Network Working Group M. Handley (ed)
Internet-Draft UCL
Expires: March 19, 2006 E. Rescorla (ed)
Network Resonance
September 15, 2005
Internet Denial of Service Considerations
draft-iab-dos-03.txt
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Abstract
This document provides an overview of possible avenues for denial-of-
service attack on Internet systems. The aim is to encourage protocol
designers and network engineers towards designs that are more robust.
We discuss partial solutions that reduce the effectiveness of
attacks, and how some solutions might inadvertently open up
alternative vulnerabilities.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. An Overview of Denial-of-Service Threats . . . . . . . . . . . 6
2.1 DoS Attacks on End-systems . . . . . . . . . . . . . . . . 6
2.1.1 Exploiting Poor Software Quality . . . . . . . . . . . 6
2.1.2 Application Resource Exhaustion . . . . . . . . . . . 7
2.1.3 Operating System Resource Exhaustion . . . . . . . . . 8
2.1.4 Triggered Lockouts and Quota Exhaustion . . . . . . . 9
2.2 DoS Attacks on Routers . . . . . . . . . . . . . . . . . . 9
2.2.1 Attacks on Routers through Routing Protocols . . . . . 10
2.2.2 IP Multicast-based DoS Attacks . . . . . . . . . . . . 11
2.2.3 Attacks on Router Forwarding Engines . . . . . . . . . 12
2.3 Attacks on Ongoing Communications . . . . . . . . . . . . 13
2.4 Attacks using the Victim's Own Resources . . . . . . . . . 14
2.5 DoS Attacks on Local Hosts or Infrastructure . . . . . . . 14
2.6 DoS Attacks on Sites though DNS . . . . . . . . . . . . . 16
2.7 DoS Attacks on Links . . . . . . . . . . . . . . . . . . . 17
2.8 DoS attacks on firewalls . . . . . . . . . . . . . . . . . 18
2.9 DoS attacks on IDS systems . . . . . . . . . . . . . . . . 19
2.10 DoS attacks on or via NTP . . . . . . . . . . . . . . . . 19
2.11 Physical DoS . . . . . . . . . . . . . . . . . . . . . . . 20
2.12 Social Engineering DoS . . . . . . . . . . . . . . . . . . 20
2.13 Legal DoS . . . . . . . . . . . . . . . . . . . . . . . . 20
2.14 Spam and Black-hole Lists . . . . . . . . . . . . . . . . 20
3. Attack Amplifiers . . . . . . . . . . . . . . . . . . . . . . 22
3.1 Methods of Attack Amplification . . . . . . . . . . . . . 22
3.2 Strategies to Mitigate Attack Amplification . . . . . . . 24
4. DoS Mitigation Strategies . . . . . . . . . . . . . . . . . . 25
4.1 Protocol Design . . . . . . . . . . . . . . . . . . . . . 25
4.1.1 Don't Hold State for Unverified Hosts . . . . . . . . 25
4.1.2 Make it Hard to Simulate a Legitimate User . . . . . . 25
4.1.3 Graceful Routing Degradation . . . . . . . . . . . . . 26
4.1.4 Autoconfiguration and Authentication . . . . . . . . . 27
4.2 Network Design and Configuration . . . . . . . . . . . . . 27
4.2.1 Redundancy and Distributed Service . . . . . . . . . . 28
4.2.2 Authenticate Routing Adjacencies . . . . . . . . . . . 28
4.2.3 Isolate Router-to-Router Traffic . . . . . . . . . . . 28
4.3 Router Implementation Issues . . . . . . . . . . . . . . . 28
4.3.1 Checking Protocol Syntax and Semantics . . . . . . . . 29
4.3.2 Consistency Checks . . . . . . . . . . . . . . . . . . 29
4.3.3 Enhance Router Robustness through Operational
Adjustments . . . . . . . . . . . . . . . . . . . . . 30
4.3.4 Proper Handling of Router Resource Exhaustion . . . . 31
4.4 End-System Implementation Issues . . . . . . . . . . . . . 31
4.4.1 State Lookup Complexity . . . . . . . . . . . . . . . 31
4.4.2 Operational Issues . . . . . . . . . . . . . . . . . . 32
5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 33
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6. Security Considerations . . . . . . . . . . . . . . . . . . . 34
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 39
A. IAB Members at the time of this writing . . . . . . . . . . . 40
Intellectual Property and Copyright Statements . . . . . . . . 41
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1. Introduction
A Denial-of-Service (DoS) attack is an attack in which one or more
machines target a victim and attempt to prevent the victim from doing
useful work. The victim can be a network server, client or router, a
network link or an entire network, an individual Internet user or a
company doing business using the Internet, an Internet Service
Provider (ISP), country, or any combination of or variant on these.
Denial of service attacks may involve gaining unauthorized access to
network or computing resources, but for the most part in this
document we focus on the cases where the denial-of-service attack
itself does not involve a compromise of the victim's computing
facilities.
Because of the closed context of the original ARPAnet and NSFnet, no
consideration was given to denial-of-service attacks in the original
Internet Architecture. As a result, almost all Internet services are
vulnerable to denial-of-service attacks of sufficient scale. In most
cases, sufficient scale can be achieved by compromising enough end-
hosts (typically using a virus, worm, or remotely controlled "bots")
or routers, and using those compromised hosts to perpetrate the
attack. Such an attack is known as a Distributed Denial of Service
attack (DDoS). However, there are also many cases where a single
well- connected end-system can perpetrate a successful DoS attack.
This document is intended to serve several purposes:
o To highlight possible avenues for attack, and by so doing
encourage protocol designers and network engineers towards designs
that are more robust.
o To discuss partial solutions that reduce the effectiveness of
attacks.
o To highlight how some partial solutions can be taken advantage of
by attackers to perpetrate alternative attacks.
This last point appears to be a recurrent theme in DoS, and
highlights the lack of proper architectural solutions. It is our
hope that this document will help initiate informed debate about
future architectural solutions that might be feasible and cost-
effective for deployment.
In addition it is our hope that this document will spur discussion
leading to architectural solutions that reduce the succeptibility of
all Internet systems to denial-of-service attacks.
We note that in principle it is not possible to distinguish between a
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sufficiently subtle DoS attack and a flash-crowd, where unexpected
heavy but non-malicious traffic has the same effect as a DoS attack.
Whilst this is true, such malicious attacks are usually more
expensive to launch than many of the crude attacks that have been
seen to date. Thus defending against DoS is not about preventing all
possible attacks, but rather is largely a question of raising the bar
sufficiently high for malicious traffic.
However, it is also important to note that not all DoS problems are
malicious. Failed links, flash crowds, misconfigured bots, and
numerous other causes can result in resource exhaustion problems, and
so the overall goal should be to be robust to all forms of overload.
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2. An Overview of Denial-of-Service Threats
In this section we will discuss a wide range of possible DoS attacks.
This list cannot be exhaustive, but the intent is to provide a good
overview of the spectrum of possibilities that need to be defended
against.
We do not provide descriptions of any attacks that are not already
publicly well documented.
2.1 DoS Attacks on End-systems
We first discuss attacks on end-systems. An end-system in this
context is typically a PC or network server, but it can also include
any communication endpoint. For example, a router also is an end-
system from the point of view of terminating TCP connections for BGP
[32] or ssh.
2.1.1 Exploiting Poor Software Quality
The simplest DoS attacks on end-systems exploit poor software quality
on the end-systems themselves, and cause that software to simply
crash. For example, buffer-overflow attacks might be used to
compromise the end-system, but even if the buffer-overflow cannot be
used to gain access, it will usually be possible to overwrite memory
and cause the software to crash. Such vulnerabilities can in
principle affect any software that uses data supplied from the
network. Thus not only might a web server be potentially vulnerable,
but it might also be possible to crash the back-end software (such as
a database) to which a web server provides data.
Software crashes due to poor coding not only affect application
software, but also the operating system kernel itself. A classic
example is the so-called "Ping of Death", which became widely known
in 1996 [10]. This exploit caused many popular operating systems to
crash when sent a single fragmented ICMP echo request packet whose
fragments totaled more than the 65535 bytes allowed in an IPv4
packet.
