Some of these tools can also be considered tools for attacking other machines. Therefore, it is advisable to test these tools on machines in our own local or private network; we should never do this with third party IPs, as these could interpret the tests as intrusions and we or our ISP may be held responsible for them and the corresponding authorities may be notified to investigate us and remove our access.
We will now briefly discuss some tools and the ways in which they can be used:
a) TripWire: this tool maintains a database of sums for checking the important files in the system.
It may serve as a preventive IDS system. We can use it to "take" a snapshot of the system, so that we can subsequently check any modification made and that it has not been corrupted by an attacker. The aim here is to protect the files in the machine itself and to avoid any changes occurring, such as those that, for example, the rootkit might have caused. Therefore, when we execute the tool again, we can check all the changes compared to the previous execution. We have to choose a subset of files that are important in the system or possible sources of attack. TripWire is proprietary, but there is a free open-source tool that is the equivalent called AIDE.
b) Nmap [Insb]: this is a tool that scans ports in large networks. It can scan from individual machines to network segments. It provides various scanning modes, depending on the system's protections. It also provides techniques with which we can determine the operating system used by remote machines. Different TCP and UDP packets may be used to test the connections. There is a graphical interface known as xnmap.
c) Wireshark [Wir] (previously called Ethereal): is a protocol analyser that captures the traffic in the network (it acts as a sniffer). It can be used to visualise the captured traffic, see the statistics and data of the individual packets and group the packets, either by origin, destination, ports or protocol. It can even reconstruct the traffic from a whole session from a Transmission Control Protocol (TCP).
d) Snort [Sno]: is an IDS system that makes it possible to analyse the traffic in real time and save logs of the messages. It can be used to analyse the protocols and search by patterns (protocol, origin, destination etc.). It can be used to detect various types of attack. Basically, it analyses the traffic in the network to detect patterns that might correspond to an attack. The system uses a series of rules to either produce a log of the situation (log) or warn the user (alert) or reject the information (drop).
e) Nessus [Nes]: detects any known vulnerabilities (by testing different intrusion techniques) and assesses the best security options for those discovered. It is a modular program that includes a series of plugins (more than 11,000) for performing the different analyses. It uses a client-server architecture, with a graphic client to show the results and the server, which carries out different tests on the machines. It has the capacity to examine whole networks. It generates reports on the results, which can be exported to different formats (HTML, for example). Up until 2005, Nessus 2 was a free tool, but the company decided to make it proprietary, in version Nessus 3. In GNU/Linux, Nessus 2 is still used, as it continues to have a GPL license and a series of plugins, which are gradually updated. Nessus 3, as a proprietary tool for GNU/Linux, is more powerful and widely used, as it is one of the most popular security tools and there is normally a free version available with plugins that are less updated than the ones in the version that is not free.
We can find many other security tools that are available. A good place to start is http://sectools.org, where the designers of Nmap maintain a list of popular tools, as voted by the users (now, a bit older list, but useful tools).