The Smurf attack is a way of generating significant computer network traffic on a victim network. This is a type of denial-of-service attack that floods a target system via spoofed broadcast ping messages.
This attack relies on a perpetrator sending a large amount of ICMP echo request (ping) traffic to IP broadcast addresses, all of which have a spoofed source IP address of the intended victim. If the routing device delivering traffic to those broadcast addresses delivers the IP broadcast to all hosts (for example via a layer 2 broadcast), most hosts on that IP network will take the ICMP echo request and reply to it with an echo reply, multiplying the traffic by the number of hosts responding. On a multi-access broadcast network, hundreds of machines might reply to each packet.[1]
In the late 1990s, many IP networks would participate in Smurf attacks (that is, they would respond to pings to broadcast addresses). Today, thanks largely to the ease with which administrators can make a network immune to this abuse, very few networks remain vulnerable to Smurf attacks.[2]
The fix is two-fold:
Another proposed solution, to fix this as well as other problems, is network ingress filtering which rejects the attacking packets on the basis of the forged source address.[4]
An example of configuring a router not to forward packets to broadcast addresses, for a Cisco router, is:
Router(config-if)# no ip directed-broadcast
(This example does not prevent a network from becoming the target of Smurf attack; it merely prevents the network from "attacking" other networks, or better said, taking part in a Smurf attack.)
A Smurf amplifier is a computer network that lends itself to being used in a Smurf attack. Smurf amplifiers act to amplify (worsen the severity of) a Smurf attack because they are configured in such a way that they generate a large number of ICMP replies to a spoofed source IP address (the victim of the attack).