While DoS attacks such as the ping-of-death are a significant
problem, they are not a significant architectural problem. Once such
an attack is discovered, the relevant code can easily be patched, and
the problem goes away. We should note though that as more and more
software becomes embedded, it is important not to lose the
possibility of upgrading the software in such systems.
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2.1.2 Application Resource Exhaustion
Network applications exist in a context that has finite resources.
In processing network traffic, such an application uses these
resources to do its intended task. However, an attacker may be able
to prevent the application from performing its intended task by
causing the application to exhaust the finite supply of a specific
resource.
The obvious resources that might be exhausted include:
o Available memory.
o The CPU cycles available.
o The disk space available to the application.
o The number of processes or threads or both that the application
is permitted to use.
o The configured maximum number of simultaneous connections the
application is permitted.
This list is clearly not exhaustive, but it illustrates a number of
points.
Some resources are self-renewing: CPU cycles fall in this category -
if the attack ceases, more CPU cycles become available.
Some resources such as disk space require an explicit action to free
up - if the application cannot do this automatically then the effects
of the attack may be persistent after the attack has ceased.
This problem has been understood for many years, and it is common
practice for logs and incoming email to be stored in a separate disk
partition (/var) on Unix systems.
Some resources are constrained by configuration: the maximum number
of processes and the maximum number of simultaneous connections are
not normally hard limits, but rather are configured limits. The
purpose of such limits is clearly to allow the machine to perform
other tasks in the event the application misbehaves. However, great
care needs to be taken to choose such limits appropriately. For
example, if a machine's sole task is to be an ftp server, then
setting the maximum number of simultaneous connections to be
significantly less than the machine can service makes the attackers
job easier. But setting the limit too high may permit the attacker
to cause the machine to crash (due to poor OS design in handling
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resource exhaustion) or permit livelock (see below), which are
generally even less desirable failure modes.
2.1.3 Operating System Resource Exhaustion
Conceptually OS resource exhaustion and application resource
exhaustion are very similar. However, in the case of application
resource exhaustion, the operating system may be able to protect
other tasks from being affected by the DoS attack. In the case of
the operating system itself running out of resources, the problem may
be more catastrophic.
Perhaps the best-known DoS attack on an operating system is the TCP
SYN- flood [8], which is essentially a memory-exhaustion attack. The
attacker sends a flood of TCP SYN packets to the victim, requesting
connection setup, but then does not complete the connection setup.
The victim instantiates state to handle the incoming connections. If
the attacker can instantiate state faster than the victim times it
out, then the victim will run out of memory that it can use to hold
TCP state, and so it cannot service legitimate TCP connection setup
attempts. This issue was exacerbated in some implementations by the
use of a small dedicated storage space for half-open connections,
which made the attack easier than it might otherwise have been. In
the case of a poorly coded operating system, running out of resources
may also cause a system crash.
An alternative TCP DoS attack is the Ack-flood [12], which is
essentially a CPU exhaustion attack on the victim. The attacker
floods the victim with TCP packets pretending to be from connections
that have never been established. A busy server that has a large
number of outstanding connections needs to check which connection the
packet corresponds to. Some TCP implementations implemented this
search rather inefficiently, and so the attacker could use all the
victim's CPU resources servicing these packets rather than servicing
legitimate requests.
We note that strong authentication mechanisms do not mitigate against
such CPU exhaustion attacks. In fact poorly designed authentication
mechanisms using cryptographic methods can exacerbate the problem.
If such an authentication mechanism allows an attacker to present a
packet to the victim that requires relatively expensive cryptographic
authentication before the packet can be discarded, then this makes
the attacker's CPU exhaustion attack easier.
CPU exhaustion attacks can be also be exacerbated by poor OS handling
of incoming network traffic. In the absence of malicious traffic, an
ideal OS should behave as follows:
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o As incoming traffic increases, the useful work done by the OS
should increase until some resource (such as the CPU) is saturated.
o From this point on, as incoming traffic continues to increase the
useful work done should be constant.
However, this is often not the case. Many systems suffer from
livelock [29] where, after saturation, increasing the load causes a
decrease in the useful work done. One cause of this is that the
system spends an increasing amount of time processing network
interrupts for packets that will never be processed, and hence a
decreasing amount of time is available for the application for which
these packets were intended.
2.1.4 Triggered Lockouts and Quota Exhaustion
Many user-authentication mechanisms attempt to protect against
password guessing attacks by locking the user out after a small
number of failed authentications. If an attacker can guess or
discover a user's ID, they may be able to trigger such a mechanism,
locking out the legitimate user.
Another way to deny service using protection mechanisms is to cause a
quota to be exhausted. This is perhaps most common in the case of
small web servers being commercially hosted, where the server has a
contract with the hosting company allowing a fixed amount of traffic
per day. An attacker may be able to rapidly exhaust this quota, and
cause service to be suspended. Similar attacks may be possible
against other forms of quota.
In the absence of such quotas, if the victim is charged for their
network traffic, a financial denial-of-service may be possible.
2.2 DoS Attacks on Routers
Many of the denial-of-service attacks that can be launched against
end- systems can also be launched against the control processor of an
IP router, for example by flooding the command and control access
ports. In the case of a router, these attacks may cause the router
to stall, or may cause the router to cease processing routing
packets. Even if the router does not stop servicing routing packets,
it may become sufficiently slow that routing protocols time out. In
any of these circumstances, the consequence of routing failure is not
only that the router ceases to forward traffic, but also that it
causes routing protocol churn that may have further side effects.
An example of such a side effect is caused by BGP route flap damping
[35], which is intended to reduce global routing churn. If an
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attacker can cause BGP routing churn, route flap damping may then
cause the flapping routes to be suppressed [27]. This suppression
likely causes the networks served by those routes to become
unreachable.
A DoS attack on the router control processor might also prevent the
router being managed effectively. This may prevent actions being
taken that would mitigate the DoS attack, and it might prevent
diagnosis of the cause of the problem.
2.2.1 Attacks on Routers through Routing Protocols
In addition to their roles as end-systems, most routers run dynamic
routing protocols. The routing protocols themselves can be used to
stage a DoS attack on a router or a network of routers. This
requires the ability to send traffic from addresses which might
plausibly have generated the relevant routing messages, which is
somewhat difficult with interior routing protocols but fairly easy
with e.g., eBGP.
The simplest attack on a network of routers is to overload the
routing table with sufficiently many routes that the router runs out
of memory, or the router has insufficient CPU power to process the
routes [15]. We note that depending on the distribution and
capacities of various routers around the network, such an attack
might not overwhelm routers near to the attacking router, but might
cause problems to show up elsewhere in the network.
Some routing protocol implementations allow limits to be configured
on the maximum number of routes to be heard from a neighbor [17].
However, limits often make the problem worse rather than better, by
making it possible for the attacker to push out legitimate routes
with spoofed routes, thus creating an an easy form of DoS attack.
An alternative attack is to overload the routers on the network by
creating sufficient routing table churn that routers are unable to
process the changes. Many routing protocols allow damping factors to
be configured to avoid just such a problem. However, as with table
size, such a threshold applied inconsistently may allow the spoofed
routes to merge with legitimate routes before the mechanism is
applied, causing legitimate routes to be damped.
The simplest routing attack on a specific destination is for an
attacker to announce a spoofed desirable route to that destination.
Such a route might be desirable because it has low metric, or because
it is a more specific route than the legitimate route. In any event,
if the route is believed it will cause traffic for the victim to be
drawn towards the attacking router, where it will typically be
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discarded.
A more subtle denial-of-service attack might be launched against a
network rather than against a destination. Under some circumstances,
the propagation of inconsistent routing information can cause traffic
to loop. If an attacker can cause this to happen on a busy path, the
looping traffic might cause significant congestion, as well as not
reaching the legitimate destination.
