SAVI D. McPherson
Internet-Draft VeriSign, Inc.
Intended status: Informational F. Baker
Expires: March 12, 2011 Cisco Systems
J. Halpern
Ericsson
September 8, 2010
SAVI Threat Scope
draft-ietf-savi-threat-scope-03
Abstract
This memo discusses threats enabled by IP source address spoofing and
discusses a number of techniques aimed at mitigating those threats.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on March 12, 2011.
Copyright Notice
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Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Glossary of Terms . . . . . . . . . . . . . . . . . . . . . . 5
3. Spoofed-based Attack Vectors . . . . . . . . . . . . . . . . . 6
3.1. Blind Attacks . . . . . . . . . . . . . . . . . . . . . . 6
3.1.1. Single Packet Attacks . . . . . . . . . . . . . . . . 6
3.1.2. Flood-Based DoS . . . . . . . . . . . . . . . . . . . 6
3.1.3. Poisoning Attacks . . . . . . . . . . . . . . . . . . 7
3.1.4. Spoof-based Worm/Malware Propagation . . . . . . . . . 7
3.1.5. Reflective Attacks . . . . . . . . . . . . . . . . . . 8
3.1.6. Accounting Subversion . . . . . . . . . . . . . . . . 8
3.1.7. Other Blind Spoofing Attacks . . . . . . . . . . . . . 8
3.2. Non-Blind Attacks . . . . . . . . . . . . . . . . . . . . 9
3.2.1. Man in the Middle (MITM) . . . . . . . . . . . . . . . 9
3.2.2. Third Party Recon . . . . . . . . . . . . . . . . . . 9
4. Current Anti-Spoofing Solutions . . . . . . . . . . . . . . . 9
4.1. Host to link layer neighbor via switch . . . . . . . . . . 11
4.2. Upstream routers . . . . . . . . . . . . . . . . . . . . . 11
4.3. ISP Edge PE Router . . . . . . . . . . . . . . . . . . . . 12
4.4. ISP NNI Router to ISP NNI Router . . . . . . . . . . . . . 12
4.5. Spoofing In Local Area Network Segments . . . . . . . . . 12
4.6. Cable Modem Subscriber Access . . . . . . . . . . . . . . 13
4.7. DSL Subscriber Access . . . . . . . . . . . . . . . . . . 13
4.8. BCP 38 . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.9. Unicast RPF . . . . . . . . . . . . . . . . . . . . . . . 13
4.10. Port-based Address Binding . . . . . . . . . . . . . . . . 13
4.10.1. Manual Binding . . . . . . . . . . . . . . . . . . . . 14
4.10.2. Automated Binding . . . . . . . . . . . . . . . . . . 14
4.10.3. IEEE 802.1X . . . . . . . . . . . . . . . . . . . . . 14
4.11. Cryptographic Techniques . . . . . . . . . . . . . . . . . 14
4.12. Residual Attacks . . . . . . . . . . . . . . . . . . . . . 15
5. Topological Considerations . . . . . . . . . . . . . . . . . . 15
5.1. Address Provisioning Mechanisms . . . . . . . . . . . . . 15
5.2. LAN devices with Multiple Addresses . . . . . . . . . . . 15
5.2.1. Routers . . . . . . . . . . . . . . . . . . . . . . . 15
5.2.2. NATs . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.2.3. Multi-Instance Hosts . . . . . . . . . . . . . . . . . 16
5.2.4. Multi-LAN Hosts . . . . . . . . . . . . . . . . . . . 16
5.2.5. Firewalls . . . . . . . . . . . . . . . . . . . . . . 16
5.2.6. Mobile IP . . . . . . . . . . . . . . . . . . . . . . 17
5.2.7. Other Topologies . . . . . . . . . . . . . . . . . . . 17
5.3. IPv6 Considerations . . . . . . . . . . . . . . . . . . . 17
6. Applicability of Anti-Spoofing Solutions . . . . . . . . . . . 18
6.1. Analysis of Host Granularity Anti-Spoofing . . . . . . . . 18
7. Existing Techniques for IP Source Address Validation . . . . . 19
8. Deployment Considerations . . . . . . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
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10. Security Considerations . . . . . . . . . . . . . . . . . . . 21
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
12.1. Normative References . . . . . . . . . . . . . . . . . . . 22
12.2. Informative References . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Overview
The Internet Protocol, specifically IPv4 [RFC0791] and IPv6
[RFC2460], 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.
