Archive of CERT general posting, CERT Advisory CA-98.01 - smurf

06/01/98, CERT Advisory CA-98.01 - smurf
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Subject: CERT Advisory CA-98.01 - smurf
From: CERT Advisory <>
Date: Mon, 5 Jan 1998 15:15:29 -0500
Organization: CERT(sm) Coordination Center - +1 412-268-7090


CERT* Advisory CA-98.01.smurf
Original issue date: Jan. 05, 1998
Last revised: --

Topic: "smurf" IP Denial-of-Service Attacks
- -----------------------------------------------------------------------------

This advisory is intended primarily for network administrators responsible for
router configuration and maintenance.

The attack described in this advisory is different from the denial-of-service
attacks described in CERT advisory CA-97.28.

The CERT Coordination Center has received reports from network service
providers (NSPs), Internet service providers (ISPs), and other sites of
continuing denial-of-service attacks involving forged ICMP echo request
packets (commonly known as "ping" packets) sent to IP broadcast
addresses. These attacks can result in large amounts of ICMP echo reply
packets being sent from an intermediary site to a victim, which can cause
network congestion or outages. These attacks have been referred to as "smurf"
attacks because the name of one of the exploit programs attackers use to
execute this attack is called "smurf."

The CERT/CC urges you to take the steps described in Section III to reduce the
potential that your site can be used as the origination site (Sec. III.C) or
an intermediary (Sec. III.A.) in this attack. Although there is no easy
solution for victim sites, we provide some recommendations in Sec. III.B.

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

- -----------------------------------------------------------------------------

I.   Description

The two main components to the smurf denial-of-service attack are the use of
forged ICMP echo request packets and the direction of packets to IP broadcast

The Internet Control Message Protocol (ICMP) is used to handle errors and
exchange control messages. ICMP can be used to determine if a machine on the
Internet is responding. To do this, an ICMP echo request packet is sent to a
machine. If a machine receives that packet, that machine will return an ICMP
echo reply packet. A common implementation of this process is the "ping"
command, which is included with many operating systems and network software
packages. ICMP is used to convey status and error information including
notification of network congestion and of other network transport
problems. ICMP can also be a valuable tool in diagnosing host or network

On IP networks, a packet can be directed to an individual machine or broadcast
to an entire network. When a packet is sent to an IP broadcast address from a
machine on the local network, that packet is delivered to all machines on that
network. When a packet is sent to that IP broadcast address from a machine
outside of the local network, it is broadcast to all machines on the target
network (as long as routers are configured to pass along that traffic).

IP broadcast addresses are usually network addresses with the host portion of
the address having all one bits. For example, the IP broadcast address for the
network is If you have subnetted your class A network
into 256 subnets, the IP broadcast address for the 10.50 subnet would be Network addresses with all zeros in the host portion, such as, can also produce a broadcast response.

In the "smurf" attack, attackers are using ICMP echo request packets directed
to IP broadcast addresses from remote locations to generate denial-of-service
attacks. There are three parties in these attacks: the attacker, the
intermediary, and the victim (note that the intermediary can also be a

The intermediary receives an ICMP echo request packet directed to the IP
broadcast address of their network. If the intermediary does not filter ICMP
traffic directed to IP broadcast addresses, many of the machines on the
network will receive this ICMP echo request packet and send an ICMP echo reply
packet back. When (potentially) all the machines on a network respond to this
ICMP echo request, the result can be severe network congestion or outages.

When the attackers create these packets, they do not use the IP address of
their own machine as the source address. Instead, they create forged packets
that contain the spoofed source address of the attacker's intended victim. The
result is that when all the machines at the intermediary's site respond to the
ICMP echo requests, they send replies to the victim's machine. The victim is
subjected to network congestion that could potentially make the network
unusable. Even though we have not labeled the intermediary as a "victim," the
intermediary can be victimized by suffering the same types of problem that the
"victim" does in these attacks.

Attackers have developed automated tools that enable them to send these
attacks to multiple intermediaries at the same time, causing all of the
intermediaries to direct their responses to the same victim. Attackers have
also developed tools to look for network routers that do not filter broadcast
traffic and networks where multiple hosts respond. These networks can the
subsequently be used as intermediaries in attacks.

