V6OPS B. Carpenter
Internet-Draft Univ. of Auckland
Intended status: Informational March 31, 2011
Expires: October 2, 2011
Advisory Guidelines for 6to4 Deployment
draft-ietf-v6ops-6to4-advisory-00
Abstract
This document provides advice to network operators about deployment
of the 6to4 technique for automatic tunneling of IPv6 over IPv4. It
is principally addressed to Internet Service Providers, including
those that do not yet support IPv6, and to Content Providers. The
intention of the advice is to minimise both user dissatisfaction and
help desk calls.
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
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents 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 "work in progress."
This Internet-Draft will expire on October 2, 2011.
Copyright Notice
Copyright (c) 2011 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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Principles of Operation . . . . . . . . . . . . . . . . . . . 4
2.1. Router 6to4 . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Anycast 6to4 . . . . . . . . . . . . . . . . . . . . . . . 5
3. Problems Observed . . . . . . . . . . . . . . . . . . . . . . 5
4. Advisory Guidelines . . . . . . . . . . . . . . . . . . . . . 9
4.1. Vendor Issues . . . . . . . . . . . . . . . . . . . . . . 9
4.2. Consumer ISPs, and enterprise networks, that do not
support IPv6 in any way . . . . . . . . . . . . . . . . . 10
4.2.1. 6to4 as the first step to IPv6 operation . . . . . . . 12
4.3. Consumer ISPs, and enterprise networks, that do
support IPv6 . . . . . . . . . . . . . . . . . . . . . . . 12
4.4. Transit ISPs and Internet Exchange Points . . . . . . . . 12
4.5. Content providers and their ISPs . . . . . . . . . . . . . 13
5. Tunnels Managed by ISPs . . . . . . . . . . . . . . . . . . . 15
6. Security Considerations . . . . . . . . . . . . . . . . . . . 15
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
9. Change log . . . . . . . . . . . . . . . . . . . . . . . . . . 16
10. Informative References . . . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18
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1. Introduction
A technique for automatic tunneling of IPv6 over IPv4, intended for
situations where a user may wish to access IPv6-based services via a
network that does not support IPv6, was defined a number of years
ago. It is known as 6to4 [RFC3056], [RFC3068] and is quite widely
deployed in end systems, especially desktop and laptop computers.
Also, 6to4 is supported in a number of popular models of CPE routers,
some of which have it enabled by default, leading to quite widespread
unintentional deployment by end users.
Unfortunately, experience shows that the method has some problems in
current deployments that can lead to connectivity failures. These
failures either cause long retry delays or complete failures for
users trying to connect to services. In many cases, the user may be
quite unaware that 6to4 is in use, and when the user contacts a help
desk, in all probability the help desk is unable to correctly
diagnose the problem. Anecdotally, many help desks simply advise
users to disable IPv6, thus defeating the whole purpose of the
mechanism, which was to encourage early adoption of IPv6.
There is additional discussion of operational issues in
[I-D.vandevelde-v6ops-harmful-tunnels]. The main goal of the present
document is to offer advice to network operators on how to deal with
this situation more constructively than by disabling 6to4. It
briefly describes the principle of operation, then describes the
problems observed, and finally offers specific advice on the
available methods of avoiding the problems. Note that some of this
advice applies to ISPs that do not yet support IPv6, since their
customers and help desks are significantly affected in any case.
Other advice applies to content providers.
We do not discuss here details of this situation that are mainly
outside the scope of network operators:
1. Operating system preferences between IPv4 and IPv6 when both
appear to be available [I-D.ietf-6man-rfc3484-revise].
2. Ensuring that application software deals gracefully with
connectivity problems [I-D.wing-v6ops-happy-eyeballs-ipv6].
3. Some content providers have chosen to avoid the problem by hiding
their IPv6 address except from customers of pre-qualified
networks [I-D.ietf-v6ops-v6-aaaa-whitelisting-implications].
A companion document [insert reference to successor of
draft-troan-v6ops-6to4-to-historic] reclassifies 6to4 as Historic.
However, this will not remove the millions of existing hosts and
customer premises equipments that implement 6to4. Hence, the advice
in this document remains necessary.
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2. Principles of Operation
There are two variants of 6to4 which are referred to here as "Router
6to4" and "Anycast 6to4". To understand Anycast 6to4, it is
necessary first to understand Router 6to4.