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.
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.
Solutions such as 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
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. The local application of the principle of BCP 38 fortunately
has local benefit, even before BCP 38 is fully deployed. It also
helps get the Internet towards a state where BCP 38 is more widely
followed.
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.
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
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topologies in which they're introduced is critical when assessing the
threats that exist with source address spoofing.
The aim of this document is to provide some additional details
regarding spoofed-based threat vectors, and discuss implications of
various network topologies.
2. Glossary of Terms
The following acronyms and terms are used throughout this memo.
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 forging datagram header contents at the Link or
Network Layer
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. Spoofed-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 spoofed-based
attack vectors exist: blind attacks and non-blind attacks. The
following sections provide some information of blind and non-blind
attacks.
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 datapath between the attack source(s) and the target
systems. The result is that they have no access to legitimate 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 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.
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 forwarding a packet (e.g., TCP SYN) to a target system that
contains both a source and destination address of that target system.
The target system would "lock up" when creating connection state
associated with the packet, or would get stuck in a state where it
continuously replies to itself.
Another class of a single packet attacks involves an attacker
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.
3.1.2. Flood-Based DoS
Flooding-based DoS attack vectors are particularly effective attacks
on the Internet today. They usually entail flooding a large number
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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 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
discriminate if the sources are not directly connected to the local
routing system.
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.
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.
3.1.3. Poisoning Attacks
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 poinsonings as
above, with more versatility.
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.
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3.1.5. Reflective Attacks
DNS reflective amplification attacks are a particularly potent DoS
attack vector on the Internet today. Like other amplification
attacks, an attack source generates a packet with a source-spoofed
address mapping to that of the target system. The packet, upon
receipt by the victim or some intermediate node, typically elicits a
large reply message, which is directed to the target of the attack.
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.
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.
3.1.6. Accounting Subversion
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.
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. 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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3.2. Non-Blind Attacks
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.
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 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.
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 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 map to it's own MAC address. If the port to which the
attacker is connected were to implement policy that binds a single
Link Layer and IP address tuple to that port upon initial
provisioning, spoofed packets, at the Link Layer and/or Network
Layer, would be discarded on ingress.
3.2.2. Third Party Recon
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.
4. Current Anti-Spoofing Solutions
The first requirement is to eliminate datagrams with spoofed
addresses from the Internet. Identifying and dropping datagrams
whose source address is incompatible with the Internet topology at
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sites where the relationship between the source address and topology
can be checked can eliminate such datagrams. For example, 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 appropriate for the device at the layer 2
switch port (the address was assigned to a, and perhaps the,
system that uses that port)
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).
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.
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:
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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 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.
4.1. 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 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.
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.
4.2. 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 downstream
router from which the datagram was received.
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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 [RFC3704].
4.3. 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
verify this.
4.4. 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 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 and use that to advertise the level of
trust to its BGP peers.
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.
4.5. Spoofing In Local Area Network Segments
On Link Layer segments where multiple hosts reside, or a single MAC
address can be associated with a port or interface, if a binding
between a hardware address (e.g., MAC address) and corresponding IP
address(es) are not provisioned via some means (either manual or
dynamic mechanisms), then an attacker may exploit attack vectors that
enable MITM or other spoof-based attacks.
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4.6. Cable Modem Subscriber Access
Cable Modem Termination Systems (CMTS) employ DOCSIS Media Access
Control (MAC) domains, which are similar to Ethernet LAN
environments.
