/etc/services is a plain ASCII file providing a mapping between friendly textual names for internet services, and their underlying assigned port numbers and protocol types. Every networking program should look into this file to get the port number (and protocol) for its service. Port numbers are assigned by the IANA (Internet Assigned Numbers Authority), and their current policy is to assign both TCP and UDP protocols when assigning a port number. Therefore, most entries will have two entries, even for TCP only services. Port numbers below 1024 (so-called 'low numbered' ports) can only be bound to by root (see bind(2), tcp(7), and udp(7).) This is so that clients connecting to low numbered ports can trust that the service running on the port is the standard implementation, and not a rogue service run by a user of the machine. Well-known port numbers specified by the IANA are normally located in this root only space. The presence of an entry for a service in the services file does not necessarily mean that the service is currently running on the machine. See inetd.conf(5) for the configuration of Internet services offered. Note that not all networking services are started by inetd(8), and so won't appear in inetd.conf(5). In particular, news (NNTP) and mail (SMTP) servers are often initialised from the system boot scripts.