226 Part II . Running the Show delivered
226 Part II . Running the Show delivered from some other computer to your Linux box, the remote system must first establish a network connection with your system. Your computer receives the connection request, examines it, sees it labeled for port 25, and thus knows that the connection should be handed to the program that handles e-mail (which happens to be sendmail). I mentioned that SMTP uses the TCP protocol. Some services use UDP, the User Datagram Protocol. All you really need to know about TCP and UDP (for the purpose of this security discussion) is that they provide different ways of packaging the information sent over a network connection. A TCP connection provides error detection and retransmission of lost data. UDP doesn t check to ensure that the data arrived complete and intact; it is meant as a fast way to send non-critical information. Disabling Network Services Although there are hundreds of services (with official port numbers listed in /etc/services) that potentially could be available and subject to attack on your Linux system, in reality only a few dozen services are installed and only a handful of those are on by default. In Fedora and RHEL systems, most network services are started by either the xinetd process or by a start-up script in the /etc/init.d directory. Other Linux systems use the inetd process instead of xinetd. xinetd and inetd are daemons that listen on a great number of network port numbers. When a connection is made to a particular port number, xinetd or inetd automatically starts the appropriate program for that service and hands the connection to it. For xinetd, the configuration file /etc/xinetd.conf is used to provide default settings for the xinetd server. The directory /etc/xinetd.d contains files telling xinetd what ports to listen on and what programs to start (the inetd daemon, alternatively, uses only the /etc/inetd.conf file). Each file in /etc/xinetd.d contains configuration information for a single service, and the file is usually named after the service it configures. For example, to enable the rsync service, edit the rsync file in the /etc/xinetd.d directory and look for a section similar to the following: service rsync { disable = yes socket_type = stream wait = no user = root server = /usr/bin/rsync server_args = –daemon log_on_failure += USERID } Note that the first line of this example identifies the service as rsync. This exactly matches the service name listed in the /etc/services file, causing the service to
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