For a more detailed description of the "smurf" attack, please consult
this document:

    "The Latest in Denial of Service Attacks: 'Smurfing':
        Description and Information to Minimize Effects"
    Author: Craig Huegen <>

II.  Impact

Both the intermediary and victim of this attack may suffer degraded network
performance both on their internal networks or on their connection to the
Internet. Performance may be degraded to the point that the network cannot be

A significant enough stream of traffic can cause serious performance
degradation for small and mid-level ISPs that supply service to the
intermediaries or victims. Larger ISPs may see backbone degradation and
peering saturation.

III. Solution

A. Solutions for the Intermediary

1. Disable IP-directed broadcasts at your router.

One solution to prevent your site from being used as an intermediary in this
attack is to disable IP-directed broadcasts at your router. By disabling these
broadcasts, you configure your router to deny IP broadcast traffic onto your
network from other networks. In almost all cases, IP-directed broadcast
functionality is not needed.

Appendix A contains details on how to disable IP-directed broadcasts for some
router vendors. If your vendor is not listed, contact that vendor for

You should disable IP-directed broadcasts on all of your routers. It is not
sufficient to disable IP-directed broadcasts only on the router(s) used for
your external network connectivity. For example, if you have five routers
connecting ten LANs at your site, you should turn off IP-directed broadcasts
on all five routers.

2. Configure your operating system to prevent the machine from responding to
   ICMP packets sent to IP broadcast addresses.

If an intruder compromises a machine on your network, the intruder may try to
launch a smurf attack from your network using you as an intermediary. In this
case, the intruder would use the compromised machine to send the ICMP echo
request packet to the IP broadcast address of the local network. Since this
traffic does not travel through a router to reach the machines on the local
network, disabling IP-directed broadcasts on your routers is not sufficient to
prevent this attack.

Some operating systems can be configured to prevent the machine from
responding to ICMP packets sent to IP broadcast addresses. Configuring
machines so that they do not respond to these packets can prevent your
machines from being used as intermediaries in this type of attack.

Appendix A also contains details on how to disable responding to ICMP packets
sent to IP broadcast addresses on some operating systems. If your operating
system is not listed, contact your vendor for instructions.

B. Solutions for the Victim

Unfortunately, there is no easy solution for victims receiving the
potentially large number of ICMP echo reply packets. ICMP echo reply traffic
(the traffic from the intermediary) could be blocked at the victim's router;
however, that will not necessarily prevent congestion that occurs between the
victim's router and the victim's Internet service provider. Victims receiving
this traffic may need to consult with their Internet service provider to
temporarily block this type of traffic in the ISP's network.

Additionally, victims in this position should contact the intermediaries and
inform them of the attack and of the steps described in the previous
section. (Please refer them to or for the most recent version of this

Victims can use the "whois" command to obtain contact information for
the sites. More information on using whois is available in

C. Solution for the Site Where Attacks Originate

We recommend filtering outgoing packets that contain a source address from a
different network.

Attacks like the smurf attack rely on the use of forged packets, that is,
packets for which the attacker deliberately falsifies the origin address. With
the current IP protocol technology, it is impossible to eliminate IP-spoofed
packets. However, you can use filtering to reduce the likelihood of your
site's networks being used to initiate forged packets.

As we mentioned in CERT advisory CA-97.28 on Teardrop and Land
denial-of-service attacks, the best current method to reduce the number of
IP-spoofed packets exiting your network is to install filtering on your
routers that requires packets leaving your network to have a source
address from your internal network. This type of filter prevents a source
IP-spoofing attack from your site by filtering all outgoing packets that
contain a source address from a different network.

A detailed description of this type of filtering is available in the
Internet Draft "Network Ingress Filtering: Defeating Denial of Service
Attacks which employ IP Source Address Spoofing" by Paul Ferguson of
Cisco Systems, Inc. and Daniel Senie of Blazenet, Inc. We recommend it
to both Internet Service Providers and sites that manage their own
routers. The document is currently available at

Note that although this document is labeled as an IETF "working draft," the
content is complete and it is being proposed as an Informational RFC. Because
this is an Internet Draft, it will only be available under the above URL for
several months. When the document is available as an RFC or otherwise, we will
update the URL in this advisory.


Appendix A - Vendor Information

Below is a list of the vendors who have provided information for this
advisory. We will update this appendix as we receive additional information.
If you do not see your vendor's name, the CERT/CC did not hear from that
vendor. Please contact the vendor directly.

Cray Research - A Silicon Graphics Company
Current versions of Unicos and Unicos/mk do not have the ability to
reject ICMP requests send to broadcast addresses.  We are tracking
this problem through SPR 709733.

