|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
NAME | DESCRIPTION | FILES | SEE ALSO | COLOPHON |
|
|
|
services(5) File Formats Manual services(5)
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 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:
service-name port/protocol [aliases ...]
where:
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 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. For example, 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
/etc/services
The Internet network services list
<netdb.h>
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).
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 services(5)
Pages that refer to this page: getaddrinfo(3), getnameinfo(3), getservent(3), getservent_r(3), pcap-filter(7), nscd(8), rpc.rquotad(8), rsyslogd(8)
|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|