Network Working Group R. Van Rein
Internet-Draft ARPA2.net
Intended status: Standards Track October 2, 2015
Expires: April 4, 2016
Declaring Kerberos Realm Names in DNS (_kerberos TXT)
draft-vanrein-dnstxt-krb1-06
Abstract
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
guesswork. DNS can make this process more flexible, provided that
DNSSEC is used to ensure authenticity of resource records.
Status of This Memo
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This Internet-Draft will expire on April 4, 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 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 guessing; the client's KDC may or may not be involved in
this process. Through DNS, the static mappings could be made more
dynamic, and be moved from local client configuration into the hands
of the party administrating a server's presence in DNS. This permits
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
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 how to publish and process Kerberos realm
names using a hitherto non-standardised use of TXT resource records.
These records hold a case-sensitive string with the realm name.
Multiple TXT records may be defined as alternatives in contexts that
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welcome this; they suggest iterating over them 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 one or more Kerberos realm names. 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 same RDATA as the following generic RDATA section that is also
used 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 MUST 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 not specify a realm name, then they can mention an invalid
realm name such as an empty <character-string>.
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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 the corresponding 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 its 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 imap.example.org would be:
_kerberos.imap.example.org. IN TXT "EXAMPLE.ORG"
The RDATA for this TXT record has already been shown above, in the
generic RDATA section notation.
As an alternative, the 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:
_imap._tcp.example.org. IN SRV 10 10 143 imap.example.org.
_kerberos.example.org. 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 guess, in which
case this facility is likely to be helpful. Since TXT records in
general pose no constraints on the relation between the contained
realm name and the DNS name at which it is found, it may be useful
for mapping of DNS name to a Kerberos realm name to map the realm
name back to a DNS name and compare the result.
3. Querying Kerberos Realm Names
This sections defines a procedure for finding Kerberos realm names
for the servers, 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 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. In addition, their possible shared use by many
domains would mean that they may have to specify many alternatives,
where it is not even certain that clients will be able to process
more than one such alternative. The domain name is closer to the
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client's context and provides a better-targeted location for the 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 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 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
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.
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 TXT record located at the
server name is also going to be helpful.
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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 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 is always attached 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
TXT records is preferred, a useful functionality test would be to
access the targeted applications from Kerberos clients without static
configuration.
Zones that intend to provide applications with Kerberos realm names
through TXT records MUST be protected by DNSSEC to make them usable
to those applications.
Note that 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
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 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 query DNS as specified
above, and that will assume that a service is running under the
client's realm. The cached DNS records, their validation and
possibly realm-crossover caching at the KDC can all benefit the
response time for future lookups by other Kerberos clients.
The use of 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 clearer path, with less guessing.
The definition of TXT records under a domain rather than under a
server host name (for cases where a choice exists, such as for
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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
Kerberos.
6. Privacy Considerations
The information contained in a TXT record for a domain is barely more
than the domain's DNS name already holds; the 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 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 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 security issue with such misguided use of TXT
records.
The general point is that the use of DNSSEC makes Kerberos rely on
the party that publishes the 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 TXT record is the same party that publishes the
service's records, namely the 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 TXT
record does not add any new abusive powers.
When an external attacker would be permitted to spoof a 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
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complete the attack. This has been mitigated by strictly requiring
Secure validation results from a DNSSEC-aware resolver for all TXT
records.
8. IANA Considerations
None.
9. References
9.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[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,
<http://www.rfc-editor.org/info/rfc2782>.
[RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record
(RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
2003, <http://www.rfc-editor.org/info/rfc3597>.
[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,
<http://www.rfc-editor.org/info/rfc4033>.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
DOI 10.17487/RFC4120, July 2005,
<http://www.rfc-editor.org/info/rfc4120>.
[RFC4343] Eastlake 3rd, D., "Domain Name System (DNS) Case
Insensitivity Clarification", RFC 4343, DOI 10.17487/
RFC4343, January 2006,
<http://www.rfc-editor.org/info/rfc4343>.
[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,
<http://www.rfc-editor.org/info/rfc6806>.
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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,
<http://www.rfc-editor.org/info/rfc3655>.
[RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name
System", RFC 4592, DOI 10.17487/RFC4592, July 2006,
<http://www.rfc-editor.org/info/rfc4592>.
[RFC6840] Weiler, S., Ed. and D. Blacka, Ed., "Clarifications and
Implementation Notes for DNS Security (DNSSEC)", RFC 6840,
DOI 10.17487/RFC6840, February 2013,
<http://www.rfc-editor.org/info/rfc6840>.
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.
Author's Address
Rick van Rein
ARPA2.net
Haarlebrink 5
Enschede, Overijssel 7544 WP
The Netherlands
Email: rick@openfortress.nl
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