One document matched: draft-ietf-savi-threat-scope-05.xml
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<rfc category="info" docName="draft-ietf-savi-threat-scope-05"
ipr="trust200902">
<front>
<title abbrev="">SAVI Threat Scope</title>
<author fullname="Danny McPherson" initials="D." surname="McPherson">
<organization>VeriSign, Inc.</organization>
<address>
<postal>
<street></street>
<city></city>
<code></code>
<region></region>
<country></country>
</postal>
<email>dmcpherson@verisign.com</email>
</address>
</author>
<author fullname="Fred Baker" initials="F.J." surname="Baker">
<organization>Cisco Systems</organization>
<address>
<postal>
<street></street>
<city></city>
<code></code>
<region></region>
<country></country>
</postal>
<email>fred@cisco.com</email>
</address>
</author>
<author fullname="Joel M. Halpern" initials="J.M." surname="Halpern">
<organization>Ericsson</organization>
<address>
<postal>
<street></street>
<city></city>
<code></code>
<region></region>
<country></country>
</postal>
<email>joel.halpern@ericsson.com</email>
</address>
</author>
<date day="11" month="April" year="2011" />
<area>Internet Area</area>
<workgroup>SAVI</workgroup>
<abstract>
<t>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.</t>
</abstract>
<!--
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</note>
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<note title="Requirements">
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in <xref
target="RFC2119"></xref>.</t>
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</front>
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<section anchor="intro" title="Overview">
<t>The Internet Protocol, specifically <xref
target="RFC0791">IPv4</xref> and <xref target="RFC2460">IPv6</xref>,
employ 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.</t>
<t>At the IP Network Layer, or Internet Layer, there is typically no
required transactional state when communicating with other hosts on the
network. Hosts generating packets for transmission have the opportunity
to spoof (forge) the source address of packets which they transmit.</t>
<t>Source address verification is necessary in order to detect and
reject spoofed packets and contribute to the overall security of IP
networks. In particular, source address verification techniques enable
detection and rejection of 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.</t>
<t>Solutions such as <xref target="RFC2827">BCP 38</xref> 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
verification 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.</t>
<t>Hence, in order to try to get better behavior, it is helpful
to look for an application like BCP 38, but one which 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 locally more specific protection, with the
benefit of better local behavior and local traceability, while also
providing better compliance with the cases dealt with by BCP 38.</t>
<t>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 needs to be able to
track from the IP address sourcing the attack to the particular
machine within the enterprise that is the source. This both that the
information be useable (logging), and that the information be
accurate, i.e. that no other machine could have been using that address.</t>
<t>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.</t>
<t>The aim of this document is to provide some additional details
regarding spoofed-based threat vectors, and discuss implications of
various network topologies.</t>
</section>
<section anchor="glossary" title="Glossary of Terms">
<t>The following acronyms and terms are used throughout this memo.<list
style="hanging">
<t hangText="BGP:">The Border Gateway Protocol, used to manage
routing policy between large networks.</t>
<t hangText="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.</t>
<t hangText="IP Address:">An Internet Protocol Address, whether IPv4
or IPv6.</t>
<t hangText="ISP:">Internet Service Provider. Any person or company
that delivers Internet service to another.</t>
<t hangText="MAC Address:">An Ethernet Address or comparable IEEE
802 series address.</t>
<t hangText="NNI Router:">Network to Network Interface Router. This
router interface faces a similar system operated by another ISP or
other large network.</t>
<t hangText="PE Router:">Provider Edge Router. This router faces
a customer of an ISP.</t>
<t hangText="Spoofing:">The act of forging datagram header contents
at the Link or Network Layer</t>
<t hangText="TCP:">The Transmission Control Protocol, used on end
systems to manage data exchange.</t>
<t hangText="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.</t>
</list></t>
</section>
<section anchor="attacks" title="Spoofed-based Attack Vectors">
<t>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 spoofed-based attack
vectors exist: blind attacks and non-blind attacks. The following sections
provide some information of blind and non-blind attacks.</t>
<section anchor="blind" title="Blind Attacks">
<t>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 datapath between the attack source(s) and the target
systems. The result is that they have no access to legitimate source
and target systems.</t>
<section anchor="spdos" title="Single Packet Attacks">
<t>One type of blind attacks, which we'll refer to here as "single
packet DoS attacks", involves an attacking system injecting spoofed
information into the network which results in either 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.</t>
<t>An example of an attack that can cause a receiving system to
crash is a LAND attack. A LAND attack packet 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. It would also contain a single value for 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 it 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.</t>
<t>Another form of blind attack is a RST probe <xref target="RFC0793"/>. The
attacker sends a series of packets to a destination which 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 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 occuring.</t>
</section>
<section anchor="flood-dos" title="Flood-Based DoS">
