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services - Internet network services list
services is a plain ASCII file providing a mapping between human-
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 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 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:
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 service-name.
port is the port number (in decimal) to use for this service.
protocol 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.
aliases is an optional space or tab separated list of other names for
this service. Again, the names are case sensitive.
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:
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
# 22 - unassigned
The Internet network services list
Definition of _PATH_SERVICES
listen(2), endservent(3), getservbyname(3), getservbyport(3),
getservent(3), setservent(3), inetd.conf(5), protocols(5), inetd(8)
Assigned Numbers RFC, most recently RFC 1700, (AKA STD0002)
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be found at http://www.kernel.org/doc/man-pages/.