2.1. Router 6to4
Router 6to4 is the original version, documented in [RFC3056]. The
model assumes that a user site operates native IPv6, but that its ISP
provides no IPv6 service. The site border router acts as a 6to4
router. If its external global 32-bit IPv4 address is V4ADDR, the
site automatically inherits the IPv6 prefix 2002:V4ADDR::/48. (The
explanation in RFC 3056 is somewhat confusing, as it refers to the
obsolete "Top Level Aggregator" terminology.) The prefix 2002:
V4ADDR::/48 will be used and delegated for IPv6 service within the
user site.
Consider two such site border routers, with global IPv4 addresses
192.0.2.170 and 190.0.2.187, and therefore inheriting the IPv6
prefixes 2002:c000:2aa::/48 and 2002:c000:2bb::/48 respectively. The
routers can exchange IPv6 packets by encapsulating them in IPv4 using
protocol number 41, and sending them to each other at their
respective IPv4 addresses. In fact, any number of 6to4 routers
connected to the IPv4 network can directly exchange IPv6 packets in
this way.
Some 6to4 routers are also configured as "Relay routers." They
behave as just described, but in addition they obtain native IPv6
connectivity with a normal IPv6 prefix. They announce an IPv6 route
to 2002::/16. For example, assume that the 6to4 router at
190.0.2.187 is a relay router, whose address on the 6to4 side is
2002:c000:2bb::1. Suppose that a host with the 6to4 address 2002:
c000:2aa::123 sends an IPv6 packet to a native IPv6 destination such
as 2001:db8:123:456::321. Assume that the 6to4 router at 192.0.2.170
has its IPv6 default route set to 2002:c000:2bb::1, i.e. the relay.
The packet will be delivered to the relay, encapsulated in IPv4.
After decapsulation, the relay will forward the packet into native
IPv6 for delivery. When the remote host replies, the packet (source
2001:db8:123:456::321, destination 2002:c000:2aa::123) will find a
route to 2002::/16 and hence be delivered to a 6to4 relay. The
process will be reversed and the packet will be encapsulated and
forwarded to the 6to4 router at 192.0.2.170 for final delivery.
Note that this process does not require the same relay to be used in
both directions. The outbound packet will go to whichever relay is
configured as the default IPv6 router at the source router, and the
return packet will go to whichever relay is announcing a route to
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2002::/16 in the vicinity of the remote IPv6 host.
There are of course many further details in RFC 3056, most of which
are irrelevant to current operational problems.
2.2. Anycast 6to4
Router 6to4 assumes that 6to4 routers and relays will be managed and
configured cooperatively. In particular, 6to4 sites need to find a
relay router willing to carry their outbound traffic, which becomes
their default IPv6 router (except for 2002::/16). The objective of
the anycast variant, defined in [RFC3068], is to avoid any need for
such configuration. The intention was to make the solution available
for small or domestic users, even those with a single host or simple
home gateway rather than a border router. This is achieved quite
simply, by defining 192.88.99.1 as the default IPv4 address for a
6to4 relay, and therefore 2002:c058:6301:: as the default IPv6 router
address for a 6to4 site.
Since Anycast 6to4 implies a default configuration for the user site,
it does not require any particular user action. It does require an
IPv4 anycast route to be in place to a relay at 192.88.99.1. As with
Router 6to4, there is no requirement that the return path goes
through the same relay.
3. Problems Observed
It should be noted that Router 6to4 was not designed to be an
unmanaged solution. Quite the contrary: RFC 3056 contains a number
of operational recommendations intended to avoid routing issues. In
practice, there are few if any deployments of Router 6to4 following
these recommendations. Mostly, Anycast 6to4 has been deployed. In
this case, the user site (either a single host or a small broaband
gateway) discovers that it doesn't have native IPv6 connectivity, but
that it does have a global IPv4 address and can resolve AAAA queries,
and therefore assumes that it can send 6to4 packets to 192.88.99.1.
Empirically, 6to4 appears to suffer from a significant level of
connection failure; see
<https://labs.ripe.net/Members/emileaben/6to4-how-bad-is-it-really>
and <http://www.potaroo.net/ispcol/2010-12/6to4fail.html>. In
experiments conducted on a number of dual stack web servers, the TCP
connection failure rate has been measured. In these experiments, the
client's connection attempt to a server was considered to have failed
when the server received a TCP SYN packet and sent a SYN/ACK packet
in response, but received no ACK packet to complete the initial TCP
3-way handshake. The experiment conducted by Aben recorded a failure
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rate of between 9% and 20% of all 6to4 connection attempts. The
experiment conducted by Huston has recorded a failure rate of between
9% and 19% of all 6to4 clients. In this latter experiment it was
further noted that between 65% to 80% of all 6to4 clients who failed
to connect using 6to4 were able to make a successful connection using
IPv4, while the remainder did not make any form of IPv4 connection
attempt, successful or otherwise, using the mapped IPv4 address as a
source address. No connection attempts were recorded by the server
using embedded RFC1918 IPv4 addresses.
