"Smurf" Attack Information


Craig A. Huegen

Last Update:  Tue Feb  8 17:47:36 PST 2000

New additions: 

* Removed "smurf update" section -- no longer valid given distributed DoS

Editor's plea: *please* distribute this information freely, and abide by
my redistribution requirements (see the very end) when doing so.  It's
important that these attacks be minimized, and communication is the only
way to help with this.


The information here provides in-depth information regarding "smurf" and
"fraggle" attacks, with a focus on Cisco routers and how to reduce the
effects of the attack.  Some information is general and not related to an
organization's particular vendor of choice; however, it is written with a
Cisco router focus.  No confirmation has been made to the effects on other
vendors' equipment; however, others have provided me with information for
various vendors, which is provided in the document.  See the
"Acknowledgements" section below for the sources and contact information. 
I am happy to accept information from other colleagues or other vendors
who are willing to provide information about other vendors' products in
relation to this topic. 

This paper is always being updated as I receive more information about
attacks and work with ways to minimize impact. 


The "smurf" attack, named after its exploit program, is one of the most
recent in the category of network-level attacks against hosts.  A
perpetrator sends a large amount of ICMP echo (ping) traffic at IP broadcast
addresses, all of it having a spoofed source address of a victim.  If the
routing device delivering traffic to those broadcast addresses performs
the IP broadcast to layer 2 broadcast function noted below, most hosts on
that IP network will take the ICMP echo request and reply to it with an
echo reply each, multiplying the traffic by the number of hosts
responding.  On a multi-access broadcast network, there could potentially
be hundreds of machines to reply to each packet.

The "smurf" attack's cousin is called "fraggle", which uses UDP echo
packets in the same fashion as the ICMP echo packets; it was a simple
re-write of "smurf".

Currently, the providers/machines most commonly hit are IRC servers and
their providers.

There are two parties who are hurt by this attack...  the intermediary
(broadcast) devices--let's call them "amplifiers", and the spoofed address
target, or the "victim".  The victim is the target of a large amount of
traffic that the amplifiers generate.

Let's look at the scenario to paint a picture of the dangerous nature of
this attack.  Assume a co-location switched network with 100 hosts, and
that the attacker has a T1.  The attacker sends, say, a 768kb/s stream of
ICMP echo (ping) packets, with a spoofed source address of the victim, to
the broadcast address of the "bounce site".  These ping packets hit the
bounce site's broadcast network of 100 hosts; each of them takes the packet
and responds to it, creating 100 ping replies out-bound.  If you multiply
the bandwidth, you'll see that 76.8 Mbps is used outbound from the "bounce
site" after the traffic is multiplied.  This is then sent to the victim (the
spoofed source of the originating packets).


Several sites have been established to do both active and passive scanning
of networks to determine whether or not directed-broadcast is enabled. is a site which actively scans the IPv4 address
space and mails network contacts with information on how to disable them. is a site which will test scan your
network and allow you to enter a known smurf amplifier site.


The perpetrators of these attacks rely on the ability to source spoofed
packets to the "amplifiers" in order to generate the traffic which causes
the denial of service.

In order to stop this, all networks should perform filtering either at the
edge of the network where customers connect (access layer) or at the edge
of the network with connections to the upstream providers, in order to
defeat the possibility of source-address-spoofed packets from entering
from downstream networks, or leaving for upstream networks.

Paul Ferguson of cisco Systems and Daniel Senie of BlazeNet have written
an RFC pertaining to this topic.  See:

for more information and examples on this subject.

Additionally, router vendors have added or are currently adding options
to turn off the ability to spoof IP source addresses  by checking the
source address of a packet against the routing table to ensure the return
path of the packet is through the interface it was received on.

Cisco has added this feature to the current 11.1CC branch, used by many
NSP's, in an interface command '[no] ip verify unicast reverse-path'.

See the "other vendors" section for 3Com information regarding this feature.


This attack relies on the router serving a large multi-access broadcast
network to frame an IP broadcast address (such as into a
layer 2 broadcast frame (for Ethernet, FF:FF:FF:FF:FF:FF).  RFC 1812,
"Requirements for IP Version 4 Routers", Section 5.3.5, specifies: 

   A router MAY have an option to disable receiving network-prefix-
   directed broadcasts on an interface and MUST have an option to
   disable forwarding network-prefix-directed broadcasts.  These options
   MUST default to permit receiving and forwarding network-prefix-
   directed broadcasts.

Generally, with IP providers and IP applications as we know them today,
this behavior should not be needed, and it is recommended that
directed-broadcasts be turned off, to suppress the effects of this attack. 