In the past there have been cases where different generations of
routers interpreted a routing protocol specification differently. In
particular, BGP specifies that in the case of an error, the BGP
peering should be dropped. However, if some of the routers in a
network treat a particular route as valid and other routes treat the
route as invalid, then it may be possible to inject a BGP route at
one point in the Internet and cause peerings to be dropped at many
other places in the Internet. Unlike many of the examples above,
while such an issue might be a serious short-term problem, this is
not a fundamental architectural problem. Once the problem is
understood, deploying patched routing code can permanently solve the
issue.
2.2.2 IP Multicast-based DoS Attacks
There are essentially two forms of IP multicast: "traditional" IP
multicast (ASM), as specified in RFC 1112 [19] where multiple sources
can send to the same multicast group, and source-specific multicast
(SSM) where the receiver must specify both the IP source address and
the group address. The two forms of multicast provide rather
different DoS possibilities.
With ASM, an attacker can simply send to multiple multicast groups.
Routing protocols such as PIM-SM [21], MSDP [28] and DVMRP [38] then
have to instantiate routing state to ensure that the traffic goes to
the group receivers and not to non-receivers. Thus ASM is
particularly vulnerable to DoS attacks causing both multicast routing
table explosion (and hence control processor memory exhaustion) and
multicast forwarding table exhaustion (and hence forwarding card
memory exhaustion or thrashing).
ASM also permits an attacker to send traffic to the same group as
legitimate traffic, potentially causing network congestion and
denying service to the legitimate group.
SSM does not permit senders to send to arbitrary groups unless a
receiver has requested the traffic. Thus sender-based attacks on
multicast routing state are not possible with SSM. However, as with
ASM, a receiver can still join a large number of multicast groups
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causing routers to hold a large amount of multicast routing state,
potentially causing memory exhaustion and hence denial-of-service to
legitimate traffic.
With IPv6, hosts are required to send ICMP Packet Too Big or
Parameter Problem messages under certain circumstances, even if the
destination address is a multicast address. If the attacker can
place himself in the appropriate position in the multicast tree, a
packet with an unknown but mandatory Destination Option, for
instance, could generate a very large number of responses to the
claimed sender.
With IPv4 the same problem exists with multicast ICMP Echo Request
packets, but these are somewhat easier to filter.
2.2.3 Attacks on Router Forwarding Engines
Router vendors implement many different mechanisms for packet
forwarding, but broadly speaking they fall into two categories: ones
that use a forwarding cache, and ones that do not. With a forwarding
cache, the forwarding engine does not hold the full routing table,
but rather holds just the currently active subset of the forwarding
table.
Many modern routers use a loosely coupled architecture, where one or
more control processors handle the routing protocols, and communicate
over an internal network link to special-purpose forwarding engines,
which actually forward the data traffic. In such architectures it
may be possible for an attacker to overwhelm the communications link
between the control processor and the forwarding engine. This is
possible because the forwarding engines support very high speed
links, and the control processor simply cannot handle a similar rate
of traffic.
There may be many ways in which an attacker can trigger communication
between the forwarding engines and the control processor. The
simplest way is for the attacker to simply send to the router's IP
address, but this should in principle be relatively easy to prevent
using filtering on the forwarding engines. Another way might be to
cause the router to forward data packets using the "slow path". This
involves sending packets that require special attention from the
forwarding router; if the forwarding engine is not smart enough to
perform such forwarding, then it will typically pass the packet to
the control processor. In a router using a forwarding cache, it may
be possible to overload the internal communications by thrashing the
forwarding cache. Finally, any form of data-triggered communication
between the forwarding engine and the control processor might cause
such a problem. Certain multicast routing protocols including PIM-SM
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contain many such data triggered events that could potentially be
problematic.
The effects of overloading such internal communications are hard to
predict, and very implementation-dependent. One possible effect
might be that the forwarding table in the forwarding engine gets out
of synchronization with the routing table in the control processor
that reflects what the routing protocols believe is happening. This
might cause traffic to be dropped or to loop.
Finally, if an attacker can generate traffic that causes a router to
auto-install access control list (ACL) entries, perhaps by triggering
a response from an intrusion detection system, then it may be
possible to exhaust the ACL resources on the router. This might
prevent future attacks from being filtered, or worse, cause ACL
processing to be handled by the route processor.
2.3 Attacks on Ongoing Communications
Instead of attacking the end-system itself, it is also possible for
an attacker to disrupt ongoing communications. If an attacker can
observe a TCP connection, then it is relatively easy for them to
spoof packets to either reset that connection or to de-synchronize it
so that no further progress can be made [25]. Such attacks are not
prevented by transport or application-level security mechanisms such
as TLS [20] or ssh, because the authentication takes place after TCP
has finished processing the packets.
If an attacker cannot observe a TCP connection, but can infer that
such a connection exists, it is theoretically possible to reset or
desynchronize that connection by spoofing packets into the
connection. However, this might require an excessively large number
of spoofed packets to guess both the port of the active end of the
TCP connection (in most cases the port of the passive end is
predictable) and the currently valid TCP sequence numbers. However,
as some operating systems have poorly implemented predictable
algorithms for selecting either the dynamically selected port or the
TCP initial sequence number [40] [9], then such attacks have been
found to be feasible [30]. Advice as to how to reduce the
vulnerability in the specific case of TCP is available in [34].
An attacker might be able to significantly reduce the throughput of a
connection by sending spoofed ICMP source quench packets, although
most modern operating systems should ignore such packets. However,
care should be taken in the design of future transport and signaling
protocols to avoid the introduction of similar mechanisms that could
be exploited.
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2.4 Attacks using the Victim's Own Resources
Instead of directly overloading the victim, it may be possible to
cause the victim or a machine on the same subnet as the victim to
overload itself.
An example of such an attack is documented in [7], where the attacker
spoofs the source address on a packet sent to the victim's UDP echo
port. The source address is that of another machine that is running
a UDP chargen server (a chargen server sends a character pattern back
to the originating source). The result is that the two machines
bounce packets back and forth as fast as they can, overloading either
the network between them or one of the end-systems itself.
2.5 DoS Attacks on Local Hosts or Infrastructure
There are a number of attacks that might only be performed by a local
attacker.
An attacker with access to a subnet may be able to prevent other
local hosts from accessing the network at all by simply exhausting
the address pool allocated by a DHCP server. This requires being
able to spoof the MAC address of an ethernet or wireless card, but
this is quite feasible with certain hardware and operating systems.
An alternative DHCP-based attack is simply to respond faster than the
legitimate DHCP server, and to give out an address that is not useful
to the victim.
These sorts of bootstrapping attacks tend to be difficult to avoid
because most of the time trust relationships are established after IP
communication has already been established.
Similar attacks are possible through ARP spoofing [4]; an attacker
can respond to ARP requests before the victim and prevent traffic
from reaching the victim. Some brands of ethernet switch allow an
even simpler attack - simply send from the victim's MAC address, and
the switch will redirect traffic destined for the victim to the
attacker's port.
It may be possible to cause broadcast storms [4] on a local LAN by
sending a stream of unicast IP packets to the broadcast MAC address -
some hosts on the LAN may then attempt to forward the packets to the
correct MAC address greatly amplifying the traffic on the LAN.
802.11 wireless networks provide many opportunities to deny service
to other users. In some cases, the lack of defenses against DoS was
a deliberate choice--because 802.11 operates on unlicensed spectrum
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it was assumed that there would be sources of interference and that
producing intentional radio-level jamming would be trivial. Thus,
the amount of DoS protection possible at higher levels was minimal.
Nevertheless, some of the weaknesses of the protocols against more
sophisticated attacks are worth noting. The most prominent of these
is that association is unprotected, thus allowing rogue APs to
solicit notifications that would otherwise have gone to legitimate
APs.
