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A DNS flood attack is a form of Distributed Denial-of-Service (DDoS) attack that targets the Domain Name System (DNS). It works by flooding a DNS server with an extremely high number of queries to overload it, which disrupts its normal operation. The goal is to block legitimate user queries and affect the accessibility of websites or online services.
How does a DNS flood attack work?
A DNS flood attack works by overwhelming a DNS server with a huge number of requests. As with other types of DDoS attack, the aim is to overload the server, exhaust its resources, and prevent it from processing legitimate requests.
- Selecting the target
The attacker selects one or more DNS servers as targets. These can be public DNS servers or servers that support a specific website or service.
- Using botnets
Attackers often use a botnet – a network of infected devices (computers, IoT devices, etc.) that can be remotely controlled by the attackers. The botnet makes it possible to generate massive amounts of traffic from thousands of IP addresses.
- Flooding with requests
The DNS flood attack is carried out by sending massive numbers of DNS requests that are intended to overwhelm the server. These requests can take various forms:
- Random subdomain requests: The attacker sends requests for non-existent subdomains. This forces the server to constantly process new requests that are not stored in the cache.
- Legitimate requests at a high frequency: Many requests for popular domains are sent so that the server processes the same request repeatedly.
- Manipulated headers: Attacks use DNS packets with fake headers or sender addresses to increase the load and disguise the origin of the requests.
- Overloading the DNS server
The server is overloaded by the large number of requests, each of which requires a certain amount of processing power. Eventually, the attack’s data volume will completely exhaust the available bandwidth of the server or network and cause delays or crashes. Random subdomain requests render the server’s DNS cache ineffective, as new requests that are not cached are constantly being made.
- Blocking legitimate users
Since the server is operating at full capacity, it can no longer process legitimate requests from normal users. This results in:
- Delays or timeout errors for requests.
- Partial or complete unavailability of websites or services that rely on the DNS server.
- Amplification attacks
Attackers can combine a DNS flood attack with a DNS amplification attack, in which small queries are sent that generate large responses. This further increases traffic, putting even more strain on the server and network connection.
What are the effects of a DNS flood attack?
A DNS flood attack can have a significant impact on both a company’s technical infrastructure and business operations.
Technical impact
- Overloading the DNS server: The DNS server is completely overwhelmed by the flood of requests and is unable to process legitimate requests.
- Service interruptions: Websites and online services that rely on the DNS server become unavailable. Users receive error messages such as “DNS server not found” or “timeout”.
- Network congestion: The enormous traffic generated by the attack can exhaust the entire network bandwidth of a company or data center, which also affects other services.
Business impact
- Loss of revenue: For businesses that depend on online services (e.g., e-commerce sites), attacks can result in significant revenue losses because customers are unable to access their website.
- Loss of productivity: Internal systems that rely on DNS, such as email servers or cloud-based applications, can fail, affecting employee productivity.
- Recovery: Resolving an attack and restoring systems takes time and technical resources.
Impact on users
- Restricted access: Users cannot access websites, services or applications that use the attacked DNS server.
- Loss of time: Business partners or customers who depend on the affected services may also be harmed.
Security-related effects
- Diversion for further attacks: DNS flood attacks can also be used to divert the resources of security teams while attackers exploit other vulnerabilities in the targeted system.
- Loss of sensitive data: When a DNS flood attack is combined with other attack techniques (e.g., through exploits or data exfiltration), confidential information can be compromised.
- Damage to infrastructure: Repeated attacks can affect the stability of the IT infrastructure and cause long-term damage to the systems.
Damage to image and reputation
- Loss of customer trust: Repeated outages or unavailability of services can erode customer confidence in a company’s reliability.
- Negative publicity: Publicly disclosed attacks can result in a loss of reputation, especially for companies that promise high security standards.
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How can you protect yourself against such attacks?
Effective protection against DNS flood attacks requires a combination of preventive measures, monitoring, and resilient infrastructure. The first step is to continuously monitor DNS traffic to detect unusual patterns or sudden increases in requests at an early stage. Traffic monitoring tools and anomaly detection systems that can identify and report suspicious activity can help with this.
Another important measure is rate limiting, which limits the number of DNS requests from a single IP address. This prevents malicious requests from completely exhausting a server’s resources. In addition, systems can be implemented to automatically block requests from known botnets.
Increasing the resilience of the DNS server is also important. Caching mechanisms reduce the load on the server by allowing frequent queries to be answered locally. Using an anycast network disperses DNS queries across multiple geographically distributed servers, preventing any single server from being overloaded. Load balancing systems can also help to distribute traffic evenly across available resources.
The configuration of the DNS server itself is also crucial. It should be ensured that the server cannot be misused as a relay for DNS amplification attacks. This includes disabling open recursion and minimizing the size of DNS responses. Implementing DNSSEC (Domain Name System Security Extensions) also protects the integrity of DNS data from tampering.
In addition to these technical measures, a solid disaster recovery plan is essential. Companies should create contingency plans that define steps for damage control and recovery in the event of an attack. Backup systems and alternative DNS servers ensure that operations can continue, even in the event of an attack.
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