Best real-time network monitoring and intrusion detection tools


In today’s digital landscape, ensuring the security and reliability of computer networks has become a paramount concern. The ever-evolving threat landscape demands vigilant measures to detect unauthorized access attempts, network failures, and potential vulnerabilities. However, constant manual monitoring by system administrators is neither practical nor efficient.

To address this challenge, a range of open-source tools has emerged, offering real-time monitoring and intrusion detection capabilities. These tools empower network administrators to proactively identify and respond to anomalies, breaches, and performance issues, thereby fortifying network security and stability.

This article delves into the realm of network security by exploring a selection of open-source tools designed to provide real-time insights into network activities. We will examine the features and functionalities of these tools, shedding light on their roles in detecting intrusions, monitoring service health, and safeguarding web applications. By gaining a comprehensive understanding of these tools, network administrators can strategically implement them to bolster their network’s defenses and ensure seamless operations.


Snort is a packet sniffer and logger that can be used as a lightweight network intrusion detection system. It features rule-based logging and can perform protocol analysis, content searching, and packet matching. It can be used to detect a variety of attacks and probes, such as stealth port scans, CGI attacks, SMB probes, OS fingerprinting attempts, and many other kinds of anomalous traffic patterns. Snort has a real-time alert capability that can notify administrators about problems as they occur using a variety of methods. Installing and running Snort is not trivial, and depending on the amount of network traffic, it will likely require a dedicated monitoring machine with considerable resources. Fortunately, Snort is well-documented and has a strong user community. By implementing a comprehensive Snort rule set, you can identify unexpected behavior that would otherwise mysteriously consume your Internet bandwidth.

Apache: mod_security

ModSecurity is an open-source intrusion detection and prevention engine for web applications. This security tool is also known as a web application firewall. ModSecurity increases web application security by protecting web applications from known and unknown attacks. It can be used on its own or as a module in the Apache web server. Several sources provide updated mod_security rules that help protect against the latest security exploits. One excellent resource is GotRoot, which maintains a vast and frequently updated repository of rules. Web application security is crucial in defending against attacks on your web server, which could result in the theft of valuable or personal data or the server being used to launch attacks or send spam to other Internet users. Such intrusions can seriously reduce your available bandwidth and damage the Internet as a whole.


Nagios is a program that monitors hosts and services on your network, notifying you immediately when problems arise. It can send notifications via email, SMS, or by running a script and will send notifications to the relevant person or group depending on the nature of the problem. Nagios runs on Linux or BSD and provides a web interface to show up-to-the-minute system status. Nagios is extensible and can monitor the status of virtually any network event. It performs checks by running small scripts at regular intervals and compares the results against an expected response. This approach allows for much more sophisticated checks than a simple network probe. For example, while ping may tell you that a machine is up and nmap may report that a TCP port responds to requests, Nagios can actually retrieve a web page or make a database request and verify that the response is not an error. Nagios can even notify you when bandwidth usage, packet loss, machine room temperature, or other network health indicators cross a particular threshold. This advanced notification can provide you with early warnings of network problems, often enabling you to respond before users have a chance to complain.


Zabbix is an open-source real-time monitoring tool that is somewhat of a hybrid between Cacti and Nagios. It uses an SQL database for data storage, has its own graph rendering package, and performs all the functions you would expect from a modern real-time monitor, such as SNMP polling and instant notification of error conditions. Zabbix is released under the GNU General Public License.


Ngrep provides most of GNU grep’s pattern matching features but applies them to network traffic. It currently recognizes IPv4 and IPv6, TCP, UDP, ICMP, IGMP, PPP, SLIP, FDDI, Token Ring, and much more. As it makes extensive use of regular expression matches, it is a tool suited for advanced users or those with a good knowledge of regular expressions. However, you don’t necessarily need to be a regex expert to make basic use of ngrep.

Pattern matches can be constrained further to match particular protocols, ports, or other criteria using BPF filters. This is the filter language used by common packet sniffing tools, such as tcpdump and snoop.


Nmap is a network diagnostic tool for showing the state and availability of network ports on a network interface. A common use is to scan a network host on a TCP/IP network to determine what ports are open, thereby allowing one to create a “map” of the network services that the machine provides. The nmap tool accomplishes this by sending specially crafted packets to a target network host and observing the responses. For example, a web server with an open port 80 but no running web server will respond differently to an nmap probe than one that not only has the port open but is also running httpd.

Similarly, you will get a different response from a port that is simply shut off versus one that is open on a host but blocked by a firewall.

Over time, nmap has evolved from being a simple port scanner to something that can detect OS versions, network drivers, the type of NIC hardware being used by an interface, driver versions, etc. In addition to scanning individual machines, it can also scan entire networks of hosts. This means that nmap can potentially be used by malicious network users as a way to “scope out” a system before attacking it. Like many diagnostic tools, nmap can be used for both good and ill, and network administrators would do well to be aware of both sides.


Zenmap is a cross-platform GUI for nmap, which runs under Linux, Windows, Mac OS X, BSD, etc. It can be downloaded from the site as well.


Somewhat between nmap and tcpdump, netcat is another diagnostic tool for probing and exploring ports and connections on a network. It takes its name from the UNIX cat(1) utility, which simply reads out whatever file you ask it to. Similarly, netcat reads and writes data across any arbitrary TCP or UDP port. The netcat utility is not a packet analyzer but works on the data (payload) contained in the packets.