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CERT(R) Advisory CA-99-17 Denial-of-Service Tools

   Original release date: December 28, 1999
   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
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 is developing a patch, as described in Appendix A. This advisory
   will be updated when the patch is available.
   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

   We've reproduced the problem in our lab and we are working now to
   create a fix that can be easily distributed to our customers. The
   problem only affects customers running our most recent release of
   networking software on machines that are continuously attached to the
   While most Macintosh customers are not affected by this problem, we
   are moving quickly to put a solution in place.

   [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, and
   Jim Duncan and John Bashinski of Cisco Systems for input and technical
   assistance used in the construction of this advisory.
   This document is available from:
CERT/CC Contact Information

   Email: This email address is being protected from spambots. You need JavaScript enabled to view it.
          Phone: +1 412-268-7090 (24-hour hotline)
          Fax: +1 412-268-6989
          Postal address:
          CERT Coordination Center
          Software Engineering Institute
          Carnegie Mellon University
          Pittsburgh PA 15213-3890
   CERT personnel answer the hotline 08:00-20:00 EST(GMT-5) / EDT(GMT-4)
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Getting security information

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   Copyright 1999 Carnegie Mellon University.
   Conditions for use, disclaimers, and sponsorship information can be
   found in
   * "CERT" and "CERT Coordination Center" are registered in the U.S.
   Patent and Trademark Office.
   Any material furnished by Carnegie Mellon University and the Software
   Engineering Institute is furnished on an "as is" basis. Carnegie
   Mellon University makes no warranties of any kind, either expressed or
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   does not make any warranty of any kind with respect to freedom from
   patent, trademark, or copyright infringement.
   Revision History
December 28, 1999:  Initial release

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