4.7. 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 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.
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.
4.8. 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.
4.9. 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].
4.10. Port-based Address Binding
Much of the work SAVI appears to be initially targeting is aimed at
minimizing source address spoofing in the LAN. In particular, if
mechanisms can be defined to accommodate configuration of port
binding information for IP and MAC layer addresses in LAN
environments, a large portion of the spoofing threat space in the LAN
can be marginalized.
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However, establishing this binding is not trivial, and varying across
both topologies type and address allocation mechanisms.
4.10.1. 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 to a single IP 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.
4.10.2. Automated Binding
For IPv4 the primary and very widely used automated binding technique
is DHCP based address assignment. Controlling where authoritative
information can be sourced, coupled with sniffing and enforcing the
assignments is an effective technique.
For IPv6, there are two common automated address binding 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, binding establishment needs to be tied to
the state machines for these protocols, and appropriate message
sniffing and enforcement. For DHCPv6 based techniques, it is also
necessary to use classification techniques to ensure that responses
come from authoritative sources.
4.10.3. IEEE 802.1X
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.
4.11. Cryptographic Techniques
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 ND, SEND is not widely
used to date. While DNSsec will significantly help protect DNS from
spoof based poisoning attacks, it will probably be sufficiently long
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for truly widespread use that other protections can be usefully
deployed as well.
4.12. 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 Considerations
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 a topology considerations perspective, when attempting
port-MAC-IP bindings, host connected to switch ports that may have
one or more IP addresses, or certainly, devices that forward packets
from other network segments, are problematic.
5.2.1. Routers
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.
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5.2.2. NATs
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.
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, which internally run multiple physical or logical hosts.
When the device is a blade server, with internal blades each hosting
a 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.
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 IP and MAC addresses, and essentially an internal
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.
5.2.4. Multi-LAN Hosts
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.
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.
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
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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 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.
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 packet which 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 which indirectly complicate
the spoofing analysis. IPv6 introduces and recommends the use of
stateless address autoconfiguration (often referred to as SLAAC).
This allows hosts to determine their IP prefix, select an ID, 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 ID 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. But it does have implications
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for switch cost; the switch needs to store more addresses and so
needs more memory.
6. Applicability of Anti-Spoofing Solutions
The above sections covered a number of security threats. Not all
these threats can be prevented by anti-spoofing techniques. However,
all of these threats can be ameliorated to some degree. We can look
at three categories of effect we can achieve:
Prevention: Some of the threats described above explicitly require
that a host send packets using some other active hosts IP address
as a source. Anti-Spoofing measures can prevent these attacks.
Impediment: Many of the attacks above, such as some kinds of DoS
attacks, can be conducted more easily if the attacking host can
use multiple different IP addresses. Depending upon the kind of
anti-spoofing available, the scope of such false addresses, or
even their use, may be prevented, hindering the attacker even if
the attack is not completely prevented.
Traceability: Even when attacks cannot be prevented, the ability to
reliably trace an attack allows appropriate responses, and thereby
also creates an environment which discourages attacks instead of
encouraging them. Thus, ensuring that even attacks which are not
dependent upon spoofing cannot use source address spoofing to hide
their origin is extremely important.
For example, sites which deploy BCP 38 cannot be the source of
attacks which rely on spoofing the source site from which an attack
was launched. Wide deployment of BCP 38 would also simplify the task
of tracking attacks back to their actual origin.
6.1. 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
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breaches are internal, this can be valuable. One example of this is
that such techniques should mitigate internal attacks on the site
routing system.
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.
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.
7. Existing Techniques for IP Source Address Validation
Existing techniques for IP source address validation are
insufficient. While each technique has its own shortcomings, the
following list of general categories of reasons include some of the
deficiencies of all existing technique:
False negatives: Techniques may yield false negatives, thus enabling
an attacker to select an IP source address that is spoofed, but
still passes IP source address validation.
