L4 ACK

TCP ACK flood

High-rate ACK packets force stateful firewalls and load balancers to track sessions that may never have completed a legitimate handshake.

How it works

  1. ACK packets arrive without matching SYN state on some devices.
  2. Stateful inspection tables fill with orphan or mismatched entries.
  3. Can bypass naive SYN-only mitigations while still consuming resources.

Packet flow (illustrative)

Burst of ACK packets toward open or filtered ports.

Illustrative flow — not a live capture.

Flag ACK
Engarde metric PPS, state table
Layer L4

What to watch in Engarde

  • Firewall session count under ACK-only traffic.
  • Asymmetric routing interactions with scrubbing centers.

Running this simulation

Select TCP attack with ACK flag in Engarde DDoS. Compare with SYN flood report on same target/port.

Mitigation perspective

Stateful firewall tuning, ACK storm detection, and hybrid mitigation validation.