Network Working Group E. Nordmark
Request for Comments: 4218 Sun Microsystems
Category: Informational T. Li
October 2005
Threats Relating to IPv6 Multihoming Solutions
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document lists security threats related to IPv6 multihoming.
Multihoming can introduce new opportunities to redirect packets to
different, unintended IP addresses.
The intent is to look at how IPv6 multihoming solutions might make
the Internet less secure; we examine threats that are inherent to all
IPv6 multihoming solutions rather than study any specific proposed
solution. The threats in this document build upon the threats
discovered and discussed as part of the Mobile IPv6 work.
Table of Contents
1. Introduction ....................................................2
1.1. Assumptions ................................................3
1.2. Authentication, Authorization, and Identifier Ownership ....4
2. Terminology .....................................................5
3. Today's Assumptions and Attacks .................................6
3.1. Application Assumptions ....................................6
3.2. Redirection Attacks Today ..................................8
3.3. Packet Injection Attacks Today .............................9
3.4. Flooding Attacks Today ....................................10
3.5. Address Privacy Today .....................................11
4. Potential New Attacks ..........................................13
4.1. Cause Packets to Be Sent to the Attacker ..................13
4.1.1. Once Packets Are Flowing ...........................13
4.1.2. Time-Shifting Attack ...............................14
4.1.3. Premeditated Redirection ...........................14
4.1.4. Using Replay Attacks ...............................15
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4.2. Cause Packets to Be Sent to a Black Hole ..................15
4.3. Third Party Denial-of-Service Attacks .....................16
4.3.1. Basic Third Party DoS ..............................17
4.3.2. Third Party DoS with On-Path Help ..................18
4.4. Accepting Packets from Unknown Locators ...................19
4.5. New Privacy Considerations ................................20
5. Granularity of Redirection .....................................20
6. Movement Implications? .........................................22
7. Other Security Concerns ........................................23
8. Security Considerations ........................................24
9. Acknowledgements ...............................................24
10. Informative References ........................................25
Appendix A: Some Security Analysis ................................27
1. Introduction
The goal of the IPv6 multihoming work is to allow a site to take
advantage of multiple attachments to the global Internet, without
having a specific entry for the site visible in the global routing
table. Specifically, a solution should allow hosts to use multiple
attachments in parallel, or to switch between these attachment points
dynamically in the case of failures, without an impact on the
transport and application layer protocols.
At the highest level, the concerns about allowing such "rehoming" of
packet flows can be called "redirection attacks"; the ability to
cause packets to be sent to a place that isn't tied to the transport
and/or application layer protocol's notion of the peer. These
attacks pose threats against confidentiality, integrity, and
availability. That is, an attacker might learn the contents of a
particular flow by redirecting it to a location where the attacker
has a packet recorder. If, instead of a recorder, the attacker
changes the packets and then forwards them to the ultimate
destination, the integrity of the data stream would be compromised.
Finally, the attacker can simply use the redirection of a flow as a
denial of service attack.
This document has been developed while considering multihoming
solutions architected around a separation of network identity and
network location, whether or not this separation implies the
introduction of a new and separate identifier name space. However,
this separation is not a requirement for all threats, so this
taxonomy may also apply to other approaches. This document is not
intended to examine any single proposed solution. Rather, it is
intended as an aid to discussion and evaluation of proposed
solutions. By cataloging known threats, we can help to ensure that
all proposals deal with all of the available threats.
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As a result of not analyzing a particular solution, this document is
inherently incomplete. An actual solution would need to be analyzed
as part of its own threat analysis, especially in the following
areas:
1) If the solution makes the split between locators and identifiers,
then most application security mechanisms should be tied to the
identifier, not to the locator. Therefore, work would be needed
to understand how attacks on the identifier mechanism affect
security, especially attacks on the mechanism that would bind
locators to identifiers.
2) How does the solution apply multihoming to IP multicast?
Depending on how this is done, there might be specific threats
relating to multicast that need to be understood. This document
does not discuss any multicast-specific threats.
3) Connection-less transport protocols probably need more attention.
They are already difficult to secure, even without a
locator/identifier split.
1.1. Assumptions
This threat analysis doesn't assume that security has been applied to
other security relevant parts of the Internet, such as DNS and
routing protocols; but it does assume that, at some point in time, at
least parts of the Internet will be operating with security for such
key infrastructure. With that assumption, it then becomes important
that a multihoming solution would not, at that point in time, become
the weakest link. This is the case even if, for instance, insecure
DNS might be the weakest link today.
This document doesn't assume that the application protocols are
protected by strong security today or in the future. However, it is
still useful to assume that the application protocols that care about
integrity and/or confidentiality apply the relevant end-to-end
security measures, such as IPsec, TLS, and/or application layer
security.
For simplicity, this document assumes that an on-path attacker can
see packets, modify packets and send them out, and block packets from
being delivered. This is a simplification because there might exist
ways (for instance, monitoring capability in switches) that allow
authenticated and authorized users to observe packets without being
able to send or block the packets.
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In some cases it might make sense to make a distinction between
on-path attackers, which can monitor packets and perhaps also inject
packets, without being able to block packets from passing through.
On-path attackers that only need to monitor might be lucky and find a
non-switched Ethernet in the path, or use capacitive or inductive
coupling to listen on a copper wire. But if the attacker is on an
Ethernet that is on the path, whether switched or not, the attacker
can also employ Address Resolution Protocol/Neighbor Discovery
(ARP/ND) spoofing to get access to the packet flow which allows
blocking as well. Similarly, if the attacker has access to the wire,
the attacker can also place a device on the wire to block. Other
on-path attacks would be those that gain control of a router or a
switch (or gain control of one of the endpoints), and most likely
those would allow blocking as well.
So the strongest currently known case where monitoring is easier than
blocking, is when switches and routers have monitoring capabilities
(for network management or for lawful intercept) where an attacker
might be able to bypass the authentication and authorization checks
for those capabilities. However, this document makes the simplifying
assumption treat all on-path attackers the same by assuming that such
an attacker can monitor, inject, and block packets. A security
analysis of a particular proposal can benefit from not making this
assumption, and look at how on-path attackers with different
capabilities can generate different attacks perhaps not present in
today's Internet.