<t>Flooding-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.</t>
<t>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 traceback. In particular, there may be one or more
attacking sources beyond your network 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 to be addressed by BCP 38 deployment, which would not be
in scope for this document. However, as noted earlier, deployment of
SAVI can help remediate lack of BCP 38, and even when BCP 38 is
deployed can help provide useful information for responding to such
attacks.</t>
<t>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.</t>
<t>If an attacker can inject messages for a protocol which
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 which are harder to trace (such as with
spoofed addresses) are of course of more concern.</t>
<t>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, or order to disrupt the legitimate owner(s)
access to the target system.</t>
</section>
<section anchor="poison-dos" title="Poisoning Attacks">
<t>While poison 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.</t>
<t>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, enable MITM or other
attacks. Network level attacks that could involve single packet DoS
include 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 ND cache. The ability
to forge a source address can also be helpful in causing a DNS
cache to accept and use incorrect information.</t>
</section>
<section anchor="propagation"
title="Spoof-based Worm/Malware Propagation">
<t>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 also as a
problem which SAVI can help track back to find infected systems.</t>
</section>
<section anchor="reflection" title="Reflective Attacks">
<t>Reflective amplifications 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.</t>
<t>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 infrastructure in early 2006 was on the
order of 76:1. The result is that just 15 attacking sources with
512Kbps of upstream attack bandwidth could generate one Gbps of
response attack traffic towards a target system.</t>
<t>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 (spoofed) ICMP echo request from an
attacker, and generating a large amount of ICMP echo response
traffic directed towards a target system. They were particularly
effective in large campus LAN environments where 50k or more hosts
might reside on a single subnet.</t>
</section>
<section anchor="beancount" title="Accounting Subversion">
<t>If an attacker wishes to distribute content or other material in
a manner that employs protocols that require only uni-directional
flooding and generate no end-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.</t>
</section>
<section anchor="other-blind" title="Other Blind Spoofing Attacks">
<t>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. 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.</t>
</section>
</section>
<!-- end section 3.1 -->
<section anchor="sighted-attack" title="Non-Blind Attacks">
<t>Non-blind attacks often involve mechanisms such as eavesdropping on
connection, resetting state so that new connections may be hijacked,
and an array of other attack vectors. Perhaps the most common of these
attack vectors is known as man in the middle attacks. In this
case, we are concerned not with an attacker who can modify a stream,
but rather one who has access to information from the stream, and uses
that to launch his own attacks.</t>
<section anchor="mitm" title="Man in the Middle (MITM)">
<t>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 often times 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.</t>
<t>For example, an attacker with access to a host on 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 <xref
target="RFC0826"></xref> 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 map to it's own
MAC address. Similar vulnerabilities exist in <xref target="RFC4861">IPv6 NDP</xref>.</t>
</section>
<section anchor="sighted-recon" title="Third Party Recon">
<t>Another example of sighted 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 middle box, they would
be able to identify what IP layer filtering policies may be in place
to protect that system.</t>
</section>
</section>
<!-- end section 3.2 -->
</section>
<!-- end section 3 -->
<section anchor="current" title="Current Anti-Spoofing Solutions">
<t>The first requirement is to eliminate datagrams with spoofed
IP addresses from the Internet. Identifying and dropping datagrams whose
source address is incompatible with the Internet topology at sites where
the relationship between the source address and topology can be checked
can eliminate such datagrams. For example, Internet devices can confirm
that: <list style="symbols">
<t>The IP source address is appropriate for the lower layer address
(they both identify the same system)</t>
<t>The IP source address is appropriate for the device at the layer
2 switch port (the address was assigned to a, and perhaps the,
system that uses that port)</t>
<t>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).</t>
</list></t>
<t>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.</t>
<figure anchor="figure1"
title="Points where an address can be validated">
<artwork align="center"><![CDATA[
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 _.-'
`----------''
]]></artwork>
</figure>
<t><xref target="figure1"></xref> illustrates five related paths
where a source address can be validated: <list style="symbols">
<t>host to switch, including host to host via the switch</t>
<t>Host to enterprise CPE Router</t>
<t>Enterprise CPE Router to ISP edge PE Router, and the reverse</t>
<t>ISP NNI Router to ISP NNI Router</t>
</list></t>
<t>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 and
the prefix aggregation in place between the spoofing detection and the
source of the datagram.</t>
<section anchor="places" title="Topological Locations for Enforcement">
<t>There are a number of kinds of places, which one might call
topological locations, where solutions may or should be deployed.</t>
<section anchor="host" title="Host to link layer neighbor via switch">
<t>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 verified against each other.
