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Re: Routing loop attacks using IPv6 tunnels - the 6rd case
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:
(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
each ISP. The only constraint is that routes toward the ISP that
advertised must not include this address.
In view of your study and in my understanding, it should be completed
"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
- 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.
(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
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
- 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 22.214.171.124 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 126.96.36.199 nor any of its equivalent IPv4 unicast addresses."
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).
- 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.
- 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.
Le 17 août 09 à 17:21, Gabi Nakibly a écrit :
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/
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.
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