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Original issue date: February 8, 1996
Last revised: September 24, 1997
Updated copyright statement

A complete revision history is at the end of this file.
The CERT Coordination Center has received reports of programs that launch denial-of-service attacks by creating a "UDP packet storm" either on a system or between two systems. An attack on one host causes that host to perform poorly. An attack between two hnosts can cause extreme network congestion in addition to adversely affecting host performance.

The CERT staff recommends disabling unneeded UDP services on each host, in particular the chargen and echo services, and filtering these services at the firewall or Internet gateway.

Because the UDP port denial-of-service attacks typically involve IP spoofing, we encourage you to follow the recommendations in advisory CA-96.21.

We will update this advisory as we receive additional information. Please check advisory files regularly for updates that relate to your site

I. Description

When a connection is established between two UDP services, each of which produces output, these two services can produce a very high number of packets that can lead to a denial of service on the machine(s) where the services are offered. Anyone with network connectivity can launch an attack; no account access is needed.

For example, by connecting a host's chargen service to the echo service on the same or another machine, all affected machines may be effectively taken out of service because of the excessively high number of packets produced. In addition, if two or more hosts are so connected, the intervening network may also become congested and deny service to all hosts whose traffic traverses that network.

II. Impact

Anyone with network connectivity can cause a denial of service. This attack does not enable them to gain additional access.

III. Solution

We recommend taking all the steps described below.

1. Disable and filter chargen and echo services.

This attack is most readily exploited using the chargen or echo services, neither of which is generally needed as far as we are aware. We recommend that you disable both services on the host and filter them at the firewall or Internet gateway.

To disable these services on a host, it is necessary to edit the inetd configuration file and cause inetd to begin using the new configuration. Exactly how to do this is system dependent so you should check your vendor's documentation for inetd(8); but on many UNIX systems the steps will be as follows:

  1. Edit the inetd configuration file (e.g. /etc/inetd.conf).
  2. Comment out the echo, chargen, and other UDP services not used.
  3. Cause the inetd process to reread the configuration file (e.g., by sending it a HUP signal).

2. Disable and filter other unused UDP services.

To protect against similar attacks against other services, we recommend:

- disabling all unused UDP services on hosts and
- blocking at firewalls all UDP ports less than 900 with the exception of specific services you require, such as DNS (port 53).

3. If you must provide external access to some UDP services, consider using a proxy mechanism to protect that service from misuse.

Techniques to do this are discussed in Chapter 8, "Configuring Internet Services," in _Building Internet Firewalls_ by Chapman and Zwicky (see Section IV below).

4. Monitor your network.

If you do provide external UDP services, we recommend monitoring your network to learn which systems are using these services and to monitor for signs of misuse. Tools for doing so include Argus, tcpdump, and netlog.

Argus is available from
MD5 (argus-1.5.tar.gz) = 9c7052fb1742f9f6232a890267c03f3c

Note that Argus requires the TCP wrappers to install:
MD5 (tcp_wrappers_7.2.tar.Z) = 883d00cbd2dedd9bfc783b7065740e74

tcpdump is available from
MD5 (tcpdump-3.0.2.tar.Z) = c757608d5823aa68e4061ebd4753e591

Note that tcpdump requires libpcap, available at
MD5 (libpcap-0.0.6.tar.Z) = cda0980f786932a7e2eebfb2641aa7a0

netlog is available from
MD5 (netlog-1.2.tar.gz) = 1dd62e7e96192456e8c75047c38e994b

5. Take steps against IP spoofing.

Because IP spoofing is typically involved in UDP port denial-of-service attacks, we encourage you to follow the guidance in advisory CA-95:01, available from

IV. Sources of further information about packet filtering

For a general packet-filtering recommendations, see

For in-depth discussions of how to configure your firewall, see

Firewalls and Internet Security: Repelling the Wily Hacker
William R. Cheswick and Steven M. Bellovin
Addison-Wesley Publishing Company, 1994
ISBN 0-201-63357

Building Internet Firewalls
Brent Chapman and Elizabeth D. Zwicky
O'Reilly & Associates, Inc., 1995
ISBN 1-56592-124-0

The CERT Coordination Center staff thanks Peter D. Skopp of Columbia University for reporting the vulnerability and Steve Bellovin of AT&T Bell Labs for his support in responding to this problem.



Cisco Alert Summary:

Cisco Security Guide

Silicon Graphics Inc.

SGI acknowledges CERT Advisory CA-96.01 and is currently investigating. No further information is available at this time.

Copyright 1996, 1997 Carnegie Mellon University.

Revision History
Sep. 24, 1997 Updated copyright statement
Feb. 14, 1997 Introduction - updated the IP spoofing reference to CA-96.21.
              Updates section - added pointers to CISCO documents. 
Aug. 30, 1996 Information previously in the README was inserted into
              the advisory.
Feb. 23, 1996 Updates section - added information from Silicon Graphics, Inc. 
Feb. 21, 1996 Solution, Sec. III.4 - added new URL for Argus.
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