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Internet Engineering Task Force (IETF) D. McPherson
Request for Comments: 6959 VeriSign, Inc.
Category: Informational F. Baker
ISSN: 2070-1721 Cisco Systems
J. Halpern
Ericsson
May 2013
Source Address Validation Improvement (SAVI) Threat Scope
Abstract
The Source Address Validation Improvement (SAVI) effort aims to
complement ingress filtering with finer-grained, standardized IP
source address validation. This document describes threats enabled
by IP source address spoofing both in the global and finer-grained
context, describes currently available solutions and challenges, and
provides a starting point analysis for finer-grained (host
granularity) anti-spoofing work.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6959.
McPherson, et al. Informational [Page 1]
RFC 6959 SAVI Threat Scope May 2013
Copyright Notice
Copyright (c) 2013 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
described in the Simplified BSD License.
Table of Contents
1. Overview ........................................................3
2. Glossary of Terms ...............................................5
3. Spoof-Based Attack Vectors ......................................6
3.1. Blind Attacks ..............................................6
3.1.1. Single-Packet Attacks ...............................6
3.1.2. Flood-Based DoS .....................................7
3.1.3. Poisoning Attacks ...................................8
3.1.4. Spoof-Based Worm/Malware Propagation ................8
3.1.5. Reflective Attacks ..................................8
3.1.6. Accounting Subversion ...............................9
3.1.7. Other Blind Spoofing Attacks ........................9
3.2. Non-blind Attacks ..........................................9
3.2.1. Man in the Middle (MITM) ............................9
3.2.2. Third-Party Recon ..................................10
3.2.3. Other Non-blind Spoofing Attacks ...................10
4. Current Anti-spoofing Solutions ................................11
4.1. Topological Locations for Enforcement .....................13
4.1.1. Host to Link-Layer Neighbor via Switch .............13
4.1.2. Upstream Switches ..................................13
4.1.3. Upstream Routers ...................................14
4.1.4. ISP Edge PE Router .................................14
4.1.5. ISP NNI Router to ISP NNI Router ...................15
4.1.6. Cable Modem Subscriber Access ......................15
4.1.7. DSL Subscriber Access ..............................15
4.2. Currently Available Tools .................................16
4.2.1. BCP 38 .............................................16
4.2.2. Unicast RPF ........................................16
4.2.3. Port-Based Address Binding .........................16
4.2.4. Cryptographic Techniques ...........................17
4.2.5. Residual Attacks ...................................18
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5. Topological Challenges Facing SAVI .............................18
5.1. Address Provisioning Mechanisms ...........................18
5.2. LAN Devices with Multiple Addresses .......................18
5.2.1. Routers ............................................18
5.2.2. NATs ...............................................19
5.2.3. Multi-instance Hosts ...............................19
5.2.4. Multi-LAN Hosts ....................................20
5.2.5. Firewalls ..........................................20
5.2.6. Mobile IP ..........................................20
5.2.7. Other Topologies ...................................21
5.3. IPv6 Considerations .......................................21
6. Analysis of Host Granularity Anti-spoofing .....................21
7. Security Considerations ........................................22
7.1. Privacy Considerations ....................................23
8. Acknowledgments ................................................24
9. References .....................................................24
9.1. Normative References ......................................24
9.2. Informative References ....................................24
1. Overview
The Internet Protocol, specifically IPv4 [RFC0791] and IPv6
[RFC2460], employs a connectionless hop-by-hop packet forwarding
paradigm. A host connected to an IP network that wishes to
communicate with another host on the network generates an IP packet
with source and destination IP addressing information, among other
options.
At the IP network layer, or Internet layer, there is typically no
required transactional state when communicating with other hosts on
the network. In particular, the network does not track any state
about the hosts using the network. This is normally a benefit.
However, as a consequence of this, hosts generating packets for
transmission have the opportunity to spoof (forge) the source address
of packets that they transmit, as the network does not have any way
to tell that some of the information is false.
Source address validation is necessary in order to detect and reject
spoofed IP packets in the network, and contributes to the overall
security of IP networks. This document deals with the subset of such
validation done by the network based on observed traffic and policy.
Such source address validation techniques enable detection and
rejection of many spoofed packets, and also implicitly provide some
assurances that the source address in an IP packet is legitimately
assigned to the system that generated the packet.
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Solutions such as those described in BCP 38 [RFC2827] provide
guidelines for one such technique for network ingress filtering.
However, if these techniques are not implemented at the ingress point
of the IP network, then the validity of the source address cannot be
positively ascertained. Furthermore, BCP 38 only implies source
address validation at the Internet layer and is most often
implemented on IP subnetwork address boundaries. One of the
difficulties in encouraging the deployment of BCP 38 is that there is
relatively little benefit until it is very widely deployed, which is
not yet the case.
Hence, in order to try to get better behavior, it is helpful to look
for an application like that described in BCP 38, but one that can be
applied locally and give locally beneficial results. The local
benefit would provide a reason for the site to deploy, while moving
the Internet as a whole towards an environment where BCP 38 is widely
effected. SAVI is aimed at providing more specific protection
locally, with the benefit of better local behavior and, in
conjunction with appropriate logging, better local traceability,
while also providing better compliance with the cases dealt with by
BCP 38.
