A SYN/ACK DoS attack is a type of Denial-of-Service (DoS) attack that targets the TCP handshake process, which is how computers establish reliable connections over the internet. Let’s break it down step by step 🧩
🔄 The TCP Handshake: A Quick Primer
Before two devices communicate over TCP, they go through a three-step handshake:
- SYN: The client sends a synchronization request to the server.
- SYN-ACK: The server acknowledges and responds with its own synchronization.
- ACK: The client confirms, and the connection is established.
💥 What Is a SYN/ACK DoS Attack?
In a SYN Flood attack, the attacker sends a flood of SYN requests to a server but never completes the handshake with the final ACK. This leaves the server waiting and allocating resources for each half-open connection.
In a SYN/ACK scan or DoS attack, the attacker may also send SYN-ACK packets directly, which is unusual because SYN-ACKs are normally sent by servers—not clients. This can confuse the target system or firewall, potentially causing it to misallocate resources or misinterpret the traffic as legitimate.
🧨 Why Is It Dangerous?
- Resource exhaustion: The server keeps ports open, waiting for ACKs that never arrive.
- Firewall probing: SYN/ACK scans can be used to detect open ports or firewall rules.
- Stealthy behavior: These packets can bypass basic filters since they resemble normal traffic.
🛡️ How to Protect Against SYN/ACK DoS Attacks
- Enable SYN cookies: A technique that prevents resource allocation until the handshake is complete.
- Use firewalls with DoS protection: These can detect and block abnormal SYN/ACK patterns.
- Rate limiting: Restricts how many connection requests are allowed per IP.
- Keep firmware updated: Routers and firewalls often patch vulnerabilities that attackers exploit.
If you’re seeing logs like “DoS attack: ACK Scan” or “SYN/ACK Scan” on your router, it usually means bots are probing your network for open ports. As long as your firewall is active and your system is patched, you’re likely safe.