RFC 2644, a Best Current Practice RFC by Daniel Senie, updates RFC 1812
to state that router software must default to denying the forwarding
and receipt of directed broadcasts.

Ethernet NIC hardware (MAC-layer hardware, specifically) will only listen
to a select number of addresses in normal operation.  The one MAC address
that all devices share in common in normal operation is the media
broadcast, or FF:FF:FF:FF:FF:FF.  If a device receives a packet destined
to the broadcast link-layer address, it will take the packet and send an
interrupt for processing by the higher-layer routines.

To stop your Cisco router from converting these layer 3 broadcasts into
layer 2 broadcasts, use the "no ip directed-broadcast" interface
configuration command.  This should be configured on each interface of all

As of Cisco IOS version 12.0, "no ip directed-broadcast" is now the default
in order to protect networks by default.  "ip directed-broadcast" will be
needed if your network requires directed broadcasts to be enabled.

Other vendor information: 

* Proteon/OpenROUTE: 
  Daniel Senie ( reports that Proteon/OpenROUTE Networks
  routers have an option to turn off directed broadcasts in the IP
  Configuration menus.  The command sequence to turn them off is: 
  *CONFIG (on newer routers) or TALK 6 (on older routers)
  A restart of the router is then required.
* Nortel Networks (Bay Networks): 
  Jon Green ( reports that bugID CR33408 added the
  ability to disable network-directed broadcasts beginning in version
  12.01 rev 1 of BayRS code.
  To disable, enter: 
  bcc> config
  hostname# ip
  ip# set directed-bcast disabled
  ip# exit
  Note that this will bounce all IP interfaces.
  Greg Hankins ( reports that in BayRS 13.01
  and later, directed-broadcast is disabled by default.
  Tim Winders ( mentions that if you upgrade
  to BayRS 13.01+ from 12.01, directed-broadcasts are not disabled.
* 3Com NETBuilder products: 
  Mike Kouri ( reports that all 3Com NETBuilders have
  an option to keep the router from forwarding the directed broadcasts.
  The command sequence to disable the forwarding is: 
  SETDefault -IP CONTrol = NoFwdSubnetBcast
  Additionally, 3Com NETBuilder products running version 9.1 or later can
  be configured to discard source-spoofed packets: 
  SETDefault !port -FireWall CONTrol = (Filter, DenySrcSpoofing)
  3Com states in the web page (listed below) that this command
  "Specifies whether packets are subject to source-spoofing checks. This is a
  CPU-intensive option and generally results in performance degradation. You
  should disable this option except on interfaces where external, untrusted
  traffic is received.  The source address of incoming packets is checked
  against the routing table.  If the routing information shows that the
  source address is unreachable, or reachable on different interfaces,
  then it is a SrcSpoofing attack."
* Lucent (Ascend): 
  Will Pierce ( reports that on Maxes or Pipelines
  running TAOS 6.0.0 or higher, you can go to the Ethernet->Mod Config menu
  and set both "Reply DirectedBcast Ping" and "Forward Directed Bcast" to
  "No".  For the Max TNT, there is an example at
  on page 40.  TNT versions 2.0.0 and higher support this.
* Cabletron SmartSwitch Router (Yago/SSR): 
  Greg Hankins ( reports directed-broadcast is
  disabled by default, and can be enabled by entering the global command
  "ip enable directed-broadcast".
* Foundry Networks: 
  Greg Hankins ( reports that hardware running
  Foundry's routing software can be configured to disable
  directed-broadcasts with the global or per-interface "no ip
  directed-broadcast" command.
* Redback Networks: 
  Justin Streiner ( reports that on the SMS-500
  and SMS-1000 access switches, there is no support for directed
  broadcasts unless used in conjunction with DHCP, and they are not
  forwarded by default.
* Extreme Networks: 
  Aurobindo Sundaram ( reports that you
  can disable IP broadcast forwarding on Extreme's Summit 1 switches
  by using the following commands: 
  disable ipforwarding broadcast all
  disable ipforwarding broadcast vlan vlan-name
* ArrowPoint Communications: 
  Greg Hankins ( reports that directed-broadcasts
  can be disabled by using the "no ip subnet-broadcast" global
  configuration command.
* SGI IRIX as a router: 
  Mike O'Connor ( reports that IRIX has been configured
  by default to not forward the directed-broadcasts when used as a router.
  The tunable for this is in /var/sysgen/master.d/bsd.