It should be noted that while BCP 38 directs providers to provide
protection from spoofed prefixes, it is clearly desirable for
enterprise operators to provide that protection more locally, and
with better traceability. This allows the enterprise to be a better
Internet participant and to quickly detect and remedy problems when
they occur. For example, when an enterprise receives a report of an
attack originating within that enterprise, the operational staff
desires to be able to track from the IP address sourcing the attack
to the particular machine within the enterprise that is the source.
This is typically simpler and more reliable than other techniques,
such as trying to find the attack in ongoing outbound traffic. To do
this, the enterprise needs usable address assignment and usage
information (appropriate logging), as well as accurate information
(SAVI), to determine that no other machine could have been using that
address.
Also, there is a possibility that in a LAN environment where multiple
hosts share a single LAN or IP port on a switch or router, one of
those hosts may spoof the source addresses of other hosts within the
local subnet. Understanding these threats and the relevant
topologies in which they're introduced is critical when assessing the
threats that exist with source address spoofing.
This document provides additional details regarding spoof-based
threat vectors and discusses implications of various network
topologies.
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2. Glossary of Terms
The following acronyms and terms are used throughout this memo.
Binding Anchor: The relationship used by a device performing source
address enforcement to perform the validation and enforcement.
Examples in different situations include Layer 2 addresses or
physical ports.
BGP: The Border Gateway Protocol, used to manage routing policy
between large networks.
CPE Router: Customer Premises Equipment router. The router on the
customer premises, whether owned by the customer or the provider.
Also called the Customer Edge, or CE, router.
IP Address: An Internet Protocol address, whether IPv4 or IPv6.
ISP: Internet Service Provider. Any person or company that delivers
Internet service to another.
MAC Address: An Ethernet address or comparable IEEE 802 series
address.
NNI Router: Network-to-Network Interface router. This router
interface faces a similar system operated by another ISP or other
large network.
PE Router: Provider Edge router. This router faces a customer of an
ISP.
Spoofing: The act of sending a datagram header whose contents at the
link layer or network layer do not match the network policies and
activities on address assignment or claiming. Generally, this
corresponds to sending messages with source network or link-layer
information that is assigned to or currently properly claimed by
some other devices in the network.
TCP: The Transmission Control Protocol, used on end systems to
manage data exchange.
uRPF: Unicast Reverse Path Forwarding. A procedure in which the
route table, which is usually used to look up destination
addresses and route towards them, is used to look up the source
address and ensure that one is routing away from it. When this
test fails, the event may be logged, and the traffic is commonly
dropped.
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3. Spoof-Based Attack Vectors
Spoofing is employed on the Internet for a number of reasons, most of
which are in some manner associated with malicious or otherwise
nefarious activities. In general, two classes of spoof-based attack
vectors exist: blind attacks and non-blind attacks. The following
sections provide some information on blind and non-blind attacks;
these sections also include information on attacks where the spoofing
is primarily intended to interfere with tracing the attacks, as well
as attacks where spoofing the source address is a necessary component
to the damage or interference.
3.1. Blind Attacks
Blind attacks typically occur when an attacker isn't on the same
local area network as a source or target, or when an attacker has no
access to the data path between the attack source(s) and the target
systems. In this situation, the attacker has no access to the source
and target systems.
3.1.1. Single-Packet Attacks
One type of blind attacks, which we'll refer to here as "single-
packet DoS (Denial of Service) attacks", involves an attacking system
injecting spoofed information into the network, which either results
in a complete crash of the target system, or in some manner poisons
some network configuration or other information on a target system so
as to impact network or other services.
An example of an attack that can cause a receiving system to crash is
what is called a LAND (Local Area Network Denial) attack. A LAND
attack would consist of an attacking system sending a packet (e.g.,
TCP SYN) to a target system that contains both a source and
destination address of that target system. The packet would also
contain a single value for the port number, used as both the source
and destination port number. Certain target systems will then "lock
up" when creating connection state associated with the packet or
would get stuck in a state where a target system continuously replies
to itself. As this is an attack that relies on bugs in the target,
it is possible, but by no means certain, that this threat is no
longer viable.
Another form of blind attack is a RST (reset) probe ([RFC4953],
Section 2.3). The attacker sends a series of packets to a
destination that is engaged in a long-lived TCP session. The packets
are RST packets, and the attacker uses the known source and
destination addresses and port numbers, along with guesses at the
sequence number. If he can send a packet close enough to the right
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value, in theory he can terminate the TCP connection. While there
are various steps that have been developed to ameliorate this attack,
preventing the spoofing of source addresses completely prevents the
attack from occurring.
3.1.2. Flood-Based DoS
Flood-based DoS attack vectors are particularly effective attacks on
the Internet today. They usually entail flooding a large number of
packets towards a target system, with the hopes of either exhausting
connection state on the target system, consuming all packet
processing capabilities of the target or intermediate systems, or
consuming a great deal of bandwidth available to the target system
such that they are essentially inaccessible.
