Internet DRAFT - draft-vanrein-dnstxt-krb1
draft-vanrein-dnstxt-krb1
Network Working Group R. Van Rein
Internet-Draft ARPA2.net
Intended status: Informational 8 November 2022
Expires: 12 May 2023
Declaring Kerberos Realm Names in DNS (_kerberos TXT)
draft-vanrein-dnstxt-krb1-11
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
guesses. DNS can make this process more flexible, provided that
DNSSEC is used to assure authenticity of resource records.
Status of This Memo
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This Internet-Draft will expire on 12 May 2023.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Defining _kerberos TXT Resource Records . . . . . . . . . . . 3
3. Publishing Kerberos Realm Names . . . . . . . . . . . . . . . 5
4. Querying Kerberos Realm Names . . . . . . . . . . . . . . . . 5
5. Efficiency Considerations . . . . . . . . . . . . . . . . . . 6
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
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.
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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. This has been informally
described and practiced, but generally considered insecure; adding
DNSSEC means that much of this existing practice can now be trusted.
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 unknown resource
record types [RFC3597]:
\# 12 ( 0b 45 58 41 4d 50 4c 45 2e 4f 52 47 )
The REALMNAME represents a Kerberos realm name [Section 6.1 of
[RFC4120]], 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 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
<character-string>.
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Though any style of realm name may be published as _kerberos TXT, it
is common for realm names in Kerberos to follow the domain style
[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. Even for domain-style realm
names, there is no required relationship (such as partial overlap)
between the realm name and the DNS name at which a TXT record is
found.
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.
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.
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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 it is an existing practice; the particular use
specified here is distinguished from comments in TXT records by
always prefixing a _kerberos label to a DNS name. An example
declaration of realm name EXAMPLE.ORG for a server named
imap.example.org would be:
imap.example.org. IN AAAA 2001:db8::143
_kerberos.imap.example.org. IN TXT "EXAMPLE.ORG"
The RDATA for this TXT record is shown above, in the generic RDATA
section notation.
3. Publishing Kerberos Realm Names
Zones that intend to provide applications with Kerberos realm names
through _kerberos TXT records SHOULD protect them with DNSSEC.
Operators SHOULD NOT define more than one valid realm name for a
given domain or host name.
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.
4. Querying Kerberos Realm Names
This section 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. This specification does not impose any
restriction on the additional use of other-than-DNS methods for for
obtaining a realm name.
When applications know their server host name, perhaps because it is
mentioned in a URL or in a ticket as a service principal name, or
when applications know a domain name for which they intend to learn
the realm name, they resolve the TXT record in DNS for the name,
prefixed with a _kerberos label.
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
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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
denial.
Only the first lt;character-string> of a _kerberos TXT record is
considered; any further ones are silently ignored under this
specification. In addition, invalid realm names such as they empty
string are silently ignored.
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.
5. Efficiency Considerations
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
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
clients.
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6. Privacy Considerations
This specification barely publishes new information in DNS, with the
exception of markation of Kerberised services. When this is
considered unattractive from a privacy viewpoint, it may be better to
rely on the existing static tables for spreading this information in
a more controlled manner.
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.
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
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, <https://www.rfc-editor.org/info/rfc1035>.
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[RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record
(RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
2003, <https://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,
<https://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,
<https://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,
<https://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,
<https://www.rfc-editor.org/info/rfc6806>.
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,
<https://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,
<https://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,
<https://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 provided useful input.
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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
Rick van Rein
ARPA2.net
Haarlebrink 5
Enschede
Email: rick@openfortress.nl
Van Rein Expires 12 May 2023 [Page 9]