The document assumes that an off-path attacker can neither see
packets between the peers (for which it is not on the path) nor block
them from being delivered. Off-path attackers can, in general, send
packets with arbitrary IP source addresses and content, but such
packets might be blocked if ingress filtering [INGRESS] is applied.
Thus, it is important to look at the multihoming impact on security
both in the presence and absence of ingress filtering.
1.2. Authentication, Authorization, and Identifier Ownership
The overall problem domain can be described using different
terminology.
One way to describe it is that it is necessary to first authenticate
the peer and then verify that the peer is authorized to control the
locators used for a particular identifier. While this is correct, it
might place too much emphasis on the authorization aspect. In this
case, the authorization is conceptually very simple; each host (each
identifier) is authorized to control which locators are used for
itself.
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Hence, there is a different way to describe the same thing. If the
peer can somehow prove that it is the owner of the identifier, and
the communication is bound to the identifier (and not the locator),
then the peer is allowed to control the locators that are used with
the identifier. This way to describe the problem is used in [OWNER].
Both ways to look at the problem, hence both sets of terminology, are
useful when trying to understand the problem space and the threats.
2. Terminology
link - a communication facility or medium over which nodes
can communicate at the link layer, i.e., the layer
immediately below IPv6. Examples are Ethernets
(simple or bridged); PPP links; X.25, Frame Relay,
or ATM networks; and Internet (or higher) layer
"tunnels", such as tunnels over IPv4 or IPv6 itself.
interface - a node's attachment to a link.
address - an IP layer name that has both topological
significance (i.e., a locator) and identifies an
interface. There may be multiple addresses per
interface. Normally an address uniquely identifies
an interface, but there are exceptions: the same
unicast address can be assigned to multiple
interfaces on the same node, and an anycast address
can be assigned to different interfaces on different
nodes.
locator - an IP layer topological name for an interface or a
set of interfaces. There may be multiple locators
per interface.
identifier - an IP layer identifier for an IP layer endpoint
(stack name in [NSRG]), that is, something that
might be commonly referred to as a "host". The
transport endpoint name is a function of the
transport protocol and would typically include the
IP identifier plus a port number. There might be
use for having multiple identifiers per stack/per
host.
An identifier continues to function regardless of
the state of any one interface.
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address field
- the source and destination address fields in the
IPv6 header. As IPv6 is currently specified, these
fields carry "addresses". If identifiers and
locators are separated, these fields will contain
locators.
FQDN - Fully Qualified Domain Name [FYI18]
3. Today's Assumptions and Attacks
The two interesting aspects of security for multihoming solutions are
(1) the assumptions made by the transport layer and application layer
protocols about the identifiers that they see, and (2) the existing
abilities to perform various attacks that are related to the
identity/location relationship.
3.1. Application Assumptions
In the Internet today, the initiating part of applications either
starts with a FQDN, which it looks up in the DNS, or already has an
IP address from somewhere. For the FQDN to perform IP address
lookup, the application effectively places trust in the DNS. Once it
has the IP address, the application places trust in the routing
system delivering packets to that address. Applications that use
security mechanisms, such as IPSEC or TLS, have the ability to bind
an address or FQDN to cryptographic keying material. Compromising
the DNS and/or routing system can result in packets being dropped or
delivered to an attacker in such cases, but since the attacker does
not possess the keying material, the application will not trust the
attacker, and the attacker cannot decrypt what it receives.
At the responding (non-initiating) end of communication today, we
find that the security configurations used by different applications
fall into approximately five classes, where a single application
might use different classes of configurations for different types of
communication.
The first class is the set of public content servers. These systems
provide data to any and all systems and are not particularly
concerned with confidentiality, as they make their content available
to all. However, they are interested in data integrity and denial of
service attacks. Having someone manipulate the results of a search
engine, for example, or prevent certain systems from reaching a
search engine would be a serious security issue.
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The second class of security configurations uses existing IP source
addresses from outside of their immediate local site as a means of
authentication without any form of verification. Today, with source
IP address spoofing and TCP sequence number guessing as rampant
attacks [RFC1948], such applications are effectively opening
themselves for public connectivity and are reliant on other systems,
such as firewalls, for overall security. We do not consider this
class of configurations in this document because they are in any case
fully open to all forms of network layer spoofing.
The third class of security configurations receives existing IP
source addresses, but attempt some verification using the DNS,
effectively using the FQDN for access control. (This is typically
done by performing a reverse lookup from the IP address, followed by
a forward lookup and verifying that the IP address matches one of the
addresses returned from the forward lookup.) These applications are
already subject to a number of attacks using techniques like source
address spoofing and TCP sequence number guessing since an attacker,
knowing this is the case, can simply create a DoS attack using a
forged source address that has authentic DNS records. In general
this class of security configurations is strongly discouraged, but it
is probably important that a multihoming solution doesn't introduce
any new and easier ways to perform such attacks. However, we note
that some people think we should treat this class the same as the
second class of security configurations.
The fourth class of security configurations uses cryptographic
security techniques to provide both a strong identity for the peer
and data integrity with or without confidentiality. Such systems are
still potentially vulnerable to denial of service attacks that could
be introduced by a multihoming solution.
Finally, the fifth class of security configurations uses
cryptographic security techniques, but without strong identity (such
as opportunistic IPsec). Thus, data integrity with or without
confidentiality is provided when communicating with an
unknown/unauthenticated principal. Just like the first category
above, such applications can't perform access control based on
network layer information since they do not know the identity of the
peer. However, they might perform access control using higher-level
notions of identity. The availability of IPsec (and similar
solutions) together with channel bindings allows protocols (which, in
themselves, are vulnerable to man-in-the-middle (MITM) attacks) to
operate with a high level of confidentiality in the security of the
identification of the peer. A typical example is the Remote Direct
Data Placement Protocol (RDDP), which, when used with opportunistic
IPsec, works well if channel bindings are available. Channel
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bindings provide a link between the IP-layer identification and the
application protocol identification.
A variant of the fifth class are those that use "leap-of-faith"
during some initial communication. They do not provide strong
identities except where subsequent communication is bound to the
initial communication, providing strong assurance that the peer is
the same as during the initial communication.
The fifth class is important and its security properties must be
preserved by a multihoming solution.
The requirement for a multihoming solution is that security be no
worse than it is today in all situations. Thus, mechanisms that
provide confidentiality, integrity, or authentication today should
continue to provide these properties in a multihomed environment.