Because these attacks require no reply from the target system and
require no legitimate transaction state, attackers often attempt to
obfuscate the identity of the systems that are generating the attack
traffic by spoofing the source IP address of the attacking traffic
flows. Because ingress filtering isn't applied ubiquitously on the
Internet today, spoof-based flooding attack vectors are typically
very difficult to trace back. In particular, there may be one or
more attacking sources beyond a network's border, and the attacking
sources may or may not be legitimate sources; it's difficult to
determine if the sources are not directly connected to the local
routing system. These attacks might be seen as primarily needing to
be addressed by BCP 38 deployment, which is not in scope for this
document. However, as noted earlier, deployment of SAVI can help
remediate lack of BCP 38 deployment, and even when BCP 38 is
deployed, SAVI can help provide useful information for responding to
such attacks.
Common flood-based DoS attack vectors today include SYN floods, ICMP
floods, and IP fragmentation attacks. Attackers may use a single
legitimate or spoofed fixed attacking source address, although
frequently they cycle through large swaths of address space. As a
result, mitigating these attacks on the receiving end with source-
based policies is extremely difficult.
If an attacker can inject messages for a protocol that requires
control-plane activity, it may be possible to deny network control
services at a much lower attack level. While there are various forms
of protection deployed against this, they are by no means complete.
Attacks that are harder to trace (such as with spoofed addresses) are
of course of more concern.
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Furthermore, the motivator for spoof-based DoS attacks may actually
be to encourage the target to filter explicitly on a given set of
source addresses, in order to disrupt access to the target system by
legitimate owner(s).
3.1.3. Poisoning Attacks
While poisoning attacks can often be done with single packets, it is
also true that a stream of packets can be used to find a window where
the target will accept the incorrect information. In general, this
can be used to perform broadly the same kinds of poisonings as above,
with more versatility.
One important class of poisoning attacks are attacks aimed at
poisoning network or DNS cache information, perhaps to simply break a
given host's connection or to enable MITM (Man in the Middle) or
other attacks. Network-level attacks that could involve single-
packet DoS include Address Resolution Protocol (ARP) cache poisoning
and ICMP redirects. The most obvious example, which depends upon
falsifying an IP source address, is an on-link attacker poisoning a
router's ARP or Neighbor Discovery (ND) cache. The ability to forge
a source address can also be helpful in causing a DNS cache to accept
and use incorrect information.
3.1.4. Spoof-Based Worm/Malware Propagation
Self-propagating malware has been observed that spoofs its source
address when attempting to propagate to other systems. Presumably,
this was done to obfuscate the actual source address of the infected
system. This attack is important both in terms of an attack vector
that SAVI may help prevent and as a problem that SAVI can help solve
by tracing back to find infected systems.
3.1.5. Reflective Attacks
Reflective amplification attacks -- wherein a sender sends a single
packet to an intermediary, resulting in the intermediary sending a
large number of packets, or much larger packets, to the target -- are
a particularly potent DoS attack vector on the Internet today. Many
of these attacks rely on using a false source address, so that the
amplifier attacks the target by responding to the messages.
DNS is one of the common targets of such attacks. The amplification
factor observed for attacks targeting DNS root and other top-level
domain name infrastructures in early 2006 was on the order of 72:1
[VRSN-REPORT]. The result was that just 27 attacking sources with
512 kbps of upstream attack bandwidth could generate 1 Gbps of
response attack traffic towards a target system.
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Smurf attacks employ a similar reflective amplification attack
vector, exploiting traditional default IP-subnet-directed broadcast
address behaviors that would result in all the active hosts on a
given subnet responding to a (spoofed) ICMP echo request from an
attacker and generating a large amount of ICMP echo response traffic
directed towards a target system. These attacks have been
particularly effective in large campus LAN environments where 50K or
more hosts might reside on a single subnet.
3.1.6. Accounting Subversion
If an attacker wishes to distribute content or other material in a
manner that employs protocols that require only unidirectional
flooding and generate no end-to-end transactional state, they may
desire to spoof the source IP address of that content in order to
avoid detection or accounting functions enabled at the IP layer.
While this particular attack has not been observed, it is included
here to reflect the range of power that spoofed addresses may have,
even without the ability to receive responses.
3.1.7. Other Blind Spoofing Attacks
Other blind spoofing attacks might include spoofing in order to
exploit source routing or other policy-based routing implemented in a
network. It may also be possible in some environments to use
spoofing techniques to perform blind or non-blind attacks on the
routers in a site or in the Internet. There are many techniques to
mitigate these attacks, but it is well known that there are
vulnerabilities in this area.
3.2. Non-blind Attacks
Non-blind attacks often involve mechanisms such as eavesdropping on
connections, resetting state so that new connections may be hijacked,
and an array of other attack vectors. Perhaps the most common of
these attack vectors are known as man-in-the-middle attacks. In this
case, we are concerned not with an attacker who can modify a stream,
but rather with one who has access to information from the stream and
uses that information to launch his own attacks.
3.2.1. Man in the Middle (MITM)
Connection hijacking is one of the more common man-in-the-middle
attack vectors. In order to hijack a connection, an attacker usually
needs to be in the forwarding path and oftentimes employs TCP RST or
other attacks in order to reset a transaction. The attacker may have
already compromised a system that's in the forwarding path, or they
may wish to insert themselves in the forwarding path.
