Sending a flood of DNS requests to a server constitutes a DNS flood attack. Since DNS uses UDP, no hand-shake process is involved. A flood of DNS requests can tie down the resources of a DNS infrastructure and create a DoS condition.
DNS servers like other Internet resources are prone to DoS attacks. DNS uses UDP queries for name resolution; so a full circuit is never established (as contrasted with TCP), thus making DoS attacks difficult to trace and block. DNS flood works by sending a flood of rapid DNS requests from multiple machines, thereby giving the server more traffic than it can handle and resulting in slower response time for legitimate requests.
Consider the following example where a DNS flood is run from one machine and the DNS server queried from another machine. The attacking machine sends different types of DNS packets, each with a different spoofed source port to the target. To assess the impact of this attack on the victim's performance, the attacker first clears his local cache from another machine and then queries the target name server. Clearing the local cache ensures the resolver gets the information from the server and not locally. The attacker then stops the attack and queries the target name server once again from another machine, after clearing the cache. The queries during the attack and after the attack prove a significant performance impact on the victim server. If this attack was multiplied from a number of machines, the impact would be even greater.