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Network Working Group                                  Dave Thaler
INTERNET-DRAFT                                           Microsoft
November 19, 2001                         Jun-ichiro itojun Hagino
Expires May 2002                           IIJ Research Laboratory

                   IPv6 Stateless DNS Discovery

                       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-

Internet Drafts are 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 a "work in progress".

     The list of current Internet-Drafts can be accessed at

     The list of Internet-Draft Shadow Directories can be accessed at


This document specifies the steps a host takes in deciding how to
autoconfigure information (other than the host's name itself)
required for name resolution in IP version 6.  The
autoconfiguration process includes determining whether such
information should be obtained through the stateless mechanism,
the stateful mechanism, or both.  This document defines the
process for acquiring a list of DNS server addresses, a domain
search path, and the domain name of the host via a stateless

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mechanism.  The details of autoconfiguration using the stateful
protocol are specified elsewhere.

Copyright Notice

Copyright (C) The Internet Society (2001).  All Rights Reserved.

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1.  Introduction

RFC 2462 [ADDRCONF] specifies "OtherConfigFlag" as a per-interface
variable, which is set from the value of the "O" ("Other stateful
configuration") flag in Router Advertisements received.  When
OtherConfigFlag is set on an interface, information related to
name resolution is obtained using the stateful autoconfiguration
mechanism.  However, when OtherConfigFlag is not set, it does not
describe how to obtain such information.  This is the purpose of
this document.

For a host to effectively resolve names of other hosts, and
potentially allow resolution of its name to be performed, the
following parameters are typically required:

 o One or more addresses of Domain Name Service (DNS) [RFC1034,
   RFC1035] servers.  The function of name-to-address resolution
   (or vice versa) in IP is performed by DNS, which requires that
   at least one DNS Server be known and reachable by a device
   desiring to perform name resolution.

 o A per-interface domain name of the host itself, and is
   equivalent to the Domain Name option in [DHCP].  This can be
   used when Multicast DNS is enabled, and the host responds to
   queries for its own name, as well as when DNS Dynamic Update is
   enabled.  Depending on the implementation, the per-interface
   domain name may or may not be the same thing as the primary
   domain name of the host.

 o Search path.  It is currently common practice for the search
   path to be computed by a device based on its domain name(s).
   However, a DHCP option [DOMSEARCH] has been proposed, and so
   search path configuration is likely to be a requirement in

 o mDNS-enabled flag.  This parameter controls whether Multicast
   DNS [MDNS] should be enabled on the host's interface.

A design team recommendation [DDDT] contains an analysis of the
requirements and solution space, which was used as the basis for
this document.  One result of this analysis was that there is a
spectrum of configuration/discovery mechanisms.  At one end, a
single protocol is used to configure/discovery all types of
information.  This style works well in an administered environment
where a network administrator wants to have a central point of

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control, and apply policies, etc.  At the other end, each protocol
is self-configuring, rather than depending on any other protocol
or server.  This style provides optimal fate sharing, and works
well in zero-configuration environments such as adhoc networks and
simple networks without network administrator staff.  The former
mechanism is used with the "Other stateful configuration" flag is
used, and this draft provides the benefits (and limitations) of
the latter approach when "Other stateful configuration" is not

2.  Overview

A set of three well-known site-local IPv6 addresses are reserved
for autodiscovery of DNS servers.  These addresses may be used as
unicast addresses, assigned to different servers, or as anycast
addresses with one of them being assigned to all DNS servers in
the site, or any combination of anycast and unicast addresses.  In
any case, host routes are propagated in the site's routing tables.
This document proposes that these three addresses be
fec0:0:0:ffff::1, fec0:0:0:ffff::2, and fec0:0:0:ffff::3.  This
list of three addresses may be hardcoded into a host.

Furthermore, we define two levels of functionality, as follows.

3.  Level 1 Compliance

Level 1 compliance entails using the three addresses above for
actual name resolution.  It provides only rudimentary
functionality.  In particular, it does not provide the ability to
have separate configuration for hosts on different subnets, nor to
provide hosts with a domain name other than one for which the DNS
server is authoritative.

3.1.  DNS Server Configuration

Level 1 functionality requires no DNS server configuration other
than assigning one of the well-known addresses to one of the
server's interfaces.  A host route must then be injected into the
routing domain, e.g., by configuring a static host route on the
server's router, and redistributing it into the intra-domain
routing protocol.

