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  • CERT Advisory CA-1999-17 Denial-of-Service Tools

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Original release date: December 28, 1999
Last Updated: March 3, 2000
Source: CERT/CC

A complete revision history is at the end of this file.

Systems Affected

  • All systems connected to the Internet can be affected by denial-of-service attacks. Tools that run on a variety of UNIX and UNIX-like systems and Windows NT systems have recently been released to facilitate denial-of-service attacks. Additionally, some MacOS systems can be used as traffic amplifiers to conduct a denial-of-service attack.

I. Description

New Distributed Denial-of-Service Tools

Recently, new techniques for executing denial-of-service attacks have been made public. A tool similar to Tribe FloodNet (TFN), called Tribe FloodNet 2K (TFN2K) was released. Tribe FloodNet is described in

Like TFN, TFN2K is designed to launch coordinated denial-of-service attacks from many sources against one or more targets simultaneously. It includes features designed specifically to make TFN2K traffic difficult to recognize and filter, to remotely execute commands, to obfuscate the true source of the traffic, to transport TFN2K traffic over multiple transport protocols including UDP, TCP, and ICMP, and features to confuse attempts to locate other nodes in a TFN2K network by sending "decoy" packets.

TFN2K is designed to work on various UNIX and UNIX-like systems and Windows NT.

TFN2K obfuscates the true source of attacks by spoofing IP addresses. In networks that employ ingress filtering as described in [1], TFN2K can forge packets that appear to come from neighboring machines.

Like TFN, TFN2K can flood networks by sending large amounts of data to the victim machine. Unlike TFN, TFN2K includes attacks designed to crash or introduce instabilities in systems by sending malformed or invalid packets. Some attacks like this are described in

Also like TFN, TFN2K uses a client-server architecture in which a single client, under the control of an attacker, issues commands simultaneously to a set of TFN2K servers. The servers then conduct the denial-of-service attacks against the victim(s). Installing the server requires that an intruder first compromise a machine by different means.

Asymmetric traffic from MacOS 9

MacOS 9 can be abused by an intruder to generate a large volume of traffic directed at a victim in response to a small amount of traffic produced by an intruder. This allows an intruder to use MacOS 9 as a "traffic amplifier," and flood victims with traffic. According to [3], an intruder can use this asymmetry to "amplify" traffic by a factor of approximately 37.5, thus enabling an intruder with limited bandwidth to flood a much larger connection. This is similar in effect and structure to a "smurf" attack, described in

Unlike a smurf attack, however, it is not necessary to use a directed broadcast to achieve traffic amplification.

II. Impact

Intruders can flood networks with overwhelming amounts of traffic or cause machines to crash or otherwise become unstable.

III. Solution

The problem of distributed denial-of-service attacks is discussed at length in [2], available at

Managers, system administrators, Internet Service Providers (ISPs) and Computer Security Incident Response Teams (CSIRTs) are encouraged to read this document to gain a broader understanding of the problem.

For the ultimate victim of distributed denial-of-service attacks

Preparation is crucial. The victim of a distributed denial-of-service attack has little recourse using currently available technology to respond to an attack in progress. According to [2]:

The impact upon your site and operations is dictated by the (in)security of other sites and the ability of of a remote attackers to implant the tools and subsequently to control and direct multiple systems worldwide to launch an attack.

Sites are strongly encouraged to develop the relationships and capabilities described in [2] before you are a victim of a distributed denial-of-service attack.

For all Internet Sites

System and network administrators are strongly encouraged to follow the guidelines listed in [2]. In addition, sites are encouraged to implement ingress filtering as described in [1]. CERT/CC recommends implementing such filtering on as many routers as practical. This method is not foolproof, as mentioned in [1]:

While the filtering method discussed in this document does absolutely nothing to protect against flooding attacks which originate from valid prefixes (IP addresses), it will prohibit an attacker within the originating network from launching an attack of this nature using forged source addresses that do not conform to ingress filtering rules.

Because TFN2K implements features designed specifically to take advantage of the granularity of ingress filtering rules, the method described in [1] means that sites may only be able to determine the network or subnet from which an attack originated.

Sites using manageable hubs or switches that can track which IP addresses have been seen at a particular port or which can restrict which MAC addresses can be used on a particular port may be able to further identify which machine(s) is responsible for TFN2K traffic. For further information, consult the documentation for your particular hub or switch.

The widespread use of this type of filtering can significantly reduce the ability of intruders to use spoofed packets to compromise or disrupt systems.

Preventing your site from being used by intruders

TFN2K and similar tools rely on the ability of intruders to install the client. Preventing your system from being used to install the client will help prevent intruders from using your systems to launch denial-of-service attacks (in addition to whatever damage they may cause to your systems).

Popular recent attacks can be found at

Sites are encouraged to regularly visit this page and address any issues found there.

For the "Mac Attack"

Apple has developed a patch, as described in Appendix A. Please see the information there.

Appendix A contains information provided by vendors for this advisory. We will update the appendix as we receive or develop more information. If you do not see your vendor's name in Appendix A, the CERT/CC did not hear from that vendor. Please contact your vendor directly.

Appendix A. Vendor Information

Apple Computer

OT Tuner 1.0 switches off an option in Open Transport that would cause a Macintosh to respond to certain small network packets with a large Internet Control Message Protocol (ICMP) packet. This update prevents Macintosh computers from being the cause of certain types of Denial of Service (DOS) issues.

The update is available from our software update server at

In addition, it will soon be available via the automatic update feature that is part of Mac OS 9.


[1] RFC2267, Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing , P. Ferguson, D. Senie, The Internet Society, January, 1998, available at

[2] Results of the Distributed-Systems Intruder Tools Workshop, The CERT Coordination Center, December, 1999, available at

[3] The "Mac Attack," a Scheme for Blocking Internet Connections, John A. Copeland, December, 1999, available at Temporary alternate URL:
The CERT Coordination Center thanks Jeff Schiller of the Massachusetts Institute of Technology, Professor John Copeland and Jim Hendricks of the Georgia Institute of Technology, Jim Ellis of Sun Microsystems, Wietse Venema of IBM, Rick Forno of Network Solutions, Inc., Dave Dittrich of the University of Washington, Steve Bellovin of AT&T, Jim Duncan and John Bashinski of Cisco Systems, and MacInTouch for input and technical assistance used in the construction of this advisory.

Copyright 1999 Carnegie Mellon University.

Revision History
December 28, 1999: Initial release
December 28, 1999: Added information regarding a patch from Apple
March 3, 2000: Updated link to apple web page.
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