Internet DRAFT - draft-hallambaker-xptr
draft-hallambaker-xptr
Internet Engineering Task Force P. Hallam-Baker
Internet-Draft VeriSign Inc
Intended status: Informational June 29, 2007
Expires: December 31, 2007
XPTR: DNS Prefix Pointer
draft-hallambaker-xptr-00
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
The DNS XPTR resource record is defined. An XPTR record may be used
in combination with prefixed DNS records to create the effect of
wildcarding and to simplify management where prefixed records are
employed on an extended scale.
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Table of Contents
1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Resolving Prefix records with XPTR . . . . . . . . . . . . . . 4
3.1. Basic Prefix Resolution . . . . . . . . . . . . . . . . . . 4
3.2. Reverse Prefix Resolution . . . . . . . . . . . . . . . . . 4
4. Using XPTR . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Wildcards . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Policy Administration . . . . . . . . . . . . . . . . . . . 6
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
8. Normative References . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . . . 8
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1. Definitions
The following definitions are used in this document:
DNS Resource Record A DNS Resource Record as defined in [TBS]
Prefixed Record A DNS Resource Record in which one or more labels
contain characters that are not valid DNS host names. [TBS]
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Introduction
Prefixed resource records, first introduced in the SRV specification
provide a means of extending the DNS without allocating a new
resource record
The principle disadvantage faced when using prefixed DNS records is
that the existing DNS specifications do not provide a 'midpoint
wildcard' of the form _prefix.*.example.com. While a DNS server can
implement such a wildcard in response to a DNS query as a 'synthetic'
wildcard this usage is not compatible with the mechanisms of DNSSEC
or Zone Transfer.
While support for wildcarding of prefixed records has not been
considered an essential requirement in service discovery applications
such as SRV and NAPTR, wildcarding is considered an essential
requirement for publication of protocol policy statements. In
particular the ability to make policy statements of the form 'All
mail from *.example.com is signed' is frequently a requirement.
While such a requirement could be satisified by issuing separate DNS
RRs for each protocol policy advertisement, this approach is only
acceptable if the number of policy advertisements is expected to be
small. While the number of official prefix registrations is small,
informal registrations number in excess of 500 in June 2007. This
number is likely to rise rapidly as the use of Web Services
increases.
The DNS resource record is expressed as a fixed 16 bit field giving
65,336 possible values. An architecture which limits the Internet to
65,336 possible protocols for machine-machine interaction is not
sustainable.
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3. Resolving Prefix records with XPTR
XPTR allows a resolution algorithm to be defined that supports the
use of wildcards in conjunction with prefixed DNS records by
introducing an additional step of indirection. Although a wildcard
cannot be applied to prefixed DNS record itself, a wildcard can be
applied to the XPTR indirection record.
3.1. Basic Prefix Resolution
The basic XPTR prefix resolution (basic) algorithm MAY be specified
as the means of resolving a particular DNS record prefix.
The basic resolution algorithm resolves a triple consisting of a DNS
node, a DNS prefix label and a DNS record type. The resolution
algorithm returns either the record requested or the result ?not
present?.
The resolution algorithm always produces a result in a maximum of
three steps when applied to DNS nodes in the forward DNS. The
requestor first looks for a prefix record at the query node itself.
If this search fails the requestor looks for a PREPTR record at the
query node and if this is found:
Record BasicPrefixResolve (
String node, String prefix, RecordType record)
1. Record F1 = Lookup (prefix + "." + node, record)
If (F1 <> NIL) Return F1
2. Record F2 = Lookup (node, PREPTR)
If (F2 = NIL) Return NIL
3. Record F3 = Lookup (prefix + "." + F2.domain)
Return F3
3.2. Reverse Prefix Resolution
In most cases an Internet service is identified by means of a domain
name. In certain circumstances it is desirable to perform service
and policy discovery by means of the IP address. This requirement is
most likely to occur in protocols for real time reporting of security
incidents where the IP address of the source of attack is known with
some degree of certainty, but not a domain name.