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For example, an attacker with access to a host on a LAN segment may
wish to redirect all the traffic on the local segment destined for a
default gateway address (or all addresses) to itself in order to
perform man-in-the-middle attacks. In order to accomplish this in
IPv4, the attacker might transmit gratuitous ARP [RFC0826] messages
or ARP replies to the Ethernet broadcast address ff:ff:ff:ff:ff:ff,
notifying all the hosts on the segment that the IP address(es) of the
target(s) now maps to its own Layer 2 address. The source IP address
in this case is spoofed. Similar vulnerabilities exist in the IPv6
ND protocol [RFC4861], although the multicast requirements of the
IPv6 ND protocol make this harder to perform with the same
generality.
3.2.2. Third-Party Recon
Another example of a non-blind attack is third-party reconnaissance.
The use of spoofed addresses, while not necessary for this, can often
provide additional information and helps mask the traceability of the
activity. The attack involves sending packets towards a given target
system and observing either target or intermediate system responses.
For example, if an attacker were to source spoof TCP SYN packets
towards a target system from a large set of source addresses and
observe responses from that target system or some intermediate
firewall or other middlebox, they would be able to identify what
IP-layer filtering policies may be in place to protect that system.
3.2.3. Other Non-blind Spoofing Attacks
There are presumably many other attacks that can be performed based
on the ability to spoof source addresses while seeing the target.
Among other attacks, if there are multiple routers on-link with
hosts, a host may be able to cause problems for the routing system by
replaying modified or unmodified routing packets as if they came from
another router.
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4. Current Anti-spoofing Solutions
The goal of this work is to reduce datagrams with spoofed IP
addresses from the Internet. This can be aided by identifying and
dropping datagrams whose source address binding is incompatible with
the Internet topology and learned information. This can be done at
sites where the relationship between the source address and topology
and binding information can be checked. For example, with these
bindings, in many networks Internet devices can confirm that:
o The IP source address is appropriate for the lower-layer address
(they both identify the same system).
o The IP source address is explicitly identified as appropriate for
the physical topology; for example, the source address is
appropriate for the Layer 2 switch port through which the datagram
was received.
o The prefix to which the IP source address belongs is appropriate
for the part of the network topology from which the IP datagram
was received (while the individual system may be unknown, it is
reasonable to believe that the system is located in that part of
the network).
In general, this involves two kinds of inspection. The primary
action is checking the source IP address in the IP header of IP
packets. In order to support such checking, the claimed or assigned
IP addresses in messages concerned with such claims or assignments
(IP ARP Requests and Responses, DHCP Replies, IPv6 ND Duplicate
Address Detection (DAD) messages, etc.) must also be examined and,
where appropriate, verified. SAVI is not concerned with verifying IP
addresses in the contents of arbitrary higher-level protocol
messages.
Filtering points farther away from the source of the datagram can
make decreasingly authoritative assertions about the validity of the
source address in the datagram. Nonetheless, there is value in
dropping traffic that is clearly inappropriate and in maintaining
knowledge of the level of trust one can place in an address.
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Edge Network 1 CPE-ISP _.------------.
_.----------------. Ingress/ ISP A `--.
,--'' `---. ,' `.
,' +----+ +------+ +------+ `. / +------+ +------+ \\
( |Host+--+Switch+--+ CPE +---)-(---+ PE +- - - -+ NNI | )
`. +----+ +------+ |Router| ,' \\ |Router| |Router| /
`---. Host-neighbor +------+' `.+------+ +--+---+,'
`----------------'' '--. |_.-'
`------------'|
|
Edge Network 2 ISP-ISP Ingress |
_.----------------. _.----------.|
,--'' `---. ,-'' |--.
,' +----+ +------+ +------+ `. ,+------+ +--+---+.
( |Host+--+Switch+--+ CPE +---)---+-+ PE +- - - -+ NNI | \\
`. +----+ +------+ |Router| ,' ( |Router| |Router| )
`---. +------+' \\+------+ +------+ /
`----------------'' `. ,'
'--. ISP B _.-'
`----------''
Figure 1: Points Where an Address Can Be Validated
Figure 1 illustrates five related paths where a source address can be
validated:
o Host to switch, including host to host via the switch
o Host to enterprise CPE router
o Enterprise CPE router to ISP edge PE router, and the reverse
o ISP NNI router to ISP NNI router
In general, datagrams with spoofed IP addresses can be detected and
discarded by devices that have an authoritative mapping between IP
addresses and the network topology. For example, a device that has
an authoritative table between link-layer and IP addresses on a link
can discard any datagrams in which the IP address is not associated
with the link-layer address in the datagram. The degree of
confidence in the source address depends on where the spoofing
detection is performed, as well as the prefix aggregation in place
between the spoofing detection and the source of the datagram.
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4.1. Topological Locations for Enforcement
There are a number of kinds of places, which one might call
topological locations, where solutions may or should be deployed. As
can be seen in the details below, as the point of enforcement moves
away from a single cable attached directly to the host being
validated, additional complications arise. It is likely that fully
addressing many of these cases may require additional coordination
mechanisms across the device that covers the disparate paths.
4.1.1. Host to Link-Layer Neighbor via Switch
The first point at which a datagram with a spoofed address can be
detected is on the link to which the source of the datagram is
connected. At this point in the network, the source link-layer and
IP addresses are both available and can be validated against each
other, and potentially against the physical port being used. A
datagram in which the IP source address does not match the
corresponding link-layer address can be discarded. Of course, the
trust in the filtering depends on the trust in the method through
which the mappings are developed. This mechanism can be applied by a
first-hop router, or switch on the link. The first-hop switch has
the most precise information for this.
