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INTERNET-DRAFT Ken Hornstein
<draft-ietf-krb-wg-krb-dns-locate-02.txt> NRL
February 28, 2001 Jeffrey Altman
Expires: August 28, 2001 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-
krb-wg-krb-dns-locate-02.txt>, and expires on August 28, 2001.
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with out-of-date information and scaling problems, especially when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 February 28, 2001
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.
DNS vs. Kerberos - Case Sensitivity of Realm Names
In Kerberos, realm names are case sensitive. While it is strongly
encouraged that all realm names be all upper case this recommendation
has not been adopted by all sites. Some sites use all lower case
names and other use mixed case. DNS on the other hand is case insen-
sitive for queries but is case preserving for responses to TXT
queries. Since "MYREALM", "myrealm", and "MyRealm" are all different
it is necessary that only one of the possible combinations of upper
and lower case characters be used. This restriction may be lifted in
the future as the DNS naming scheme is expanded to support non-ASCII
names.
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, and Target have the standard mean-
ing as defined in RFC 2052.
As per RFC 2052 the Port number should be the value assigned to "ker-
beros" by the Internet Assigned Number Authority (88).
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.
Hornstein, Altman [Page 2]
RFC DRAFT February 28, 2001
_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 - Kerberos password changing server location information
Kerberos password changing server [KERB-CHG] location is to be stored
using the DNS SRV RR [RFC 2052]. The format of this RR is as fol-
lows:
Service.Proto.Realm TTL Class SRV Priority Weight Port Target
The Service name for the password server is always "_kpasswd".
The Proto MUST be "_udp".
The Realm is the Kerberos realm that this record corresponds to.
TTL, Class, SRV, Priority, Weight, and Target have the standard mean-
ing as defined in RFC 2052.
As per RFC 2052 the Port number should be the value assigned to
"kpasswd" by the Internet Assigned Number Authority (464).
Overview - Kerberos admin server location information
Kerberos admin 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 the admin server is always "_kerberos-adm".
The Proto can be either "_udp" or "_tcp". If these records are to be
used, a "_tcp" record MUST be included. If the Kerberos admin imple-
mentation supports UDP transport, a "_udp" record SHOULD be included.
The Realm is the Kerberos realm that this record corresponds to.
TTL, Class, SRV, Priority, Weight, and Target have the standard mean-
ing as defined in RFC 2052.
As per RFC 2052 the Port number should be the value assigned to
"kerberos-adm" by the Internet Assigned Number Authority (749).
Note that there is no formal definition of a Kerberos admin protocol,
so the use of this record is optional and implementation-dependent.
Hornstein, Altman [Page 3]
RFC DRAFT February 28, 2001
Example - Kerberos administrative server location information
These are DNS records for a Kerberos realm ASDF.COM. It has one
administrative server, kdc1.asdf.com.
_kerberos-adm._tcp.ASDF.COM. IN SRV 0 0 749 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"
_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:
Hornstein, Altman [Page 4]
RFC DRAFT February 28, 2001
_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.
Current practice for REALM to KDC mapping 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 vulnerabili-
ties that already exist in the common practice of using hostnames for
KDC locations.
Current practice for HOSTNAME to REALM mapping is to provide a local
configuration of mappings of hostname or domain name to realm which
are then mapped to KDCs. But this again is vulnerable to spoofing
via CNAME records that point to hosts in other domains. This has the
same effect as when a TXT record is spoofed. In a realm with no
cross-realm trusts this is a DoS attack. However, when cross-realm
trusts are used it is possible to redirect a client to use a comprom-
ised realm.
This is not an exploit of the Kerberos protocol but of the Kerberos
trust model. The same can be done to any application that must
resolve the hostname in order to determine which domain a non-FQDN
belongs to.
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 information on a client should
Hornstein, Altman [Page 5]
RFC DRAFT February 28, 2001
be interpretted as permission to use DNS.
Expiration
This Internet-Draft expires on August 28, 2001.
References
[RFC1510]
The Kerberos Network Authentication System; Kohl, Newman; Sep-
tember 1993.
[RFC1035]
Domain Names - Implementation and Specification; Mockapetris;
November 1987
[RFC2782]
A DNS RR for specifying the location of services (DNS SRV); Gul-
brandsen, Vixie; Feburary 2000
[KERB-CHG]
Kerberos Change Password Protocol; Horowitz;
ftp://ds.internic.net/internet-drafts/draft-ietf-cat-kerb-chg-
password-02.txt
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 6]