False positives: Techniques may yield false positives, thereby
causing interruption or denial of service to hosts that use
legitimate IP source addresses.
Non-trivial configuration: Requirements for non-trivial
configuration imply expenditures and pose a risk for
misconfiguration, which may again lead to false positives or false
negatives. Both may discourage operators from employing a given
technique.
Proprietary: Procurement policies of organizations oftentimes
require that devices purchased use techniques that are
standardized rather than proprietary. Such policies, for good or
ill, hinder or prevent the deployment of proprietary techniques.
The only standardized technique for IP source address validation is
ingress filtering [RFC2827]. This calls for routers to check whether
the prefix of a to-be-forwarded packet's IP source address is amongst
a list of prefixes considered legitimate for the interface through
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which the packet arrives. Packets that fail this check are
discarded.
Ingress filtering may yield false negatives in a deterministic
manner. Packets with a legitimate IP source address prefix, but a
spoofed interface identifier, pass ingress filtering checks. Also,
packets with an illegitimate IP source address prefix pass the checks
as long as the prefix is from the list of prefixes considered
legitimate, if more than one prefix is considered as legitimate on
the ingress interface..
Ingress filtering implementations that require manual establishment
of the list of legitimate prefixes cause considerable configuration
overhead. Unicast Reverse Path Forwarding (uRPF) mitigates this
issue by automatically deriving the list of legitimate prefixes from
a router's forwarding table: A to-be-forwarded packet's IP source
address prefix is considered legitimate if the packet is coming
through an interface via which return traffic would be routed. On
the other hand, Unicast Reverse Path Forwarding may yield false
positives, in particular for hosts and networks with multiple,
topologically separate Internet attachments [RFC3704].
Consequently, there is a need for an IP source address validation
technique that avoids false negatives and false positives, that can
be set up with no or only trivial configuration, and that has been
standardized. The development of such a technique is the goal of the
proposed Source Address Validation Improvements (SAVI) working group
in the Internet Engineering Task Force.
8. Deployment Considerations
From a global Internet perspective, deployment of anti- spoofing
techniques tends to suffer from a "tragedy of the commons" situation.
That is, there is a general consensus that everyone should implement
anti-spoofing measures, yet individual organizations don't want to
bear the cost of deployment themselves, often because demonstrating
direct tangible return on investment is not possible. Upon analysis,
it often seems apparent that local implementation of anti-spoofing
measures is of more benefit to the "greater Internet" than the local
network domain itself. A similar situation occurs with de-
aggregation of Internet routing information for multi-homing and
traffic engineering purposes, as well as the lack of explicit inter-
domain routing prefix filters on the Internet.
Until network operators begin to accept that their local design
choices have global implications, and act upon this responsibility,
the problem is not going to go away.
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Ideally, with additional work in the areas of SAVI to ease deployment
and management burdens, the deployment cost to operators will
decrease and more wide-scale deployment will continue. Furthermore,
application of SAVI-like techniques provides more obvious benefits to
network administrators that are concerned with MITM and similar
attacks.
As mentioned earlier, there are local security benefits to the
deployment of SAVI security mechanisms. This may help motivate the
deployment of tools with widespread benefit.
9. IANA Considerations
This memo asks the IANA for no new parameters.
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.
10. Security Considerations
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.
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 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).
Until every Internet-connected network implements source address
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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.
11. Acknowledgements
A portion of the primer text in this document came directly from
[I-D.baker-sava-operational], authored by Fred Baker and Ralph Droms.
Many thanks to Christian Vogt and Suresh Bhogavilli for contributing
text and a careful review of this document.
12. References
12.1. 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.
12.2. Informative References
[I-D.baker-sava-operational]
Baker, F. and R. Droms, "IPv4/IPv6 Source Address
Verification", draft-baker-sava-operational-00 (work in
progress), June 2007.
[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.
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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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