The SSID field provides effectively no defense against this kind of
attacks. Unless encryption is enabled, it is trivial to announce the
presence of a base station (or even of an ad-hoc mode host) with the
same network name (SSID) as the legitimate basestation. Even adding
authentication and encryption a la 802.1X and 802.11i may not help
much in this respect. The SSID space is unmanaged, so everyone's
free to put anything they want in the SSID field. Most host stacks
don't deal gracefully with this. Moreover, SSIDs are very often set
to the manufacturer's default, making them highly predictable.
Some 802.11 basestations have limited memory for the number of
associations they can support. If this is exceeded, they may drop
all associations. In an attempt to forestall this problem, some APs
advertise their load so as to enable stations to choose APs that are
less loaded. However, crude implementations of these algorithms can
result in instability.
Finally, as the authentication in 802.11 takes place at a
comparatively high level in the stack, it is possible to simply
deauthenticate or disassociate the victim from the basestation, even
if WEP is in use [26]. Bellardo and Savage [3] describe some simple
remedies that reduce the effectiveness of such attacks. While IEEE
802.11w will protect Deauthenticate or Disassociate frames, this
attack is still possible via forging of Association frames.
What all these attacks have in common is that they exploit
vulnerabilities in the link auto-configuration mechanisms. In a
wireless network it is necessary for a station to detect the presence
of APs in order to choose which one to connect to. In 802.11 this is
handled via the Beacon and Probe Request/Response mechanisms.
The Beacon cannot easily be encrypted, because the station needs to
utilize them prior to authentication in order to discover which APs
it may wish to communicate with. Since authentication can only occur
after interpreting the Beacon, an encrypted Beacon would present a
chicken-egg problem: you can't obtain a key to decrypt the Beacon
until completing authentication, and you may not be able to figure
out which AP to authenticate with prior to decrypting the Beacon.
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As a result, discussions of Beacon frame security have largely
focused on authentication of Beacon frames, not encryption. Even
here, solutions are difficult. While it may be possible to for a
station to validate a Beacon *after* authentication (either by
checking a MIC computed with the group key provided by the AP, or
verifying the Beacon parameters during the 4-way handshake), doing so
*before* authentication may require synchronization of keys between
APs within an SSID.
2.6 DoS Attacks on Sites though DNS
In today's Internet, DNS is of sufficient importance that if access
to a site's DNS servers is denied, the site is effectively
unreachable, even if there is no actual communication problem with
the site itself.
Many of the attacks on end-systems described above can be perpetrated
on DNS servers. As servers go, DNS servers are not particularly
vulnerable to DoS. So long as a DNS server has sufficient memory, a
modern host can usually respond very rapidly to DNS requests for
which it is authoritative. This was demonstrated in October 2002
when the root nameservers were subjected to a very large DoS attack
[36]. A number of the root nameservers have since been replicated
using anycast [1] to further improve their resistance to DoS.
However it is important for authoritative servers to have relaying
disabled, or it is possible for an attacker to force the DNS servers
to hold state [39].
Many of the routing attacks can also be used against DNS servers by
targeting the routing for the server. If the DNS server is co-
located with the site for which is authoritative, then the fact that
the DNS server is also unavailable is of secondary importance.
However, if all the DNS servers are made unavailable, this may cause
email to that site to bounce rather than being stored while the mail
servers are unreachable, so distribution of DNS server locations is
important.
Causing network congestion on links to and from a DNS server can have
similar effects to end-system attacks or routing attacks, causing DNS
to fail to obtain an answer, and effectively denying access to the
site being served.
We note that if an attacker can deny external access to all the DNS
for a site, this will not only cause email to that site to be
dropped, but will also cause email from that site to be dropped.
This is because recent versions of mail transfer agents such as
sendmail will drop email if the mail originates from a domain that
does not exist. This is a classic example of unexpected
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consequences. Sendmail performs this check as an anti-spam measure,
and spam itself can be viewed as a form of DoS attack. Thus
defending against one DoS attack opens up the vulnerability that
allows another DoS attack. If a receiving implmentation is using
DNS-based blackholing, an attacker can also mount a DoS attack by
attacking the blackhole server.
Finally, a data corruption attack is possible if a site's nameserver
is permitted to relay requests from untrusted third parties [39].
The attacker issues a query for the data he wishes to corrupt, and
the victim's nameserver relays the request to the authoritative
nameserver. The request contains a 16-bit ID that is used to match
up the response with the request. If the attacker spoofs sufficient
response packets from the authoritative nameserver just before the
official response will arrive, each containing a forged response and
a different DNS ID, then there is a reasonable chance that one of the
forged responses will have the correct DNS ID. The incorrect data
will then be believed and cached by the victim's nameserver, so
giving the incorrect response to future queries. The probability of
the attack can further be increased if the attacker issues many
different requests for the same data with different DNS IDs, because
many nameserver implementations will issue relayed requests with
different DNS IDs, and so the response only has to match any one of
these request IDs [6] [33].
The use of anycast for DNS services makes it even more vulnerable to
spoofing attacks. An attacker who can convince the ISP to accept an
anycast route to his fake DNS server can arrange to receive requests
and generate fake responses. Anycast DNS also makes DoS attacks on
DNS easiet. The idea is to disable one of the DNS servers while
maintaining the BGP route to that server. This creates failures for
any client which is routed to the (now defunct) server.
2.7 DoS Attacks on Links
The simplest DoS attack is to simply send enough non-congestion-
controlled traffic that a link becomes excessively congested, and
legitimate traffic suffers unacceptably high packet loss.
Under some circumstances the effect of such a link DoS can be much
more extensive. We have already discussed the effects of denying
access to a DNS server. Congesting a link might also cause a routing
protocol to drop an adjacency if sufficient routing packets are lost,
potentially greatly amplifying the effects of the attack. Good
router implementations will prioritize the transmission of routing
packets, but this is not a total panacea. If routers are peered
across a shared medium such as ethernet, it may be possible to
congest the medium sufficiently that routing packets are still lost.
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Even if a link DoS does not cause routing packets to be lost, it may
prevent remote access to a router using ssh or SNMP. This might make
the router unmanageable, or prevent the attack being correctly
diagnosed.
The prioritization of routing packets can itself cause a DoS problem.
If the attacker can cause a large amount of routing flux, it may be
possible for a router to send routing packets at a high enough rate
that normal traffic is effectively excluded. This is however
unlikely except on low bandwidth links.
Finally, it may be possible to an attacker to deny access to a link
by causing the router to generate sufficient monitoring or report
traffic that the link is filled. SNMP traps are one possible vector
for such an attack, as they are not normally congestion controlled.
Attackers with physical access to multiple access links can easily
bring down the link. This is particularly easy to mount and
difficult to counter with wireless networks.
2.8 DoS attacks on firewalls
Firewalls are intended to defend the systems behind them against
attack. In that they restrict the traffic that can reach those
systems, they may also aid in defending against denial-of-service
attacks. However, under some circumstances the firewall itself may
also be used as a weapon in a DoS attack.
There are many different types of firewall, but generally speaking
they fall into stateful and stateless classes. The state here refers
to whether the firewall holds state for the active flows traversing
the firewall. Stateless firewalls generally can only be attacked by
attempting to exhaust the processing resources of the firewall.
Stateful firewalls can be attacked by sending traffic that causes the
firewall to hold excessive state or state which has pathological
structure.
In the case of excessive state, the firewall simply runs out of
memory, and can no longer instantiate the state required to pass
legitimate flows. Most firewalls will then fail disconnected,
causing denial-of-service to the systems behind the firewall.
In the case of pathological structure, the attacker sends traffic
that causes the firewall's data structures to exhibit worst case
behaviour. An example of this would be when the firewall uses hash
tables to look up forwarding state, and the attacker can predict the
hash function used. The attacker may then be able to cause a large
amount of flow state to hash to the same bucket, which causes the
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firewall's lookup performance to change from O(1) to O(n), where n is
the number of flows the attacker can instantiate [18]. Thus the
attacker can cause forwarding performance to degrade to the point
where service is effectively denied to the legitimate traffic
traversing the firewall.
