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. The C library
routines getservent(3), getservbyname(3),
getservbyport(3), setservent(3), and endservent(3)
support querying this file from programs.
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
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
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
/usr/include/netdb.h. This is usually set to /etc/services.
Each line describes one service, and is of the form:
service-name port/protocol [aliases
is the friendly name the service is known by and looked up
under. It is case sensitive. Often, the client program is named after the
is the port number (in decimal) to use for this
is the type of protocol to be used. This field should match
an entry in the protocols(5) file. Typical values include
tcp and udp.
is an optional space or tab separated list of other names
for this service (but see the BUGS section below). Again, the names are
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 inter-operability 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 behaviour should not be relied on.)
As a backwards compatibility feature, the slash (/) between the port
number and protocol name can in fact be either a slash or a comma (,).
Use of the comma in modern installations is depreciated.
This file might be distributed over a network using a network-wide naming
service like Yellow Pages/NIS or BIND/Hesiod.