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denial-of-service attack| DoS attack| distributed denial-of-service attack| Attack

A denial-of-service attack (DoS attack) or distributed denial-of-service attack (DDoS attack) is an attempt to make a computer resource unavailable to its intended users. Although the means to carry out, motives for, and targets of a DoS attack may vary, it generally consists of the concerted efforts of a person or people to prevent an Internet site or service from functioning efficiently or at all, temporarily or indefinitely. Perpetrators of DoS attacks typically target sites or services hosted on high-profile web servers such as banks, credit card payment gateways, and even root nameservers.
One common method of attack involves saturating the target (victim) machine with external communications requests, such that it cannot respond to legitimate traffic, or responds so slowly as to be rendered effectively unavailable. In general terms, DoS attacks are implemented by either forcing the targeted computer(s) to reset, or consuming its resources so that it can no longer provide its intended service or obstructing the communication media between the intended users and the victim so that they can no longer communicate adequately.
Denial-of-service attacks are considered violations of the IAB's Internet proper use policy, and also violate the acceptable use policies of virtually all Internet Service Providers.


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.

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 future.
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 order.
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 them.

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 online.
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 one.
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 attacker.

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 http://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 http://www.nanog.org/mtg-9910/robert.html. The paper is available from http://www.us.uu.net/gfx/projects/security/centertrack.pdf.
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 http://www.fbi.gov/nipc/trinoo.htm.
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 http://www.washington.edu/People/dad/.
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 http://www.sans.org/y2k/DDoS.htm, and the Ingress Filtering article is at http://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 http://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 http://staff.washington.edu/dittrich/talks/cert/.
The results of that workshop are available at http://www.cert.org/reports/dsit_workshop.pdf, and CERT also issued an Advisory about this attack at http://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 http://www.cisco.com/warp/public/707/newsflash.html#overview.

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 ftp://vic.cc.purdue.edu/pub/tools/unix/lsof/.
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 http://www.openssh.org/.
Third, install packet filtering. Packet filtering comes with recent Linux releases, and is available for most other OSes. IPFilter http://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 filtering.

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 found the online location for the CenterTrack paper.
Lew Perin helped with suggestions about elaborating the discussion of securing servers.
Jeff Moore provided many helpful suggestions that substantially improved this paper.
Patrick W. Gilmore 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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