.\" This manpage is Copyright (C) 1996 Austin Donnelly <and1000@cam.ac.uk>,
.\" with additional material Copyright (c) 1995 Martin Schulze
.\"     <joey@infodrom.north.de>
.\"
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.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
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.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
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.\" manual page may be incorrect or out-of-date.  The author(s) assume no
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.\"
.\"   This manpage was made by merging two independently written manpages,
.\"   one written by Martin Schulze (18 Oct 95), the other written by
.\"   Austin Donnelly, (9 Jan 96).
.\"
.\" Thu Jan 11 12:14:41 1996 Austin Donnelly  <and1000@cam.ac.uk>
.\"   * Merged two services(5) manpages
.\"
.TH SERVICES 5 2020-04-11 "Linux" "Linux Programmer's Manual"
.SH NAME
services \- Internet network services list
.SH DESCRIPTION
.B 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
.BR getservent (3),
.BR getservbyname (3),
.BR getservbyport (3),
.BR setservent (3),
and
.BR endservent (3)
support querying this file from programs.
.PP
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.
.PP
Port numbers below 1024 (so-called "low numbered" ports) can be
bound to only by root (see
.BR bind (2),
.BR tcp (7),
and
.BR 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.
.PP
The presence of an entry for a service in the
.B services
file does not necessarily mean that the service is currently running
on the machine.
See
.BR inetd.conf (5)
for the configuration of Internet services offered.
Note that not all
networking services are started by
.BR inetd (8),
and so won't appear in
.BR inetd.conf (5).
In particular, news (NNTP) and mail (SMTP) servers are often
initialized from the system boot scripts.
.PP
The location of the
.B services
file is defined by
.B _PATH_SERVICES
in
.IR <netdb.h> "."
This is usually set to
.IR /etc/services "."
.PP
Each line describes one service, and is of the form:
.IP
\f2service-name\ \ \ port\f3/\f2protocol\ \ \ \f1[\f2aliases ...\f1]
.TP
where:
.TP
.I service-name
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
.IR service-name "."
.TP
.I port
is the port number (in decimal) to use for this service.
.TP
.I protocol
is the type of protocol to be used.
This field should match an entry
in the
.BR protocols (5)
file.
Typical values include
.B tcp
and
.BR udp .
.TP
.I aliases
is an optional space or tab separated list of other names for this
service.
Again, the names are case
sensitive.
.PP
Either spaces or tabs may be used to separate the fields.
.PP
Comments are started by the hash sign (#) and continue until the end
of the line.
Blank lines are skipped.
.PP
The
.I service-name
should begin in the first column of the file, since leading spaces are
not stripped.
.I service-names
can be any printable characters excluding space and tab.
However, a conservative choice of characters should be used to minimize
compatibility problems.
For example, a\-z, 0\-9, and hyphen (\-) would seem a
sensible choice.
.PP
Lines not matching this format should not be present in the
file.
(Currently, they are silently skipped by
.BR getservent (3),
.BR getservbyname (3),
and
.BR getservbyport (3).
However, this behavior should not be relied on.)
.PP
.\" The following is not true as at glibc 2.8 (a line with a comma is
.\" ignored by getservent()); it's not clear if/when it was ever true.
.\"   As a backward compatibility feature, the slash (/) between the
.\"   .I port
.\"   number and
.\"   .I protocol
.\"   name can in fact be either a slash or a comma (,).
.\"   Use of the comma in
.\"   modern installations is deprecated.
.\"
This file might be distributed over a network using a network-wide
naming service like Yellow Pages/NIS or BIND/Hesiod.
.PP
A sample
.B services
file might look like this:
.PP
.in +4n
.EX
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
.EE
.in
.SH FILES
.TP
.I /etc/services
The Internet network services list
.TP
.I <netdb.h>
Definition of
.B _PATH_SERVICES
.\" .SH BUGS
.\" It's not clear when/if the following was ever true;
.\" it isn't true for glibc 2.8:
.\"    There is a maximum of 35 aliases, due to the way the
.\"    .BR getservent (3)
.\"    code is written.
.\"
.\" It's not clear when/if the following was ever true;
.\" it isn't true for glibc 2.8:
.\"    Lines longer than
.\"    .B BUFSIZ
.\"    (currently 1024) characters will be ignored by
.\"    .BR getservent (3),
.\"    .BR getservbyname (3),
.\"    and
.\"    .BR getservbyport (3).
.\"    However, this will also cause the next line to be mis-parsed.
.SH SEE ALSO
.BR listen (2),
.BR endservent (3),
.BR getservbyname (3),
.BR getservbyport (3),
.BR getservent (3),
.BR setservent (3),
.BR inetd.conf (5),
.BR protocols (5),
.BR inetd (8)
.PP
Assigned Numbers RFC, most recently RFC\ 1700, (AKA STD0002).