On a truly shared medium link, such as classic Ethernet, the best
that can be done is to validate the link-layer and IP addresses
against the mappings. When the link is not shared, such as when the
hosts are connected through a switch, the source host can be
identified precisely based on the port through which the datagram is
received or the Layer 2 address if it is known to the switch. Port
identification prevents transmission of malicious datagrams whose
link-layer and IP addresses are both spoofed to mimic another host.
Other kinds of links may fall at different places in this spectrum,
with some wireless links having easier ways of identifying individual
devices than others, for example.
4.1.2. Upstream Switches
In many topologies, there can be additional switches between the
host-attached switch and the first router in the network. In these
cases, additional issues can arise that affect SAVI operations. If
the bridging topologies that connect the switches change, or if the
Link Aggregation Control Protocol (LACP) [IEEE802.1AX], the Virtual
Router Redundancy Protocol (VRRP), or link management operations
change the links that are used to deliver traffic, the switch may
need to move the SAVI state to a different port, or the state may
need to be moved or reestablished on a different switch.
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4.1.3. Upstream Routers
Beyond the first-hop router, subsequent routers may additionally
filter traffic from downstream networks. Because these routers do
not have access to the link-layer address of the device from which
the datagram was sent, they are limited to confirming that the source
IP address is within a prefix that is appropriate for a downstream
router from which the datagram was received.
Options include the use of simple access lists or the use of Unicast
Reverse Path Forwarding (uRPF). Access lists are generally
appropriate only for the simplest cases, as management can be
difficult. Strict uRPF accepts the source address on a datagram if
and only if it comes from a direction that would be rational to send
a datagram directed to the address, which means that the filter is
derived from routing information. These filtering procedures are
discussed in more detail in [RFC3704].
In many cases, this router has access to information about what IP
prefixes are to be used on a given subnet. This might be because it
delegated that prefix through DHCP or monitored such a delegation.
It may have advertised that prefix in IPv6 Neighbor Discovery Router
Advertisement messages, or monitored such an advertisement. These
can be seen as generalizations of the access lists above. When the
topology permits, the router can enforce that these prefixes are used
by the hosts.
4.1.4. ISP Edge PE Router
An obvious special case of the discussion is with an ISP PE router,
where it provides its customer with access. BCP 38 specifically
encourages ISPs to use ingress filtering to limit the incidence of
spoofed addresses in the network.
The question that the ISP must answer for itself is the degree to
which it trusts its downstream network. A contract might be written
between an ISP and its customer requiring that the customer apply the
procedures of network ingress filtering to the customer's own
network, although there's no way upstream networks would be able to
validate this.
Conversely, if the provider has assigned a single IP address to the
customer (for example, with IPv4 NAT in the CPE), PE enforcement of
BCP 38 can be on the full address, simplifying many issues.
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4.1.5. ISP NNI Router to ISP NNI Router
The considerations explicitly related to customer networks can also
be applied to neighboring ISPs. An interconnection agreement might
be written between two companies requiring that network ingress
filtering policy be implemented on all customer connections. ISPs
might, for example, mark datagrams from neighboring ISPs that do not
sign such a contract or demonstrably do not behave in accordance with
it as 'untrusted'. Alternatively, the ISP might place untrusted
prefixes into a separate BGP community [RFC4271] and use that to
advertise the level of trust to its BGP peers.
In this case, uRPF is less effective as a validation tool, due to
asymmetric routing. However, when it can be shown that spoofed
addresses are present, the procedure can be applied.
Part of the complication here is that in the abstract, it is very
difficult to know what addresses should appear in packets sent from
one ISP to another. Hence, packet-level filtering and enforcement
are very difficult at this point in the network. Whether one views
this as specific to the NNI, or a general property of the Internet,
it is still a major factor that needs to be taken into account.
4.1.6. Cable Modem Subscriber Access
Cable Modem Termination Systems (CMTS) employ Data Over Cable Service
Interface Specification (DOCSIS) Media Access Control (MAC) domains.
These share some properties with general switched networks, as
described above in Section 4.1.1, and some properties with DSL access
networks, as described below in Section 4.1.7. They also often have
their own provisioning and monitoring tools that may address some of
the issues described here.
4.1.7. DSL Subscriber Access
While DSL subscriber access can be bridged or routed, as seen by the
service provider's device, it is generally the case that the
protocols carry enough information to validate which subscriber is
sending packets. Thus, for ensuring that one DSL subscriber does not
spoof another, enforcement can generally be done at the aggregation
router. This is true even when there is a bridged infrastructure
among the subscribers, as DSL access generally requires all
subscriber traffic to go through the access aggregation router.
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If it is desirable to provide spoofing protection among the devices
within a residence, that would need to be provided by the CPE device,
as the ISP's router does not have enough visibility to do that. It
is not clear at this time that this problem is seen as a relevant
threat.
4.2. Currently Available Tools
There are a number of tools that have been developed, and have seen
some deployment, for addressing these attacks.