2.9 DoS attacks on IDS systems
Intrusion detection systems (IDS) suffer from similar problems to
firewalls. It may be possible for an attacker to cause the IDS to
exhaust its available processing power, to run out of memory, or to
instantiate state with pathological structure. Unlike a firewall, an
IDS will normally fail open, which will not deny service to the
systems protected by the IDS. However it may mean that subsequent
attacks that the IDS would have detected will be missed.
Some IDSs are reactive; that is on detection of a hostile event they
react to block subsequent traffic from the hostile system, or to
terminate an ongoing connection from that system. It may be possible
for an attacker to spoof packets from a legitimate system, and hence
cause the IDS to believe that system is hostile. The IDS will then
cause traffic from the legitimate system to be blocked, hence denying
service to it. The effect can be particularly bad if the legitimate
system is a router, DNS server, or other system whose performance is
essential for the operation of a large number of other systems.
2.10 DoS attacks on or via NTP
Network time servers are generally not considered security-critical
services, but under some circumstances NTP servers might be used to
perpetrate a DoS attack.
The most obvious such attack is to DoS the NTP servers themselves.
Many end systems have rather poor clock accuracy and so, without
access to network time, their clock will naturally drift. This can
cause problems with distributed systems that rely on good clocks.
For example one commonly used revision control system can fail if it
perceives the modification timestamp to be in the future.
If the NTP servers relied on by a host can be subverted, either
through compromising or impersonating them, then the attacker may be
able to control the host's system clock. This can cause many
unexpected consequences, including the premature expiry of dated
resources such as encryption or authentication keys. This in turn
can prevent access to other more critical services.
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2.11 Physical DoS
The discussion thus far has centered on denial-of-service attacks
perpetrated using the network. However, computer systems are only as
resilient as the weakest link. It may be easier to deny service by
causing a power failure, by cutting network cables, or by simply
switching a system off, and so physical security is at least as
important as network security. Physical attacks can also serve as
entry points for non-physical DoS, for instance by reducing the
resources available to deal with overcapacity.
2.12 Social Engineering DoS
The weakest link may also be human. In defending against DoS, the
possibility of denial-of-service through social engineering should
not be neglected, such as convincing an employee to make a
configuration change that prevents normal operation.
2.13 Legal DoS
Computer systems cannot be considered in isolation from the social
and legal systems in which they operate. This document focuses
primarily on the technical issues, but we note that "cease and
desist" letters, government censorship, and other legal mechanisms
also touch on denial- of-service issues.
2.14 Spam and Black-hole Lists
Unsolicited commercial email, also known as "spam", can effectively
cause denial-of-service to email systems. While the intent is not
denial-of-service, the large amount of unwanted mail can waste the
recipient's time, or cause legitimate email to fail to be noticed
amongst all the background noise. If spam filtering software is
used, some level of false positives is to be expected, and so these
messages are effectively denied service.
One mechanism to reduce spam is the use of black-hole lists. The IP
addresses of dial-up ISPs or mail servers used to originate or relay
spam are added to black-hole lists. The recipients of mail choose to
consult these lists and reject spam if it originates or is relayed by
systems on the list. One significant problem with such lists is that
it may be possible for an attacker to cause a victim to be black-hole
listed, even if the victim was not responsible for relaying spam.
Thus the black-hole list itself can be a mechanism for effecting a
DoS attack. Note that every black-hole list has its own policy
regarding additions, and some are less susceptible to this DoS attack
than others. Consumers of black-hole list technology are advised to
investigate these policies before they subscribe. Similar
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considerations apply to feeds of bad BGP bad route advertisements.
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3. Attack Amplifiers
Many of the attacks described above rely on sending sufficient
traffic to overwhelm the victim. Such attacks are made much easier
by the existence of "attack amplifiers", where an attacker can send
traffic from the spoofed source address of the victim and cause
larger responses to be returned to the victim. A detailed discussion
of such reflection attacks can be found in [31].
3.1 Methods of Attack Amplification
The simplest such attack was the "smurf" attack [11], where an ICMP
echo request packet with the spoofed source address of the victim is
sent to the subnet-broadcast address of a network to be used as an
amplifier. Every system on that subnet then responds with an ICMP
echo response that returns to the victim. Smurf attacks are no
longer such a serious problem, as these days routers usually drop
such packets and end-systems do not respond to them.
An alternative form of attack amplifier is typified by a DNS
reflection attack. An attacker sends a DNS request to a DNS server
requesting resolution of a domain name. Again the source address of
the request is the spoofed address of the victim. The request is
carefully chosen so that the size of the response is significantly
greater than the size of the request, thereby providing the
amplification. As an aside, it is interesting to note that the
largest DNS responses tend to be those incorporating DNSsec
authentication information. This attack amplifier can only be used
by an attacker with the ability to spoof the source address of the
victim. However, we note that if the victim's DNS server is
configured to relay requests from external clients, it may be
possible to cause it to congest its own incoming network link.
Another variant of attack amplifier involves amplification through
retransmission. This is typified by a TCP amplification attack known
as "bang.c". The attacker sends a spoofed TCP SYN with the source
address of the victim to a arbitrary TCP server. The server will
respond with a SYN|ACK which is sent to the victim, and when no final
ACK is received to complete the handshake, the SYN|ACK will be
retransmitted a number of times. Typically this attack uses a very
large list of arbitrarily chosen servers as reflectors. For the
attack to be successful, the reflector must not receive a RST from
the victim in response to the SYN|ACK - however if the attack traffic
sufficiently overwhelms the server or access link to the server, then
packet loss will ensure that many reflectors do not receive a RST in
response to their SYN|ACK, and so continue to retransmit. The attack
can be exacerbated by firewalls that silently drop the incoming SYN|
ACK without sending a RST.
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Care must also be taken with services that relay requests. If an
attacker can send a request to a proxy, and that proxy now attempts
to connect to a victim whose address is chosen by the attacker then,
if the proxy repeatedly resends the request when receiving no answer,
this can also serve as an attack amplifier.
Another variant of amplification occurs in protocols that include,
within the protocol payload, an IP address or name of host to which
subsequent messages should be sent. An example of such as protocol
is the Session Initiation Protocol (SIP), which carries a payload
defined by the Session Description Protocol (SDP). The SDP payload
of the SIP message conveys the IP address and port to which media
packets, typically encoded using the Real Time Transport Protocol
(RTP), are sent.
To launch this attack, an attacker sends a protocol message, and sets
the IP address within the payload to point to the attack target. The
recipient of the message will generate subsequent traffic to that IP
address. Depending on the protocol, this attack can provide
substantial amplification properties. In the specific case of SIP,
if a caller makes calls to high bandwidth media sources (such as a
video server or streaming audio server), a single SIP INVITE packet,
typically a few hundred bytes, can result in a nearly continuous
stream of media packets at rates anywhere from a few kbits per second
up to megabits per second. This particular attack is called the
"voice hammer".
Unlike the other techniques described above, this technique does not
require the attacker to modify packets or even spoof their source IP
address. This makes it easier to launch.
This attack is prevented through careful protocol design. Protocols
should, whenever possible, avoid including IP addresses or hostnames
within protocol payloads as addresses to which subsequent messaging
should be sent. Rather, when possible, messages should be sent to
the source IP from which the protocol packet came. If such a design
is not possible, the protocol should include a handshake whereby it
can be positively determined that the protocol entity at that IP
address or hostname does, in fact, wish to receive that subsequent
messaging. That handshake itself needs to be lightweight (to avoid
being the source of another DoS attack), and secured against the
spoofing of the handshake response.
Finally, a somewhat similar attack is possible with some protocols
where where one message leads to another message that is not sent as
a reply to the source address of the first message. This can be an
issue with protocols to enable mobility for example, and might permit
an attacker to avoid ingress filtering. Such protocols are
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notoriously difficult to get right.