3.2. Redirection Attacks Today
This section enumerates some of the redirection attacks that are
possible in today's Internet.
If routing can be compromised, packets for any destination can be
redirected to any location. This can be done by injecting a long
prefix into global routing, thereby causing the longest match
algorithm to deliver packets to the attacker.
Similarly, if DNS can be compromised, and a change can be made to an
advertised resource record to advertise a different IP address for a
hostname, effectively taking over that hostname. More detailed
information about threats relating to DNS are in [DNS-THREATS].
Any system that is along the path from the source to the destination
host can be compromised and used to redirect traffic. Systems may be
added to the best path to accomplish this. Further, even systems
that are on multi-access links that do not provide security can also
be used to redirect traffic off of the normal path. For example, ARP
and ND spoofing can be used to attract all traffic for the legitimate
next hop across an Ethernet. And since the vast majority of
applications rely on DNS lookups, if DNSsec is not deployed, then
attackers that are on the path between the host and the DNS servers
can also cause redirection by modifying the responses from the DNS
servers.
In general, the above attacks work only when the attacker is on the
path at the time it is performing the attack. However, in some cases
it is possible for an attacker to create a DoS attack that remains at
least some time after the attacker has moved off the path. An
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example of this is an attacker that uses ARP or ND spoofing while on
path to either insert itself or send packets to a black hole (a
non-existent L2 address). After the attacker moves away, the ARP/ND
entries will remain in the caches in the neighboring nodes for some
amount of time (a minute or so in the case of ARP). This will result
in packets continuing to be black-holed until ARP entry is flushed.
Finally, the hosts themselves that terminate the connection can also
be compromised and can perform functions that were not intended by
the end user.
All of the above protocol attacks are the subject of ongoing work to
secure them (DNSsec, security for BGP, Secure ND) and are not
considered further within this document. The goal for a multihoming
solution is not to solve these attacks. Rather, it is to avoid
adding to this list of attacks.
3.3. Packet Injection Attacks Today
In today's Internet the transport layer protocols, such as TCP and
SCTP, which use IP, use the IP addresses as the identifiers for the
communication. In the absence of ingress filtering [INGRESS], the IP
layer allows the sender to use an arbitrary source address, thus the
transport protocols and/or applications need some protection against
malicious senders injecting bogus packets into the packet stream
between two communicating peers. If this protection can be
circumvented, then it is possible for an attacker to cause harm
without necessarily needing to redirect the return packets.
There are various levels of protection in different transport
protocols. For instance, in general TCP packets have to contain a
sequence that falls in the receiver's window to be accepted. If the
TCP initial sequence numbers are random, then it is very hard for an
off-path attacker to guess the sequence number close enough for it to
belong to the window, and as a result be able to inject a packet into
an existing connection. How hard this is depends on the size of the
available window, whether the port numbers are also predictable, and
the lifetime of the connection. Note that there is ongoing work to
strengthen TCP's protection against this broad class of attacks
[TCPSECURE]. SCTP provides a stronger mechanism with the
verification tag; an off-path attacker would need to guess this
random 32-bit number. Of course, IPsec provides cryptographically
strong mechanisms that prevent attackers, on or off path, from
injecting packets once the security associations have been
established.
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When ingress filtering is deployed between the potential attacker and
the path between the communicating peers, it can prevent the attacker
from using the peer's IP address as source. In that case, the packet
injection will fail in today's Internet.
We don't expect a multihoming solution to improve the existing degree
of prevention against packet injection. However, it is necessary to
look carefully at whether a multihoming solution makes it easier for
attackers to inject packets because the desire to have the peer
present at multiple locators, and perhaps at a dynamic set of
locators, can potentially result in solutions that, even in the
presence of ingress filtering, make packet injection easier.
3.4. Flooding Attacks Today
In the Internet today there are several ways for an attacker to use a
redirection mechanism to launch DoS attacks that cannot easily be
traced to the attacker. An example of this is to use protocols that
cause reflection with or without amplification [PAXSON01].
Reflection without amplification can be accomplished by an attacker
sending a TCP SYN packet to a well-known server with a spoofed source
address; the resulting TCP SYN ACK packet will be sent to the spoofed
source address.
Devices on the path between two communicating entities can also
launch DoS attacks. While such attacks might not be interesting
today, it is necessary to understand them better in order to
determine whether a multihoming solution might enable new types of
DoS attacks.
For example, today, if A is communicating with B, then A can try to
overload the path from B to A. If TCP is used, A could do this by
sending ACK packets for data that it has not yet received (but it
suspects B has already sent) so that B would send at a rate that
would cause persistent congestion on the path towards A. Such an
attack would seem self-destructive since A would only make its own
corner of the network suffer by overloading the path from the
Internet towards A.
A more interesting case is if A is communicating with B and X is on
the path between A and B, then X might be able to fool B to send
packets towards A at a rate that is faster than A (and the path
between A and X) can handle. For instance, if TCP is used, then X
can craft TCP ACK packets claiming to come from A to cause B to use a
congestion window that is large enough to potentially cause
persistent congestion towards A. Furthermore, if X can suppress the
packets from A to B, it can also prevent A from sending any explicit
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"slow down" packets to B; that is, X can disable any flow or
congestion control mechanism such as Explicit Congestion Notification
[ECN]. Similar attacks can presumably be launched using protocols
that carry streaming media by forging such a protocol's notion of
acknowledgement and feedback.
An attribute of this type of attack is that A will simply think that
B is faulty since its flow and congestion control mechanisms don't
seem to be working. Detecting that the stream of ACK packets is
generated from X and not from A might be challenging, since the rate
of ACK packets might be relatively low. This type of attack might
not be common today because, in the presence of ingress filtering, it
requires that X remain on the path in order to sustain the DoS
attack. And in the absence of ingress filtering an attacker would
need either to be present on the path initially and then move away,
or to be able to perform the setup of the communication "blind",
i.e., without seeing the return traffic (which, in the case of TCP,
implies guessing the initial sequence number).
The danger is that the addition of multihoming redirection mechanisms
might potentially remove the constraint that the attacker remain on
the path. And with the current, no-multihoming support, using
end-to-end strong security at a protocol level at (or below) this
"ACK" processing would prevent this type of attack. But if a
multihoming solution is provided underneath IPsec that prevention
mechanism would potentially not exist.
