What is an NXDOMAIN Attack?
An NXDOMAIN DDoS attack is a DNS-based attack that targets a DNS server with a large volume of queries for non-existent domains. When the DNS server receives a request for a domain that doesn’t exist, it returns an NXDOMAIN response, indicating that the queried domain could not be found. The attack’s objective is to flood the DNS server with numerous such requests, overwhelming its resources and causing it to either fail or perform poorly under the traffic load.
In essence, the attacker exploits the DNS system’s functionality to deliberately trigger NXDOMAIN responses from the server, consuming significant server resources, including CPU power, memory, and bandwidth. This can lead to denial of service, preventing legitimate requests from being processed.
How the NXDOMAIN Attack Works
To better understand the NXDOMAIN attack, it’s important to first grasp how DNS works and how NXDOMAIN responses are generated.
DNS Query Process: When a user tries to visit a website by entering a domain name (e.g., example.com) into their browser, a DNS query is sent to the DNS server to resolve that domain into an IP address. If the DNS server can’t find the requested domain in its cache or records, it checks with other DNS servers to resolve the query.
NXDOMAIN Response: If the DNS server ultimately cannot find the domain (for example, if it doesn’t exist or there is a typo), it returns an NXDOMAIN response. This response informs the client that the requested domain does not exist.
In an NXDOMAIN DDoS attack, the attacker exploits this mechanism by flooding the DNS server with a large number of queries for non-existent domains, such as randomdomain12345.com, hoping to generate NXDOMAIN responses for each query. As a result, the server spends excessive resources processing these invalid queries, which depletes its capacity to respond to legitimate requests.
The Impact of NXDOMAIN Attacks
The consequences of an NXDOMAIN DDoS attack can be significant, particularly for organizations that rely on DNS servers to maintain their online presence. The main impacts of an NXDOMAIN attack include:
Service Disruption: By overwhelming DNS servers with invalid requests, the server becomes incapable of processing legitimate queries, causing service outages for users trying to access affected websites or services.
Resource Exhaustion: DNS servers are forced to expend valuable computational resources (CPU, memory, and bandwidth) to handle the flood of invalid queries, resulting in degraded performance or crashes.
Network Congestion: The high volume of traffic generated by the attack can also lead to congestion in the network, which impacts the availability and performance of other services, especially for businesses with limited bandwidth.
Increased Operational Costs: Organizations may incur additional costs related to mitigating the attack, such as purchasing additional server resources, implementing DDoS protection, or hiring third-party services to filter traffic.
Effective ways to Mitigate NXDOMAIN DDoS Attack
To defend against NXDOMAIN attacks, organizations must adopt a combination of preventative measures, traffic analysis, and DDoS mitigation strategies. Below are some key steps that can help mitigate the impact of NXDOMAIN attacks:
Implement DNS Rate Limiting
Rate limiting restricts the number of requests a DNS server can process from a single source within a given period. By limiting the frequency of DNS queries from each IP address, organizations can reduce the risk of being overwhelmed by a flood of requests during an attack.
Deploy DNS Traffic Filtering
Traffic filtering solutions can help detect and block malicious traffic, such as NXDOMAIN queries from known attack sources. Security systems such as Intrusion Detection Systems (IDS) can analyse incoming traffic to identify suspicious patterns and filter out attack traffic before it reaches the DNS server.
Use a DNS Firewall
A DNS firewall can block or rate-limit DNS requests based on threat intelligence, reducing the risk of being impacted by NXDOMAIN and other DNS-based attacks. These firewalls can be configured to reject queries for non-existent domains or queries from suspicious IP addresses.
Use DNS Caching and Load Balancing
DNS caching reduces the load on DNS servers by storing resolved DNS queries for a period of time. This minimizes the number of DNS lookups required for repeated queries, reducing the server’s vulnerability to attack. Additionally, load balancing techniques can distribute traffic across multiple DNS servers, ensuring that no single server becomes overwhelmed during an attack.
Adopt Anycast for DNS
Anycast is a routing method where DNS servers are deployed across multiple geographic locations, with the same IP address assigned to each server. During a DDoS attack, the traffic is routed to the closest server, which helps distribute the load and improves resiliency against attacks.
Monitor DNS Traffic
Continuous monitoring of DNS traffic allows security teams to detect any unusual spikes in requests or the presence of invalid domain queries. Early detection of these patterns can help to identify an ongoing NXDOMAIN attack and initiate countermeasures before it causes significant damage.
