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 be bound to only by root (see bind(2), tcp(7), and udp(7)). This is so 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 initialized from the system boot scripts.
The location of the services file is defined by _PATH_SERVICES in <netdb.h>. This is usually set to /etc/services.
Each line describes one service, and is of the form:
Either spaces or tabs may be used to separate the fields.
Comments are started by the hash sign (#) and continue until the end of the line. Blank lines are skipped.
The service-name should begin in the first column of the file, since leading spaces are not stripped. service-names can be any printable characters excluding space and tab. However, a conservative choice of characters should be used to minimize compatibility problems. E.g., a-z, 0-9, and hyphen (-) would seem a sensible choice.
Lines not matching this format should not be present in the file. (Currently, they are silently skipped by getservent(3), getservbyname(3), and getservbyport(3). However, this behavior should not be relied on.)
This file might be distributed over a network using a network-wide naming service like Yellow Pages/NIS or BIND/Hesiod.
A sample services file might look like this:
netstat 15/tcp qotd 17/tcp quote msp 18/tcp # message send protocol msp 18/udp # message send protocol chargen 19/tcp ttytst source chargen 19/udp ttytst source ftp 21/tcp # 22 - unassigned telnet 23/tcp