Voici le post de Elias Levy le 11 février 2000 dans la liste Bugtraq (abonnez-vous là)
à propos des DoS.


Unless you lived under a rock for the past week you've heard of the
distributed denial of service attacks (DDOS) against some of the
top Internet web sites. In particular these attacks were of the
bandwidth consumption type. Some of the network providers involved
claim to have been upwards of 1 Gb/s traffic.

These attacks are not knew. David Dittrich has performed an excellent
job dissecting these attacks and the tools that enable them. You
can find David's analysis of these tools at:

* Trinoo

* Tribe Flood Network

* Stacheldraht

A number of meetings have been held to discuss the attacks and search
for possible solutions. These include CERT Distributed-Systems Intruder
Tools Workshop back in November and the two recent Birds of Feather (BOF)
sessions organized by the ICSA at the RSA and NANOG conferences.

You can find David's notes on the CERT workshop at:

You can find the results of the CERT workshop at:

CERT has also issued an advisory last month on the problem:

These attack are made possible because of fundamental design decisions
at the IP protocol level. It does not provide strong authentication of
the source of a packet, and it only provides a best effort service
with no resource allocation protocol.

To date no one has come up with a "silver bullet" solution to the problem.
That being said, the are a number of options to mitigate it. I'll present
a few and would like to hear what other suggestions you may have.

Network Ingress Filtering:

All network access providers should implement network ingress filtering
to stop any of their downstream networks from injecting packets with
faked or "spoofed" addressed into the Internet.

Although this does not stop an attack from occurring it does make it
much easier to track down the source of the attack and terminate it

For information on network ingress filtering read RFC 2267:

Rate Limit Some Network Traffic:

A number of routers in the market today have features that allow you
you limit the amount of bandwidth some type of traffic can consume.
This is sometimes referred to as "traffic shaping".

In Cisco IOS software this feature is called Committed Access Rate (CAR).
CAR allows you to enforce a bandwidth policy against network traffic
that matches an access list.

This can be used in a proactive way if you know most of your network
traffic will be of some particular type. For example if you are running
a web farm you can configure the system such as any web traffic gets
as much bandwidth as it requires while limiting all other traffic to
smaller manageable rate.

It can also be used in a reactive way if you can craft an access rule
that will match some of the network traffic using by the DDOS attack.
For example if the attack is employing ICMP packets or TCP SYN packets
you could configure the system to specificly limit the bandwidth those
types of packets will be allowed to consume. This will allow some of
these packets which may belong to legitimate network flows to go through.

Because of the avalanche effect of the DDOS attacks for this option to
be effective it must be deployed as depth into the network as possible
(closer to the source of the attack packets). You may need to ask your
network access provider to implement these filters for you in their
routers. This will not be possible for many organizations for a number
of reasons.

Furthermore, DDOS attack tools can generate random packets such as that
matching them with a set of access list rules can become difficult unless
you do so by using negative space (by defining normal traffic and assuming
everything else is DDOS traffic).

To find out more about CAR go to:

Intrusion Detection Systems

Use an Intrusion Detection System to detect attackers that are communicating
with the "slave", "master" or "agent" machines. This will allow you
to know whether some machine in your network is being used to launch a known
attack but will probably not detect new variations of these attacks and the
tools that implement them.

Most IDS vendors have signatures available to detect Trinoo, TNF or
Stacheldraht network traffic.

Host Auditing Tools

A number of file scanning tools are available that attempt to detect the
existence of known DDOS tool client and server binaries in your system.
A number of host auditing tool vendors have updated their tools to include
these signatures. Just like antivirus software the tools become obsolete
as new DDOS tools are developed or modified.

The NIPC has made available a tool called "find_ddos" that searches the
filesystem for the Trinoo, TNF, TNF2K and Stacheldraht DDOS tools. The
tool is available for Solaris (Sparc and Intel) and Linux (Intel)
operating systems. Beware that the NIPC does not provide source code
to this program. You can find the program at:

Network Auditing Tools

A number of network scanning tools are available that attempt to detect
the presence of DDOS agents running on hosts on your network. A number
of network auditing tool vendors have updated their products to include
the tests. Just like antivirus software these tools become obsolete
as new DDOS tools are developed or modified.

Dave Dittrich, Marcus Ranum, and others have developed "gag". This tool
detects Stacheldraht agents.  Its available for Unix systems. You can
find the program at:

Dave Dittrich, Marcus Ranum, George Weaver, David Brumley, and others have
developed "dds". This tool detects Trinoo, TFN and Stacheldraht agents. You
can find the program at:

Automated Network Tracing Tools

Tracing streams of packets with faked or "spoof" address through
the network is a time consuming tasks that requires the cooperation
of all networks carrying the traffic and that must be completed
while the attack is in progress.

If you recall when SYN flooding DOS attacks became fashionable back in
1997 MCI developed a tool called DoSTracker that automated a lot of
the work required for them to trace the source of an attack through
their network.

Tools need to be developed to automate the tracing process within
a network under the same authority as well as tools that can request
traces to be performed across network authority boundaries.

I'll buy the first person to write such tool a case of Red Hook
Extra Special Bitter (ESB).

Emergency Data Centers

One can also think of these attacks as some type of natural disaster. It
is common to have as part of your disaster contingency plans access
to an offsite emergency data center that can be brought online in a short
period of time to resume partial or full operational capacity. Organizations
with enough resources should consider such site as a mitigating factor
to the risk of a DDOS attack.


A number of insurance companies are now providing computer and computer
security related policies. Under some circumstances this may provide
a better return on investment (ROI) that some of the other measures
presented here.

Example of companies providing some type of computer security insurance are:

If you have any other suggestions that might mitigate the DDOS attacks
let us know.

Elias Levy


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