4.2.1. BCP 38
If BCP 38 [RFC2827] is implemented in LAN segments, it is typically
done so on subnetwork boundaries and traditionally relates only to
network-layer ingress filtering policies. The result is that hosts
within the segment cannot spoof packets from address space outside of
the local segment itself; however, they may still spoof packets using
sources' addresses that exist within the local network segment.
4.2.2. Unicast RPF
Unicast RPF is a crude mechanism to automate definition of BCP 38
style filters based on routing table information. Its applicability
parallels that of BCP 38, although deployment caveats exist, as
outlined in [RFC3704].
4.2.3. Port-Based Address Binding
Much of the work of SAVI is initially targeted at minimizing source
address spoofing in the LAN. In particular, if mechanisms can be
defined to accommodate configuration of port binding information for
IP, either to a port, to an unchangeable or authenticated MAC
address, or to other credentials in the packet such that an impostor
cannot create the needed values, a large portion of the spoofing
threat space in the LAN can be marginalized.
However, establishing this binding is not trivial and varies across
both topology types and address allocation mechanisms.
4.2.3.1. Manual Binding
Binding of a single link-layer and network-layer address to a port
may initially seem trivial. However, two primary areas exist that
can complicate such techniques. In particular, these areas involve
topologies where more than a single IP-layer address may be
associated with a MAC address on a given port, or where multiple
hosts are connected via a single physical port. Furthermore, if one
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or more dynamic address allocation mechanisms such as DHCP are
employed, then some mechanism must exist to associate those IP-layer
addresses with the appropriate link-layer ports as addresses are
allocated or reclaimed.
4.2.3.2. Automated Binding
For IPv4, the primary and very widely used automated address
assignment technique is DHCP-based address assignment. This can be
coupled with filtering policies that control which hosts can
originate DHCP replies. Under such circumstances, SAVI switches can
treat DHCP replies as authoritative sources of IP address binding
information. By eavesdropping on the DHCP exchanges, the SAVI switch
can create the bindings needed for address usage enforcement.
For IPv6, there are two common automated address assignment
techniques. While there are many variations and details, for
purposes of understanding the threats and basic responses, these are
Stateless Address Autoconfiguration (SLAAC) and DHCP-based IPv6
address assignment. For DHCP-based IPv6 address assignment, the
techniques above are applicable and suitable.
When SLAAC is used for IPv6 address assignment, the switches can
observe the duplicate address detection messages and use those to
create the enforcement bindings. This enables the switches to ensure
that only properly claimed IP addresses are used for data traffic.
It does not enforce that these addresses are assigned to the hosts,
since SLAAC does not have a notion of address assignment.
4.2.3.3. IEEE 802.1x
IEEE 802.1x is an authentication protocol that permits a network to
determine the identity of a user seeking to join it and apply
authorization rules to permit or deny the action. In and of
themselves, such tools confirm only that the user is authorized to
use the network, but they do not enforce what IP address the user is
allowed to use. It is worth noting that elements of 802.1x may well
be useful as binding anchors for SAVI solutions.
4.2.4. Cryptographic Techniques
MITM and replay attacks can typically be mitigated with cryptographic
techniques. However, many of the applications today either don't or
can't employ cryptographic authentication and protection mechanisms.
ARP for IPv4 does not use such protection. While Secure Neighbor
Discovery (SEND) provides such protection for the IPv6 ND protocol,
SEND is not widely used to date. Usage of such techniques is outside
the scope of this document.
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While DNSSEC will significantly help protect DNS from the effects of
spoof-based poisoning attacks, such protection does not help protect
the rest of the network from spoofed attacks.
4.2.5. Residual Attacks
It should be understood that not all combinations of network,
service, and enforcement choices will result in a protectable
network. For example, if one uses conventional SLAAC in a switched
network, but tries to only provide address enforcement on the routers
on the network, then the ability to provide protection is severely
limited.
5. Topological Challenges Facing SAVI
As noted previously, topological components and address allocation
mechanisms have significant implications on what is feasible with
regard to link-layer address and IP address port bindings. The
following sections discuss some of the various topologies and address
allocation mechanisms that proposed SAVI solutions should attempt to
address.
5.1. Address Provisioning Mechanisms
In a strictly static environment, configuration management for access
filters that map link-layer and network-layer addresses on a specific
switch port might be a viable option. However, most networks,
certainly those that accommodate actual human users, are much more
dynamic in nature. As such, mechanisms that provide port-MAC-IP
bindings need to accommodate dynamic address allocation schemes
enabled by protocols such as DHCP, DHCPv6 for address allocation, and
IPv6 Stateless Address Autoconfiguration.
5.2. LAN Devices with Multiple Addresses
From the perspective of network topology, consider hosts connected to
switch ports that may have one or more IP addresses, and devices that
forward packets from other network segments. It is much harder to
enforce port-MAC-IP bindings on traffic from such hosts and devices
than for traffic from more simply connected devices.
5.2.1. Routers
Routers are the most obvious examples of devices for which it is
problematic to implement port-MAC-IP bindings. Routers not only
originate packets themselves and often have multiple interfaces, but
also forward packets from other network segments. As a result, it's
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difficult for port-MAC-IP binding rules to be established a priori,
because it's likely that many addresses and IP subnets should be
associated with the port-MAC in question.
