A smurf attack is a Layer 3 reflection attack vector, one of the amplification classes a thorough DDoS test is built to exercise. The attacker sends ICMP echo requests to a network's broadcast address while spoofing the victim's IP as the source. Every host on that network replies to the victim at once, multiplying a single packet into a flood proportional to the number of responding hosts. It is the historical archetype of broadcast amplification.
Why it matters in DDoS testing
Smurf attacks are largely mitigated today because routers no longer forward directed broadcasts by default (per RFC 2644), but the pattern (spoof the source, abuse a multiplier) is the template every modern reflection vector follows. A test that exercises reflection confirms upstream absorption and source-address validation (BCP 38) hold across the vector family, not just the specific protocol of the day. The amplification lineage from smurf to memcached is traced in Understanding DDoS Attack Vectors.