Network Working Group R. Van Rein
Internet-Draft ARPA2.net
Intended status: Standards Track November 13, 2014
Expires: May 17, 2015
Kerberos Realm Descriptors in DNS (KREALM)
draft-vanrein-dnstxt-krb1-01
Abstract
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
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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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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
specifications.
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
syntax:
KREALM-DATA ::= SEQUENCE {
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
it.
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
service.
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
[RFC4282].
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
responses.
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
example.com. IN KREALM "MBgxFjAUFgVyZWFsbQwLRVhBTVBMRS5DT00="
Written out, the RDATA holds the following DER representation:
SEQUENCE
SET
SEQUENCE
IA5STRING realm
UTF8STRING EXAMPLE.COM
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 ftp.example.com, as far as this domain descriptor in DNS is
concerned:
ftp.example.com. IN KREALM "MAIxAA=="
The RDATA for this KREALM record encodes no tags and no values at
all:
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SEQUENCE
SET
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
optional.
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
www.example.com. IN KREALM "ME8xTTAOFgdzZXJ2aWNlDANmdHAwDxYHc2Vydmlj
ZQwESFRUUDAUFgVyZWFsbQwLRVhBTVBMRS5DT00wFBYFcmVhbG0MC0VYQU1QTEUuT1JH"
This RDATA contains the following data structure:
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SEQUENCE
SET
SEQUENCE
IA5STRING service
UTF8STRING ftp
SEQUENCE
IA5STRING service
UTF8STRING HTTP
SEQUENCE
IA5STRING realm
UTF8STRING EXAMPLE.COM
SEQUENCE
IA5STRING realm
UTF8STRING EXAMPLE.ORG
This matches the following principal names, found by iterating over
all combinations of "server" and "realm" values:
o HTTP/www.example.com@EXAMPLE.COM
o HTTP/www.example.com@EXAMPLE.ORG
o ftp/www.example.com@EXAMPLE.COM
o ftp/www.example.com@EXAMPLE.ORG
Apart from the case-insensitivity of the DNS name "www.example.com",
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
available.
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
name.
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
definition.
DISCUSS: Suggestions on the submission process for new tags are
requested.
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
ARPA2.net
Haarlebrink 5
Enschede, Overijssel 7544 WP
The Netherlands
Email: rick@openfortress.nl
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