A type of Distributed Denial of Service (DDoS) attack in
which the attacker targets one or more Domain Name System (DNS)
servers belonging to a given zone, attempting to hamper resolution of
resource records of that zone and its sub-zones.
DNS Amplification attack
In a DNS amplification attack, a large number of DNS request are sent
with a spoofed from-IP-address to one or more DNS servers. Depending
on configuration, these DNS servers will send a response back to the
IP address that the request appeared to originate from.
DNS Amplification DDoS Attack
Although, US Cert and CloudFlare both have good definitions I felt they lacked simplification. However, I have included them as citation because they include technical workings.
DNS flood attacks use the high bandwidth connections of botnets to directly overwhelm the DNS servers via DDoS attack. The volume of requests from the botnet devices overwhelms the DNS provider’s services and prevents legitimate users from accessing the provider's DNS servers.
DNS flood attacks differ from DNS amplification attacks. Unlike DNS floods, DNS amplification attacks reflect and amplify traffic off unsecured DNS servers in order to hide the origin of the attack and increase its effectiveness. DNS amplification attacks can use devices with smaller bandwidth connections to make numerous requests to unsecured DNS servers. The devices make many small requests for very large DNS records, but when making the requests, the attacker forges the return address to be that of the intended victim. The amplification allows the attacker to take out larger targets with only limited attack resources. Although DNS amplification is not reserved for low bandwidth, it is the ideal attack method if high bandwidth is not available.