3.2 Strategies to Mitigate Attack Amplification
In general, the architectural lessons to be learnt are simple:
o As far as possible, perform ingress filtering [22] [37] to
prevent source address spoofing.
o Avoid designing protocols or mechanisms that can return
significantly larger responses than the size of the request, unless a
handshake is performed to validate the client's source address. Such
a handshake needs to incorporate an unpredictable nonce that is
secure enough to mitigate the amplification effects of the protocol.
o All retransmission during initial connection setup should be
performed by the client.
o Proxies should not arbitrarily relay requests to destinations
chosen by a client.
o Avoid signaling third-party connections. Any unavoidable third-
party connections setup by a signaling protocol should incorporate
lightweight validation before sending significant data.
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4. DoS Mitigation Strategies
A general problem with DoS defense is that it is not in principle
possible to distinguish between a flash-crowd and a DoS attack.
Indeed, having your site taken down by a flash crowd is probably a
more common experience than having it DoS-ed --- so common it's
acquired its own names: being Slashdotted or Farked, after the web
sites that are common sources of flash crowds. Thus, the first line
of defense against DoS attacks must be to provision your service so
that it can handle a foreseeable legitimate peak load.
Underprovisioned sites are the easiest to take down.
Specific strategies for DoS defense fall into two broad categories:
1. Avoiding allowing attacks that are better than generic resource
consumption.
2. Minimizing the extent to which generic resource consumption
attacks crowd
In the remainder of this section, we consider specific applications
of these two approaches at a variety of levels of network system
architecture.
4.1 Protocol Design
4.1.1 Don't Hold State for Unverified Hosts
From an end-system server point of view, one simple aim is to avoid
instantiating state without having completed a handshake with the
client to validate their address, and as far as possible to push work
and stateholding to client. There are a number of techniques that
might be used to do this, including SYN-cookies [5] [2]. All client-
server protocols should probably be designed to allow such techniques
to be used, but the enabling of the mechanism should normally be at
the server's discretion to avoid unnecessary work under normal
circumstances.
4.1.2 Make it Hard to Simulate a Legitimate User
Other than having massive overcapacity, the only real defense against
resource consumption attacks is to preferentially discriminate
against attackers. The general idea is to find something that
legitimate users can do but attackers can't. The most commonly
proposed approaches include:
1. Puzzles: force the attacker to do some computation that would not
be onerous for a single user but is too expensive to do en masse.
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[2]
2. Reverse Turing Tests: specialized puzzles that are hard for
machines to do but easy for humans, thus making automated attacks
hard.[42]
3. Reachability testing: force the proposed client to demonstrate
that it can receive traffic at a given IP address. This makes it
easier to trace attackers.
All of these techniques have substantial limitations. Puzzles tend
to discriminate against legitimate users with slow computers. In
addition, the wide availability of "bots" means that attackers have
ample computing power at their disposal. There has been substantial
work in attacking Reverse Turing Tests automatically, thus making
them of limited applicability. Finally, reachability testing is
substantially weakened by bots because the attacker does not need to
hide his source address.
4.1.3 Graceful Routing Degradation
A goal with routing protocols is that of graceful degradation in
overload, and automatic recovery after the source of the overload has
been remedied. Some routing protocols satisfy this goal more than
others. Although RIP doesn't scale well, if a router runs out of
memory when receiving a RIP route, it can just drop the route and
send an infinite metric to its peers. The route will later be
refreshed, and if the original source of the problem has been
resolved, the router will now be able to process it correctly.
On the other hand, BGP is stateful in the sense that a peer assumes
you have processed or chosen to filter any route that it sent you.
There is no mechanism to refresh state in the base BGP spec, and even
the later route refresh option [16] is hard to use usefully in the
presence of overload. A BGP router that cannot store a route it
received has two choices: completely restart BGP, or shutdown one or
more peerings [15]. This means that the effects of a BGP overload
are rather more severe than they need to be, and so amplifies the
effect of any attack.
In general, few routing protocol designs actively consider the
possible behaviour of routers under overload conditions; this should
be an explicit part of future routing protocol designs. Although
precise details should clearly be left to implementors, the protocol
design needs to give them the capability to do their job properly.
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4.1.4 Autoconfiguration and Authentication
Autoconfiguration mechanisms greatly ease deployment, and are
increasingly necessary as the number of networked devices grows
beyond what can be managed manually. However, it should be
recognised that unauthenticated autoconfiguration opens up many
avenues for attack. There is a clear tension between ease of
configuration and security of configuration, especially because there
are environments in which it is desirable for units to operate with
effectively no authentication (e.g., airport hotspots). Future
autoconfiguration protocols should consider the need to allow
different end-systems to operate at different points in this spectrum
within the same autoconfiguration framework. However, this also
implies that the network elements should avoid acting for
unauthenticated hosts, instead just letting them access the network
more or less directly.
4.2 Network Design and Configuration
In general, networks should be provisioned with private, out-of-band
access to console or control ports so that such control facilities
will be available in the face of a DoS attack launched against either
the control or data plane of the (in-band) network. Typically such
out-of-band networks are provisioned on a separate infrastructure for
exactly this purpose. Out-of-band access is a crucial capability for
DoS mitigation, since many of the typical redundancy and capacity
management techniques (such as prioritizing routing or network
management traffic) fail during such attacks. In addition, many
redundancy protocols such as VRRP [RFC3768] can fail during such
attacks as they may be unable to keep adjacencies alive.
There are several default configuration settings that can also be
exploited to generate several of the attacks outlined in this
document. For example, some vendors may have features such as IP
redirect, directed broadcast, and proxy ARP enabled by default.
Similar defaults, such as publicly readable SNMP [RFC3411]
communities (e.g., "public") can be used to reveal otherwise
confidential information to a prospective attacker. Finally, other
unauthenticated configuration management protocols such as TFTP
[RFC1350] should be avoided if possible; at the very least access to
TFTP configuration archives should be protected and TFTP should be
filtered at administrative boundaries. Finally, since many of the
password encryption techniques used by router vendors are reversible,
keeping such passwords on a configuration archive (as part of a
configuration file), even in the encrypted form written by the
router, can lead to unauthorized access if the archive is
compromized.
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4.2.1 Redundancy and Distributed Service
A basic principle of designing systems to handle failure to have
redundant servers which can take over when one fails. This is
equally true in the case of DoS attacks, which often focus on a given
server and/or link. If service delivery points can be distributed
across the network, then it becomes much harder to attack the entire
service. In particular, this makes attacks on a single network link
more difficult.
4.2.2 Authenticate Routing Adjacencies
In general, cryptographic authentication mechanisms are too costly to
form the main part in DoS prevention. However, routing adjacencies
are too important to risk an attacker being able to inject bad
routing information, which can affect more than the router in
question. Additional non-cryptographic mechanisms should then be
used to avoid arbitrary end-systems being able to cause the router to
spend CPU cycles on validating authentication data.
For BGP, at the very least, this implies the use of TCP MD5 [24] or
IPsec authentication, combined with the GTSM [23] to prevent EBGP
association with non-immediate neighbours. In future, this will
likely imply better authentication of the routing information itself.
4.2.3 Isolate Router-to-Router Traffic
As far as is feasible, router-to-router traffic should be isolated
from data traffic. How this should be implemented depends on the
precise technologies available, both in the router and at the link-
layer. The goal should be that failure of the link for data traffic
should also cause failure for the routing traffic, but that an
attacker cannot directly send packets to the control processor of the
routers.
A downside of this is that some diagnostic techniques (such as
pinging consecutive routers to find the source of a delay) may no
longer be possible. Ideally, alternative mechanisms (which do not
open up additional avenues for DoS) should be designed to replace
such lost techniques.
4.3 Router Implementation Issues
Because a router can be considered as an end-system, it can
potentially benefit from all the prevention mechanisms prescribed for
end-system implementation. However one basic distinction between a
router and a host is that the former implements routing protocols and
forwards data, which in turn lead to additional router specific
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implementation considerations. The issues described below are meant
to be illustrative and not exhaustive.
