Chapter 6 . Securing Linux 225 Fortunately, there (Web host music)
Chapter 6 . Securing Linux 225 Fortunately, there are many tools and techniques for combating intrusion attacks. This section discusses the most common break-in methods and the tools available to protect your system. Although the examples shown are specific to Fedora and other Red Hat Linux systems, the tools and techniques are generally applicable to any Linux or UNIX-like operating system. Evaluating Access to Network Services Fedora, Red Hat Linux, and its UNIX kin provide many network services, and with them many avenues for cracker attacks. You should know these services and how to limit access to them. What do I mean by a network service? Basically, I am referring to any task that the computer performs that requires it to send and receive information over the network using some predefined set of rules. Routing e-mail is a network service. So is serving Web pages. Your Linux box has the potential to provide thousands of services. Many of them are listed in the /etc/services file. Look at a snippet of that file: # /etc/services: # service-name port/protocol [aliases …] [# comment] chargen 19/tcp ttytst source chargen 19/udp ttytst source ftp-data 20/tcp ftp-data 20/udp # 21 is registered to ftp, but also used by fsp ftp 21/tcp ftp 21/udp fsp fspd ssh 22/tcp # SSH Remote Login Protocol ssh 22/udp # SSH Remote Login Protocol telnet 23/tcp telnet 23/udp # 24 - private mail system smtp 25/tcp mail After the comment lines, you will notice three columns of information. The left column contains the name of each service. The middle column defines the port number and protocol type used for that service. The rightmost field contains an optional alias or list of aliases for the service. As an example, examine the last entry in the file snippet. It describes the SMTP (Simple Mail Transfer Protocol) service, which is the service used for delivering e-mail over the Internet. The middle column contains the text 25/tcp, which tells you that the SMTP protocol uses port 25 and uses the Transmission Control Protocol (TCP) as its protocol type. What exactly is a port number? It is a unique number that has been set aside for a particular network service. It allows network connections to be properly routed to the software that handles that service. For example, when an e-mail message is
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