Distributed Reflected Denial of Service (DRDoS) is an attack on a server or website in which the attacker does not attack his victim directly, but the victim is attacked by a third party. In DRDoS attacks, the attacker sends requests to Internet services, which answer them.
Since the attacker uses IP spoofing to specify a false Internet address, namely that of his victim, the Internet services answer the queries to the spoofed address. The response thus represents a DoS attack on the part of the Internet services. Since the source address is false, tracing is impossible.