4.3.1 Checking Protocol Syntax and Semantics
Protocol syntax defines the formation of the protocol messages and
the rules of exchanges. The questions addressed by protocol syntax
checking includes, but is not limited to, the following:
1. Who sent the message?
2. Does the content conform to the protocol format?
3. Was the message sent with correct timing?
The first step in protocol syntax verification is to ensure that an
incoming message was sent by a legitimate party. There are multiple
ways to perform this check. One can verify the source IP address and
even the MAC address of the message. Utilizing the fact that eBGP
peers are normally directly connected, one can also check the TTL
value in a packet and discard any BGP updates packet whose TTL is
less than some maximum value (typically max TTL - 1) [23].
Cryptographic authentication should also be used whenever available
to verify that an incoming message is indeed from an expected sender.
For BGP, at the very least, this implies the use of TCP MD5 [24] or
IPsec authentication.
In addition to the sender verification, it is also important to check
the syntax of a received routing message, as opposed to assuming that
all messages came in a correct format. It happened in the past that
routers crashed upon receiving ill-formed routing messages. Such
faults will be prevented by performing rigorous syntax checking.
4.3.2 Consistency Checks
Protocol semantics defines the meaning of the message content, the
interpretation of the values, and the actions to be taken according
to the content. Here is a simple example of using semantic checking.
When a link failure causes a router in AS A to send a peer router B a
withdrawal message for prefix P, B should make sure that any
alternative path it finds to reach P does not go through A. This
simple check is shown to significantly improve BGP convergence time
in many cases [45].
Another example of using semantic checking against false routing
injection is described in [47]. The basic idea is to attach to the
route announcement for prefix P a list of the valid origin ASes. Due
to the rich connectivity in today's Internet topology, a remote AS
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will receive routing updates from multiple different paths and can
check to see whether each update carries the identical origin AS
list. Although a false origin may announce reachability to P, or
alter the origin AS list, it would be difficult, if not impossible,
to block the correct updates from propagating out, thus remote ASes
can detect the existence of false updates by observing the
inconsistency of the received origin AS lists for P. Research studies
show that the "allowed origin list" test can effectively detect
majority of falsely originated updates.
Generally speaking, verifying the validity of BGP routes can be
challenging because BGP is policy driven and policies of individual
ISPs are not known in most cases. But assuming policies do not
change in short time scale, in principle one could verify new updates
against observed routes from recent past, which reflect the routing
policies in place. Research work is needed to explore this
direction.
Note that while the above steps are all fairly simple and don't
really "bullet-proof" the protocol, each adds some degree of
protection. As such, the combination of the above techniques can
result in an efffective reduction in the probability of undetected
faults.
4.3.3 Enhance Router Robustness through Operational Adjustments
There exist a number of configuration tunings that can enhance
robustness of BGP operations. One example is to let BGP peers
coordinate the setting of a limit on the number of prefixes which one
BGP speaker will send to its peer [46]. Although such check does not
validate the prefix owned by each peer, it can prevent false
announcements of large number of invalid routes. Had all BGP routers
been configured with this simple checking earlier, several large
scale routing outages in the past could have been prevented. Note,
however, that care must be taken with hard limits of this type
because they can be used to mount a DoS because implementations often
discard excess routes indiscriminately, thus potentially causing
black-holing of correct routes.
Another example of useful configuration tuning is to adjust the BGP's
KeepAlive and Hold Timer values to minimize BGP peering session
resets. Previous measurements show that heavy traffic load, such as
those caused by Worms, can cause BGP KeepAlive messages to be delayed
or dropped, which in turn cause BGP peering session break down. Such
load induced session breaks and re-establishments can lead to
excessive amount of BGP updates during the periods when stable
routing is mostly needed.
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4.3.4 Proper Handling of Router Resource Exhaustion
In addition to the resource exhaustion problems that are generally
apply to all end systems, as described in Section 2, router
implementations must also take special care in handling resource
exhaustions when they occur in order to keep the router operating
despite the problem. For example under normal operations a router
does not require a large cache to hold outstanding ARP requests
because the replies are normally received within a few milliseconds.
However certain conditions can lead to ARP cache exhaustion, for
example during a virus attack where many packets are sent to non-
existing IP addresses, thus there are no ARP replies to the requests
for those addresses. Such phenomenon happened in the past and led to
routers stop packet forwarding.
Routers collectively operate as the brain of the Internet. Thus a
smooth functioning of all the routers is vitally important. Our goal
is to minimize routers' load on the control plane and maximize the
correctness of routing protocol operations.
4.4 End-System Implementation Issues
4.4.1 State Lookup Complexity
Any system that instantiates per-connection state should take great
care to implement the state-lookup mechanisms in such a way that
performance can not be controlled by the attacker. One way to
achieve this is to use hash-tables where the hash mechanism is keyed
in such a way that the attacker cannot instantiate a large number of
flows in the same hash bucket.
4.4.1.1 Avoid Livelock
Most operating systems use network interrupts to receive data from
the network, which is a good solution if the host spends only a small
amount of its time handling network traffic. However, this leaves
the host open to livelock [29], where under heavy load the OS spends
all its time handling interrupts and no time doing the work needed to
handle the traffic at the application level. Server operating
systems should consider using network polling at times of heavy load,
rather that being interrupt-driven, and should be carefully
architected so that as far as reasonably possible, traffic received
by the OS is processed to completion, or very cheaply discarded.
4.4.1.2 Use Unpredictable Values for Session IDs
Most recent TCP implementations use fairly good random mechanisms for
allocating the TCP initial sequence numbers. In general, any
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dynamically allocated value used purely to identify a communications
session should be allocated using an unpredictable mechanism, as this
increases the search space for an attacker that wishes to disrupt
ongoing communications. Thus the dynamically allocated port of the
active end of a TCP connection might also be randomly allocated.
With DNS, the ID which is used to match responses with requests
should also be randomly generated. However, as the ID field is only
16 bits, the protection is rather limited, especially in the face of
birthday attacks.
4.4.2 Operational Issues
4.4.2.1 Eliminate Bad Traffic Early
Many DoS attacks are generic bandwidth consumption attacks that
operate by clogging the link that connects the victim server to the
Internet. Filtering these attacks at the server does no good because
the traffic has already traversed the link which is the scarce
resource. Such flows need to be filtered at some point closer to the
attacker. Where possible, operators should filter out obviously bad
traffic. In particular, they should perform ingress filtering [22].
4.4.2.2 Establish a Monitoring Framework
Network operators are strongly encouraged to establish a monitoring
framework to detect and log abnormal network activity. One can not
defend against an attack one doesn't detect or understand. Such
monitoring tools can be used to set a baseline of "normal" traffic,
and can be used to determine:
o Aberrant flows.
o Type and source of the aberrant flows
This is extremely helpful when responding to DDoS or a flash crowd,
and should be in place prior to the event.
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5. Conclusions
In this document we have highlighted possible avenues for DoS attack
on networks and networked systems, with the aim of encouraging
protocol designers and network engineers towards designs that are
more robust. We have discussed partial solutions that reduce the
effectiveness of attacks, and highlighted how some partial solutions
can be taken advantage of by attackers to perpetrate alternative
attacks.
Our focus has primarily been on protocol and network architecture
issues, but there are many things that network and service operators
can do to lessen the threat. Further advice and information for
network operators can be found in [13] [37] [14].
It is our hope that this document will spur discussion leading to
architectural solutions that reduce the succeptibility of all
Internet systems to denial-of-service attacks.
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6. Security Considerations
This entire document is about security.
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7. Acknowledgements
We are very grateful to Vern Paxson, Paul Vixie, Rob Thomas, Dug
Song, George Jones, Jari Arkko, and Geoff Huston for their
constructive comments on earlier versions of this document.
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8. References
[1] J. Abley, "Hierarchical Anycast for Global Service Distribution",
http://www.isc.org/tn/isc-tn-2003-1.txt
[2] T. Aura, P. Nikander, J. Leiwo, "DOS-resistant authentication
with client puzzles", In B. Christianson, B. Crispo, and M. Roe,
editors, Proceedings of the 8th International Workshop on Security
Protocols, Lecture Notes in Computer Science, Cambridge, UK, April
2000.
