gabi,
Thanks for your quick answer.
Further remarks in line.
Le 19 août 09 à 12:12, Gabi Nakibly a écrit :
Remi,
See my comments inline (<gn>).
Gabi
From: Rémi Després <remi.desp...@free.fr>
To: Gabi Nakibly <gnaki...@yahoo.com>
Cc: v6ops <v6...@ops.ietf.org>; 6man 6man <ipv6@ietf.org>;
sec...@ietf.org; Mark Townsley <towns...@cisco.com>
Sent: Tuesday, August 18, 2009 8:00:42 PM
Subject: Re: Routing loop attacks using IPv6 tunnels - the 6rd case
Hi Gabi,
First, thanks to you and your colleagues for this research, and for
the clear presentation of its results.
In my understanding, your contribution is important for transition
solutions to be carefully selected, and where needed improved.
This mail is to complement the analysis with what applies to 6rd.
For those who don't know it, 6rd, like 6to4, ISATAP and Teredo, is
an automatic tunnel mechanism in actual use for IPv6 across IPv4
clouds.
With it, service providers can offer native IPv6 to their customers
while using for this their existing IPv4 infrastructures.
Publication of the RFC that describes it, RFC 5569, has been
delayed since May for a reason related to intellectual property
rights applicable to independent submissions.
But the draft on which 6rd is based is still available, and a new
draft to extend its applicability is also available:
- tools.ietf.org/html/draft-despres-6rd-03
- tools.ietf.org/html/draft-townsley-ipv6-6rd-01
<gn>
I must admit that this is the first time I read the spec of 6rd so
forgive me if I miss something.
</gn>
(1) Case of ISPs that operate 6rd relays and no 6to4 relays (and
neither Teredo relays nor ISP-infrastructure NATs)
In its sec. 3, draft-despres-6rd-03 says:
<<<
The IPv4 anycast address of 6rd relays may be chosen
independently by
each ISP. The only constraint is that routes toward the ISP that
are
advertised must not include this address.
>>>
In view of your study and in my understanding, it should be
completed with:
"Also, the ISP must not forward toward the global IPv4 global
Internet packets having this address as source."
With this, an ISP that operates 6rd relays but operates neither
6to4 relays nor Teredo relays nor NATs is immune to the routing
loop attack because:
- An IPv6 packet forwarded to the IPv6 Internet by a 6rd relay
cannot come back to an IPv4 interface of a 6rd relay of the same
ISP: there is no IPv4 route back to the ISP for its 6rd anycast
address.
- An IPv6 packet received from the IPv6 Internet by a 6rd relay
cannot be sent back to the IPv4 global Internet: the source address
of its IPv4 encapsulating packet is the 6rd anycast address, which
prevents it from reaching the IPv4 global Internet.
Note that, if interfaces of the ISP to the IPv4 global Internet are
already subject to ingress filtering (packets received by the
global Internet are discarded if there is no reverse path available
for them), the added sentence is not necessary. It is just just a
double precaution for cases where such ingress filtering doesn't
apply.
<gn>
I agree with you that above check will work. However, I might
choose another way here: the relay must make the following two checks:
1) When an IPv6 packet is received from the IPv6 Internet the 6rd
relay must ensure before encapsulation that the intended IPv4
destination address belongs to one of the ISP's clients (I assume
it can make this check easily). This way no IPv6 packet received
from the IPv6 Internet will be relayed to a 6to4 relay (and then
back to the 6rd relay through the IPv6 Internet).
The problem is that this check is not easy "in general" because ISPs
typically have their IPv4 prefixes allocated one by one as they
increase the number of their clients.
2) When an encapsulated packet is received from the IPv4 network
side the 6rd relay must check that the IPv6 destination does not
include its own IPv4 address. For example the IPv6 destination
address must not be: 2002:<IPv4 address of 6rd relay>::/48. This
will prevent the packet from ever reaching back the 6rd relay
through its IPv4 interface
The problem is that to be general, and as you noted, this test
depends on a knowledge of all formats used to embed IPv4 addresses in
IPv6 addresses. When a new format is introduced, a security weakness
therefore holds until all relays are upgraded to support it.
