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A SYN flood is a denial-of-service attack that targets TCP servers by leaving many connection attempts unfinished. The server waits for the final step of each connection handshake, and enough half-open connections can consume resources needed to accept legitimate traffic.
How a SYN flood works
TCP normally establishes a connection through a three-step handshake:
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- A client sends a SYN segment to request a connection.
- The listening server replies with SYN-ACK and keeps some connection state while it waits.
- The client sends an ACK, completing the handshake.
In a SYN flood, many connection attempts do not reach that final step. The server retains incomplete, or half-open, connections until they time out. If these occupy enough of the resources available for new connections, legitimate attempts may be delayed, ignored, or displaced. The precise effect depends on the TCP implementation and how it handles its connection backlog. IETF RFC 4987 and CISA’s DDoS Quick Guide describe this mechanism.
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The defining target is TCP connection-handling capacity at a server, rather than data stored on it. A SYN flood is a denial-of-service method; its purpose is to interfere with service availability, not inherently to steal information or gain access. CISA classifies TCP SYN packet floods as OSI Layer 4 attacks.
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The term can also refer to a flood that strains network packet-processing capacity. RFC 4987 distinguishes that from attacks aimed at the TCP implementation itself. The packets may look similar, but the constrained resource and resulting impact are different, so the label alone does not establish what has been exhausted.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How SYN flood defenses differ
There is no single universal configuration recommendation: RFC 4987 is an informational IETF memo from August 2007, not an Internet standard. It surveys several defense families whose suitability depends on where they are deployed and on compatibility and operational needs.
- SYN cookies: The server encodes connection information in its SYN-ACK sequence number so it can avoid retaining normal per-connection state while waiting for the final ACK. The RFC notes potential compatibility trade-offs with some TCP options and application data carried in SYN segments.
- SYN caches: These handle pending connections using bounded state rather than the usual full per-connection state. Behavior and details depend on the implementation.
- Filtering: Network filters can discard attack traffic before it reaches the host, if they are positioned and configured to do so.
- Proxies: A proxy can handle or mediate connection attempts upstream of the protected server, but requires a suitable network deployment.
For an organization running a public-facing service, defensive next steps are to identify whether the pressure is at the TCP endpoint or elsewhere in the network, then check current operating-system and network-provider documentation for available mitigations and their trade-offs. The RFC explains the mechanisms; it should not be treated as current platform-specific setup guidance.
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