There have been several possible reasons offered for this form of
6to4 connection failure. One is the use of private IPv4 addresses
embedded in the 6to4 address, making the return path for the 6to4
tunnel infeasible, and the second is the use of local filters and
firewalls that drop incoming IP packets that use IP protocol 41. If
the former case were prevalent it would be reasonable to expect that
a significant proportion of failed 6to4 connections would use
embedded IPv4 addresses that are either drawn from the private use
(RFC 1918) address ranges, contrary to RFC 3056, or from addresses
that are not announced in the Internet's IPv4 inter-domain routing
table. Neither case was observed to any significant volume in the
experiments conducted by Huston. Furthermore, the experimental
conditions were varied to use a return 6to4 tunnel with either the
native IPv4 source address of the dual stack server or an IPv4 source
address of 192.88.99.1. No change in the 6to4 connection failure
rate was observed between these two configurations; however, other
operators have reported significant problems when replying from the
native address, caused by stateful firewalls at the user site. Given
that the server used its own 6to4 relay for the return path, the only
difference in the IP packet itself between the successful IPv4
connections and the failed 6to4 connections was the IP protocol
number, which was 6 (TCP) for the successful IPv4 connections and 41
(IPv6 payload) for the failed 6to4 connections. The inference from
these experiments is that one likely reason for the high connection
failure rate for 6to4 connections is the use of local filters close
to the end-user that block incoming packets with protocol 41.
In a dual stack context this connection failure rate was effectively
masked by the ability of the client system to recover from the
failure and make a successful connection using IPv4. In this case
the only effect on the client system was a delay in making the
connection of between 7 and 20 seconds as the client's system timed
out on the 6to4 connection attempts (see
[I-D.wing-v6ops-happy-eyeballs-ipv6]).
This experience and further analysis shows that specific operational
problems with Anycast 6to4 include:
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1. Outbound Black Hole: 192.88.99.1 does not generate 'destination
unreachable' but in fact packets sent to that address are
dropped. This can happen due to routing or firewall
configuration, or even because the relay that the packets happen
to reach contains an ACL such that they are discarded.
This class of problem arises because the user's ISP is accepting
a route to 192.88.99.0/24 despite the fact that it doesn't go
anywhere useful. Either the user site or its ISP is dropping
outbound Protocol 41 traffic, or the upstream operator is
unwilling to accept incoming 6to4 packets from the user's ISP.
The latter is superficially compatible with the design of Router
6to4 (referred to as "unwilling to relay" in RFC 3056). However,
the simple fact of announcing a route to 192.88.99.0/24 in IPv4,
coupled with the behavior described in RFC 3068, amounts to
announcing a default route for IPv6 to all 6to4 sites that
receive the IPv4 route. This violates the assumptions of RFC
3056.
The effect of this problem on users is that their IPv6 stack
believes that it has 6to4 connectivity, but in fact all outgoing
IPv6 packets are black-holed. The prevalence of this problem is
hard to measure, since the resulting IPv6 packets can never be
observed from the outside.
2. Inbound Black Hole: In this case, 6to4 packets sent to
192.88.99.1 are correctly delivered to a 6to4 relay, and reply
packets are returned, but they are dropped by an inbound Protocol
41 filter. As far as the user is concerned, the effect is the
same as the previous case: IPv6 is a black hole. Many enterprise
neworks are believed to be set up in this way. Connection
attempts due to this case can be observed by IPv6 server
operators, in the form of SYN packets from addresses in 2002::/16
followed by no response to the resulting SYN/ACK. From the
experiments cited above, this appears to be a significant problem
in practice.
3. No Return Relay: If the Outbound Black Hole problem does not
occur, i.e. the outgoing packet does reach the intended native
IPv6 destination, the target system will send a reply packet, to
2002:c000:2aa::123 in our example above. Then 2002::/16 may or
may not be successfully routed. If it is not routed, the packet
will be dropped (hopefully with 'destination unreachable').
According to RFC 3056, an unwilling relay "MUST NOT advertise any
2002:: routing prefix into the native IPv6 domain"; therefore,
conversely, if this prefix is advertised the relay must relay
packets regardless of source and destination. However, in
practice the problem arises that some relays reject packets that
they should relay, based on their IPv6 source address.