There is one case study where this will stop intended behavior: In the
case where samba (an SMB server for UNIX) or NT is used to "remote
broadcast" into a LAN workgroup so that the workstations on that LAN can
see the server, this will prevent the LAN machines from seeing the remote
server.  This is *only* in the case where there is no WINS server (WINS is
routed unicast) and a "remote broadcast" is being used--it's a rare but
notable condition.

(Editor's note:  I welcome any comments as to what else breaks without
the support for directed-broadcast.)

Additionally, hosts can be patched to refuse to respond to broadcasted
ICMP echo packets.  RFC 1122, "Requirements for Internet Hosts --
Communications Layer", Section, states: 

   An ICMP Echo Request destined to an IP broadcast or IP
   multicast address MAY be silently discarded.

      This neutral provision results from a passionate debate
      between those who feel that ICMP Echo to a broadcast
      address provides a valuable diagnostic capability and
      those who feel that misuse of this feature can too
      easily create packet storms.

Because of this, most IP stack implementors have chosen to implement the
default support provision, which is to reply to an ICMP Echo Request.
As mentioned in the paragraph from the RFC (above), it is perfectly legal
for a host to silently discard ICMP echos.  Several patches have been
found floating about in mailing lists for disabling response to broadcast
ICMP echos for the freely-available UNIX systems.

In the case of the smurf or fraggle attack, each host which supports this
behavior on a broadcast LAN will happily reply with an ICMP or UDP (smurf
or fraggle, respectively) echo-reply packet toward the spoofed source
address, the victim.

The following section contains information to configure hosts *not* to
respond to ICMP echo requests to broadcast addresses.

IBM has provided a setting in AIX 4.x to disable responses to broadcast
addresses.  It is not available in AIX 3.x.  Use the "no" command to turn
it off or on.  NOTE: On AIX 4.x responses are DISABLED by default.
        no -o bcastping=0         # disable bcast ping responses (default)

Solaris can be set not to respond to ICMP echo requests.  Add the
following line to your /etc/rc2.d/S69inet startup: 
        ndd -set /dev/ip ip_respond_to_echo_broadcast 0
If you're using Solaris as a router, you can configure it not to
forward directed broadcasts by placing the following line in
your /etc/rc2.d/S69inet startup:
	ndd -set /dev/ip ip_forward_directed_broadcasts 0

Starting with version 2.2.5, FreeBSD's IP stack does not respond to icmp
echo requests destined to broadcast and multicast addresses by default.
The sysctl parameter for this functionality is net.inet.icmp.bmcastecho. 
Beginning with version 3.x, FreeBSD makes this option configurable in
the /etc/rc.conf file with an option under the miscellaneous network
configuration section.

Under NetBSD, directed broadcasts can be disabled by using the sysctl
        sysctl -w net.inet.ip.directed-broadcast=0

Under Linux, one can use the CONFIG_IP_IGNORE_ECHO_REQUESTS variable to
completely ignore ICMP echo requests.  Of course, this violates RFC 1122.
"ipfw" can be used from Linux to block broadcast echos, a la: 

Any system with ipfw can be protected by adding rules such as: 
        ipfwadm -I -a deny -P icmp -D -S 0/0 0 8
        ipfwadm -I -a deny -P icmp -D -S 0/0 0 8
(replace and with your base network number
and broadcast address, respectively)

To protect a host against "fraggle" attacks on most UNIX machines, one
should comment the lines which begin with "echo" and "chargen" in
/etc/inetd.conf and restart inetd.


The amount of bandwidth and packets per second (pps) that can be generated
by this attack is quite large.  With a 200-host LAN, I was able to
generate over 80 Mbps traffic at around 35 Kpps toward my target--a
pretty significant amount.  The victims receive this because traffic is
multiplied by the number of hosts on the broadcast network used (in this
case, with a 200-host network, I was only required to send 400 Kbps
to the broadcast address--less than one-third of a T1).

Many hosts cannot process this many packets per second; many hosts are
connected to 10 Mbps Ethernet LANs where more traffic than wire speed
is sent.  Therefore, the ability to drop these packets at the network
border, or even before it flows down the ingress pipes, is desired.

Cisco routers have several "paths" which packets can take to be routed; 
each has a varying degree of overhead.  The slowest of these is "process" 
switching.  This is used when a complex task is required for processing
packets.  The other modes are variations of a fast path--each of them with
a set of advantages and disadvantages.  However, they're all handled at
interrupt level (no process-level time is required to push these packets). 

