The port numbers on which certain standard services are offered are defined in the RFC 1700 Assigned Numbers. The /etc/services file enables server and client programs to convert service names to these numbers -ports. The list is kept on each host and it is stored in the file /etc/services. Only the "root" user is allowed to make modification in this file and it is rare to edit the /etc/services file to make change since it already contains the more common service names to port numbers. To improve security, we can immunize this file to prevent unauthorized deletion or addition of services. To immunize the /etc/services file, use the command:

           [root@deep] /#chattr  +i /etc/services