Smurf is a DoS attack in which the target host is overwhelmed by ICMP echo replies from its own network. Smurf starts by sending an ICMP echo request (ping) to the broadcast IP address on the target's subnet. The ICMP echo request is spoofed to contain the target's IP address in the source address field of the echo request's IP header. Then, when other hosts on the subnet receive the echo requests, they all direct their replies to the target host.
Smurf is defeated by modern router and firewall settings that prohibit the forwarding of ICMP echo requests to broadcast addresses.