In IOS versions (even the most recent), access-list denies are handled at
the process (slow) level, because they require an ICMP unreachable to be
generated to the originating host.  All packets were sent to the process
level automatically to be handled this way.

Under a recent code change (Cisco bug ID CSCdj35407--integrated in version
11.1(14)CA and later 11.1CA, 11.1CC, 11.1CE, and 12.0 trains), packets
denied by an access-list will be dropped at the interrupt (fast) level, with
the exception of 2 packets per second per access-list deny line. These 2
packets per second will be used to send the "ICMP unreachable via
administrative block" messages.  This assumes that you don't want to log
the access-list violations (via the "log" or "log-input"  keywords).  The
ability to rate-limit "log-input" access-list lines (in order to more
easily log these packets) is currently being integrated;  see the section
below on tracing spoofed packet attacks for information on logging.

Filtering ICMP echo reply packets destined for your high-profile machines
at the ingress interfaces of the network border routers will then permit
the packets to be dropped at the earliest possible point.  However, it
does not mean that the network access pipes won't fill, as the packets
will still come down the pipe to be dropped at the router.  It will,
however, take the load off the system being attacked.  Keep in mind that
this also denies others from being able to ping from that machine (the
replies will never reach the machine).

For those customers of providers who use Cisco, this may give you some
leverage with the providers' security teams to help save your pipes by
filtering before the traffic is sent to you.

An additional technology you can use to protect your machines is to use
committed access rate, or CAR.  CAR is a functionality that works
with Cisco Express Forwarding, found in 11.1CC, 11.1CE, and 12.0.  It
allows network operators to limit certain types of traffic to specific
sources and/or destinations.

For example, a provider has filtered its IRC server from receiving
ICMP echo-reply packets in order to protect it, but  many attackers are
now attacking other customer machines or network devices in order to
fill some network segments.

The provider above chose to use CAR in order to limit all ICMP echo
and echo-reply traffic received at the borders to 256 Kbps.  An example

! traffic we want to limit
access-list 102 permit icmp any any echo
access-list 102 permit icmp any any echo-reply
! interface configurations for borders
interface Serial3/0/0
 rate-limit input access-group 102 256000 8000 8000 conform-action transmit exceed-action drop

This limits ICMP echo and echo-reply traffic to 256 Kbps with a small
amount of burst.  Multiple "rate-limit" commands can be added to an
interface in order to control other kinds of traffic as well.

The command "show interface [interface-name] rate-limit" will show the
statistics for rate-limiting; "clear counters [interface-name]" will
clear the statistics for a fresh look.

CAR can also be used to limit TCP SYN floods to particular hosts --
without impeding existing connections.  Some attackers have started
using very high streams of TCP SYN packets in order to harm systems
once again.

Here is an example which limits TCP SYN packets directed at host to 8 kbps or so: 

! We don't want to limit established TCP sessions -- non-SYN packets
access-list 103 deny tcp any host established
! We do want to limit the rest of TCP (this really only includes SYNs)
access-list 103 permit tcp any host
! interface configurations for network borders
interface Serial3/0/0
 rate-limit input access-group 103 8000 8000 8000 conform-action transmit exceed-action drop

Currently, CAR is only available for 7200 and 7500 series routers.
Additional platform support is planned in 12.0.

Additionally, CAR can be used to set IP precedence; this is beyond
the scope of this paper.  Consult for more information
on the uses of CAR.


Tracking these attacks can prove to be difficult, but is possible with
coordination and cooperation from providers.  This section also assumes
Cisco routers, because I can speak only about the abilities of Cisco to
log/filter packets and what impact it may have.

Today, logging packets which pass through or get dropped in an ACL is
possible; however, all packets with the "log" or "log-input" ACL options
are sent to process level for logging.  For a large stream of packets,
this could cause excessive CPU problems.  For this reason, tracking
attacks via IOS logging today is limited to either lower bandwidth attacks
(smaller than 10k packets per second).  Even then, the number of log
messages generated by the router could overload a syslog server. 

Cisco bug ID CSCdj35856 addresses this problem.  It has been integrated
into IOS version 11.1CA releases beginning with 11.1(14.1)CA (a
maintenance interim release), and makes it possible to log packets at
defined intervals and to process logged packets not at that interval in
the fast path.  I will update this page with version numbers as the
releases are integrated.

Some information on logging: 

In later 11.1 versions, a new keyword was introduced for ACL logging: 
"log-input".  A formatted ACL line utilizing the keyword looks like this: 

access-list 101 permit icmp any any echo log-input

When applied to an interface, this line will log all ICMP ping packets
with input interface and MAC address (for multi-access networks). 
Point-to-point interfaces will not have a MAC address listed. 

