What Is DNS Flood Attack?

DNS (Domain Name System), the most critical internet service and key component which allows you to connect to websites by converting the human-readable domain names to the unique IP addresses where the sites are stored. Chat services, Email services, and social networks depend on DNS to support 24/7 settling IP addresses into hostnames.

With such importance, DNS becomes one of the critical attack vectors. Without realizing this, most organizations overlooked this component when securing their infrastructure. As DNS is often found outdated, completely vulnerable, or without proper protections, DNS attacks are increasingly happening.

Here we are going to discuss DNS flood, one of the most popular DNS-based attacks, which affect your company.

What is DNS Flood?

Like other Internet resources, DNS is also highly prone to DDoS (Distributed Denial of Service) attacks.

DNS flooding is a symmetric DDoS attack. When a DNS server is flooded in a DDoS attack, the attack attempts to exhaust server resources with floods of IP addresses. The main goal of the DNS flood DDoS attack is to overload the victim server and make it not able to serve DNS requests since the available resources are affected by the hosted DNS zones.

How Does DNS Flood Attack Works?

To execute a DNS flood attack against a DNS server, the hacker often uses botnets to run a script from multiple servers. These scripts bombard malformed packets against a DNS service. The victim can’t distinguish which packets are from real clients and which aren’t since the attacker spoof all packet details including source IP.

This way the UDP Flood exhaust the server resources as well as the bandwidth of the victim server. The result?


When the legitimate client visits a website and he doesn’t have its IP cached, his DNS requests won’t be able to pass through due to the competing malicious requests. Because the overwhelming volume of requests exhausts the DNS server resources, it won’t have the capacity available to send him the IP address he is looking for.

Moreover, this attack is quite difficult to trace with deep analysis since each request appears legitimate.


The DNS NXDOMAIN attack is another common type of DNS Flood attack, which involves attackers sending a large volume of requests to a victim server requesting for records, which are invalid or non-existent. It results in expending the resources of the DNS server and fills the cache with invalid requests -eventually hampers the response time for legitimate requests and halting all the DNS resolution services.

DNS Flood Attack Mitigation Approaches

If the hackers employ an abundant number of IP addresses, they can bypass most of the anomaly detection algorithms. This makes the DNS flood attack mitigation process quite difficult sometimes.

However, still, there are different approaches you can take to prevent this attack:

  1. Keep Your Resolver Private – Make sure your own resolver is not open to external users. It is recommended to restrict its usage only to your network users to prevent its cache from being contaminated by attackers outside your company.
  2. Use DDoS Mitigation Service – Wherever you keep your DNS servers, they are prone to a DDoS attack, which can make your services unreachable and cause business disruptions. To prevent DNS DDoS Flooding, it is best to use DDoS mitigation services from a trusted partner like Indusface whose fully managed DDoS protection service can help to block some of the unwanted traffic and ensure your DNS services remain reachable.
  3. Effective Patch Management Solution – it is a critical tool in DNS flood attack mitigation. Cybercriminals love taking the benefits of loopholes and vulnerabilities in software. Hence, it is vital to run patches as soon as possible. Keeping name servers patched and up to date can prevent them from being abused by known vulnerabilities.
  4. Use A Dedicated DNS Server – Small organizations typically host their DNS server along with their application servers due to cost constraints. However, it increases the chances of DNS flood DDoS attacks. It is always recommended to run your DNS services on a dedicated server.
  5. Conduct a DNS Audit – Over time organizations tend to forget about their old subdomains. Some of them may be using outdated software or vulnerable to exploitation. Frequent auditing of DNS zones will provide an insight on DNS-related vulnerabilities, helping you understand what requires to be addressed.


“Cybersecurity is only as strong as the weakest link” – if the weakest link is the DNS system, especially a part, which is outside your control, it is more prone to DNS flood DDoS attack. Hence, realize the risk of DNS attacks and take proactive mitigation approaches. Otherwise, you may experience the serious consequences of data theft, business disruption, or worse.