The provided string alert tcp any any -> 192.168.100.0/24 21 (msg: ““FTP on the network!””;) is a classic example of a rule for a Network Intrusion Detection System (NIDS), specifically in the format used by Snort and Suricata. The rule's components are standard for an IDS: an action (alert), protocol (tcp), source and destination IP/port, a direction operator (->), and rule options that include a descriptive message (msg). The purpose of this rule is to detect TCP traffic destined for the FTP port (21) on a specific subnet and generate an alert, which is the primary function of an IDS.

A. A firewall IPTable: iptables rules use a different syntax, employing flags like -A (append), -p (protocol), and -j (jump) with targets such as ACCEPT or DROP.

B. FTP Server rule: FTP server configuration files define server behavior, user access, and directory permissions, not network traffic inspection rules in this format.

C. A Router IPTable: Router configurations, such as Cisco Access Control Lists (ACLs) or iptables on Linux-based routers, use a distinct and different syntax to permit or deny traffic.

1. Snort 3 Users Manual: The official documentation for Snort

a widely-used IDS

specifies the rule syntax. The "Rule Headers" section details the structure: Action Protocol SrcIP SrcPort Direction DstIP DstPort (Options). The question's rule perfectly matches this documented format. (Source: Snort.org

Snort 3 Users Manual

Chapter 4: Rules

Section 4.1: Rule Headers).

2. University of Virginia

CS 4457: Computer Networks Courseware: Lecture slides on "Network Security" explicitly show examples of Snort rules. Slide 23 presents the "Anatomy of a Snort Rule

" which is identical to the structure in the question

contrasting it with firewall rules. (Source: University of Virginia

School of Engineering and Applied Science

CS 4457

Lecture 23: Network Security

Slide 23).

3. Linux iptables Man Page: The official manual page for iptables provides the authoritative syntax for its rules. The format is iptables [-t table] {-A|-C|-D} chain rule-specification

which is structurally different from the rule provided in the question. (Source: iptables(8) — Linux manual page).