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Whether the native IPv6 destination has no route to 2002::/16, or
it turns out to have a route to an unwilling relay, the effect is
the same: all return IPv6 packets are black-holed. While there
is no direct evidence of the prevalence of this problem, it
certainly exists in practice.
4. Large RTT: In the event that none of the above three problems
applies, and a two-way path does in fact exist between a 6to4
host and a native host, the round trip time may be quite large
and variable since the paths to the two relays are unmanaged and
may be complex. Overloaded relays might also cause highly
variable RTT.
5. PMTUD Failure: A common link MTU size observed on the Internet
today is 1500 bytes. However, when using 6to4 the path MTU is
less than this due to the encapsulation header. Thus a 6to4
client will normally see a link MTU that is less than 1500, but a
native IPv6 server will see 1500. Path MTU Discovery does not
always work, and this can lead to connectivity failures. Even if
a TCP SYN/ACK exchange works, TCP packets with full size payloads
may simply be lost. These failures are disconcerting even to an
informed user, since a standard 'ping' from the client to the
server will succeed, because it generates small packets, and the
successful SYN/ACK exchange can be traced. Also, the failure may
occur on some paths but not others, so a user may be able to
fetch web pages from one site, but only ping another.
6. Reverse DNS Failure: Typically a 6to4-addressed host will not
have a reverse DNS delegation. If reverse DNS is used as a
pseudo-security check, it will fail.
7. Bogus Address Failure: By design, 6to4 does not work and will not
activate itself if the available V4ADDR is a private address
[RFC1918]. However it will also not work if the available V4ADDR
is a "bogon", i.e. a global address that is being used by the
operator as a private address. A common case of this is a legacy
wireless network using 1.1.1.0/24 as if it was a private address.
In this case, 6to4 will assume it is connected to the global
Internet, but there is certainly no working return path.
This failure mode will also occur if an ISP is operating a
Carrier Grade NAT between its customers and the Internet, and is
using global public address space as if it were private space to
do so.
8. Faulty 6to4 Implementations: It has been reported that some 6to4
implementations attempt to activate themselves even when the
available IPv4 address is an RFC 1918 address. This is in direct
contradiction to RFC 3056, and will produce exactly the same
failure mode as Bogus Address Failure. It is of course outside
the ISP's control.
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9. Difficult Fault Diagnosis: The existence of all the above failure
modes creates a problem of its own: very difficult fault
diagnosis, especially if the only symptom reported by a user is
slow access to web pages, caused by a long timeout before
fallback to IPv4. Tracking down anycast routing problems and
PMTUD failures is particularly hard.
The practical impact of the above problems, which are by no means
universal as there is considerable successful use of Anycast 6to4,
has been measured at a fraction of 1% loss of attempted connections
to content servers (see <http://www.fud.no/ipv6/>). While this seems
low, it amounts to a significant financial impact for content
providers. Also, end users frustrated by the poor response times
caused by fall-back to IPv4 connectivity
[I-D.wing-v6ops-happy-eyeballs-ipv6] are considered likely to
generate help desk calls with their attendant costs.
A rather different operational problem caused incidentally by 6to4 is
that, according to observations made by Tim Chown and James Morse at
the University of Southampton, rogue Router Advertisements [RFC6104]
predominantly convey a 2002::/16 prefix. This appears to be due to
misbehaviour by devices acting as local IPv6 routers or connection-
sharing devices but issuing RA messages on the wrong interface.
4. Advisory Guidelines
There are several types of operator involved, willingly or
unwillingly, in the Anycast 6to4 scenario and they will all suffer if
things work badly. There is a clear incentive for each of them to
take appropriate action, as described below.
This document avoids formal normative language, because it is highly
unlikely that the guidelines apply universally. Each operator will
make its own decisions about which of the following guidelines are
useful in its specific scenario.
4.1. Vendor Issues
Although this document is aimed principally at operators, there are
some steps that implementers and vendors of 6to4 should take.
1. Some vendors of routers, including customer premises equipment,
have not only included support for 6to4 in their products, but
have enabled it by default. This is bad practice - it should
always be a conscious decision by a user to enable 6to4. Many of
the above problems only occur due to unintentional deployment of
6to4.
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2. Similarly, host operating systems should not enable Anycast 6to4
by default; it should always be left to the user to switch it on.
3. Any 6to4 implementation that attempts to activate itself when the
available IPv4 address is an RFC 1918 address is faulty and needs
to be updated.
