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
Linux systems and their 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.
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Securing Linux 6
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.
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