Network Working Group Alain Durand
INTERNET-DRAFT SUN Microsystems, inc.
August 21, 2002 Jun-ichiro itojun Hagino
Expires February 2002 IIJ Research Laboratory
Dave Thaler
Microsoft
Well known site local unicast addresses for DNS resolver
<draft-ietf-ipv6-dns-discovery-06.txt>
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
This documents specifies a method for nodes to find a DNS resolver
with minimum configuration in the network and without running a
discovery protocol on the nodes. This method is to be used in last
resort, when no other information about the addresses of DNS
resolvers is available.
Copyright notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
1. Introduction
RFC 2462 [ADDRCONF] provides a way to autoconfigure nodes with one or
more IPv6 address and default routes.
However, for a node to be fully operational on a network, many other
parameters are needed, such as the address of a DNS resolver, mail
relays, web proxies, etc. Except for name resolution, all the other
services are usually described using names, not addresses, such as
smtp.myisp.net or webcache.myisp.net. For obvious bootstrapping
reasons, a node needs to be configured with the IP address (and not
the name) of a DNS resolver. As IPv6 addresses look much more
complex than IPv4 ones, there is some incentive to make this
configuration as automatic and simple as possible.
Although it would be desirable to have all configuration parameters
configured/discovered automatically, it is common practice in IPv4
today to ask the user to do manual configuration for some of them by
entering server names in a configuration form. So, a solution that
will allow for automatic configuration of the DNS resolver is seen as
an important step forward in the autoconfiguration story.
The intended usage scenario for this proposal is a home or enterprise
network where IPv6 nodes are plugged/unplugged with minimum
management and use local resources available on the network to
autoconfigure. This proposal is also usefull in cellular networks
where all moble devices are included within the same site.
2. Pre-configuration vs discovery
Some of the discussions in the past around DNS server discovery have
been trying to characterize the solution space into stateless versus
stateful or server-oriented versus severless. It is not absolutely
clear how much state if any needs to be kept to perform DNS server
discovery, and, although the semantic differences between a router
and a server are well understood from a conceptual perspective, the
current implementations tend to blur the picture. In another attempt
to characterize different approaches, one can look at how much
intelligence a client needs to have in order to use the service.
One avenue is to ask the IPv6 node to participate in a discovery
protocol, such as SLP or DHCP, learn the address of the server and
send packets to this server. Another one is to pre-configure (hard-
code) a local scope address on the IPv6 node and let it send packets
directly to this address, with the underlying assumption that the
routing system will forward them to the right place. This document
explores this later avenue of pre-configuration that does not require
participation of the end node in the DNS resolver discovery
mechanism.
The mechanism described here is to be used as a last resort, in the
absence of any information about the addresses of DNS resolvers. If
other configuration mechanisms are available, they should be tried
first.
Note to implementors:
Implementing only the mechanism described in this memo may end up
causing some interoperability problems when operating in networks
where no DNS resolver is configured with the well known addresses.
Thus, it is recommended to implement also other mechanisms for
overriding this default, for example: manual configuration, L2
mechanisms and/or DHCPv6.
3. Reserved prefix and addresses
The basic idea of this proposal is to reserve three well known IPv6
site local addresses for DNS resolvers and then have the routing
system forward the DNS request to them.
IPv6 stub-resolvers implementing this proposal are pre-configured
with those three IPv6 addresses as DNS resolver.
Local DNS resolvers should be configured with one of those three
addresses to enable clients to switch from one to the other if one
fails.
A solution to enable clients to reach the DNS resolvers is to inject
host routes in the local routing system. Examples of methods for
injecting host routes and a brief discussion of their fate sharing
properties are presented here:
a) Manual injection of routes by a router on the same subnet.
If the node running the DNS resolver goes down, the router may or
may not be notified and keep announcing the route.
b) Running a routing protocol on the same node running the DNS
resolver.
If the process running the DNS resolver dies, the routing protocol
may or may not be notified and keep annoucing the route.
c) Running a routing protocol within the same process running the
DNS resolver.
If the DNS resolver and the routing protocol run in separated
threads, similar concerns as above are true.
d) Having an "announcement" protocol that the DNS resolver could
use to advertize the host route to the nearby router. Details of
such a protocols are out of scope of this document, but something
similar to [MLD] is possible.
An alternate solution is to configure a link with the well known
prefix and position the three DNS resolvers on that link. The
advantage of this method is that host routes are not necessary , the
well known prefix is advertized to the routing system by the routers
on the link. However, in the event of a problem on the physical link,
all resolvers will become unreachable.
IANA considerations for this prefix are covered in Section 6.
4. Site local versus global scope considerations
The rationales for having a site local prefix are:
-a) Using a site local prefix will ensure that the traffic to the
DNS resolver stays local to the site. This will prevent the DNS
requests from accidentally leaking out of the site. However, the
local resolver can implement a policy to forward DNS resolution of
non-local addresses to an external DNS resolver.
-b) Reverse DNS resolution of site local addresses is only
meaningful within the site. Thus, making sure that such queries
are first sent to a DNS resolver located within the site perimeter
increase their likelyhood of success.
5. Examples of use
This section presents example scenarios showing how the mechanism
described in this memo can co-exist with other techniques, namely
manual configuration and DHCPv6 discovery.
5.1 Simple case, general purpose DNS resolver
This example shows the case of an enterprise or a cellular network
that manages a full flavor general purpose DNS resolver and a large
number of nodes running DNS stub resolvers. The DNS resolver is
performing (and caching) all the recursive queries on behalf of the
stub resolvers. Those stub resolvers are either manually configured
with the IPv6 address of the resolver or with one (or several) of the
well known site local unicast addresses defined in this memo.