Here's an example of the log entry for a multi-access network (FDDI, Ether): 

Sep 10 23:17:01 PDT: %SEC-6-IPACCESSLOGDP: list 101 permitted icmp (FastEthernet1/0 0060.3e2f.6e41) -> (8/0), 5 packets

Here's an example of the log entry for a point-to-point network: 

Sep 10 23:29:00 PDT: %SEC-6-IPACCESSLOGDP: list 101 permitted icmp (BRI0 *PPP*) -> (8/0), 1 packet

Substituting "log" for "log-input" will eliminate the incoming interface
and MAC address from the log messages. 

We'll use the first log entry to demonstrate how to go from here.  This
log entry means the packet came in on FastEthernet1/0, from MAC address
0060.3e2f.6e41, destined for  From here, you can use "show ip
arp" (if needed) to determine the IP address for the MAC address, and go
to the next hop for tracing or contact the necessary peer (in the case of
an exchange point).  This is a hop-by-hop tracing method. 

Example of "show ip arp" used to find next hop: 

netlab#show ip arp 0060.3e2f.6e41
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet            32   0060.3e2f.6e41  ARPA   FastEthernet1/0

As you can see, is the next hop where the packets came from
and we should go there to continue the tracing process, utilizing the same
ACL method.  By doing this, you can track the spoof attack backwards. 

While this is general information on tracking spoofed packets, it must be
noted that the victims of a smurf/fraggle attack get packets from the listed
source in the packets; i.e., they receive echo-reply packets truly from the
source listed in the IP header.  This information should be used by the
amplifiers or intermediaries to track the spoofed echo _request_ packets
back to their source (the perpetrator).


Two other denial of service attacks frequently encountered are TCP SYN
floods, and UDP floods aimed at diagnostic ports on hosts. 

TCP SYN attacks consist of a large number of spoofed TCP connection set-up
messages aimed at a particular service on a host.  Older TCP
implementations cannot handle many faked connection set-up packets, and
will not allow access to the victim service.

The most common form of UDP flooding directed at harming networks is an
attack consisting of a large number of spoofed UDP packets aimed at
diagnostic ports on network devices.  This attack is also known as the
"pepsi" attack (again named after the exploit program), and can cause
network devices to use up a large amount of CPU time responding to these

To get more information on minimizing the effects of these two attacks,

Defining Strategies to Protect Against TCP SYN
  Denial of Service Attacks

Defining Strategies to Protect Against UDP Diagnostic
  Port DoS Attacks


One ISP has reported that, spread across three routers (2 RSP2 and 1
RSP4), the fast drop code eliminated a sustained 120 Mbps smurf
attack and kept the network running without performance problems.

As always, your mileage may vary. 


Thanks to all those who helped review and provide input to the paper, as
well as sanity checking.

Referenced documents:

RFC-1122, "Requirements for Internet Hosts - Communication Layers";
  R.T. Braden; October 1989.

RFC-1812, "Requirements for IP Version 4 Routers"; F. Baker; June 1995.

RFC-2267, "Network Ingress Filtering: Defeating Denial of Service Attacks
  which employ IP Source Address Spoofing"; P. Ferguson, D. Senie;
  January 1998.

RFC-2644, "Changing the Default for Directed Broadcasts in Routers";
  D. Senie; August 1999.

Defining Strategies to Protect Against TCP SYN
  Denial of Service Attacks

Defining Strategies to Protect Against UDP Diagnostic
  Port DoS Attacks

Cisco command documention to turn off directed broadcasts

3Com command documentation to turn off directed broadcasts

3Com command documentation to disable source spoofing


Permission to duplicate this information is granted under these terms: 

1.  My name and e-mail address remains on the information as a target for
    questions and identification of the source
2.  My disclaimer appears on the information at the bottom
3.  Feel free to add extra information from other discussions, etc., but
    please ensure the correct attribution is made to the author.  Also
    provide Craig Huegen ( a copy of your additions.
4.  Please help disseminate this information to other network
    administrators who are affected by these attacks.

If you have questions, I will be happy to answer them to the best of my


I'm speaking about this as an interested party only.  All text in this
paper was written by me; I speak/write for no one but myself.  No vendors
have officially confirmed/denied any of the information contained herein.
All research for this paper is being done purely as a matter of
self-interest and desire to help others minimize effects of this attack. 

Craig A. Huegen

Craig A. Huegen /
This page automatically incorporates the latest smurf.txt, available from