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.</t>
<t>On a truly shared medium link, such as classic Ethernet,
the best that can be
done is to verify 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 MAC 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.</t>
<t>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.</t>
</section>
<section anchor="upstream" title="Upstream routers">
<t>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 downstream router
from which the datagram was received.</t>
<t>Options include the use of simple access lists or the use of
unicast reverse path filtering (uRPF). Access lists are generally
appropriate only for the simplest cases, as management can be
difficult. Strict Unicast RPF 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 <xref target="RFC3704"></xref>.</t>
</section>
<section anchor="edge-pe" title="ISP Edge PE Router">
<t>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.</t>
<t>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 verify
this.</t>
<t>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.</t>
</section>
<section anchor="nni-nni" title="ISP NNI Router to ISP NNI Router">
<t>The considerations explicitly related to customer networks can also
be applied to neighboring ISPs. An interconnection agreement might be
written between two companies requiring network ingress filtering
policy be implemented on all customers 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 <xref target="RFC4271"/> and use that to advertise the level of
trust to its BGP peers.</t>
<t>In this case, uRPF is less effective as a verification tool, due to
asymmetric routing. However, when it can be shown that spoofed
addresses are present, the procedure can be applied.</t>
<t>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
is 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.</t>
</section>
<section anchor="cable" title="Cable Modem Subscriber Access">
<t>Cable Modem Termination Systems (CMTS) employ DOCSIS Media Access
Control (MAC) domains. These share some properties with
general switched networks, as described above in <xref
target="host"></xref>, some properties with DSL access networks, as
described below in <xref target="dsl"></xref>. They also often have
their own provisioning and monitoring tools which may address some of
the issues described here. </t>
</section>
<section anchor="dsl" title="DSL Subscriber Access">
<t>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 verify 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.</t>
<t>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 ISPs 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.</t>
</section>
</section>
<section anchor="currentTools" title="Currently Available
Tools">
<t>There are a number of tools which have been developed, and
have seen some deployment, for addressing these attacks.</t>
<section anchor="bcp38" title="BCP 38">
<t>If <xref target="RFC2827">BCP 38</xref> 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.</t>
</section>
<section anchor="urpf" title="Unicast RPF">
<t>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 <xref target="RFC3704"></xref>.</t>
</section>
<section anchor="port-binding" title="Port-based Address Binding">
<t>Much of the work of SAVI is initially targeting
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 unforgeable MAC
address, or to other unforgeable credentials in the packet, a
large portion of the spoofing threat space in the LAN can be
marginalized.</t>
<t>However, establishing this binding is not trivial, and varying
across both topologies type and address allocation mechanisms.</t>
<section anchor="manual" title="Manual Binding">
<t>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 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.</t>
</section>
<section anchor="auto-bind" title="Automated Binding">
<t>For IPv4 the primary and very widely used automated
address assignment
technique is DHCP based address assignment. Controlling where
authoritative information can be sourced, coupled with sniffing
and enforcing the assignments is an effective technique,
which can in many networks automatically provide sufficient binding information.</t>
<t>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 DHCPv6
based address assignment. In both cases, SAVI binding establishment
needs to be tied to the state machines for these
address assignment protocols, with
appropriate message sniffing and enforcement. For DHCPv6 based
techniques, it is also necessary to use classification techniques
to ensure that responses which are trusted actually
come from authoritative sources.</t>
</section>
<section anchor="IEEE8021x" title="IEEE 802.1x">
<t>IEEE 802.1x is an authentication protocol that permits a network
to determine the identity of a system 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 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.</t>
</section>
</section>
<section anchor="crypto" title="Cryptographic Techniques">
<t>Needless to say, 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 SEND provides such protection for IPv6 NDP, SEND is not widely used
to date.</t>
<t>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.</t>
</section>
<section anchor="residual" title="Residual Attacks">
<t>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.</t>
</section>
</section>
</section>
<!-- end of 4.0 -->
<section anchor="topology" title="Topological Challenges Facing SAVI">
<t>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.</t>
<section anchor="provisioning" title="Address Provisioning Mechanisms">
<t>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.</t>
</section>
<section anchor="multi-address"
title="LAN devices with Multiple Addresses">
<t>From a topology considerations perspective, when attempting
port-MAC-IP bindings, hosts connected to switch ports that may have one
or more IP addresses, or certainly, devices that forward packets from
other network segments, present traffic that is much harder to
make subject to such bindings and enforcement.</t>
<section anchor="Routers" title="Routers">
<t>The most obvious example of devices that are problematic when
attempting to implement port-MAC-IP bindings is that of routers.