4. 6to4 implementations should adopt updated IETF recommendations on
address selection [I-D.ietf-6man-rfc3484-revise].
5. 6to4 router or connection-sharing implementations must avoid
issuing rogue RAs [RFC6104]. Specifically, where 6to4 is being
enabled by a host for Internet connection sharing purposes, and
the host supports [RFC4191], then it should set the Router
Advertisement router preference bits to 11 (low preference).
4.2. Consumer ISPs, and enterprise networks, that do not support IPv6
in any way
To reduce the negative impact of Anycast 6to4 deployed (probably
unknowingly) by users, and consequent user dissatisfaction and help
desk calls, such ISPs should check in sequence:
1. Does the ISP have a route to 192.88.99.1? (This means an
explicit route, or knowledge that the default upstream provider
has an explicit route. A default route doesn't count!)
2. If so, is it functional and stable?
3. If so, is the ping time reasonably short?
4. If so, does the relay willingly accept 6to4 traffic from the
ISP's IPv4 prefixes? (Note that this is an administrative as
well as a technical question - is the relay's operator willing to
accept the traffic?)
Unless the answer to all these questions is 'yes', subscribers will
be no worse off, and possibly better off, if the route to 192.88.99.1
is blocked and generates an IPv4 'destination unreachable'. There is
little operational experience with this, however.
Some implementations also perform some form of 6to4 relay
qualification. For example, one host implementation (Windows) tests
the Protocol 41 reachability by sending an ICMPv6 echo request with
Hop Limit=1 to the relay, expecting a response or Hop Limit exceeded
error back. Lack of any response indicates that the 6to4 relay does
not work so 6to4 is turned off [Savola].
A more constructive approach for such an ISP is to seek out a transit
provider who is indeed willing to offer outbound 6to4 relay service,
so that the answer to each of the questions above is positive.
In any case, such ISPs should always allow protocol 41 through their
network and firewalls. Not only is this a necessary condition for
6to4 to work, but it also allows users who want to use a configured
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IPv6 tunnel service to do so.
Some operators, particularly enterprise networks, silently block
Protocol 41 on security grounds. Doing this on its own is bad
practice, since it contributes to the problem and harms any users who
are knowingly or unknowingly attempting to run 6to4. The strategic
solution is to deploy native IPv6, making Protocol 41 redundant. In
the short term, experimentation could be encouraged by allowing
Protocol 41 for certain users, while returning appropriate ICMP
responses as mentioned above. Unfortunately, if this is not done,
the 6to4 problem cannot be solved.
Operators should never use "bogon" address space such as the example
of 1.1.1.0/24 for customers, since IPv4 exhaustion means that all
such addresses are likely to be in real use in the near future.
(Also see [I-D.ietf-intarea-shared-addressing-issues].) An operator
that is unable to immediately drop this practice should ensure that
192.88.99.1 generates IPv4 'destination unreachable'. It has been
suggested that they could also run a dummy 6to4 relay at that address
which always returns ICMPv6 'destination unreachable' as a 6to4
packet. However, these techniques are not very effective, since most
current end-user 6to4 implementations will ignore them.
If an operator is providing legitimate global addresses to customers
(neither RFC 1918 nor bogon addresses), and also running Carrier
Grade NAT (Large Scale NAT) between this address space and the global
address space of the Internet, then 6to4 cannot work properly. Such
an operator should also take care to return 'destination unreachable'
for 6to4 traffic. Alternatively, they could offer untranslated
address space to the customers concerned.
A customer who is intentionally using 6to4 may also need to create
AAAA records, and the operator should be able to support this, even
if the DNS service itself runs exclusively over IPv4. However,
customers should be advised to consider carefully whether their 6to4
service is sufficiently reliable for this.
Operators could, in principle, offer reverse DNS support for 6to4
users [RFC5158], although this is not straightforward for domestic
customers.
Paradoxically, operators in this category should consider whether
they need to defend themselves against rogue IPv6 RA messages
[RFC6105], since such messages may appear from devices seeking to
operate as 6to4 routers.
Finally, enterprise operators who have complete administrative
control of all end-systems may choose to disable 6to4 in those
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systems as an integral part of their plan to deploy IPv6.
4.2.1. 6to4 as the first step to IPv6 operation
Some IPv4 operators have chosen to install a 6to4 relay, connected
via an IPv6-in-IPv4 tunnel to an IPv6 operator, as a first step
before native IPv6 deployment. The routing guidelines in Section 4.4
would apply. However, offering genuine IPv6 service to interested
customers, even if tunneled, would generally be a better first step.
