Distributed Denial of Service

Distributed Denial of Service

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  • Glavni Administrator
  • Predrag Damnjanović
  • SysAdmin i programer
  • Pridružio: 17 Apr 2003
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Distributed Denial of Service Attacks have recently emerged as one of
the most newsworthy, if not the greatest, weaknesses of the Internet.
This paper attempts to explain how they work, why they are hard to
combat today, and what will need to happen if they are to be brought
under control. It is divided into four sections.

The first is an overview of the current situation. The second is a detailed
description of exactly how this attack works, and why it is hard to
cope with today; of necessity it includes a description of how the
Internet works today. The third section describes the short-term
prospects, what can be done today to help alleviate this problem; and
the final section describes the long-term picture, what will change to
bring this class of problem under control, if not eliminate it
entirely. And finally there are some appendices: a bibliography,
giving references to original research work and announcements; a brief
article on securing servers; and acknowledgements for the many people
who helped make this paper possible.

Copyright 2000 OVEN Digital. You may reproduce this document as long
as this copyright notice is left intact.

1. Overview

Distributed Denial of Service (DDoS) attacks are a relatively new
development; they first appeared in the summer last year, and were
first widely discussed a couple of months ago. During the week of
February 7th through 11th, 2000, we saw them emerge as a major new
category of attack on the Internet. They took out many sites,
including Yahoo, Buy.com, eBay, Amazon, Datek, E*Trade, and CNN. The
victims were unreachable for several hours each.

What*s worse, there*s no current prospect of either tracking the
perpetrators down, or of preventing similar attacks in the near

This was a major event, covered in the major news media. They have
done an excellent job in their coverage; as far as it has gone, their
coverage has been accurate. The problem is, their coverage hasn*t been
sufficiently detailed to explain why we cannot track down the people
committing these attacks, and why we can*t defend against them.
There*s a good reason for these omissions: the attack is subtle, and
understanding how it works well enough to understand why we can*t cope
today, and what will have to change before we can, requires a more
detailed explanation of how the Internet is constructed than the mass
media are prepared to deliver to their audiences.

A brief note on usage: the network where these attacks are taking
place is called the **Internet**, with a capital **I**; it is the
public network shared by people all over the world. An **internet**,
with a lower-case **i**, is a collection of networks interconnected;
many organizations have private internets. The Internet is the result
of inter-connecting a gigantic number of private internets.

2. Detailed explanation of DDoS attacks

DDoS attacks involve breaking into hundreds or thousands of machines
all over the Internet. Then the attacker installs DDoS software on
them, allowing them to control all these burgled machines to launch
coordinated attacks on victim sites. These attacks typically exhaust
bandwidth, router processing capacity, or network stack resources,
breaking network connectivity to the victims.

So the perpetrator starts by breaking into weakly-secured computers,
using well-known defects in standard network service programs, and
common weak configurations in operating systems. On each system, once
they break in, they perform some additional steps. First, they install
software to conceal the fact of the break-in, and to hide the traces
of their subsequent activity. For example, the standard commands for
displaying running processes are replaced with versions that fail to
display the attacker*s processes. These replacement tools are
collectively called a **rootkit**, since they are installed once you
have **broken root**, taken over system administrator privileges, to
keep other **root users** from being able to find you. Then they
install a special process, used to remote-control the burgled machine.
This process accepts commands from over the Internet, and in response
to those commands it launches an attack over the Internet against some
designated victim site. And finally, they make a note of the address
of the machine they*ve taken over. All these steps are highly
automated. A cautious intruder will begin by breaking into just a few
sites, then using them to break into some more, and repeating this
cycle for several steps, to reduce the chance they are caught during
this, the riskiest part of the operation. By the time they are ready
to mount the kind of attacks we*ve seen recently (gigabytes per second
of traffic dumped on Yahoo, according to reports in SANS) they have
taken over thousands of machines and assembled them into a DDoS
network; this just means they all have the attack software installed
on them, and the attacker knows all their addresses (stored in a file
on their control system).

Now comes time for the attack. The attacker runs a single command,
which sends command packets to all the captured machines, instructing
them to launch a particular attack (from a menu of different varieties
of flooding attacks) against a specific victim. When the attacker
decides to stop the attack, they send another single command.

