DNS Amplification DDoS Attacks: How to Understand and Fight Back
DNS Amplification DDoS Attacks: What You Need to Know
DNS Amplification DDoS attacks are a sneaky type of cyber assault. They exploit open DNS resolvers to flood a target server or network with overwhelming traffic. This is a kind of reflection-based volumetric DDoS attack, which can make even the toughest network infrastructures buckle under pressure.
How Do DNS Amplification Attacks Work?
These attacks take advantage of the difference in bandwidth between the attacker and the targeted server. Here’s the kicker: small queries can generate massive responses, creating a tidal wave of data. When a botnet sends these requests, the amplification effect skyrockets. This makes it tough to trace the attacker and cranks up the attack traffic to boot.
The attacker tweaks the request to get the biggest possible response from DNS resolvers. The target then gets hit with a super-sized version of the attacker’s initial traffic, drowning the network in fake traffic. The DNS resolvers return responses that are way larger than the incoming packets. When the source IP address is swapped with the victim’s, the server sends a massive data packet to the victim, leading to a denial of service.
The Stages of a DNS Amplification Attack
- Exploiting Vulnerable Endpoints: The attacker uses a compromised endpoint to send UDP packets with spoofed IP addresses to a DNS resolver.
- Spoofing the Victim’s IP: The spoofed address in the packets points to the victim’s real IP address.
- Requesting Maximum Response: Each UDP packet asks the DNS resolver for the largest possible response.
- Sending Amplified Responses: The DNS resolver, trying to be helpful, sends a large response packet to the spoofed IP address.
- Overwhelming the Target: The victim’s IP address receives the response, and the network infrastructure is flooded with traffic from hundreds of thousands of bots, leading to a denial of service.
Fighting Back Against DNS Amplification Attacks
Defending against DNS amplification attacks directly on your server is a tough nut to crack. Even if your server is the target, the real damage is done to the surrounding infrastructure. The massive traffic volume can overwhelm your Internet Service Provider (ISP) or other infrastructure providers.
In some cases, the ISP might have to blackhole all traffic directed to the victim’s IP address to protect other systems. Their first priority is to protect themselves, after all. Besides off-site protection services like Cloudflare DDoS protection, the best mitigation strategies are usually implemented at the ISP or infrastructure level.
Open DNS resolvers are a crucial part of DNS amplification attacks. Poorly configured DNS resolvers exposed to the Internet are all an attacker needs to launch an attack. Ideally, DNS resolvers should only serve devices from a trusted domain. In reflection-based attacks, open DNS resolvers respond to queries from any source, allowing for misuse. The first step in blocking any type of amplification attack is restricting the DNS resolver to respond only to queries from trusted sources.
Another solution is for the ISP to reject any internal traffic where the source IP address is spoofed. Since the attacker’s botnet must send UDP requests with a spoofed source IP address to the victim’s IP address, the ISP can mitigate the attack by rejecting any internal traffic that seems to come from outside the network. If a packet sent from within the network appears to come from outside, it’s likely a spoofed packet and should be rejected.
