerberos client wishes to use a
   Kerberized service on the host foo.asdf.com.  It would first query:

   _kerberos.foo.asdf.com. IN TXT

   Finding no match, it would then query:

   _kerberos.asdf.com. IN TXT

   And find an answer of ASDF.COM.  This would be the realm that
   foo.asdf.com resides in.

   If another Kerberos client wishes to use a Kerberized service on the
   host salesserver.asdf.com, it would query:

   _kerberos.salesserver.asdf.com IN TXT

   And find an answer of SALES.ASDF.COM.

Security considerations

   As DNS is deployed today, it is an unsecure service.  Thus the infor-
   mation returned by it cannot be trusted.  However, the use of DNS to
   store this configuration information does not introduce any new secu-
   rity risks to the Kerberos protocol.

   Current practice is to use hostnames to indicate KDC hosts (stored in
   some implementation-dependent location, but generally a local config
   file).  These hostnames are vulnerable to the standard set of DNS
   attacks (denial of service, spoofed entries, etc).  The design of the
   Kerberos protocol limits attacks of this sort to denial of service.
   However, the use of SRV records does not change this attack in any
   way.  They have the same vulnerabilities that already exist in the
   common practice of using hostnames for KDC locations.

   The same holds true for the TXT records used to indicate the domain
   name to realm mapping.  Current practice is to configure these map-
   pings locally.  But this again is vulnerable to spoofing via CNAME
   records that point to hosts in other domains.  This has the same
   effect as a spoofed TXT record.

   While the described protocol does not introduce any new security
   risks to the best of our knowledge, implementations SHOULD provide a
   way of specifying this information locally without the use of DNS.
   However, to make this feature worthwhile a lack of any configuration

Hornstein, Altman                                               [Page 4]

RFC DRAFT                                                  June 21, 1999

   information on a client should be interpretted as permission to use
   DNS.

Expiration

   This Internet-Draft expires on December 21, 1999.

References

   [RFC1510]
        The Kerberos Network Authentication System; Kohl, Newman; Sep-
        tember 1993.

   [RFC1035]
        Domain Names - Implementation and Specification; Mockapetris;
        November 1987

   [RFC2052]
        A DNS RR for specifying the location of services (DNS SRV); Gul-
        brandsen, Vixie; October 1996

Authors' Addresses

   Ken Hornstein
   US Naval Research Laboratory
   Bldg A-49, Room 2
   4555 Overlook Avenue
   Washington DC  20375 USA

   Phone: +1 (202) 404-4765
   EMail: kenh@cmf.nrl.navy.mil

   Jeffrey Altman
   The Kermit Project
   Columbia University
   612 West 115th Street #716
   New York NY 10025-7799 USA

   Phone: +1 (212) 854-1344
   EMail: jaltman@columbia.edu

Hornstein, Altman                                               [Page 5]
