Security Strategies in Linux Platforms and Applications, 3rd Edition

Checking for Open Network Ports

This section focuses on basic open-source commands related to network ports—specifically the nmap, lsof, telnet, and netstat commands. Many of the tools discussed in this chapter take advantage of these commands.

Related to these commands is netcat, the nc command, which is discussed later in this chapter. More extensive tools such as Nessus and OpenVAS, a fork of Nessus, are also covered later in this chapter.

The telnet Command

One reason the telnet command still exists on the systems of many security administrators is its utility. It’s an easy way to check for open ports over a network. It’s worth noting this command is the telnet client and not the telnet server, which is used to accept remote connections ...