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A SYN flood is a denial-of-service attack on a TCP server. It sends connection-opening requests that leave handshakes unfinished, causing the server to retain incomplete-connection state while it waits. If that state or related connection resources become scarce, legitimate connection attempts may be delayed, ignored, or displaced.
How a SYN flood works
- A client starts a TCP connection by sending a SYN segment to a listening server.
- The server answers with a SYN-ACK and waits for the client’s final ACK, retaining some connection state during that wait.
- In a SYN flood, many handshakes are left incomplete. The accumulated half-open connections put pressure on finite host or connection-backlog resources.
The sequence is described in CISA’s DDoS Quick Guide and in the IETF’s RFC 4987. The precise response varies with the TCP implementation and its backlog handling; servers do not all react identically.
What the attack affects—and what it does not mean
A SYN flood is defined by denial of service against hosts running TCP server processes, not by an attempt to steal data or gain unauthorized access. Its core mechanism is the accumulation of incomplete connections and the resource pressure they create.
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Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →CISA classifies TCP SYN packet floods as OSI Layer 4 attacks. The term does not, by itself, establish that a flood is exhausting only one kind of resource: the SYN traffic can target TCP connection state, while high packet rates can also burden network packet-processing capacity. RFC 4987 distinguishes these attack goals, so the impact should be described in terms of the resource actually under pressure.
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How SYN flood defenses differ
RFC 4987, an informational IETF memo from August 2007 rather than an Internet standard, surveys several mitigation approaches. They differ in where they operate, how they handle connection state before the handshake completes, and what operational or compatibility costs they introduce.
| Approach | How it helps | Trade-off or constraint |
|---|---|---|
| Filtering | Can block or limit traffic before it reaches the TCP host. | Requires suitable network placement and filtering rules; the RFC does not prescribe a universal configuration. |
| SYN cache | Keeps bounded connection state using a different approach from ordinary per-connection handling. | Behavior depends on the implementation; no single setting or capacity is established by the RFC. |
| SYN cookies | Encode necessary connection information in the server’s SYN-ACK sequence number, avoiding normal per-connection state while awaiting the final ACK. | The RFC notes compatibility trade-offs involving some TCP options and application data carried on SYN segments. |
| Proxying | A proxy can handle or mitigate connection attempts before they reach the protected server. | Requires an appropriate proxy deployment in the traffic path. |
These are defense families, not plug-in guarantees. RFC 4987 explains their mechanisms and trade-offs but does not make standards-level configuration recommendations. For a real service, verify the current operating-system, network-provider, or proxy documentation for the applicable settings and capabilities.
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