Besides, and more important, some formats may use _ISP dependent
prefixes_ (and 6rd is already in this case!). These formats cannot be
recognized by a constant code.
This being noted, I agree that, to extend applicability of 6rd relays
to cases where ingress filtering doesn't apply, and to deal more
simply with the case of 6rd ISPs that also operate 6to4 relays, 6rd
relays SHOULD do as you propose:
*In 6rd relays, packets received on the IPv4 side should be discarded
if their IPv6 destinations are 6to4 addresses containing the ISP 6rd
anycast address.*
This way all the checks are done only at the 6rd relay and not in
other IPv4 border routers of the ISP which should not be aware of
the 6rd deployment.
Note that IPv4 border routers of the ISP need not to be aware of the
6rd in particular.
They only have to make sure that ingress filtering "in general"
applies to packets sent toward the global Internet.
(If their downstream neighbors do ingress filtering as they should,
these border routers have nothing specific to do. In cases where this
isn't sure though, they should better prevent source address spoofing
by filtering themselves source addresses for which they have no
reverse path.)
</gn>
(2) Case of ISPs that operate 6rd relays AND 6to4 relays (but
neither Teredo relays nor ISP-infrastructure NATs)
In its sec. 5 on security, draft-despres-6rd-03 says:
<<<
o RELAY PACKETS TOWARD THE INTERNET: The IPv6 source must be a 6rd
address that matches the IPv4 source. The IPv6 destination must
not start with the ISP 6rd prefix.
...
o RELAY PACKETS FROM THE INTERNET: The IPv6 source must not be a
6rd
address of the ISP. The IPv4 destination must not be multicast,
i.e. must not start with 224/3...
>>>
In view of your study and in my understanding, it MUST be completed
with:
- after the first quoted paragraph:
"Furthermore, if the ISP also operates 6to4 relays that advertise
on the IPv6 network the 6to4 IPv6 prefix 2002::/16, the IPv4 source
must be neither the 6to4 anycast address 192.88.99.0 nor any of its
equivalent IPv4 unicast addresses."
- after the second quoted paragraph:
"Furthermore, if the ISP also operates 6to4 relays that advertise
on the IPv6 network the 6to4 IPv6 prefix 2002::/16, the IPv4
destination derived from the IPv6 destination must be neither the
IPv4 anycast address 192.88.99.0 nor any of its equivalent IPv4
unicast addresses."
<gn>
Actually, I believe that the precautions I suggested above will
work here also instead of those checks. Won't they?
As explained above, it would be 100% safe only if all embedded IPv4
addresses were guaranteed to be recognized, which is not the case.
In general, I think that checks performed on the destination
address (IPv4 or IPv6) should be more robust than checks on a
source address.
In my understanding, not always.
Each scenario has to be studied for what it is.
</gn>
With this, an ISP that operates both 6rd and 6to4 relays is also
immune to the routing-loop attack because:
- an IPv6 packet forwarded to the global Internet by 6rd relays can
come back to the ISP IPv4 network via one of the 6to4 relays of the
ISP BUT cannot be accepted again by a 6rd relay: its IPv4 source
address is then one of a 6to4 relay, which, with the first added
sentence, prevents it from being accepted by the 6rd relay.
- an IPv6 packet received from the IPv6 Internet by a 6rd relay
cannot be sent back to the IPv4 global Internet via one of the 6to4
relays: the IPv4 address derived from its IPv6 destination would
have for this to be one of a 6to4 relays, which, with the second
added sentence, prevents it from being forwarded by the 6rd relay.
