Introduction: IP Spoofing, Part III - ICMP Smurfing
By: Suhas A Desai

"Smurf" is the name of an automated program that attacks a network by exploiting IP broadcast addressing. Smurf and similar programs can cause the attacked part of a network to become "inoperable." Network nodes and their administrators to exchange information about the state of the network use ICMP.

A smurf program builds a network packet with a spoofed victim source address. The packet contains an ICMP ping message addressed to an IP broadcast address, meaning all IP addresses in a given network. If the routing device delivering traffic to those broadcast addresses performs the IP broadcast to layer 2 broadcast function, most hosts on that IP network will reply to it with an ICMP echo reply each. The echo responses to the ping message are sent back to the victim address. Enough pings and resultant echoes can flood the network making it unusable for real traffic.

A related attack is called "fraggle", simple re-write of smurf; uses UDP echo packets in the same fashion as the ICMP echo packets. The intermediary (broadcast) devices, and the spoofed victim are both hurt by this attack. The attackers 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.

One way to defeat smurfing is to disable IP broadcast addressing at each network router since it is seldom used.

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