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DNS Flood DDoS Attack: How it Works and How to Protect Yourself

October 11, 2021
Nera Besic

What Is DNS Flood Attack?

DNS Flood is a DNS attack in which cybercriminals use the Domain Name System (DNS) protocol to carry out a version of User Datagram Protocol (UDP) flood. Cybercriminals deploy valid but spoofed DNS request packets at an extremely high packet rate and create an extremely large group of source IP addresses. 

Because these look like valid requests, the target’s DNS servers start to respond to every request. The DNS server may be overwhelmed by the sheer number of requests. The DNS attack takes up great amounts of network resources that tire out the DNS infrastructure until it is taken offline, causing the target’s internet access to go down with it. 

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DNS Flood vs DNS Amplification vs UDP Flood

DNS flood attacks must be clearly distinguished from DNS amplification attacks. DNS amplification is an asymmetrical DDoS attack—it involves a cybercriminal sending a look-up query with spoofed target IP, causing the spoofed target to be the receiver of greater DNS responses. With such attacks, the cybercriminal’s aim is to saturate the network by over-taxing bandwidth capacity on an ongoing basis. 

A DNS flood  is a symmetrical DDoS attack. Such attacks aim to tire-out server-side assets (such as CPU or memory) using a flood of UDP requests, which are created by scripts running on compromised bonet machines. 

A DNS flood attack is considered a variation of the UDP flood attack, because DNS servers use the UDP protocol for name resolution. This is classified as a Layer 7 attack. For UDP-based queries (as distinct from TCP queries), the attack prevents the creation of an entire circuit, making it easier to achieve spoofing. 

Learn more in our detailed guides to DNS amplification attacks

How Does a DNS Flood Attack Work?

The Domain Name System translates between domain names that are easy to remember (for instance and website server addresses that are difficult to remember (for instance A successful attack on DNS infrastructure can thus make the internet unusable for most users. 

DNS flood attacks are a relatively new form of DNS attack, which has grown with the increase of high bandwidth IoT botnets such as Mirai.  

DNS flood attacks leverage the high bandwidth connection of various IoT devices like DVR boxes and IP cameras to bombard the DNS servers of a provider. The amount of requests from IoT devices floods the DNS provider’s services and stops valid users from gaining access to the provider’s DNS servers. 

DNS amplification attacks differ from other DNS flood attacks. DNS amplification amplifies and reflects traffic off unsecured DNS servers to conceal the origin of the attack and to increase its success. 

DNS amplification attacks send large volumes of requests to unsecured DNS servers, using devices with small bandwidth connections. The devices forward multiple small requests for extremely large DNS records, while attackers redirect the return address to the targeted victim’s address. Amplification lets cybercriminals attack larger targets using only limited resources. 

Another major type of DNS flood attack is DNS NXDOMAIN flood attack, whereby the cybercriminal floods the DNS server with requests for records which are invalid or nonexistent. The DNS server uses up all its resources searching for these records, its cache becomes full of bad requests, and in the end it has no resources to deploy legitimate requests.

DNS Flood Attack Mitigation Approaches

If the cybercriminal makes use of a huge amount of IP addresses, they may bypass various anomaly detection algorithms. This can make it difficult to mitigate DNS flood attacks. 

However, there are various approaches you can use to mitigate this kind of attack: 

  • Keep your DNS resolver private—ensure your resolver is not exposed to external users. You should restrict its usage to internal network users alone, which will prevent its cache from being contaminated by cybercriminals from outside your organization. 
  • Use a DDoS mitigation service—irrespective of where you retain your DNS servers, they are always prone to DDoS attack, which may cause your services to be unreachable and make business disruptions. To stop DNS DDoS Flooding, use a DDoS mitigation service from a trusted third party. This service may help to stop some of the unwanted traffic and make sure your DNS services stay reachable.
  • Use a patch management solution—this is an essential tool for DNS flood attack mitigation. Cybercriminals often take advantage of vulnerabilities and loopholes in software, so you need to run patches regularly. Keep name servers up-to-date and patched to prevent them from being subject to known vulnerabilities.
  • Utilize a dedicated DNS server—small organizations generally host their DNS server alongside their application server to save money, but this makes the likelihood of DNS flood DDoS attacks greater. It is best to run your DNS services on a dedicated server.
  • Carry out DNS audits—with time, organizations often forget about their outdated subdomains. You might be using old software, or software that is vulnerable to exploitation. Regular auditing of DNS zones will offer an insight into DNS-related vulnerabilities, letting you understand what needs to be addressed.

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