Cisco Systems
Cisco recommends the following configuration settings as protection against
being used as an intermediary in smurf attacks:

1. Disabling IP directed broadcast for all interfaces on which it is
   not needed. This must be done on all routers in the network, not
   just on the border routers. The command "no ip directed-broadcast"
   should be applied to each interface on which directed broadcasts
   are to be disabled.

   Very few IP applications actually need to use directed broadcasts,
   and it's extremely rare for such an application to be in use in a network
   without the knowledge of the network administrator. Nonetheless,
   as when any functionality is disabled, you should be alert for
   possible problems.

   This is the preferred solution for most networks.

2. If your network configuration is simple enough for you to create
   and maintain a list of all the directed broadcast addresses in
   your network, and if you have a well-defined perimeter separating
   your own network from potentially hostile networks, consider
   using a filter at the perimeter to prevent directed broadcasts from
   entering the network. For example, if your network number is, and you uniformly use a subnet mask of,
   then you might use a Cisco access list entry like

     access-list 101 deny ip

   Note that this is not a complete access list; it's simply a single
   entry. See the Cisco documentation for more information on configuring
   access lists. The best place to apply such a filter is usually on
   the incoming side of each router interface that connects to the
   potentially hostile network.

   This solution may be administratively infeasible for networks using
   variable-length subnet masks, or which have complex external
   connectivity. There is also some possibility that legitimate
   directed broadcasts may be being sent into your network from the
   outside, especially if you're working in a research environment.

In addition to these protections against being used as an intermediary in a
smurf attack, Cisco recommends that you take steps to prevent users within
your own network from launching such attacks. For "stub" networks which do
not provide transit connectivity (most corporate and institutional
networks, many smaller ISPs), this is usually best done by installing
filters at the network perimeter to prevent any packets from leaving
your network unless their IP source addresses actually lie within
your network's address space. For the example network above, you might
place the following entry in the incoming access lists on the interface(s)
facing your internal network:

   access-list 101 permit ip
   access-list 101 deny ip

FreeBSD, Inc.
In FreeBSD 2.2.5 and up, the tcp/ip stack does not respond to icmp
echo requests destined to broadcast and multicast addresses by default. This
behaviour can be changed via the sysctl command via
mib net.inet.icmp.bmcastecho.

IBM Corporation
- -----
There is a network attribute called "bcastping" that controls whether
or not responses to ICMP echo packets to the broadcast address are
allowed.  A value of zero turns off responses and a value of one
turns them on.  The default is zero (i.e., by default AIX version 4
is not vulnerable to the described denial-of-service attack).

Use the following command to check the value of the bcastping

   $ no -o bcastping

Use the following command to turn off responses to ICMP broadcast
packets (as root):

   # no -o bcastping=0

- -----
The "bcastping" attribute does not exist in version 3.

IBM and AIX are registered trademarks of International Business Machines

Livingston Enterprises, Inc.
Livingston Enterprises products discard any ICMP packets directed to
broadcast addresses, so we protect against this form of attack.  No
special configuration is required.

The NetBSD Project
Under NetBSD you can disable directed broadcast with this command,
as root:

	# sysctl -w net.inet.ip.directed-broadcast=0

- -----------------------------------------------------------------------------
The CERT Coordination Center thanks Craig A. Huegen. Much of the content in
this advisory has been derived from his document on "smurf" attacks. The CERT
Coordination Center also thanks Paul Ferguson and Daniel Senie for providing
information on network ingress filtering, and John Bashinski of Cisco for his
- -----------------------------------------------------------------------------

If you believe that your system has been compromised, contact the CERT
Coordination Center or your representative in the Forum of Incident Response
and Security Teams (see

CERT/CC Contact Information
- ----------------------------

Phone    +1 412-268-7090 (24-hour hotline)
                CERT personnel answer 8:30-5:00 p.m. EST(GMT-5) / EDT(GMT-4)
                and are on call for emergencies during other hours.

Fax      +1 412-268-6989

Postal address
         CERT Coordination Center
         Software Engineering Institute
         Carnegie Mellon University
         Pittsburgh PA 15213-3890

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Getting security information
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- ---------------------------------------------------------------------------

Copyright 1998 Carnegie Mellon University. Conditions for use, disclaimers,
and sponsorship information can be found in and .
If you do not have FTP or web access, send mail to with
"copyright" in the subject line.

*CERT is registered in the U.S. Patent and Trademark Office.

- ---------------------------------------------------------------------------

This file:

Revision history

Version: 2.6.2


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