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A host route must then be injected into the site's routing
infrastructure.  Route injection can be done via any of several
methods, including:

a) Run the server on a router, and configure it to inject the host

b) Run a routing protocol on the server, and configure it to
   inject the host route.  Note that this requires that the server
   and its router(s) must run the same routing protocol, at least
   for communication between the router(s) and the server(s) on
   the link.  However, a server does not need to participate fully
   in the routing protocol, it only needs to be able to inject

c) Run multiple servers on the same link(s), and configure their
   local router(s) to inject host routes for the well-known
   address into the site's routing infrastructure.  Running
   multiple servers on the same link provides robustness to the
   failure of a server, while routing provides robustness to the
   loss of routers and other links.  There may still be some
   failures, however, such as a unidirectional failure of the
   router's interface, which are not handled by this option.

d)  Modify the routers on the link to periodically solicit (using
    Neighbor Discovery) the well-known address, and inject the
    host route based on whether a reply is received.

e)  Use a host-to-router protocol for communicating anycast group
    joins to routers.  There is now work in progress [HOST-
    ANYCAST] in this regard.

3.2.  Host Behavior

The host sets its DNS server list equal to the set of three
addresses listed above.  The search path is not discovered, but is
generated from the host's domain name(s) as is currently common
practice.  Multicast DNS is enabled only if no servers respond.
If desired, a per-interface domain name can be obtained by sending
a query (with the Recursion Desired (RD) bit set), doing a reverse
lookup for the well-known site-local IPv6 destination address, and
extracting the domain name from the NS record in the reply.

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4.  Level 2 Compliance

Level 2 compliance allows site administrators to have site-wide
defaults for all name resolution parameters, and optionally to
have subnet-specific overrides.  However, it defines a new DNS
record type to hold name resolutioin configuration information.
In this way, DNS becomes self-configuring.

4.1.  DNS Server Configuration

A new record type, CFG, is defined, with a syntax as follows:
<owner> <class> <ttl> CFG "<attribute name>=<attribute value>"

The set of attribute names is described below.  This set may be
extended by other RFCs, but any new attributes MUST be specific to
name resolution.

The DNS server list is expressed with a string of the form
"dnsservers=<address>[,<address>]*" where the attribute value is a
comma-separated list of one or more addresses (either IPv4 or
IPv6) in string literal format suitable for passing to

The domain name to use is expressed with a string of the form
"domainname=<domain>" where the attribute value is the domain name
the client should use.

The domain suffix search path is expressed with a string of the
form "searchpath=<domain>[,<domain>]*" where the attribute value
is a comma-separated list of domain suffixes.

The mDNS enabled flag is expressed with a string of the form
"mdnsenabled=<value>" where the attribute value is either "true"
or "false".

Name resolution information can be expressed as defaults for the
entire site, as well as per-subnet overrides if desired.  To
express site defaults, the record owner used is a wildcard, namely
*.local.arpa.  The format of per-subnet overrides is described in
the next section.

[NOTE WELL: the use of "local.arpa" and the CFG record syntax
shown above are just placeholders until DNS experts figure out

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what the right thing is.]

Each of the attributes described herein are optional, and any
combination may be used, except that only one record per
attributename should be present per owner (site or subnet) string.

*.local.arpa   IN   CFG   "dnsservers=fec0:0:1::1,fec0:0:2::2"
*.local.arpa   IN   CFG   "domainname=example.com"
*.local.arpa   IN   CFG   "searchpath=foo.example.com,
*.local.arpa   IN   CFG   "mdnsenabled=true"

                  Table 1: Example configuration

The DNS server must also be assigned one of the well-known site-
local addresses, and a host route must be injected into the site's
routing infrastructure, e.g. using one of the methods described
above in Section 3.1.

4.2.  Host Behavior

When an interface comes up, and a host determines that the
OtherConfigFlag on the interface is off, then it takes the
following actions.

The host constructs a DNS query for CFG records for
"<subnet>.local.arpa.", where <subnet> is constructed from an
onlink prefix as follows:

1) Determine the onlink prefix to use.  Any onlink site-local
   prefix is used, if present.  Otherwise, any onlink global
   prefix is used.  If no other onlink prefixes are present (e.g.,
   if no routers are present), the link-local prefix is used as a
   last resort.

2) Convert the subnet prefix to a string by taking the lower case
   string literal representation, with no zero compression, and
   replacing all colons with underscores.  Table 2 below shows
   some examples.  This notation is used so that it uses only one
   token, is unique per prefix, and is human readable.

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Prefix                     String
-----------------          --------------------------------------
fec0:0:0:1::/64            fec0_0000_0000_0001.local.arpa
3ffe:ffff:0:1234::/64      3ffe_ffff_0000_1234.local.arpa
fe80::/64                  fe80_0000_0000_0000.local.arpa

                     Table 2: Example queries

Once the query is formed, the host initially sends out the query
using UDP to each discovery address in turn until a reply is
received, or until the end of the list is reached.  To avoid
implosion problems when an entire site reboots such as after a
power outage, the first request should wait 3 seconds for a reply,
with the wait period doubling for each subsequent request.

Since the destination address may be an anycast address, the reply
will necessarily come from a different address.  The host must not
discard the reply simply because the source address is different.
A more detailed discussion of this issue can be found in

Upon receiving a response, the host parses the CFG records and
acts on the information as follows.

