"PRSD attacks are a type of DDoS attack used by threat actors to disrupt DNS record lookups by flooding a DNS server with large amounts of DNS queries against non-existing subdomains," says the report. BleepingComputer reports that the FBI says that the state voter registration website was not affected by the DDoS siege due to properly set up rate-limiting on the target's DNS servers.

A high volume of DNS requests consistent with a PRSD DDoS attack hit the DNS server of the voter registration website over a month, with short periods of time where the amount of DNS requests increased tenfold, says the report. 

"The requests occurred over the course of at least one month in intervals of approximately two hours, with request frequency- peaking around 200,000 DNS requests during a period of time when less than 15,000 requests were typical for the targeted website," the FBI explains.

The DNS requests had source IP addresses belonging to recursive DNS servers, obfuscating the originating host(s) or attacker, and were largely for non-existent subdomains of the targeted website, says BleepingComputer. 

The FBI provided potential targets with a series of precautionary measures to be taken to successfully mitigate DDoS attacks, to include:

• Implement an incident response plan, including a DDoS mitigation strategy, and practice this plan prior to an actual incident.
• If the incident response plan involves external organizations, ensure the appropriate contacts with the external organizations are established prior to an incident.
• Enable automated patches for your operating system, Web browser(s), and software to the extent possible. When necessary, manually apply as soon as possible.
• Maintain a timeline of attacks, recording all relevant details.