4.3. Consumer ISPs, and enterprise networks, that do support IPv6
Once an operator does support IPv6 service, whether experimentally or
in production, it is almost certain that users will get better
results using this service than by continuing to use 6to4.
Therefore, these operators are encouraged to advise their users to
disable 6to4 and they should not create DNS records for any 6to4
addresses.
Such an operator may automatically fall into one of the following two
categories (transit provider or content provider), so the guidelines
in Section 4.4 or in Section 4.5 will apply instead.
Operators in this category should make sure that no routers are
unintentionally or by default set up as active 6to4 relays.
Unmanaged 6to4 relays will be a source of problems.
Operators in this category should consider whether they need to
defend themselves against rogue RA messages [RFC6105].
4.4. Transit ISPs and Internet Exchange Points
We assume that transit ISPs and IXPs have IPv6 connectivity. To
reduce the negative impact of Anycast 6to4 on all their client
networks, it is strongly recommended that they each run an Anycast
6to4 relay service. This will have the additional advantage that
they will terminate the 6to4 IPv4 packets, and can then forward the
decapsulated IPv6 traffic according to their own policy. Otherwise,
they will blindly forward all the encapsulated IPv6 traffic to a
competitor who does run a relay.
It is of critical importance that routing to this service is
carefully managed:
1. The IPv4 prefix 192.88.99.0/24 must be announced only towards
client IPv4 networks whose outbound 6to4 packets will be
accepted.
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2. The IPv6 prefix 2002::/16 must be announced towards native IPv6.
The relay must accept all traffic towards 2002::/16 that reaches
it, so the scope reached by this announcement should be carefully
planned. It must reach all client IPv6 networks of the transit
ISP or IXP. If it reaches a wider scope, the relay will be
offering a free ride to non-clients.
3. The evidence is mixed, but it seems best to ensure that when the
relay sends 6to4 packets back towards a 6to4 user, they should
have 192.88.99.1 as their IPv4 source address (not the relay's
unicast IPv4 address). This is to avoid problems if the user is
behind a stateful firewall that drops inbound packets from
addresses that have not been seen in outbound traffic.
4. The relay should be capable of responding correctly to ICMPv6
echo requests encapsulated in IPv4 protocol 41, typically with
outer destination address 192.88.99.1 and inner destination
address 2002:c058:6301::. (As noted previously, some 6to4 hosts
are known to send echo requests with Hop Limit = 1, which allows
them to rapidly detect the presence or absence of a relay in any
case, but operators cannot rely on this behaviour.)
5. Protocol 41 must not be filtered in any IPv4 network or
firewalls.
6. As a matter of general practice, which is essential for 6to4 to
work well, IPv6 PMTUD must be possible, which means that ICMPv6
must not be blocked anywhere [RFC4890]. This also requires that
the relay has a sufficiently high ICMP error generation
threshold. For a busy relay, a typical default rate limit of 100
packets per second is too slow. On a busy relay, 1000pps or more
might be needed. If ICMPv6 "Packet too Big" error messages are
rate-limited, users will experience PMTUD failure.
7. The relay must have adequate performance, and since load
prediction is extremely hard, it must be possible to scale it up
or, perhaps better, to replicate it as needed. Since the relay
process is stateless, any reasonable method of load sharing
between multiple relays will do.
8. The relay must of course be connected directly to global IPv4
space, with no NAT.
Operators in this category should make sure that no routers are
unintentionally or by default set up as active 6to4 relays.
Unmanaged 6to4 relays will be a source of problems.
4.5. Content providers and their ISPs
We assume that content providers and their ISPs have IPv6
connectivity, and that content servers are dual stacked. There is a
need to avoid the situation where a client host, configured with
Anycast 6to4, suceeds in sending an IPv6 packet to the server, but
the 6to4 return path fails as described above. To avoid this, there
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must be a locally positioned 6to4 relay. Large content providers are
advised to operate their own relays, and ISPs should do so in any
case. There must be a 2002::/16 route from the content server to the
relay. As noted in the previous section, the corresponding route
advertisement must be carefully scoped, since any traffic that
arrives for 2002::/16 must be relayed.
Such a relay may be dedicated entirely to return traffic, in which
case it need not respond to the 6to4 anycast address.
Nevertheless, it seems wisest to ensure that when the relay sends
6to4 packets back towards a 6to4 user, they should have 192.88.99.1
as their IPv4 source address (not the relay's unicast IPv4 address).
