Internal Service
In the Unix environment, it is a program handled internally by inetd and executes in the background. Services such as echo, time, and discard are
fairly trivial, but are useful for testing. They should be disabled when tests
are not in progress, since attackers could use internal services for denial of
service attacks.
In the /etc/inetd.conf file, internal services contain the term internal as the last entry on the line.
See also External Service.