CERT® Advisory CA-1996-01 UDP Port Denial-of-Service AttackOriginal 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 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. DescriptionWhen 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. ImpactAnyone with network connectivity can cause a denial of service. This attack does not enable them to gain additional access.
III. SolutionWe 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:
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
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
Note that Argus requires the TCP wrappers to install:
tcpdump is available from
Note that tcpdump requires libpcap, available at
netlog is available from
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 filteringFor 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
Building Internet Firewalls
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.
CiscoCisco Alert Summary:
Silicon Graphics Inc.SGI acknowledges CERT Advisory CA-96.01 and is currently investigating. No further information is available at this time.
This document is available from: http://www.cert.org/advisories/CA-1996-01.html
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Copyright 1996, 1997 Carnegie Mellon University.
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.