Are IDSs similar to firewalls?

No, Intrusion Detection Systems (IDSs) are not similar to firewalls; they serve distinct functions in network security. Firewalls act as a first line of defense, enforcing access control policies by inspecting network traffic and blocking unauthorized connections based on predefined rules. For example, a firewall might block all incoming traffic to port 23 (Telnet) if it's not explicitly allowed.

An IDS, conversely, primarily monitors network or system activities for malicious or anomalous behavior that indicates a potential intrusion. It does not typically block traffic itself but rather detects, logs, and alerts administrators to suspicious events. As described in works on intrusion detection, IDSs analyze traffic that has already passed through the firewall or internal system activities for signs of attack patterns or deviations from normal behavior. For instance, an IDS might detect a port scan on an internal network segment, even if the initial connection was allowed by a firewall, or identify a known signature of a malware attempting to communicate.

The fundamental difference lies in their operational model: firewalls prevent unauthorized access based on policy rules, while IDSs detect intrusions that might bypass firewalls or originate from within the network. They are complementary security tools, with firewalls providing prevention and IDSs offering detection and visibility into potential threats that circumvent initial defenses or emerge internally. This distinction is discussed in various security contexts, including J.M. Kizza's Guide to Computer Network Security.