Note: RFC 3068, where the 6to4 anycast address is introduced, says
that "each 6to4 relay router that advertise the 6to4 anycast prefix
MUST also provide an equivalent IPv4 unicast address". Whether this
is really important in practice is IMHO unclear. On the other hand,
if this MUST is dispensed with, the above security precaution can
be implemented in 6rd relays without a need to handle a variable
number of addresses, and to administratively configure them (with
the associated risks of human errors).
<gn>
If you do the check on the destination address you can avoid this
administrative configuration altogether.
Agreed for packets forwarded to the IPv6 side (see above).
Now, for packets from the IPv6 Internet, rather than checking that
the embedded IPv4 destination has one of the ISP allocated prefixes,
there is a better check I hadn't seen before sending the previous e-
mail:
*In 6rd relays, packets received on the IPv6 side should be discarded
if their source addresses are 6to4 addresses containing the ISP 6rd
anycast address.*
The above administrative configuration is thus unnecessary, and the
6rd relay cannot participate in a routing loop attack even if its ISP
also operates 6to4 relays.
NOTE: As a matter of fact, a source address check can also be used to
improve the Mitigation Measures you proposed for 6to4, ISATAP and
Teredo relays:
*In your three forwarding conditions, it would be sufficient to add
"or source" after each occurrence of "destination".*
Thus, each of these relays becomes protected against rooting loop
attacks via any other 6to4, ISATAP, and Teredo relay, even if this
relay doesn't make the new check on destination addresses.
</gn>
To conclude:
- Without needing to modify 6to4 relays, ISATAP relays, and Teredo
relays, ISPs that support 6rd and don't support 6to4 appear to be
already protected against routing loop attacks if ingress filtering
is operational at their interfaces to the IPv4 global Internet.
With an additional simple precaution in 6rd relays, they can also
be immune in the absence of such filtering.
<gn>
I fully agree.
Thanks.
Thoughts on the proposal to improve your mitigation measures?
Regards,
RD
</gn>
- A necessary additional security precaution against routing-loop
attacks is now identified for ISPs that support 6rd and that,
having started with 6to4, wish to keep it for backward
compatibility. Thanks again for your analysis which made it possible.
Best regards,
RD
Le 17 août 09 à 17:21, Gabi Nakibly a écrit :
> Hi all,
> I would like to draw the attention of the list to some research
results which my colleague and I at the National EW Research &
Simulation Center have recently published. The research presents a
class of routing loop attacks that abuses 6to4, ISATAP and Teredo.
The paper can be found at: http://www.usenix.org/events/woot09/tech/
full_papers/nakibly.pdf
>
> Here is the abstract:
> IPv6 is the future network layer protocol for the Internet. Since
it is not compatible with its predecessor, some interoperability
mechanisms were designed. An important category of these mechanisms
is automatic tunnels, which enable IPv6 communication over an IPv4
network without prior configuration. This category includes ISATAP,
6to4 and Teredo. We present a novel class of attacks that exploit
vulnerabilities in these tunnels. These attacks take advantage of
inconsistencies between a tunnel's overlay IPv6 routing state and
the native IPv6 routing state. The attacks form routing loops which
can be abused as a vehicle for traffic amplification to facilitate
DoS attacks. We exhibit five attacks of this class. One of the
presented attacks can DoS a Teredo server using a single packet.
The exploited vulnerabilities are embedded in the design of the
tunnels; hence any implementation of these tunnels may be
vulnerable. In particular, the attacks were tested against the
ISATAP, 6to4 and Teredo implementations of Windows Vista and
Windows Server 2008 R2.
>
> I think the results of the research warrant some corrective
action. If this indeed shall be the general sentiment of the list,
I will be happy write an appropriate I-D. The mitigation measures
we suggested in the paper are the best we could think of to
completely eliminate the problem. However they are far from perfect
since they would require tunnel implementations to be updated in
case new types of automatic tunnels are introduced.
>
> Your comments are welcome.
>
> Gabi
>
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