What is Inline Mode?

Understanding Inline Mode

Inline mode is a deployment configuration in which a security device—such as a firewall, intrusion prevention system (IPS), or data loss prevention system—sits directly in the path of network traffic. Every packet passes through the device, allowing it to inspect, modify, and block traffic in real time before it reaches its destination.

In inline mode the device is physically and logically in the traffic flow, typically bridging two interfaces so traffic must traverse it. Because it controls forwarding, it can drop malicious packets, reset connections, and enforce policy actively rather than just observing. This contrasts with passive or promiscuous (tap/SPAN) mode, where a device receives a copy of traffic and can detect and alert but cannot stop an attack in progress—the difference between an IPS (inline, blocking) and a traditional IDS (passive, alerting).

For security, inline mode is what enables prevention rather than mere detection: threats can be stopped at the moment they cross the boundary. The tradeoff is that an inline device becomes part of the critical path, so it must be sized for throughput and latency and designed for resilience—failures can break connectivity unless fail-open hardware or high-availability pairs are used. Tuning is also vital, because false positives in inline mode block legitimate traffic, not just generate noise.

For example, a company deploys an IPS inline between its internet edge firewall and the internal network. When an attacker sends an exploit attempt matching a known signature, the IPS recognizes the malicious packets as they pass through and drops them before they reach the vulnerable server, also resetting the session. Had the same sensor been in passive monitor mode, it would have logged an alert after the exploit already reached the target, leaving prevention dependent on a separate, slower response.