Now to go into details of the attacks. While there are variations,
they generally take a common form. The controlled machines being used
to mount the attacks send a stream of packets. For most of the
attacks, these packets are directed at the victim machine. For one
variant (called **smurf**, named after the first circulated program to
perform this attack) the packets are aimed at other networks, where
they provoke multiple echoes all aimed at the victim. To go into
further detail, some background description of the Internet is in

The Internet consists of hundreds of thousands or millions of small
networks (called Local Area Networks, or LANs), all interconnected;
attached to these LANs are many millions of separate computers. Any
of these computers can communicate with any other computer. This works
by assigning every computer an address. The addresses are structured
(organized into groups) so that special-purpose traffic-handling
computers, called routers, can direct them in the right direction to
reach their intended destination. A typical connection today may
require 15 or more hops, crossing from one LAN to another, before it
reaches its final destination. But most of these **LANs** are actually
special-purpose links within and between network transport companies.
These backbone providers handle the hard problems of routing traffic.

Looking a little closer, when one computer wants to send a message to
another, it divides it into fixed-size pieces, called **packets**.
Each of these packets is handled separately by the Internet, then the
message (if it is larger than a single packet) is reassembled at the
remote computer. So the traffic passing between machines consists
entirely of packets of data. Each of these packets has a pair of
addresses in it, called the Source and Destination IP (for Internet
Protocol) addresses. These are the addresses of the originating
machine, and the recipient. They are quite analogous to the address
and return address on an envelope, in traditional mail.

When such a packet is sent over the Internet, it is passed first to
the nearest router; commonly this router is at the point where the
local network connects to the Internet. This router is often called a
border router. In larger organizations the story may be more complex;
a large organization often assembles its own collection of LANs,
interconnected into an in-house internet, cross-connected at one or
more points (often with firewalls) with the Internet that we all know
and love. But returning to our tale, when a packet leaves a computer,
it is passed to a border router. This router passes it upstream to a
core router, which interconnects with many other core routers all over
the Internet; they pass the packet on until it reaches its
destination. The source address is normally ignored by routers; it
normally only tells the final destination machine where the request is
coming from. That*s an essential part of the problem we face today.

The packets used in today*s DDoS attacks use forged source addresses;
they are lying about where the packet comes from. The very first
router to receive the packet can very easily catch the lie; it has to
know what addresses lie on every network attached to it, so that it
can correctly route packets to them. If a packet arrives, and the
source address doesn*t match the network it*s coming from, the router
should discard the packet. This style of packet checking is called
variously Ingress or Egress filtering, depending on the point of view;
it is Egress from the customer network, or Ingress to the heart of the
Internet. If the packet is allowed past the border, catching the lie
is nearly impossible. Returning to our analogy, if you hand a letter
to a letter-carrier who delivers to your home, there*s a good chance
he could notice if the return address is not your own. If you deposit
a letter in the corner letter-box, the mail gets handled in sacks, and
routed via high-volume automated sorters; it will never again get the
close and individual attention required to make any intelligent
judgments about the accuracy of the return address. Likewise with
forged source addresses on internet packets: let them past the first
border router, and they are unlikely to be detected.

Now let*s look at the situation from the victim*s point of view. The
first thing you know, the first sign that you may have a problem, is
when thousands of compromised systems all over the world commence to
flood you with traffic, all at once. The first symptom is likely to be
a router crash, or to look a lot like one; traffic simply stops
flowing between you and the Internet. When you look more closely you
may discover that one or more targeted servers are being overloaded by
the small fraction of the traffic that actually gets delivered, but
the failures extend much further back.

So you try and find out what*s going wrong. After the first few quick
checks don*t solve the problem, you look at the traffic flowing
through your network, and about then you realize you are a victim of a
major denial of service attack. So you capture a sample of the packets
flying over your net, as many as you can. What does each packet tell
you? Well, it will have your address as its destination address, and
it will have some random number as a source address. There*s no trace
of the compromised host that is busy attacking you now. All that*s
there is a low-level, hardware address of the last router that
forwarded the packet; these low-level addresses are used to handle
distribution of packets within a LAN. So you can see what router
passed the packet to you, but nothing else. Identifying that router
may identify the Internet carrier that passed the traffic to you, if
you don*t have a complex internet of your own, within your own
organization. But either way, the next step is to capture another
packet on the other side of the forwarding router, and see where that
packet came from. Each step of the trace requires starting over,
collecting fresh evidence.