In such situations the service discovery process MAY specify the use
of the Reverse DNS. The reverse DNS is an area of the DNS space (in-
addr.arpa, ipv6.arpa). A PTR record in the reverse DNS maps an IPv4
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or IPv6 address to a DNS name.
Depending on the requirements of the service it MAY be desirable for
discovery to process XPTR records in the reverse DNS directly or to
first attempt to follow a PTR record to obtain a DNS node where basic
prefix resolution is to be performed.
Record ReversePrefixResolve(
IPAddress address, String prefix, RecordType record)
1. Record R1 = Lookup (prefix + "." +
ReverseToNode (address), record)
If (F1 <> NIL) Return F1
2. Record R2 = Lookup (node, ReverseToNode (address)
If (R2 = NIL) Return NIL
Else Return BasicPrefixResolve (R2.domain, prefix, record)
4. Using XPTR
XPTR may be used to simplfy administration of prefixed DNS records
and to permit the resolution of wildcard DNS records.
4.1. Wildcards
In order to illustrate the use of XPTR we consider the resolution of
a hypothetical Internet protocol 'NOOP'.
The discovery protocol for NOOP is specified as using SRV with the
basic prefix resolution protocol. There is no default port
assignment.
All service requests for the 'NOOP' service with SRV Prefix _noop in
the domain example.com are to be directed to the main noop server,
except for the mathematics department math.example.com which has its
own server.
The zone file is:
_noop._tcp.example.com SRV 1 1 80 noop.example.com
_noop._tcp.math.example.com SRV 1 1 80 math.example.com
h1.example.com A 10.1.1.1
h1.example.com XPTR example.com
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*.example.com XPTR example.com
Although XPTR addresses the lack of a DNS midpoint wildcard it does
not address the fact that the semantics of DNS wildcards are
considerably more restrictive than is generally convenient and a DNS
wildcard only binds to a DNS zone if there are no other records of
any type defined for that node. It is therefore necessary to specify
an XPTR record for the node h1.example.com which is outside the scope
of the wildcard.
4.2. Policy Administration
We hypothecate the existence of a protocol policy prefixes
_a._policy, _b._policy etc. to be used to specify protocol
configuration options.
The administrator of example.com has three basic types of machine;
desktops, laptops and servers. The range of services a particular
machine is allowed to offer is determined by its class. Instead of
defining the protocol configuration policy for each machine
individually the administrator specifies an XPTR record to direct
resolution to a node where the characteristics for the whole class
are defined:
_a._policy.laptop.example.com TXT "SSL=always"
_b._policy.laptop.example.com TXT "MinVersion=2.3 MaxVersion=3.4"
_a._policy.desktop.example.com TXT ""
_b._policy.desktop.example.com TXT "MinVersion=2.1 MaxVersion=3.4"
_a._policy.server.example.com TXT ""
_b._policy.server.example.com TXT "MinVersion=1.0 MaxVersion=3.4"
alice.example.com XPTR laptop.example.com
bob.example.com XPTR laptop.example.com
carol.example.com XPTR desktop.example.com
doug.example.com XPTR laptop.example.com
_b._policy.doug.example.com TXT "MinVersion=2.3 MaxVersion=3.4"
edward.example.com XPTR server.example.com
mail.example.com XPTR server.example.com
The default policy may be overriden as necessary by a policy declared
at the specific node. In this case the administrator has overriden
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policy B for doug.example.com.
5. Acknowledgements
The ideas in this document arose from extensive discussions with the
DKIM and DNSEXT working groups.
6. IANA Considerations
This document requests allocation of a DNS Resource Record for the
XPTR record.
7. Security Considerations
The XPTR record does not change the security model or field of
application of DNS. It does however make it more likely that DNS
will be used in situations where the need for robust integrity and
authenticity controls such as those provided by DNSSEC will become
more apparent.
In particular it is highly desirable for a prefixed record used to
distribute a security policy to be signed.
In cases where an XPTR directs resolution of prefixed records to a
DNS zone that is under a different administrative control regime,
administrative control and the ability to enforce security controls
is transfered to another party.
8. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Author's Address
Phillip Hallam-Baker
VeriSign Inc
Email: pbaker@verisign.com
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