Thus, the challenge for multihoming solutions is to not create
additional types of attacks in this area, or make existing types of
attacks significantly easier.
3.5. Address Privacy Today
In today's Internet there is limited ability to track a host as it
uses the Internet because in some cases, such as dialup connectivity,
the host will acquire different IPv4 addresses each time it connects.
However, with increasing use of broadband connectivity, such as DSL
or cable, it is becoming more likely that the host will maintain the
same IPv4 over time. Should a host move around in today's Internet,
for instance, by visiting WiFi hotspots, it will be configured with a
different IPv4 address at each location.
We also observe that a common practice in IPv4 today is to use some
form of address translation, whether the site is multihomed or not.
This effectively hides the identity of the specific host within a
site; only the site can be identified based on the IP address.
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In the cases where it is desirable to maintain connectivity as a host
moves around, whether using layer 2 technology or Mobile IPv4, the
IPv4 address will remain constant during the movement (otherwise the
connections would break). Thus, there is somewhat of a choice today
between seamless connectivity during movement and increased address
privacy.
Today when a site is multihomed to multiple ISPs, the common setup is
that a single IP address prefix is used with all the ISPs. As a
result it is possible to track that it is the same host that is
communication via all ISPs.
However, when a host (and not a site) is multi-homed to several ISPs
(e.g., through a General Packet Radio Service (GPRS) connection and a
wireless hot spot), the host is provided with different IP addresses
on each interface. While the focus of the multihoming work is on
site multihoming, should the solution also be applicable to host
multihoming, the privacy impact needs to be considered for this case
as well.
IPv6 stateless address auto-configuration facilitates IP address
management, but raises some concerns since the Ethernet address is
encoded in the low-order 64 bits of the IPv6 address. This could
potentially be used to track a host as it moves around the network,
using different ISPs, etc. IPv6 specifies temporary addresses
[RFC3041], which allow applications to control whether they need
long-lived IPv6 addresses or desire the improved privacy of using
temporary addresses.
Given that there is no address privacy in site multihoming setups
today, the primary concerns for the "do no harm" criteria are to
ensure that hosts that move around still have the same ability as in
today's Internet to choose between seamless connectivity and improved
address privacy, and also that the introduction of multihoming
support should still provide the same ability as we have in IPv6 with
temporary addresses.
When considering privacy threats, it makes sense to distinguish
between attacks made by on-path entities observing the packets flying
by, and attacks by the communicating peer. It is probably feasible
to prevent on-path entities from correlating the multiple IP
addresses of the host; but the fact that the peer needs to be told
multiple IP addresses in order to be able to switch to using
different addresses, when communication fails, limits the ability of
the host to prevent correlating its multiple addresses. However,
using multiple pseudonyms for a host should be able address this
case.
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4. Potential New Attacks
This section documents the additional attacks that have been
discovered that result from an architecture where hosts can change
their topological connection to the network in the middle of a
transport session without interruption. This discussion is again
framed in the context where the topological locators may be
independent of the host identifiers used by the transport and
application layer protocols. Some of these attacks may not be
applicable if traditional addresses are used. This section assumes
that each host has multiple locators and that there is some mechanism
for determining the locators for a correspondent host. We do not
assume anything about the properties of these mechanisms. Instead,
this list will serve to help us derive the properties of these
mechanisms that will be necessary to prevent these redirection
attacks.
Depending on the purpose of the redirection attack, we separate the
attacks into several different types.
4.1. Cause Packets to Be Sent to the Attacker
An attacker might want to receive the flow of packets, for instance
to be able to inspect and/or modify the payload or to be able to
apply cryptographic analysis to cryptographically protected payload,
using redirection attacks.
Note that such attacks are always possible today if an attacker is on
the path between two communicating parties, and a multihoming
solution can't remove that threat. Hence, the bulk of these concerns
relate to off-path attackers.
4.1.1. Once Packets Are Flowing
This might be viewed as the "classic" redirection attack.
While A and B are communicating X might send packets to B and claim:
"Hi, I'm A, send my packets to my new location." where the location
is really X's location.
"Standard" solutions to this include requiring that the host
requesting redirection somehow be verified to be the same host as the
initial host that established communication. However, the burdens of
such verification must not be onerous, or the redirection requests
themselves can be used as a DoS attack.
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To prevent this type of attack, a solution would need some mechanism
that B can use to verify whether a locator belongs to A before B
starts using that locator, and be able to do this when multiple
locators are assigned to A.
4.1.2. Time-Shifting Attack
The term "time-shifting attack" is used to describe an attacker's
ability to perform an attack after no longer being on the path.
Thus, the attacker would have been on the path at some point in time,
snooping and/or modifying packets; and later, when the attacker is no
longer on the path, it launches the attack.
In the current Internet, it is not possible to perform such attacks
to redirect packets. But for some time after moving away, the
attacker can cause a DoS attack, e.g., by leaving a bogus ARP entry
in the nodes on the path, or by forging TCP Reset packets based on
having seen the TCP Initial Sequence Numbers when it was on the path.
It would be reasonable to require that a multihoming solution limit
the ability to redirect and/or DoS traffic to a few minutes after the
attacker has moved off the path.
4.1.3. Premeditated Redirection
This is a variant of the above where the attacker "installs" itself
before communication starts.
For example, if the attacker X can predict that A and B will
communicate in the (near) future, then the attacker can tell B: "Hi,
I'm A and I'm at this location". When A later tries to communicate
with B, will B believe it is really A?
If the solution to the classic redirection attack is based on "prove
you are the same as initially", then A will fail to prove this to B
because X initiated communication.
Depending on details that would be specific to a proposed solution,
this type of attack could either cause redirection (so that the
packets intended for A will be sent to X) or they could cause DoS
(where A would fail to communicate with B since it can't prove it is
the same host as X).
To prevent this attack, the verification of whether a locator belongs
to the peer cannot simply be based on the first peer that made
contact.
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4.1.4. Using Replay Attacks
While the multihoming problem doesn't inherently imply any
topological movement, it is useful to also consider the impact of
site renumbering in combination with multihoming. In that case, the
set of locators for a host will change each time its site renumbers,
and, at some point in time after a renumbering event, the old locator
prefix might be reassigned to some other site.
