services — Internet network services list
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 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 ...]
service-nameis 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.
portis the port number (in decimal) to use for this service.
protocolis the type of protocol to be used. This field
should match an entry in the protocols(5)
file. Typical values include tcp and
udp.
aliasesis an optional space or tab separated list of other names for this service (but see the BUGS section below). 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.)
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.
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
There is a maximum of 35 aliases, due to the way the getservent(3) code is written.
Lines longer than BUFSIZ
(currently 1024) characters will be ignored by getservent(3), getservbyname(3), and
getservbyport(3). However,
this will also cause the next line to be mis-parsed.
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)
Guide to Yellow Pages Service
Guide to BIND/Hesiod Service
This page is part of release 2.79 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
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This manpage is Copyright (C) 1996 Austin Donnelly <and1000cam.ac.uk>, with additional material (c) 1995 Martin Schulze <joeyinfodrom.north.de> Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. 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 <and1000cam.ac.uk> * Merged two services(5) manpages |