5.2.2. NATs
Validating traffic from prefix-based and multi-address NATs is also
problematic, for the same reasons as for routers. Because they may
forward traffic from an array of addresses, validation requires
advance knowledge of the IPs that should be associated with a given
port-MAC pair.
5.2.3. Multi-instance Hosts
Another example that introduces complexities is that of multi-
instance hosts attached to a switch port. These are single physical
devices that internally run multiple physical or logical hosts. When
the device is a blade server, e.g., with internal blades each hosting
a physical machine, there is essentially a physical switch inside the
blade server. While feasible, this creates some complexity for
determining where enforcement logic can or should live.
Logically distinct hosts, such as are provided by many varieties of
virtualization logic, result in a single physical host and connect to
a single port on the Ethernet switch in the topology, actually having
multiple internal virtual machines. Each virtual machine may have
its own IP and MAC addresses. These are connected by what is
essentially (or sometimes literally) an internal LAN switch. While
this internal switch may be a SAVI enforcement point to help control
threats among the virtual hosts, or between virtual hosts and other
parts of the network, such enforcement cannot be counted on in all
implementations. If the virtual machines are interconnected by the
internal switch, then that logical device is the first switch for the
purposes of this analysis.
A further complication with multi-instance hosts is that in many
environments, these hosts may move while retaining their IP
addresses. This can be an actual relocation of the running software,
or a backup instance taking over the functions of the software. In
both cases, the IP address will appear at a new topological location.
Depending upon the protocols used, it may have the same MAC address
or a different one, and the system may or may not issue a gratuitous
ARP request after relocation. When such a move is done without
changing the MAC address, the SAVI switches will need to update their
state. While ARP may be helpful, traffic detection, switch-based
neighbor solicitation, interaction with an orchestration system, or
other means may be used.
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5.2.4. Multi-LAN Hosts
Multi-interface hosts, in particular those that are multihomed and
may forward packets from any of a number of source addresses, can be
problematic as well. In particular, if a port-MAC-IP binding is made
on each of the interfaces, and then either a loopback IP or the
address of a third interface is used as the source address of a
packet forwarded through an interface for which the port-MAC-IP
binding doesn't map, the traffic may be discarded. Static
configuration of port-MAC-IP bindings may accommodate this scenario,
although some a priori knowledge of address assignment and topology
is required.
While it is rare to use loopback addressing or to send packets out of
one interface with the source address of another, these rarities do
legitimately occur. Some servers, particularly ones that have
underlying virtualization, use loopback techniques for management.
5.2.5. Firewalls
Firewalls that forward packets from other network segments, or serve
as a source for locally originated packets, suffer from the same
issues as routers.
5.2.6. Mobile IP
Mobile IP hosts in both IPv4 and IPv6 are proper members of the site
where they are currently located. Their care-of address is a
properly assigned address that is on the link they are using, and
their packets are sent and received using that address. Thus, they
do not introduce any additional complications. (There was at one
time consideration of allowing mobile hosts to use their home address
when away from home. This was not done, precisely to ensure that
mobile hosts comply with source address validity requirements.)
Mobile hosts with multiple physical interfaces fall into the cases
above.
Mobile IP Home Agents (HAs) are somewhat more interesting. Although
they are (typically) fixed devices, they are required to send and
receive packets addressed from or to any currently properly
registered mobile node. From an analysis point of view, even though
the packets that an HA handles are actually addressed to or from the
link the HA is on, it is probably best to think of them as routers,
with a virtual interface to the actual hosts they are serving. Thus,
if the Mobile IP HA is trusted, it can itself perform IP source
address checking on the packets it forwards on behalf of mobile
nodes. This would utilize bindings established by the Mobile IP
registration mechanisms.
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5.2.7. Other Topologies
Any topology that results in the possibility that a device connected
to a switch port may forward packets with more than a single source
address for a packet that it originated may be problematic.
Additionally, address allocation schemas introduce additional
considerations when examining a given SAVI solutions space.
5.3. IPv6 Considerations
IPv6 introduces additional capabilities that indirectly complicate
the spoofing analysis. IPv6 introduces and recommends the use of
SLAAC [RFC4862]. This allows hosts to determine their IP prefix,
select an Interface Identifier (IID), and then start communicating.
While there are many advantages to this, the absence of control
interactions complicates the process of behavioral enforcement.
An additional complication is the very large IID space. Again, this
64-bit IID space provided by IPv6 has many advantages. It provides
the opportunity for many useful behaviors. However, it also means
that in the absence of controls, hosts can mint anonymous addresses
as often as they like, modulo the idiosyncrasies of the duplicate
address procedure. Like many behaviors, this is a feature for some
purposes and a problem for others. For example, without claiming the
entire IID space, an on-link attacker may be able to generate enough
IP addresses to fill the Neighbor Discovery table space of the other
Layer 3 (L3) devices on the link, including switches that are
monitoring L3 behavior. This could seriously interfere with the
ability of other devices on the link to function.
6. Analysis of Host Granularity Anti-spoofing
Applying anti-spoofing techniques at the host level enables a site to
achieve several valuable objectives. While it is likely the case
that for many site topologies and policies full source spoofing
protection is not possible, it is also true that for many sites there
are steps that can be taken that provide benefit.