Routers not only originate packets themselves and often have
multiple interfaces, but also forward packets from other network
segments. As a result, it's 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.</t>
</section>
<section anchor="nat" title="NATs">
<t>Validating traffic from Prefix-based and multi-address NATs also
becomes problematic, for
the same reasons as routers. Because they may forward traffic from
an array of address, a priori knowledge must exist providing what
IPs should be associated with a given port-MAC pair.</t>
</section>
<section anchor="vHosts" title="Multi-Instance Hosts">
<t>Another example that introduces complexities is that of
multi-instance hosts attached to a switch port. These are single
physical devices, which 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 tractable, this creates some complexity
for determining where enforcement logic can or should live.</t>
<t>Logically distinct hosts such as are provided by many varieties of
virtualization logic result in a single physical host, connect to
a single port on the Ethernet switch in the topology, actually having
multiple internal virtual machines, each with IP and MAC
addresses, and what is essentially (or sometimes literally) an
internal LAN
switch. While it may be possible for this internal switch to help
control threats among the virtual hosts, or between virtual hosts
and other parts of the network, such enforcement cannot be counted
on at this time.</t>
</section>
<section anchor="multihome" title="Multi-LAN Hosts">
<t>Multi-interface hosts, in particular those that are multi-homed
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 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 on address assignment and topology
is required.</t>
<t>While the use of loopback addressing or sending packets out one
interface with the source address from another are rare, they do
legitimately occur. Some servers, particularly ones that have underlying
virtualization, use loopback techniques for management.</t>
</section>
<section anchor="firewall" title="Firewalls">
<t>Firewalls that forward packets from other network segments, or
serve as a source for locally originated packets, suffer from the
same issues as routers.</t>
</section>
<section anchor="mip" title="Mobile IP">
<t>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.</t>
<t>Mobile IP home agents 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 a Home Agent 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.</t>
</section>
<section anchor="other-topo" title="Other Topologies">
<t>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 packet which it originated may be
problematic. Additionally, address allocation schemas introduce
additional considerations when examining a given SAVI solutions
space.</t>
</section>
</section>
<!-- end of 5.2 -->
<section anchor="ipv6" title="IPv6 Considerations">
<t>IPv6 introduces additional capabilities which indirectly complicate
the spoofing analysis. IPv6 introduces and recommends the use of
<xref target="RFC4862">SLAAC</xref>. This
allows hosts to determine their IP prefix, select an IID, and then
start communicating. While there are many advantages to this, the
absence of control interactions complicates the process of behavioral
enforcement.</t>
<t>An additional complication is the very large IID space. Again, this
64 bit ID 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 ID space, an on-link attacker may be able to generate enough IP
addresses to fill the Neighbor Discovery table space of the other L3
devices on the link, including switches which are monitoring L3
behavior. This could seriously interfere with the ability for other
devices on the link to function.</t>
</section>
</section>
<!-- end of 5.0 -->
<section anchor="analysis"
title="Analysis of Host Granularity Anti-Spoofing">
<t>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.</t>
<t>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.</t>
<t>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
what the actual source of the incident was. If address usage can be
tied to the kinds of anchors described earlier, then it is possible to
respond to security incidents.</t>
<t>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 track even those
attacks back to their source.</t>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>This memo asks the IANA for no new parameters.</t>
<t>Note to RFC Editor: This section will have served its purpose if it
correctly tells IANA that no new assignments or registries are required,
or if those assignments or registries are created during the RFC
publication process. From the authors' perspective, it may therefore be
removed upon publication as an RFC at the RFC Editor's discretion.</t>
</section>
<section anchor="Security" title="Security Considerations">
<t>This document provides limited discussion of some security threats
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 verification application side.</t>
<t>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.</t>
<t>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 user 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).</t>
<t>Until 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 must not be used as an authentication mechanism. The
only technique to unquestionably verify source addresses of a received
datagram are cryptographic authentication mechanisms such as IPsec.</t>
</section>
<section anchor="Acknowledgments" title="Acknowledgments">
<t>A portion of the primer text in this document came directly from
<xref target="I-D.baker-sava-operational"></xref>, 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.</t>
</section>
</middle>
<back>
<!-- references split to informative and normative -->
<references title="Normative References">
<?rfc include="reference.RFC.0791"?>
<?rfc include="reference.RFC.2460"?>
</references>
<references title="Informative References">
<?rfc include="reference.RFC.0793"?>
<?rfc include="reference.RFC.0826"?>
<?rfc include="reference.RFC.2827"?>
<?rfc include="reference.RFC.3704"?>
<?rfc include="reference.RFC.4271"?>
<?rfc include="reference.RFC.4861"?>
<?rfc include="reference.RFC.4862"?>
<?rfc include="reference.I-D.baker-sava-operational"?>
</references>
</back>
</rfc>
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