Every time the back-trace crosses an administrative boundary, between
you and your Internet provider, between them and the next backbone
provider on the path, all the way back to the compromised machine, you
have to enlist the aid of another team of administrators to collect
fresh evidence and carry the trace further back.

Now remember that you have to do this in thousands of directions, to
each of the thousands of compromised machines that are participating
in this attack.

Today there*s no possibility of performing more than a few back-traces
at most, in as little as a few hours. Even that would require some
luck to favor your efforts. So as long as the attacker turns their
attack off after at most a few hours, you are unlikely to find more
than a few of the thousands of machines used to launch the attack; the
remainder will remain available for further attacks. And the
compromised machines that are found will contain no evidence that can
be used to locate the original attacker; your trace will stop with

3. Immediate prospects

Here we discuss what can be done to avoid being part of the problem,
what can be done if you are the victim, what can you do to make you a
harder target to take down; and we mention the possibility of an alert
system, currently under discussion, intended to speed the process of
tracking down these attacks.

First and most important, second to nothing else: secure your servers.
This is not a complex or difficult procedure; a brief sketch of the
steps involved can be found in Appendix B of this paper; more
information is available in books on security and in many places

It*s easy to prioritize the machines to be secured, to determine which
ones need attention most urgently. At the low end, dialup machines are
the lowest worry. They may be used as relay points, as the attacker is
trying to muddy his trail to prevent being caught while breaking into
machines, but they won*t play a noticeable part in mounting an actual
attack. The traffic levels seen by Yahoo would require hundreds of
thousands of the fastest available dialup connections operating in
unison. Any machine with at least a T1 (1.5 million bits per second)
connection to the Internet is far more of a worry; and machines with
T3 (45 million bits per second) or faster connections are prime
targets for people mounting these attacks. Secure your computers. You
want to do this anyway; they are your machines, you don*t want them
broken into. But these attacks couldn*t be mounted if there weren*t
many, many thousands of poorly-secured systems with high-speed
connectivity, available to mount such an attack.

Second, ensure that packets are being filtered at the point where you
connect to the Internet, to prevent forged source addresses. This
provides protection in both directions; it prevents your machines from
being used to mount these attacks, if any are broken into, and it
prevents some attacks that might help intruders break into your
machines. If this sort of filtering were universal, these attacks
could only be leveled using legal source addresses; this would
eliminate the entire slow step-by-step back-tracing currently
required, and allow the victim to read the attacking machines*
addresses right out of the attacking packets. This would make response
far faster and easier.

And a third defensive measure prevents you from being used to mount
the smurf attacks that are part of this pattern of DDoS. Smurf attacks
send packets to a **smurf amplifier** network. This is any network
that allows such packets in. These packets come from outside the
amplifier net, but are directed to its broadcast address. Such packets
aren*t used for any legitimate purpose; they are an oversight in the
design of the internet protocol. They have a forged source address, to
direct all the replies (from all the hosts on the amplifier network)
to the victim; each such packet gets repeated by every machine in the
net, amplifying the effect of the attack. Packets directed at the
broadcast address from outside the net are called IP Directed
Broadcast packets, and should be blocked at the border. The command to
do this for Cisco routers is **no ip directed-broadcast**.

The above measures help ensure that your systems won*t be used to help
mount one of these attacks, and they are the place where you can be
most effective today. But they don*t help you defend against an attack
like this, they just ensure that you won*t inadvertently assist in

Today, there are only limited measures you can take to prevent from
becoming a victim of DDoS. You can make yourself harder to target by
distributing your website over multiple server farms, with multiple
points of contact to the Internet; completely taking your site
off-line (rather than making it seem slower) will require saturating
every connection you have. The more places you connect to the
Internet, and the more servers you have behind your connection, the
harder you are to hit. But this is an expensive recourse, unless you
are a very very large site; replicating servers and expensive Internet
connections adds up very fast.