This potentially give an attacker the ability to replay whatever
protocol mechanism was used to inform a host of a peer's locators so
that the host would incorrectly be led to believe that the old
locator (set) should be used even long after a renumbering event.
This is similar to the risk of replay of Binding Updates in [MIPv6],
but the time constant is quite different; Mobile IPv6 might see
movements every second while site renumbering, followed by
reassignment of the site locator prefix, might be a matter of weeks
or months.
To prevent such replay attacks, the protocol used to verify which
locators can be used with a particular identifier needs some replay
protection mechanism.
Also, in this space one needs to be concerned about potential
interaction between such replay protection and the administrative act
of reassignment of a locator. If the identifier and locator
relationship is distributed across the network, one would need to
make sure that the old information has been completely purged from
the network before any reassignment. Note that this does not require
an explicit mechanism. This can instead be implemented by locator
reuse policy and careful timeouts of locator information.
4.2. Cause Packets to Be Sent to a Black Hole
This is also a variant of the classic redirection attack. The
difference is that the new location is a locator that is nonexistent
or unreachable. Thus, the effect is that sending packets to the new
locator causes the packets to be dropped by the network somewhere.
One would expect that solutions that prevent the previous redirection
attacks would prevent this attack as a side effect, but it makes
sense to include this attack here for completeness. Mechanisms that
prevented a redirection attack to the attacker should also prevent
redirection to a black hole.
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4.3. Third Party Denial-of-Service Attacks
An attacker can use the ability to perform redirection to cause
overload on an unrelated third party. For instance, if A and B are
communicating, then the attacker X might be able to convince A to
send the packets intended for B to some third node C. While this
might seem harmless at first, since X could just flood C with packets
directly, there are a few aspects of these attacks that cause
concern.
The first is that the attacker might be able to completely hide its
identity and location. It might suffice for X to send and receive a
few packets to A in order to perform the redirection, and A might not
retain any state on who asked for the redirection to C's location.
Even if A had retained such state, that state would probably not be
easily available to C, thus C can't determine who the attacker was
once C has become the victim of a DoS attack.
The second concern is that, with a direct DoS attack from X to C, the
attacker is limited by the bandwidth of its own path towards C. If
the attacker can fool another host, such as A, to redirect its
traffic to C, then the bandwidth is limited by the path from A
towards C. If A is a high-capacity Internet service and X has slow
(e.g., dialup) connectivity, this difference could be substantial.
Thus, in effect, this could be similar to packet amplifying
reflectors in [PAXSON01].
The third, and final concern, is that if an attacker only need a few
packets to convince one host to flood a third party, then it wouldn't
be hard for the attacker to convince lots of hosts to flood the same
third party. Thus, this could be used for Distributed Denial-of-
Service attacks.
A third party DoS attack might be against the resources of a
particular host (i.e., C in the example above), or it might be
against the network infrastructure towards a particular IP address
prefix, by overloading the routers or links even though there is no
host at the address being targeted.
In today's Internet, the ability to perform this type of attack is
quite limited. In order for the attacker to initiate communication,
it will in most cases need to be able to receive some packets from
the peer (the potential exception being techniques that combine this
with TCP-sequence-number-guessing techniques). Furthermore, to the
extent that parts of the Internet uses ingress filtering [INGRESS],
even if the communication could be initiated, it wouldn't be possible
to sustain it by sending ACK packets with spoofed source addresses
from an off-path attacker.
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If this type of attack can't be prevented, there might be mitigation
techniques that can be employed. For instance, in the case of TCP a
partial defense can be constructed by having TCP slow-start be
triggered when the destination locator changes. (Folks might argue
that, separately from security, this would be the correct action for
congestion control since TCP might not have any congestion-relation
information about the new path implied by the new locator.)
Presumably the same approach can be applied to other transport
protocols that perform different forms of (TCP-friendly) congestion
control, even though some of them might not adapt as rapidly as TCP.
But since all congestion-controlled protocols probably need to have
some reaction to the path change implied by a locator change, it
makes sense to think about 3rd party DoS attacks when designing how
the specific transport protocols should react to a locator change.
However, this would only be a partial solution since it would
probably take several packets and roundtrips before the transport
protocol would stop transmitting; thus, an attacker could still use
this as a reflector with packet amplification. Thus, the multihoming
mechanism probably needs some form of defense against third party DoS
attacks, in addition to the help we can get from the transport
protocols.
4.3.1. Basic Third Party DoS
Assume that X is on a slow link anywhere in the Internet. B is on a
fast link (gigabits; e.g., a media server) and A is the victim.
X could flood A directly but is limited by its low bandwidth. If X
can establish communication with B, ask B to send it a high-speed
media stream, then X can presumably fake out the
"acknowledgements/feedback" needed for B to blast out packets at full
speed. So far, this only hurts X and the path between X and the
Internet. But if X could also tell B "I'm at A's locator", then X
has effectively used this redirection capability in multihoming to
amplify its DoS capability, which would be a source of concern.
One could envision rather simple techniques to prevent such attacks.
For instance, before sending to a new peer locator, perform a clear
text exchange with the claimed new locator of the form "Are you X?"
resulting in "Yes, I'm X.". This would suffice for the simplest of
attacks. However, as we will see below, more sophisticated attacks
are possible.
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4.3.2. Third Party DoS with On-Path Help
The scenario is as above, but, in addition, the attacker X has a
friend Y on the path between A and B:
----- ----- -----
| A |--------| Y |--------| B |
----- ----- -----
/
/
/
/
/
/
-----
| X |
-----
With the simple solution suggested in the previous section, all Y
might need to do is fake a response to the "Are you X?" packet, and
after that point in time Y might not be needed; X could potentially
sustain the data flow towards A by generating the ACK packets. Thus,
it would be even harder to detect the existence of Y.
Furthermore, if X is not the actual end system but an attacker
between some node C and B, then X can claim to be C, and no finger
can be pointed at X either:
----- ----- -----
| A |--------| Y |--------| B |
----- ----- -----
/
/
/
/
/
/
----- -----
| C |-------| X |
----- -----
Thus, with two attackers on different paths, there might be no trace
of who did the redirection to the 3rd party once the redirection has
taken place.