As noted above, this is to avoid problems if the user is behind a
stateful firewall that drops UDP packets from addresses that have not
been seen in outbound traffic. However, it is also necessary that
192.88.99.1 is not blocked by upstream ingress filtering - this needs
to be tested.
Without careful engineering, there is nothing to make the return path
as short as possible. It is highly desirable to arrange the scope of
advertisements for 2002::/16 such that content providers have a short
path to the relay, and the relay should have a short path to the ISP
border. Care should be taken about shooting off advertisements for
2002::/16 into BGP4; they will become traffic magnets. If every ISP
with content provider customers operates a relay, there will be no
need for any of them to be advertised beyond each ISP's own
customers.
Protocol 41 must not be filtered in the ISP's IPv4 network or
firewallls. If the relays are placed outside the content provider's
firewall, the latter may filter protocol 41 if desired.
The relay must have adequate performance, and since load prediction
is extremely hard, it must be possible to scale it up or, perhaps
better, to replicate it as needed. Since the relay process is
stateless, any reasonable method of load sharing between multiple
relays will do.
The relay must of course be connected directly to global IPv4 space,
with no NAT.
An option for content servers is to embed the relay function directly
in the content server. This is in fact trivial, since it can be
achieved by enabling a local 6to4 interface on the server, and using
it to route 2002::/16 for outbound packets. (This might not allow
use of 192.88.99.1 as the source address.) Further details are to be
found at <http://www.potaroo.net/ispcol/2010-05/v6hints.html>.
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However, in this case Protocol 41 must be allowed by the firewalls.
Content providers who do embed the relay function in this way could,
in theory, accept inbound 6to4 traffic as well. This is highly
unadvisable since, according the the rules of 6to4, they would then
have to relay traffic for other IPv6 destinations too. So they
should not be reachable via 192.88.99.1. Also, they should certainly
not create an AAAA record for their 6to4 address - their inbound IPv6
access should be native, and advertising a 6to4 address might well
lead to uRPF ingress filtering problems.
To avoid the path MTU problem described above, content servers should
also set their IPv6 MTU to a safe value. From experience, 1280 bytes
(the minimum allowed for IPv6) is recommended; again see
<http://www.potaroo.net/ispcol/2010-05/v6hints.html>. Of course,
ICMPv6 "Message too Big" must not be blocked or rate-limited anywhere
[RFC4890].
Reverse DNS delegations are highly unlikely to exist for 6to4
clients, and are by no means universal for other IPv6 clients.
Content providers (and in fact all service providers) should not rely
on them as a pseudo-security check for IPv6 clients.
Operators and content providers should make sure that no routers are
unintentionally or by default set up as active 6to4 relays.
Unmanaged 6to4 relays will be a source of problems.
5. Tunnels Managed by ISPs
There are various ways, such as tunnel brokers [RFC3053], 6rd
[RFC5969], and L2TPv2 hub-and-spoke [RFC5571], by which Internet
Service Providers can provide tunneled IPv6 service to subscribers in
a managed way, in which the subscriber will acquire an IPv6 prefix
under a normal provider-based global IPv6 prefix. Most of the issues
described for 6to4 do not arise in these scenarios. However, for
IPv6-in-IPv4 tunnels used by clients behind a firewall, it is
essential that IPv4 Protocol 41 is not blocked.
As a matter of general practice, IPv6 PMTUD must be possible, which
means that ICMPv6 "Message too Big" must not be blocked or rate-
limited anywhere [RFC4890].
6. Security Considerations
There is a general discussion of security issues for IPv6-in-IPv4
tunnels in [I-D.ietf-v6ops-tunnel-security-concerns], and
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[I-D.ietf-v6ops-tunnel-loops] discusses possible malicious loops.
[RFC3964] specifically discusses 6to4 security. In summary, tunnels
create a challenge for many common security mechanisms, simply
because a potentially suspect packet is encapsulated inside a
harmless outer packet. All these considerations apply to the
automatic mechanisms discussed in this document. However, it should
be noted that if an operator provides well managed servers and relays
for 6to4, non-encapsulated IPv6 packets will pass through well
defined points (the native IPv6 interfaces of those servers and
relays) at which security mechanisms may be applied.
A blanket recommendation to block Protocol 41 is not compatible with
mitigating the 6to4 problems described in this document.
7. IANA Considerations
This document makes no request of the IANA.