A more practical defensive measure, particularly for smaller sites, is
to discuss with your Internet connectivity provider what they would be
able to do to help you in the face of such an attack. If they don*t
already have provisions in place for rapidly tracking these attacks,
and placing filters to reduce their effect, then they need to be
developing them now. If they don*t have any current plans, you might
direct them to some of the resources described in the Bibliography,
particularly RFC 2267 on Ingress filtering, and Robert Stone*s NANOG
paper on CenterTrack (described in the next section).

But in the final analysis, the only real defense against DDoS today is
to not be sufficiently newsworthy to attract the attention of an

4. Long-term prospects

Now for the future; what will we do to eliminate this threat?

Two major developments are currently actively underway, to help
prevent DDoS attacks from remaining unmanageable. The major one is
ingress filtering. Right now, well-administered networks practice this
at their borders. If all networks were so well-administered, these
attacks could be dealt with relatively quickly; mounting a single
attack would deliver to the victim the addresses of all the conquered
machines; their owners could be notified, and filters could be put in
place near the machines to block the attacks while the machines are
being shut down and fixed. What*s more, the **smurf** variant of the
attack, which achieves an additional amplification of traffic levels
by exploiting network configuration problems elsewhere, would be
impossible. Today some routers can be told to do ingress filtering
completely automatically, and nearly all the rest can be manually
configured to do it. All routers need to be able to do this filtering,
and it needs to be enabled by default. It will be, and soon (but not
**soon** in the Internet, e-business time scale). Until today, such
filtering wasn*t considered required practice for participating in the
Internet. This has changed.

Something very similar happened a few years ago, in response to
spammers. Until relatively recently, the normal way to configure email
servers allowed **open relaying**; anyone could send a message to any
email server, and it would accept it and do its best to deliver it to
its final destination. Spammers exploited this to relay their torrents
of junk mail. Shortly thereafter, people learned how to modify email
servers to prevent open relaying; the fixed servers would accept email
from anyone for their local users, and would accept email from their
local users for anyone, but would refuse to relay email from one
stranger to another. Shortly thereafter, this configuration was
shipped as the default for all new mail servers. Once it became the
norm, the problem reduced to tracking down those people who were slow
to upgrade. The last step was to introduce blacklists, databases used
to track the remaining open email relays. Many Internet providers
subscribe to these blacklists, and reject all email sent from machines
listed on them. This provides a very very strong incentive to get your
shop straightened out; if you allow open relaying, soon you won*t be
able to send email to most of the Internet.

I expect ingress filtering to follow a very similar course. Soon it
will be the default configuration for new routers, and eventually
there will be blacklists of sites whose routers don*t provide this
protection, and people will have to fix their routers if they want to
be able to reach most of the Internet.

The other half of the solution comes in developing techniques for
rapidly tracking these attacks to their source, and notifying the
people who need to secure their broken servers, or the providers who
need to put blocks in place to shut down an attack at its many
sources. Robert Stone of UUNET presented a paper at the October NANOG
(North American Network Operators Group) entitled **CenterTrack: An IP
Overlay Network for Tracking DoS Floods**. It describes a technique
for designing a network, with a handful of extra diagnostic routers,
to allow rapid tracking of these floods to their source, even in the
face of forged source addresses. UUNET may have pioneered this
research, but anyone else who doesn*t develop comparable facilities
will find their customers fleeing to providers who can. The minimum
standard for service has just gone up.

Possibly the hardest part of the problem lies in notification. How do
you contact the people who need to help you solve one of these
problems, rapidly? It can take days to find and speak to the
responsible systems administrator for a given machine, knowing only
that machine*s address. People on the Bugtraq mailing list (described
in the Bibliography, below) are working out the design for an alert
notification system, to help speed response.

A. Bibliography

The Bugtraq mailing list carried the first public descriptions of the
tools used in this attack, a series of articles in December, 1999 by
David Dittrich. I*ll be happy to email copies of these articles. The
Bugtraq mailing list has archives available online, and subscription
info available, at www.securityfocus.com/.