A specific case of this is when X=Y, and X is located on the same LAN
as B.
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A potential way to make such attacks harder would be to use the last
received (and verified) source locator as the destination locator.
That way, when X sends the ACK packets (whether it claims to be X or
C) the result would be that the packet flow from B would switch back
towards X/C, which would result in an attack similar to what can be
performed in today's Internet.
Another way to make such attacks harder would be to perform periodic
verifications that the peer is available at the locator, instead of
just one when the new locator is received.
A third way that a multihoming solution might address this is to
ensure that B will only accept locators that can be authenticated to
be synonymous with the original correspondent. It must be possible
to securely ensure that these locators form an equivalence class. So
in the first example, not only does X need to assert that it is A,
but A needs to assert that it is X.
4.4. Accepting Packets from Unknown Locators
The multihoming solution space does not only affect the destination
of packets; it also raises the question from which sources packets
should be accepted. It is possible to build a multihoming solution
that allows traffic to be recognized as coming from the same peer
even if there is a previously unknown locator present in the source
address field. The question is whether we want to allow packets from
unverified sources to be passed on to transport and application layer
protocols.
In the current Internet, an attacker can't inject packets with
arbitrary source addresses into a session if there is ingress
filtering present, so allowing packets with unverified sources in a
multihoming solution would fail our "no worse than what we have now"
litmus test. However, given that ingress filtering deployment is far
from universal and ingress filtering typically wouldn't prevent
spoofing of addresses in the same subnet, requiring rejecting packets
from unverified locators might be too stringent.
An example of the current state are the ability to inject RST packets
into existing TCP connections. When there is no ingress filtering in
the network, this is something that the TCP endpoints need to sort
out themselves. However, deploying ingress filtering helps in
today's Internet since an attacker is limited in the set of source
addresses it can use.
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A factor to take into account to determine the "requirement level"
for this is that when IPsec is used on top of the multihoming
solution, then IPsec will reject such spoofed packets. (Note that
this is different than in the redirection attack cases where even
with IPsec an attacker could potentially cause a DoS attack.)
There might also be a middle ground where arbitrary attackers are
prevented from injecting packets by using the SCTP verification tag
type of approach [SCTP]. (This is a clear-text tag which is sent to
the peer which the peer is expected to include in each subsequent
packet.) Such an approach doesn't prevent packet injection from
on-path attackers (since they can observe the verification tag), but
neither does ingress filtering.
4.5. New Privacy Considerations
While introducing identifiers can be helpful by providing ways to
identify hosts across events when its IP address(es) might change,
there is a risk that such mechanisms can be abused to track the
identity of the host over long periods of time, whether using the
same (set of) ISP(s) or moving between different network attachment
points. Designers of solutions to multihoming need to be aware of
this concern.
Introducing the multihoming capability inherently implies that the
communicating peers need to know multiple locators for each other in
order to be resilient to failures of some paths/locators. In
addition, if the multihoming signaling protocol doesn't provide
privacy, it might be possible for 3rd parties on the path to discover
many or all the locators assigned to a host, which would increase the
privacy exposure compared to a multihomed host today.
For instance, a solution could address this by allowing each host to
have multiple identifiers at the same time and perhaps even changing
the set of identifiers that are used over time. Such an approach
could be analogous to what is done for IPv6 addresses in [RFC3041].
5. Granularity of Redirection
Different multihoming solutions might approach the problem at
different layers in the protocol stack. For instance, there have
been proposals for a shim layer inside IP, as well as transport layer
approaches. The former would have the capability to redirect an IP
address while the latter might be constrained to only redirect a
single transport connection. This difference might be important when
it comes to understanding the security impact.
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For instance, premeditated attacks might have quite different impact
in the two cases. In an IP-based multihoming solution a successful
premeditated redirection could be due to the attacker connecting to a
server and claiming to be 'A', which would result in the server
retaining some state about 'A', which it received from the attacker.
Later, when the real 'A' tries to connect to the server, the
existence of this state might mean that 'A' fails to communicate, or
that its packets are sent to the attacker. But if the same scenario
is applied to a transport-layer approach, then the state created due
to the attacker would perhaps be limited to the existing transport
connection. Thus, while this might prevent the real 'A' from
connecting to the server while the attacker is connected (if they
happen to use the same transport port number), most likely it would
not affect 'A's ability to connect after the attacker has
disconnected.
A particular aspect of the granularity question is the direction
question: will the created state be used for communication in the
reverse direction of the direction when it was created? For
instance, if the attacker 'X' suspects that 'A' will connect to 'B'
in the near future, can X connect to A and claim to be B, and then
have that later make A connect to the attacker instead of to the real
B?
Note that transport layer approaches are limited to the set of
"transport" protocols that the implementation makes aware of
multihoming. In many cases there would be "transport" protocols that
are unknown to the multihoming capability of the system, such as
applications built on top of UDP. To understand the impact of the
granularity question on the security, one would also need to
understand how such applications/protocols would be handled.
A property of transport granularity is that the amount of work
performed by a legitimate host is proportional to the number of
transport connections it creates that uses the multihoming support,
since each such connection would require some multihoming signaling.
And the same is true for the attacker. This means that an attacker
could presumably do a premeditated attack for all TCP connections to
port 80 from A to B, by setting up 65,536 (for all TCP source port
numbers) connections to server B and causing B to think those
connections should be directed to the attacker and keeping those TCP
connections open. Any attempt to make legitimate communication more
efficient (e.g., by being able to signal for multiple transport
connections at a time) would provide as much relative benefit for an
attacker as the legitimate hosts.
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The issue isn't only about the space (granularity), but also about
the lifetime component in the results of a multihoming request. In a
transport-layer approach, the multihoming state would presumably be
destroyed when the transport state is deleted as part of closing the
connection. But an IP-layer approach would have to rely on some
timeout or garbage collection mechanisms, perhaps combined with some
new explicit signaling, to remove the multihoming state. The
coupling between the connection state and multihoming state in the
transport-layer approach might make it more expensive for the
attacker, since it needs to keep the connections open.
In summary, there are issues we don't yet understand well about
granularity and reuse of the multihoming state.
6. Movement Implications?
In the case when nothing moves around, we have a reasonable
understanding of the security requirements. Something that is on the
path can be a MITM in today's Internet, and a multihoming solution
doesn't need to make that aspect any more secure.