One important class of benefit is masquerade prevention. Security
threats involving one machine masquerading as another, for example,
in order to hijack an apparently secure session, can occur within a
site with significant impact. Having mechanisms such that host-
facing devices prevent this is a significant intra-site security
improvement. Given that security experts report that most security
breaches are internal, this can be valuable. One example of this is
that such techniques should mitigate internal attacks on the site
routing system.
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A second class of benefit is related to the traceability described
above. When a security incident is detected, either within a site or
externally (and traced to the site), it can be critical to determine
the actual source of the incident. If address usage can be tied to
the kinds of anchors described earlier, this can help in responding
to security incidents.
In addition to these local observable benefits, there can be more
global benefits. For example, if address usage is tied to anchors,
it may be possible to prevent or control the use of large numbers of
anonymous IPv6 addresses for attacks, or at least to trace even those
attacks back to their source.
As described below in the security considerations, these operational
behaviors need to be evaluated in the context of the reduction in
user privacy implied if one logs traffic bindings. In particular, in
addition to the architectural trade-offs, the network administrator
must plan for the proper handling of this relevant privacy
information about his users.
7. Security Considerations
This document provides limited discussion of some security threats
that source address validation improvements will help to mitigate.
It is not meant to be all-inclusive, either from a threat analysis
perspective or from the source address validation application side.
It is seductive to think of SAVI solutions as providing the ability
to use this technology to trace a datagram to the person, or at least
end system, that originated it. For several reasons, the technology
can be used to derive circumstantial evidence, but does not actually
solve that problem.
In the Internet layer, the source address of a datagram should be the
address of the system that originated it and to which any reply is
expected to come. But systems fall into several broad categories.
Many are single-user systems, such as laptops and PDAs. Multi-user
systems are commonly used in industry, and a wide variety of
middleware systems and application servers have no users at all, but
by design relay messages or perform services on behalf of users of
other systems (e.g., SMTP and peer-to-peer file sharing).
Even if every Internet-connected network implements source address
validation at the ultimate network ingress, and assurances exist that
intermediate devices are to never modify datagram source addresses,
source addresses cannot be used as an authentication mechanism. The
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only techniques for unquestionably validating source addresses of
a received datagram are cryptographic authentication mechanisms
such as IPsec.
It must be presumed that there will be some failure modes in any SAVI
deployment, given the history of technical security mechanisms. A
possible attack to be considered by network administrators is an
inside attack probing the network for modes of spoofing that can be
accomplished. If the probes are conducted at a level below alarm
thresholds, this might allow an internal attacker to safely determine
what spoof modes he can use. Thus, the use of these techniques must
be managed in such a way as to avoid giving a false sense of security
to the network administrator.
7.1. Privacy Considerations
It should be understood that enforcing and recording IP address
bindings have privacy implications. In some circumstances, this
binding data may be considered to be personally identifying
information. In general, collecting private information about users
brings ethical and legal responsibilities to the network
administrator.
For this reason, collection and retention of logged binding
information need to be considered carefully. Prevention of spoofing
does not in itself require such retention. Analysis of immediate
events may rely on having logs of current bindings. Thus, privacy
issues can be ameliorated by removing binding logs after the binding
lifetimes expire. Logs of apparent spoof attempts are a separate
matter and may require longer retention to detect patterns of
deliberate or accidental abuse.
With operations of the type described here, the network administrator
is collecting information about where on his network the user is
active. In addition, the recorded bindings supplement address usage
information about users that is available from DHCP logs. For
example, if IPv6 SLAAC is being used, and IP to Layer 2 address
bindings are being logged, the administrator will have access to
information associating users with their IP addresses even if IPv6
privacy addresses are used.
In addition to this, care must be taken in attributing actions to
users on the basis of this sort of information. Whatever the
theoretical strength of the tools, administrators should always allow
for such information being wrong and should be careful about any
actions taken on the basis of apparent attribution. These techniques
do nothing about address spoofing from other sites, so any evaluation
of attribution also needs to allow for such cases.
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8. Acknowledgments
A portion of the primer text in this document came directly from
[SAVA], authored by Fred Baker and Ralph Droms. Many thanks to
Christian Vogt, Suresh Bhogavilli, and Pekka Savola for contributing
text and a careful review of this document.
9. References
9.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
9.2. Informative References
[IEEE802.1AX]
IEEE, "IEEE Standard for Local and metropolitan area
networks - Link Aggregation", IEEE 802.1AX, 2008.
[RFC0826] Plummer, D., "Ethernet Address Resolution Protocol: Or
converting network protocol addresses to 48.bit Ethernet
address for transmission on Ethernet hardware", STD 37,
RFC 826, November 1982.
[RFC2827] 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.
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks",
RFC 4953, July 2007.
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[SAVA] Baker, F. and R. Droms, "IPv4/IPv6 Source Address
Verification", Work in Progress, June 2007.
[VRSN-REPORT]
Silva, K., Scalzo, F., and P. Barber, "Anatomy of Recent
DNS Reflector Attacks from the Victim and Reflector Point
of View", VeriSign White Paper, April 2006.
Authors' Addresses
Danny McPherson
VeriSign, Inc.
EMail: dmcpherson@verisign.com
Fred Baker
Cisco Systems
EMail: fred@cisco.com
Joel M. Halpern
Ericsson
EMail: joel.halpern@ericsson.com
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