Craig A. Huegen

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

(Note: Most of this information is historical and out-of-date. Obviously, we have seen phenomenal growth of Denial-of-Service attacks across the Internet, leveraging security vulnerabilities to assemble networks of compromised hosts which can be used to attack sites, rather than relying upon a big pipe or amplification techniques. Most of this material is left unchanged, for reference.)

New additions:

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-prefixdirected 

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 routers.

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:

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 command:

        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 follows:

! 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 packets.

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

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.


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 knowledge.


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