Robert Stone of UUNET presented a paper at the October NANOG (North
American Network Operators Group) entitled "CenterTrack: An IP Overlay
Network for Tracking DoS Floods", describing a system developed at
UUNET to assist in tracking these attacks to their sources quickly.
The abstract is available at
www.nanog.org/mtg-9910/robert.html. The paper is available from

The FBI has published an alert, including contact info for notifying
them if you find any example of the attack in action, and tools for
spotting the attacking software, at

David Dittrich of the University of Washington, who has done most of
the published analysis of the attacking tools, has his papers and
analyses available on the Internet at

The SANS Institute has published articles on the Distributed Denial of
Service (DDoS attack) and on the ingress filtering that should be
deployed to help make it harder to implement and easier to track down
and stop. The DDoS paper is at www.sans.org/y2k/DDoS.htm, and
the Ingress Filtering article is at www.sans.org/y2k/egress.htm
(the difference between **ingress** and **egress** is which side of
the fence you are standing on; in this case different reporters have
used different terms for the same concept. In both cases the filtering
refers to ensuring that source addresses are accurate as packets leave
their originating local area networks (LANs) and emerge into the core
routers of the Internet.)

The news and discussion website Slashdot www.slashdot.org/ has
articles related to this topic, including an interview with David
Dittrich and an update of the above-mentioned FBI website.

The Internet Engineering Task Force (IETF) issued RFC 2267 in January
of 1998 entitled "Network Ingress Filtering: Defeating Denial of
Service Attacks which employ IP Source Address Spoofing".

David Dittrich*s talk for the Computer Emergency Response Team (CERT)
Distributed Intruder Tools Workshop is available online at

The results of that workshop are available at
www.cert.org/reports/dsit_workshop.pdf, and CERT also issued an
Advisory about this attack at
www.cert.org/advisories/CA-2000-01.html. These references were
cut-n-pasted from an article posted by Elias Levy, the moderator of
the Bugtraq list, to that list. The article, entitled **DDOS Attack
Mitigation**, is the best overview of the situation I*ve found online
to date, and I*ll be happy to forward a copy via email.

Cisco has a page up describing various measures that can be taken with
their equipment, to help protect against various aspects of these
attacks, at

B. Securing servers

Securing a computer system is always a tradeoff; to make it more
secure, you disable services, making it less useful, and you carefully
examine details of those services you leave running, making it more
expensive to set up and maintain. Servers however are easy to secure;
they are special-purpose machines, and need only offer a very limited
range of services. So the bulk of the effort consists solely in
disabling everything else.

First, find out everything that*s running on your server. List the
processes, or better list the network ports that have servers
listening on them. The commands to do this vary from one OS to
another; under Unix processes can be listed with **ps**, and open
network ports can be listed with **netstat**. A better tool, which
lists open network ports together with which process is listening on
each one, is **lsof**, available from

Second, disable everything but the specific processes required to
serve the content for which the machine is in use. For example, a web
server should not be listening on any of the network ports for other
services besides http (TCP port 80) or https (TCP port 443). For
remote administration and content updates, use a remote login and file
copy program with good encryption, such as ssh

Third, install packet filtering. Packet filtering comes with recent
Linux releases, and is available for most other OSes. IPFilter
coombs.anu.edu.au/~avalon/ip-filter.html works with most
versions of Unix. Packet filtering gives you two benefits. First, it
allows you to once again block off everything that doesn*t need to be
remotely accessible; this provides a second line of defense, in case
any of the services you disabled should be inadvertently re-enabled.
And second, it allows a machine to provide fine control over access to
services. For example, a web server may need to run, or to access, a
database server. That database server should not be accessible by
random strangers over the Internet, but it needs to be accessible to
the web server. This sort of control can be enforced by packet

C. Acknowledgements

I couldn*t have done this paper without all the superb research work,
published to the benefit of everyone in the industry; in particular
David Dittrich, Marcus J. Ranum, and Elias Levy have provided my most
valuable original sources.

Adam Rothschild <asr@oven.com> found the online location for the
CenterTrack paper.

Lew Perin <perin@acm.org> helped with suggestions about elaborating
the discussion of securing servers.

Jeff Moore <jbm@oven.com> provided many helpful suggestions that
substantially improved this paper.

Patrick W. Gilmore <patrick@ianai.net> corrected some technical
errors, and added the material about IP Directed Broadcast.

Emerson Tan, of Arthur Andersen, provided the reference to Cisco*s
document on preventing DDoS attacks.

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