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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Repair Windows errors before they cause bigger problemsFix Now →A SYN attack usually refers to a SYN flood: a denial-of-service attack that sends repeated TCP connection requests and leaves the target waiting for acknowledgments that do not arrive. The resulting incomplete connections can tie up resources and make it harder for legitimate users to connect. A SYN packet itself is normal; the attack is the abusive pattern and its effect.
What SYN means in a normal TCP connection
SYN is a TCP control flag used to start a connection. A normal TCP handshake has three steps:
- The client sends a SYN request to the server.
- The server replies with SYN-ACK.
- The client sends an ACK, completing the handshake so the connection can carry data.
These packets are routine parts of network operation. One SYN packet, by itself, does not indicate an attack.
How a SYN flood works
In a SYN flood, a target receives many initial SYN requests and responds to them, but many of the final ACKs never arrive. The target retains incomplete connection state while it waits or retries, then eventually releases that state after a timeout or by another mechanism. If enough incomplete requests accumulate, they can consume connection-handling resources and reduce the target’s ability to accept legitimate connections. CISA’s DDoS Quick Guide describes the attack succinctly: “SYN Flood works by establishing half-open connections to a node.”
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Some attackers may use spoofed source IP addresses, making a simple block of an apparent source address an incomplete defense. Spoofing is a possible technique, not a requirement: the defining pattern is the accumulation of incomplete handshakes, not the presence of spoofed addresses or any particular number of sources. Cloudflare’s explanation of SYN floods describes how a server may wait for ACK packets that never arrive.
What a SYN attack affects
The goal is denial of service. When connection-handling capacity is consumed, legitimate users may see slow connections, failed connection attempts, or an unavailable service. A SYN flood operates against TCP connection setup; it is different from an HTTP request flood, which targets application-level processing of web requests.
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How SYN floods can be mitigated
Mitigations differ in where they handle connection state and filtering. No single method is a guaranteed cure for every service or network design.
| Approach | Where it works | Trade-offs and scope |
|---|---|---|
| SYN cookies | At the server or TCP stack | Can avoid keeping an ordinary backlog entry while waiting for the final handshake step. Reconstructing connection state can involve losing some connection information. It is one implementation approach, not a complete guarantee. Cloudflare describes the mechanism. |
| Handshake proxying | At an intermediary or provider edge | The intermediary completes the handshake before passing the connection to the protected server, shifting some connection work away from that server. This describes Cloudflare’s stated service architecture, not a universal feature of every intermediary. Cloudflare’s overview. |
| Managed TCP flood protection | Upstream or provider network | Provider-specific controls may offer SYN flood rules, thresholds, and configuration modes. Cloudflare’s Advanced TCP Protection documentation describes options for Magic Transit customers; those settings are not universal recommendations. |
Which control fits depends on the service and network topology. The cited guidance does not establish a universal packet-rate threshold or platform-specific setting, so operators should consult current guidance from their network provider and equipment vendors rather than apply a generic threshold.
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How to distinguish an attack from normal traffic
- A SYN is expected: TCP uses SYN packets to begin ordinary connections.
- The pattern matters: A flood involves many requests whose handshakes remain incomplete, not merely seeing SYN traffic.
- Spoofing is not mandatory: Source addresses may be forged, but that is not what defines a SYN flood.
- It is not necessarily distributed: The term describes the TCP handshake abuse; it does not by itself establish how many systems are sending traffic.
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