-------------------------------------------
| |
| --------------------- |
| |manually configured| |
| |DNS stub resolver | |
| --------------------- |
| ---------- | |
| |DNS |<----------- |
| |resolver|<----------- |
| ---------- | |
| --------------------- |
| |DNS stub resolver | |
| |configured with | |
| |well known address | |
| --------------------- |
| |
-------------------------------------------
(The DNS resolver is configured to listen both on
its IPv6 address and on the well known address)
5.2 DNS forwarder
A drawback of the choice of site local scope for the reserved
addresses for DNS resolver is that, in the case of a home/small
office network connected to an ISP, DNS traffic cannot be sent
directly to the ISP DNS resolver without having the ISP and all its
customers share the same definition of site.
In this scenario, the home/small office network is connected to the
ISP router (PE) via an edge router (CPE).
-------------
/ |
-------- -------------- / |
|ISP PE| |customer CPE| / Customer |
| |===========| |====< site |
| | | | \ |
-------- -------------- \ |
\ |
-------------
The customer router CPE could be configured on its internal interface
with one of the reserved site local addresses and listen for DNS
queries. It would be configured to use one (or several) of the well
known site local unicast addresses within the ISP's site to send its
own queries to. It would act as a DNS forwarder, forwarding queries
received on its internal interface to the ISP's DNS resolver.
-------------
/ |
---------- -------------- / |
|ISP | |customer CPE| / Customer |
|DNS |===========| DNS|====< site |
|resolver| <------|---forwarder|-----\---- |
---------- -------------- \ |
\ |
-------------
In this configuration, the CPE is acting as a multi-sited router.
5.3 DNS forwarder with DHCPv6 interactions
In this variant scenario, DHCPv6 is be used between the PE and CPE to
do prefix delegation [DELEG] and DNS resolver discovery.
-------------
/ |
-------- -------------- / |
|ISP | |customer CPE| / Customer |
|DHCPv6|===========| DHCPv6|====< site |
|server| <------|------client| \ |
-------- -------------- \ |
\ |
-------------
This example will show how DHCPv6 and well known site local unicast
addresses cooperate to enable the internal nodes to access DNS.
The customer router CPE is configured on its internal interface with
one of the reserved site local addresses and listen for DNS queries.
It would act as a DNS forwarder, as in 5.2, forwarding those queries
to the DNS resolver pointed out by the ISP in the DHCPv6 exchange.
-------------
/ |
---------- -------------- / |
|ISP | |customer CPE| / Customer |
|DNS |===========| DNS|====< site |
|resolver| <------|---forwarder|-----\---- |
---------- -------------- \ |
\ |
-------------
The same CPE router could also implement a local DHCPv6 server and
advertises itself as DNS forwarder.
-------------
/ |
-------- -------------- / Customer |
|ISP PE| |customer CPE| / site |
| |===========|DHCPv6 |====< |
| | |server------|-----\---> |
-------- -------------- \ |
\ |
-------------
Within the site:
a) DHCPv6 aware clients use DHCPv6 to obtain the address of the
DNS forwarder...
-------------
/ |
---------- -------------- / Customer |
|ISP | |customer CPE| / site |
|DNS |===========| DNS|====< |
|resolver| <------|---forwarder|-----\----DHCPv6 |
---------- -------------- \ client |
\ |
-------------
(The address of the DNS forwarder is aquired via DHCPv6.)
b) other nodes simply send their DNS request to the reserved site
local addresses.
-------------
/ |
---------- -------------- / customer |
|ISP | |customer CPE| / site |
|DNS |===========| DNS|====< |
|resolver| <------|---forwarder|-----\----non DHCPv6|
---------- -------------- \ node |
\ |
-------------
(Internal nodes use the reserved site local unicast address.)
A variant of this scenario is the CPE can decide to pass the global
address of the ISP DNS resolver in the DHCPv6 exchange with the
internal nodes.
6. IANA considerations
The site local prefix fec0:0000:0000:ffff::/64 is to be reserved out
of the site local fec0::/10 prefix.
The unicast addresses fec0:000:0000:ffff::1, fec0:000:0000:ffff::2
and fec0:000:0000:ffff::3 are to be reserved for DNS resolver
configuration.
All other addresses within the fec0:0000:0000:ffff::/64 are reserved
for future use and are expected to be assigned only with IESG
approval.
7. 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 as the well-known addresses are used as unicast
addresses.
The payload can be protected using standard DNS security techniques.
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.
The use of site local addresses instead of global addresses will
ensure the DNS queries issued by host using this mechanism will not
leak out of the site.
8. References
[ADDRCONF]
Thomson, S., and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[MLD]
Deering, S., Fenner, W., Haberman, B.,
"Multicast Listener Discovery (MLD) for IPv6",
RFC2710, October 1999.
[TSIG]
Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG)",
RFC2845, May 2000.
[TKEY]
D. Eastlake, "Secret Key Establishment for DNS (TKEY RR)",
RFC2930, September 2000.
[DHCPv6]
Bound, J., Carney, M., Perkins, C., Lemon, T., Volz, B. and
Droms, R. (ed.), "Dynamic host Configuration Protocol for IPv6
(DHCPv6)", draft-ietf-dhc-dhcpv6-23 (work in progress),
Februray 2002.
[DELEG]
Troan, O., Droms, R., "IPv6 Prefix Options for DHCPv6",
draft-troan-dhcpv6-opt-prefix-delegation-00.txt (work in progress),
February 2002.
9. Authors' Addresses
Alain Durand
SUN microsystems, inc.
901 San Antonio rd UMPK 17-202
Palo Alto, CA 94303, USA.
Email: Alain.Durand@sun.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
Dave Thaler
Microsoft
One Microsoft Way
Redmond, CA 98052, USA
Email: dthaler@microsoft.com
10. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
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