If a dnsservers attribute is present, the list of server addresses
is extracted from the value.  If no such attribute is present (or
if no reply is received), an implementation-specific default list
is used.  For example:

o    an implementation MAY use an empty list (effectively
     disabling name resolution),

o    a host MAY use a DNS server list containing only the anycast
     address, subject to the limitations discussed in the next

o    a host MAY use mDNS [MDNS] only, or

o    a host MAY use some combination of the above.
In general, the list obtained is used in the same way as if the
list had been obtained (or failed to be obtained) via DHCP.

If a domainname attribute is present, the domain name is extracted
from the value.  The domain name (or lack thereof) is used in the

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same way as if the list had been obtained (or failed to be
obtained) via DHCP.

If the searchpath attribute is present, the search path is
extracted from the value.  If not present, the host uses the
search path it would use if no path had been obtained if DHCP were
in use.

If the mdnsenabled attribute is present, the value is extracted.
If not present, mDNS is not enabled.

5.  Security Considerations

Ensuring that queries reach a legitimate DNS server relies on the
security of the IPv6 routing infrastructure.  The issues here are
the same as those for protecting basic IPv6 connectivity.

IPsec/IKE can be used when the well-known addresses are used as
unicast addresses, rather than anycast addresses.  When anycast is
used, per RFC 2373 an anycast address cannot be used as a source
address, and as a result IPsec/IKE cannot be used.  However, the
payload can be protected as follows.  If the client can
preconfigure a well known private or public key then TSIG [TSIG]
can be used with the same packets presented for the query.  If
this is not the case, then TSIG keys will have to be negotiated
using [TKEY].  After the client has the proper key then the query
can be performed.

6.  IANA Considerations

The IANA should assign the following site-local IPv6 addresses to
be used as well-known addresses assigned to DNS servers:
fec0:0:0:ffff::1, fec0:0:0:ffff::2, and fec0:0:0:ffff::3.

[Note to readers: the above addresses are tentative, but the ffff
is intended to be consistent with a simultaneous proposal to
reserve the ffff SLA for use with IANA-assigned addresses such as

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7.  Acknowledgements

The IPv6 DNS Discovery Design Team [DDDT] provided recommendations
that formed the basis of this specification.  Rob Austein and the
IPv6 Working Group provided valuable feedback on the mechanism.
Aaron Schrader provided helpful comments on this draft.

8.  References

     Thomson, S., and T. Narten, "IPv6 Stateless Address
     Autoconfiguration", RFC 2462, December 1998.

     Hagino, Jun-ichiro itojun, and K. Ettikan, "An analysis of
     IPv6 anycast", draft-ietf-ipngwg-ipv6-anycast-
     analysis-00.txt, Work in progress, July 2001.

     Thaler, D., Editor, "Analysis of DNS Server Discovery
     Mechanisms for IPv6", draft-ietf-ipngwg-dns-discovery-

     D. Eastlake, "Storage of Diffie-Hellman Keys in the Domain
     Name System (DNS)", RFC 2539, March 1999.

     D. Eastlake, "Domain Name System Security Extensions", RFC
     2535, March 1999.

     B. Aboba, "DHCP Domain Search Option", draft-aboba-dhc-
     domsearch-01.txt, December 2000.

     Haberman, B., and D. Thaler, "Host-based Anycast using MLD",
     draft-haberman-ipngwg-host-anycast-00.txt, February 2001.

     Esibov, L., Aboba, B., and D. Thaler, draft-ietf-dnsext-
     mdns-03.txt, July 2001.

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     D. Eastlake, "Secret Key Establishment for DNS (TKEY RR)" RFC
     2930, September 2000.

     Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington,
     "Secret Key Transaction Authentication for DNS (TSIG)", RFC
     2845, May 2000.

9.  Authors' Addresses

Dave Thaler
One Microsoft Way
Redmond, CA 98052, USA
Email: dthaler@microsoft.com

Jun-ichiro itojun HAGINO
Research Laboratory, Internet Initiative Japan Inc.
Takebashi Yasuda Bldg.,
3-13 Kanda Nishiki-cho,
Chiyoda-ku, Tokyo 101-0054, JAPAN
Email: itojun@iijlab.net

10.  Full Copyright Statement

Copyright (C) The Internet Society (2001).  All Rights Reserved.

This document and translations of it may be copied and furnished
to others, and derivative works that comment on or otherwise
explain it or assist in its implementation may be prepared,
copied, published and distributed, in whole or in part, without
restriction of any kind, provided that the above copyright notice
and this paragraph are included on all such copies and derivative
works.  However, this document itself may not be modified in any
way, such as by removing the copyright notice or references to the
Internet Society or other Internet organizations, except as needed
for the purpose of developing Internet standards in which case the
procedures for copyrights defined in the Internet languages other
than English.

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The limited permissions granted above are perpetual and will not
be revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on

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