8. Acknowledgements
Useful comments and contributions were made by Emile Aben, Tore
Anderson, Jack Bates, Cameron Byrne, Tim Chown, Remi Despres, Jason
Fesler, Wes George, Geoff Huston, Eric Kline, Victor Kuarsingh,
Martin Levy, David Malone, Martin Millnert, Keith Moore, Gabi
Nakibly, Michael Newbery, Pekka Savola, Mark Smith, Nathan Ward,
James Woodyatt, and others.
This document was produced using the xml2rfc tool [RFC2629].
9. Change log
draft-ietf-v6ops-6to4-advisory-00: WG draft, updated with WG
comments, 2011-03-31
draft-carpenter-v6ops-6to4-teredo-advisory-03: updated with
additional security reference and addditional comments, 2011-03-12
draft-carpenter-v6ops-6to4-teredo-advisory-02: updated after further
comments, removed references to Teredo, 2011-02-24
draft-carpenter-v6ops-6to4-teredo-advisory-01: updated after WG
discussion, 2011-02-10
draft-carpenter-v6ops-6to4-teredo-advisory-00: original version,
2011-02-03
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10. Informative References
[I-D.ietf-6man-rfc3484-revise]
Matsumoto, A., Kato, J., and T. Fujisaki, "Update to RFC
3484 Default Address Selection for IPv6",
draft-ietf-6man-rfc3484-revise-02 (work in progress),
March 2011.
[I-D.ietf-intarea-shared-addressing-issues]
Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
Roberts, "Issues with IP Address Sharing",
draft-ietf-intarea-shared-addressing-issues-05 (work in
progress), March 2011.
[I-D.ietf-v6ops-tunnel-loops]
Nakibly, G. and F. Templin, "Routing Loop Attack using
IPv6 Automatic Tunnels: Problem Statement and Proposed
Mitigations", draft-ietf-v6ops-tunnel-loops-06 (work in
progress), March 2011.
[I-D.ietf-v6ops-tunnel-security-concerns]
Krishnan, S., Thaler, D., and J. Hoagland, "Security
Concerns With IP Tunneling",
draft-ietf-v6ops-tunnel-security-concerns-04 (work in
progress), October 2010.
[I-D.ietf-v6ops-v6-aaaa-whitelisting-implications]
Livingood, J., "IPv6 AAAA DNS Whitelisting Implications",
draft-ietf-v6ops-v6-aaaa-whitelisting-implications-03
(work in progress), February 2011.
[I-D.vandevelde-v6ops-harmful-tunnels]
Velde, G., Troan, O., and T. Chown, "Non-Managed IPv6
Tunnels considered Harmful",
draft-vandevelde-v6ops-harmful-tunnels-01 (work in
progress), August 2010.
[I-D.wing-v6ops-happy-eyeballs-ipv6]
Wing, D. and A. Yourtchenko, "Happy Eyeballs: Trending
Towards Success with Dual-Stack Hosts",
draft-wing-v6ops-happy-eyeballs-ipv6-01 (work in
progress), October 2010.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
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June 1999.
[RFC3053] Durand, A., Fasano, P., Guardini, I., and D. Lento, "IPv6
Tunnel Broker", RFC 3053, January 2001.
[RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
via IPv4 Clouds", RFC 3056, February 2001.
[RFC3068] Huitema, C., "An Anycast Prefix for 6to4 Relay Routers",
RFC 3068, June 2001.
[RFC3964] Savola, P. and C. Patel, "Security Considerations for
6to4", RFC 3964, December 2004.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, November 2005.
[RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering
ICMPv6 Messages in Firewalls", RFC 4890, May 2007.
[RFC5158] Huston, G., "6to4 Reverse DNS Delegation Specification",
RFC 5158, March 2008.
[RFC5571] Storer, B., Pignataro, C., Dos Santos, M., Stevant, B.,
Toutain, L., and J. Tremblay, "Softwire Hub and Spoke
Deployment Framework with Layer Two Tunneling Protocol
Version 2 (L2TPv2)", RFC 5571, June 2009.
[RFC5969] Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
Infrastructures (6rd) -- Protocol Specification",
RFC 5969, August 2010.
[RFC6104] Chown, T. and S. Venaas, "Rogue IPv6 Router Advertisement
Problem Statement", RFC 6104, February 2011.
[RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
February 2011.
[Savola] Savola, P., "Observations of IPv6 Traffic on a 6to4
Relay", ACM SIGCOMM CCR 35 (1) 23-28, 2006.
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Author's Address
Brian Carpenter
Department of Computer Science
University of Auckland
PB 92019
Auckland, 1142
New Zealand
Email: brian.e.carpenter@gmail.com
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