DDoS Attack
A Distributed Denial of Service (DDoS) attack is a malicious attempt to disrupt the normal traffic of a targeted server, service, or network by overwhelming it with a flood of Internet traffic from multiple sources. In the context of DNS, attackers often exploit DNS infrastructure through techniques like DNS amplification to generate massive amounts of traffic directed at victims.
How it works
DDoS attacks work by flooding a targeted system with so much traffic that it becomes unable to process legitimate requests. Unlike a simple DoS attack from a single source, DDoS attacks use multiple compromised systems (often thousands of infected devices forming a 'botnet') to simultaneously attack the target, making them much harder to stop and trace.
DNS is frequently exploited in DDoS attacks, particularly through a technique called DNS amplification:
**DNS Amplification Attack Process:**
1. **IP Spoofing**: The attacker sends DNS queries to open DNS resolvers, but spoofs the source IP address to appear as if the requests are coming from the victim's IP address
2. **Large Query Construction**: The attacker crafts DNS queries designed to generate the largest possible responses. Typically, they use 'ANY' queries which request all available information about a DNS zone
3. **Amplified Response**: The DNS servers, believing they're responding to legitimate requests from the victim, send large DNS responses (up to 70 times larger than the original query) to the victim's IP address
4. **Overwhelming Traffic**: The victim receives massive amounts of unsolicited DNS response traffic from many DNS servers simultaneously, overwhelming their network bandwidth and making their services inaccessible
The amplification factor makes DNS-based DDoS attacks particularly effective. A small 60-byte query can generate a 4,000-byte response, providing a 70x amplification factor. When multiplied across thousands of open DNS resolvers, this can generate devastating amounts of traffic.
Key Points
- DDoS attacks use multiple sources to flood targets, making them difficult to stop
- DNS amplification exploits open DNS resolvers to multiply attack traffic by 50x-70x
- Attackers spoof the victim's IP address to redirect DNS responses to the target
- The 'ANY' DNS query type is commonly used to maximize response size
- Modern DDoS attacks can generate hundreds of gigabits per second of traffic
Common Use Cases
- DNS Amplification Attack: Exploiting open DNS resolvers to amplify small queries into large responses directed at victims, overwhelming their network capacity
- DNS Flood Attack: Overwhelming authoritative DNS servers with millions of DNS queries, preventing them from responding to legitimate requests
- Reflection Attack: Using DNS servers as intermediaries to hide the attacker's true location while directing amplified traffic at the victim
- Infrastructure Disruption: Targeting critical DNS infrastructure like root servers or major DNS providers to disrupt internet services for large populations
code DNS Amplification Attack Example
| Type | Host / Name | Value / Points to | TTL |
|---|---|---|---|
| Query | Attacker → DNS Server | 60 bytes | — |
| Response | DNS Server → Victim | 4,000 bytes | — |
| Scale | 10,000 DNS Servers | 40 MB total | — |
| Impact | Victim Network | Overwhelmed | — |
* This example shows how a small DNS query is amplified into a large response directed at the victim.
Frequently Asked Questions
What is the difference between DoS and DDoS?expand_more
DoS (Denial of Service) involves a single attacker or source overwhelming a target, while DDoS (Distributed Denial of Service) uses multiple sources - often thousands of compromised computers (a botnet) - to attack simultaneously. DDoS attacks are much more powerful and harder to mitigate because blocking a single IP address doesn't stop the attack, and the distributed nature makes it difficult to trace back to the original attacker.
How can DNS amplification attacks be prevented?expand_more
Prevention requires action at multiple levels: DNS server operators should disable open recursive resolvers or restrict them to known clients only, implement response rate limiting (RRL), and filter out 'ANY' queries from untrusted sources. Network operators should implement ingress filtering (BCP 38) to prevent IP spoofing. Organizations can also use DDoS protection services that can absorb and filter malicious traffic before it reaches their infrastructure.
Why are open DNS resolvers a problem?expand_more
Open DNS resolvers accept queries from anyone on the internet, not just authorized clients. While this may seem helpful, it makes them exploitable for DNS amplification attacks. Attackers can abuse these open resolvers to amplify their attacks without compromising them. This is why DNS best practices recommend that recursive resolvers should only accept queries from authorized clients, not the entire internet.
Can DNSSEC prevent DDoS attacks?expand_more
No, DNSSEC was not designed to prevent DDoS attacks. DNSSEC provides authentication and integrity for DNS data, but it doesn't prevent flooding attacks. In fact, DNSSEC can potentially make amplification attacks worse because signed DNS responses are larger than unsigned ones. However, DNSSEC does prevent certain DNS-based attacks like cache poisoning that might be used in conjunction with DDoS attacks.
How large can DDoS attacks get?expand_more
Modern DDoS attacks can be massive. Some of the largest recorded DNS amplification attacks have exceeded 1 Terabit per second (Tbps) of traffic. In 2016, the Dyn DNS attack reached 1.2 Tbps and temporarily disrupted major websites including Twitter, Netflix, and Reddit. As botnets grow larger and amplification techniques improve, attack sizes continue to increase, making DDoS mitigation an ongoing arms race.