But it is more difficult to understand the requirements when hosts
are moving around. For instance, a host might be on the path for a
short moment in time by driving by an 802.11 hotspot. Would we or
would we not be concerned if such a drive-by (which many call a
"time-shifting" attack) would result in the temporarily on-path host
being able to act as a MITM for future communication? Depending on
the solution, this might be possible if the attacker causes a
multihoming state to be created in various peer hosts while the
attacker was on the path, and that state remained in the peers for
some time.
The answer to this question doesn't seem to be obvious even in the
absence of any new multihoming support. We don't have much
experience with hosts moving around that are able to attack things as
they move. In Mobile IPv6 [MIPv6] a conservative approach was taken
that limits the effect of such drive-by attacks to the maximum
lifetime of the binding, which is set to a few minutes.
With multihoming support the issue gets a bit more complicated
because we explicitly want to allow a host to be present at multiple
locators at the same time. Thus, there might be a need to
distinguish between the host moving between different locators, and
the host sending packets with different source locators because it is
present at multiple locators without any topological movement.
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Note that the multihoming solutions that have been discussed range
from such "drive-by" attacks being impossible (for instance, due to a
strong binding to a separate identifier as in HIP, or due to reliance
on the relative security of the DNS for forward plus reverse lookups
in NOID), to systems that are first-come/first-serve (WIMP being an
example with a separate ID space, a MAST approach with a PBK being an
example without a separate ID space) that allow the first host that
uses an ID/address to claim it without any time limit.
7. Other Security Concerns
The protocol mechanisms added as part of a multihoming solution
shouldn't introduce any new DoS in the mechanisms themselves. In
particular, care must be taken not to:
- create state on the first packet in an exchange, since that could
result in state consumption attacks similar to the TCP SYN
flooding attack.
- perform much work on the first packet in an exchange (such as
expensive verification)
There is a potential chicken-and-egg problem here, because
potentially one would want to avoid doing work or creating state
until the peer has been verified, but verification will probably need
some state and some work to be done. Avoiding any work does not seem
possible, but good protocol design can often delay state creation
until verification has been completed.
A possible approach that solutions might investigate is to defer
verification until there appears to be two different hosts (or two
different locators for the same host) that want to use the same
identifier. In such a case one would need to investigate whether a
combination of impersonation and DoS attack can be used to prevent
the discovery of the impersonation.
Another possible approach is to first establish communications, and
then perform verification in parallel with normal data transfers.
Redirection would only be permitted after verification was complete,
but prior to that event, data could transfer in a normal,
non-multihomed manner.
Finally, the new protocol mechanisms should be protected against
spoofed packets, at least from off-path sources, and replayed
packets.
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8. Security Considerations
In section 3, the document presented existing protocol-based
redirection attacks. But there are also non-protocol redirection
attacks. An attacker that can gain physical access to one of
- the copper/fiber somewhere in the path,
- a router or L2 device in the path, or
- one of the end systems
can also redirect packets. This could be possible, for instance, by
physical break-ins or by bribing staff that have access to the
physical infrastructure. Such attacks are out of scope of this
discussion, but are worth keeping in mind when looking at the cost
for an attacker to exploit any protocol-based attacks against
multihoming solutions; making protocol-based attacks much more
expensive to launch than break-ins/bribery type of attacks might be
overkill.
9. Acknowledgements
This document was originally produced by a MULTI6 design team
consisting of (in alphabetical order): Iljitsch van Beijnum, Steve
Bellovin, Brian Carpenter, Mike O'Dell, Sean Doran, Dave Katz, Tony
Li, Erik Nordmark, and Pekka Savola.
Much of the awareness of these threats come from the work on Mobile
IPv6 [MIPv6, NIKANDER03, AURA02].
As the document has evolved, the MULTI6 WG participants have
contributed to the document. In particular: Masataka Ohta brought
up privacy concerns related to stable identifiers. The suggestion to
discuss transport versus IP granularity was contributed by Marcelo
Bagnulo, who also contributed text to Appendix B. Many editorial
clarifications came from Jari Arkko.
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10. Informative References
[NSRG] Lear, E. and R. Droms, "What's In A Name: Thoughts from
the NSRG", Work in Progress, September 2003.
[MIPv6] Johnson, D., Perkins, C., and J. Arkko, "Mobility
Support in IPv6", RFC 3775, June 2004.
[AURA02] Aura, T. and J. Arkko, "MIPv6 BU Attacks and Defenses",
Work in Progress, March 2002.
[NIKANDER03] Nikander, P., T. Aura, J. Arkko, G. Montenegro, and E.
Nordmark, "Mobile IP version 6 Route Optimization
Security Design Background", Work in Progress, December
2003.
[PAXSON01] V. Paxson, "An Analysis of Using Reflectors for
Distributed Denial-of-Service Attacks", Computer
Communication Review 31(3), July 2001.
[INGRESS] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP
Source Address Spoofing", BCP 38, RFC 2827, May 2000.
[SCTP] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
Zhang, L., and V. Paxson, "Stream Control Transmission
Protocol", RFC 2960, October 2000.
[RFC3041] Narten, T. and R. Draves, "Privacy Extensions for
Stateless Address Autoconfiguration in IPv6", RFC 3041,
January 2001.
[DNS-THREATS] Atkins, D. and R. Austein, "Threat Analysis of the
Domain Name System (DNS)", RFC 3833, August 2004.
[FYI18] Malkin, G., "Internet Users' Glossary", RFC 1983,
August 1996.
[ECN] Ramakrishnan, K., Floyd, S., and D. Black, "The
Addition of Explicit Congestion Notification (ECN) to
IP", RFC 3168, September 2001.
[OWNER] Nikander, P., "Denial-of-Service, Address Ownership,
and Early Authentication in the IPv6 World", Security
Protocols 9th International Workshop, Cambridge, UK,
April 25-27 2001, LNCS 2467, pages 12-26, Springer,
2002.
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RFC 4218 Threats to IPv6 Multihoming Solutions October 2005
[RFC1948] Bellovin, S., "Defending Against Sequence Number
Attacks", RFC 1948, May 1996.
[PBK] Scott Bradner, Allison Mankin, Jeffrey Schiller, "A
Framework for Purpose-Built Keys (PBK)", Work in
Progress, June 2003.
