External Service
In the Unix environment, it is a separate program controlled by inetd and executes in the background. Unlike internal services, these programs are
not internal to inetd. Some examples are ftp, telnet, login, and finger.
In the /etc/inetd.conf file, external services contain the command name and arguments (such as /usr/etc/ftpd ftpd for the FTP daemon) as the last entries on the line.
See also Internal Service.