[3] J. Bellardo, S. Savage, "802.11 Denial-of-Service Attacks: Real
Vulnerabilities and Practical Solutions", Proceedings of the USENIX
Security Symposium, Washington D.C., August 2003.
[4] S.M. Bellovin, "Security Problems in the TCP/IP Protocol Suite",
Computer Communication Review, Vol. 19, No. 2, pp. 32-48, April 1989.
[5] D.J. Bernstein, "SYN Cookies", http://cr.yp.to/syncookies.html
[6] CCAIS/RNP Alertas do Cais ALR-19112002a, "Vulnerability in the
sending requests control of Bind versions 4 and 8 allows DNS
spoofing", http://www.rnp.br/cais/alertas/2002/cais- ALR-
19112002a.html
[7] CERT Advisory CA-1996-01, "UDP Port Denial-of-Service Attack",
Feb 1996.
[8] CERT Advisory CA-1996-21, "TCP SYN Flooding and IP Spoofing
Attacks", Sept 1996.
[9] CERT Advisory CA-2001-09, "Statistical Weaknesses in TCP/IP
Initial Sequence Numbers", May 2001.
[10] CERT Advisory CA-1996-26, "Denial-of-Service Attack via ping",
Dec 1996.
[11] CERT Advisory CA-1998-01, "Smurf IP Denial-of-Service Attacks",
http://www.cert.org/advisories/CA-1998-01.html, Jan 1998.
[12] CERT Incident Note IN-2000-05, "'mstream' Distributed Denial of
Service Tool", May 2000.
[13] CERT/CC - "Managing the Threat of Denial of Service Attacks",
http://www.cert.org/archive/pdf/Managing_DoS.pdf
[14] CERT/CC - "Trends in Denial of Service Attack Technology",
http://www.cert.org/archive/pdf/DoS_trends.pdf
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[15] D.F. Chang, R. Govindan, J. Heidemann, "An Empirical Study of
Router Response to Large Routing Table Load", Proceedings of the 2nd
Internet Measurement Workshop (IMW 2002), 2002.
[16] E. Chen, "Route Refresh Capability for BGP-4", RFC 2918,
September 2000
[17] Cisco Systems, "Configuring the BGP Maximum-Prefix Feature",
Cisco Document ID: 25160, http://www.cisco.com/warp/public/459/bgp-
maximum-prefix.html
[18] Scott A Crosby and Dan S Wallach, "Denial of Service via
Algorithmic Complexity Attacks", Proceedings of the USENIX Security
Symposium, Washington D.C., August 2003.
[19] S. Deering, "Host extensions for IP multicasting", RFC 1112, Aug
1989.
[20] T. Dierks, C. Allen, "The TLS Protocol, Version 1.0", RFC 2246,
Jan 1999.
[21] D. Estrin, D. Farinacci, A. Helmy, D. Thaler, S. Deering, M.
Handley, V. Jacobson, C. Liu, P. Sharma, L. Wei, "Protocol
Independent Multicast-Sparse Mode (PIM-SM): Protocol Specification",
RFC 2362, June 1998.
[22] P. Ferguson, D. Senie, "Network Ingress Filtering: Defeating
Denial of Service Attacks which employ IP Source Address Spoofing",
RFC 2827, May 2000.
[23] V. Gill, J. Heasley, D. Meyer "The Generalized TTL Security
Mechanism (GTSM)", RFC 3682, February 2004.
[24] A. Heffernan, "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
[25] Laurent Joncheray, "Simple Active Attack Against TCP", 5th
USENIX Security Symposium, 1995.
[26] M. Lough, "A Taxonomy of Computer Attacks with Applications to
Wireless", PhD thesis, Virginia Polytechnic Institute, April 2001.
[27] Z. Mao, R. Govindan, G. Varghese, R. Katz, "Route Flap Dampening
Exacerbates Internet Routing Convergence", Proceedings of ACM
SIGCOMM, 2002.
[28] D. Meyer, W. Fenner (Editors), "Multicast Source Discovery
Protocol (MSDP)", draft-ietf-msdp-spec-15.txt, Work in Progress.
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[29] J. Mogul, KK. Ramakrishnan, "Eliminating Receive Livelock in an
Interrupt-driven Kernel", ACM Transactions on Computer Systems, Vol
15, Number 3, pp. 217-252, 1997.
[30] National Infrastructure Secuity Co-ordination Center (NISCC),
Vulnerability Advisory 236929, April 2004,
http://www.uniras.gov.uk/vuls/2004/236929/
[31] V. Paxson, "An Analysis of Using Reflectors for Distributed
Denial- of-Service Attacks", Computer Communication Review 31(3),
July 2001.
[32] Y. Rekhter, T. Li, "A Border Gateway Protocol 4 (BGP-4)", RFC
1771, March 1995.
[33] Joe Stewart, "DNS Cache Poisoning - The Next Generation", Jan 27
2003, http://www.securityfocus.com/guest/17905
[34] R. Stewart (Editor), Transmission Control Protocol security
considerations, Internet Draft, April 2004, draft-ietf-tcpm-
tcpsecure-00.txt
[35] C. Villamizar, R. Chandra, R. Govindan, "BGP Route Flap
Damping", RFC 2439, November 1998.
[36] P. Vixie, G. Sneeringer, M. Schleifer, "Events of 21-Oct-2002",
http://f.root-servers.org/october21.txt
[37] P. Vixie, "Securing the Edge",
http://www.icann.org/committees/security/sac004.txt
[38] D. Waitzman, C. Partridge, S.E. Deering, "Distance Vector
Multicast Routing Protocol", RFC 1075, Nov 1988.
[39] D. Wessels, "Running An Authoritative-Only BIND Nameserver",
http://www.isc.org/tn/isc-tn-2002-2.txt
[40] M. Zalewski, "Strange Attractors and TCP/IP Sequence Number
Analysis", http://razor.bindview.com/publish/papers/tcpseq.html
[41] J. Bellardo and S. Savage, "802.11 Denial-of-Service Attacks:
Real Vulnerabilities and Practical Solutions", Proceedings of the
12th USENIX Security Symposium, August 2003.
[42] L. von Ahn, M. Blum, N. Hopper, and J. Langford. CAPTCHA: Using
hard AI problems for security. In Proceedings of Eurocrypt, 2003.
[43] The whole world disappeared? http://www.merit.edu/mail.archives/
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nanog/1998-04/msg00181.html, Apr 1998.
[44] Outage: MCI Worldcom nationwide ATM network.
http://www.merit.edu/ mail.archives/nanog/1999-02/msg00077.html, Feb
1999.
[45] D. Pei, X. Zhao, L. Wang, D. Massey, A. Mankin, F. S. Wu, and L.
Zhang. Improving BGP Conver-gence Through Assertions Approach. In
Proc. of IEEE INFOCOM, June 2002.
[46] Srikanth Chavali, Vasile Radoaca, Mo Miri, Luyuan Fang, and
Susan Hares. Peer prefix limits ex-change in bgp. http://
www.ietf.org/internet-drafts/draft-chavali-bgp-prefixlimit-01.txt,
April 2004.
[47] X. Zhao, D. Massey, A. Mankin, S.F. Wu, D. Pei, L. Wang, L.
Zhang, "BGP Multiple Origin AS (MOAS) Conflicts",
http://nanog.org/mtg-0110/lixia.html, 2001.
Authors' Addresses
Mark J. Handley (ed)
UCL
Gower Street
London WC1E 6BT
UK
Email: M.Handley@cs.ucl.ac.uk
Eric Rescorla (ed)
Network Resonance
2483 E. Bayshore #212
Palo Alto 94303
USA
Email: ekr@networkresonance.com
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Appendix A. IAB Members at the time of this writing
o Bernard Aboba
o Loa Andersson
o Brian Carpenter
o Leslie Daigle
o Patrik Faltstrom
o Bob Hinden
o Kurtis Lindqvist
o David Meyer
o Pekka Nikander
o Eric Rescorla
o Pete Resnick
o Jonathan Rosenberg
o Lixia Zhang
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