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The Smurf attack is a way of generating a lot of computer network traffic to a victim host. That is, it is a type of denial-of-service attack. Specifically, it floods a target system via spoofed broadcast ping messages.
In such an attack, a perpetrator sends a large amount of ICMP echo requests (ping) traffic to IP broadcast addresses, all of it having a spoofed source 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 twofold:
- Configure individual hosts and routers not to respond to ping requests to broadcast addresses,[1] and
- Configure routers not to forward packets directed to broadcast addresses. Until 1999, standards required routers to forward such packets by default, but in that year, the standard was changed to require the default to be not to forward.[3]
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:
no ip directed-broadcast
(Please note that 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).
References
- ^ a b "CERT Advisory CA-1998-01 Smurf IP Denial-of-Service Attacks".
- ^ For example, netscan.org (Web Archive) showed 122,945 broken networks as of Jan 25, 1999, but only 2,417 as of Jan 06, 2005.
- ^ D. Senie, "Changing the Default for Directed Broadcasts in Routers", RFC 2644, BCP 34
- ^ P. Ferguson and D. Senie, "Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing", RFC 2827, BCP 38