[NOID] Erik Nordmark, "Multihoming without IP Identifiers",
Work in Progress, July 2004.
[HIP] Pekka Nikander, "Considerations on HIP based IPv6
multi-homing", Work in Progress, July 2004.
[WIMP] Jukka Ylitalo, "Weak Identifier Multihoming Protocol
(WIMP)", Work in Progress, June 2004.
[CBHI] Iljitsch van Beijnum, "Crypto Based Host Identifiers",
Work in Progress, February 2004.
[TCPSECURE] M. Dalal (ed), "Transmission Control Protocol security
considerations", Work in Progress, November 2004.
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Appendix A: Some Security Analysis
When looking at the proposals that have been made for multihoming
solutions and the above threats, it seems like there are two
separable aspects of handling the redirection threats:
- Redirection of existing communication
- Redirection of an identity before any communication
This seems to be related to the fact that there are two different
classes of use of identifiers. One use is for:
o Initially establishing communication; looking up an FQDN to find
something that is passed to a connect() or sendto() API call.
o Comparing whether a peer entity is the same peer entity as in some
previous communication.
o Using the identity of the peer for future communication
("callbacks") in the reverse direction, or to refer to a 3rd party
("referrals").
The other use of identifiers is as part of being able to redirect
existing communication to use a different locator.
The first class of use seems to be related to something about the
ownership of the identifier; proving the "ownership" of the
identifier should be sufficient in order to be authorized to control
which locators are used to reach the identifier.
The second class of use seems to be related to something more
ephemeral. In order to redirect the existing communication to some
other locator, it seems to be sufficient to prove that the entity is
the same as the one that initiated the communication. One can view
this as proving the ownership of some context, where the context is
established around the time when the communication is initiated.
Preventing unauthorized redirection of existing communication can be
addressed by a large number of approaches that are based on setting
up some form of security material at the beginning of communication,
and later using the existence of that material for one end to prove
to the other that it remains the same. An example of this is Purpose
Built Keys [PBK]. One can envision different approaches for such
schemes with different complexity, performance, and resulting
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security such as anonymous Diffie-Hellman exchange, the reverse hash
chains presented in [WIMP], or even a clear-text token exchanged at
the initial communication.
However, the mechanisms for preventing unauthorized use of an
identifier can be quite different. One way to prevent premeditated
redirection is to simply not introduce a new identifier name space,
and instead to rely on existing name space(s), a trusted third party,
and a sufficiently secure way to access the third party, as in
[NOID]. Such an approach relies on the third party (DNS in the case
of NOID) as the foundation. In terms of multihoming state creation,
in this case premeditated redirection is as easy or as hard as
redirecting an IP address today. Essentially, this relies on the
return-routability check associated with a roundtrip of
communication, which verifies that the routing system delivers
packets to the IP address in question.
Alternatively, one can use the crypto-based identifiers such as in
[HIP] or crypto-generated addresses as in [CBHI], which both rely on
public-key crypto, to prevent premeditated attacks. In some cases it
is also possible to avoid the problem by having one end of the
communication use ephemeral identifiers as the initiator does in
[WIMP]. This avoids premeditated redirection by detecting that some
other entity is using the same identifier at the peer and switching
to use another ephemeral ID. While the ephemeral identifiers might
be problematic when used by applications, for instance due to
callbacks or referrals, note that for the end that has the ephemeral
identifier, one can skirt around the premeditated attacks (assuming
the solution has a robust way to pick a new identifier when one is in
use/stolen).
Assuming the problem can't be skirted by using ephemeral identifiers,
there seem to be 3 types of approaches that can be used to establish
some form of identity ownership:
- A trusted third party, which states that a given identity is
reachable at a given set of locators, so the endpoint reached at
one of this locators is the owner of the identity.
- Crypto-based identifiers or crypto-generated addresses where the
public/private key pair can be used to prove that the identifier
was generated by the node knowing the private key (or by another
node whose public key hashes to the same value)
- A static binding, as currently defined in IP, where you trust that
the routing system will deliver the packets to the owner of the
locator, and since the locator and the identity are one, you prove
identity ownership as a sub-product.
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A solution would need to combine elements that provide protection
against both premeditated and ongoing communication redirection.
This can be done in several ways, and the current set of proposals do
not appear to contain all useful combinations. For instance, the HIP
CBID property could be used to prevent premeditated attacks, while
the WIMP hash chains could be used to prevent on-going redirection.
And there are probably other interesting combinations.
A related, but perhaps separate aspect, is whether the solution
provides for protection against man-in-the-middle attacks with
on-path attackers. Some schemes, such as [HIP] and [NOID] do, but
given that an on-path attacker can see and modify the data traffic
whether or not it can modify the multihoming signaling, this level of
protection seems like overkill. Protecting against on-path MITM for
the data traffic can be done separately using IPsec, TLS, etc.
Finally, preventing third party DoS attacks is conceptually simpler;
it would suffice to somehow verify that the peer is indeed reachable
at the new locator before sending a large number of packets to that
locator.
Just as the redirection attacks are conceptually prevented by proving
at some level the ownership of the identifier or the ownership of the
communication context, third party DoS attacks are conceptually
prevented by showing that the endpoint is authorized to use a given
locator.
The currently known approaches for showing such authorization are:
- Return routability. That is, if an endpoint is capable of
receiving packets at a given locator, it is because he is
authorized to do so. This relies on routing being reasonably hard
to subvert to deliver packets to the wrong place. While this
might be the case when routing protocols are used with reasonable
security mechanisms and practices, it isn't the case on a single
link where ARP and Neighbor Discovery can be easily spoofed.
- Trusted third party. A third party establishes that a given
identity is authorized to use a given set of locators (for
instance the DNS).
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RFC 4218 Threats to IPv6 Multihoming Solutions October 2005
Authors' Addresses
Erik Nordmark
Sun Microsystems, Inc.
17 Network Circle
Mountain View, CA 94025
USA
Phone: +1 650 786 2921
Fax: +1 650 786 5896
EMail: erik.nordmark@sun.com
Tony Li
EMail: Tony.Li@tony.li
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RFC 4218 Threats to IPv6 Multihoming Solutions October 2005
Full Copyright Statement
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RFC, FYI, BCP