services(5)

1SERVICES(5)                Linux Programmer's Manual               SERVICES(5)
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NAME

6       services - Internet network services list
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DESCRIPTION

9       services  is  a  plain  ASCII  file  providing a mapping between human-
10       friendly textual names for internet services, and their underlying  as‐
11       signed  port  numbers  and  protocol  types.   Every networking program
12       should look into this file to get the port number  (and  protocol)  for
13       its  service.   The C library routines getservent(3), getservbyname(3),
14       getservbyport(3), setservent(3),  and  endservent(3)  support  querying
15       this file from programs.
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17       Port  numbers  are  assigned by the IANA (Internet Assigned Numbers Au‐
18       thority), and their current policy is to assign both TCP and UDP proto‐
19       cols  when  assigning a port number.  Therefore, most entries will have
20       two entries, even for TCP-only services.
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22       Port numbers below 1024 (so-called "low numbered" ports) can  be  bound
23       to  only by root (see bind(2), tcp(7), and udp(7)).  This is so clients
24       connecting to low numbered ports can trust that the service running  on
25       the port is the standard implementation, and not a rogue service run by
26       a user of the machine.  Well-known port numbers specified by  the  IANA
27       are normally located in this root-only space.
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29       The  presence  of  an entry for a service in the services file does not
30       necessarily mean that the service is currently running on the  machine.
31       See  inetd.conf(5)  for the configuration of Internet services offered.
32       Note that not all networking services are started by inetd(8),  and  so
33       won't  appear  in  inetd.conf(5).   In particular, news (NNTP) and mail
34       (SMTP) servers are often initialized from the system boot scripts.
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36       The location of the services  file  is  defined  by  _PATH_SERVICES  in
37       <netdb.h>.  This is usually set to /etc/services.
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39       Each line describes one service, and is of the form:
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41              service-name   port/protocol   [aliases ...]
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43       where:
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45       service-name
46              is  the  friendly name the service is known by and looked up un‐
47              der.  It is case sensitive.  Often, the client program is  named
48              after the service-name.
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50       port   is the port number (in decimal) to use for this service.
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52       protocol
53              is  the type of protocol to be used.  This field should match an
54              entry in the protocols(5) file.  Typical values include tcp  and
55              udp.
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57       aliases
58              is  an  optional  space or tab separated list of other names for
59              this service.  Again, the names are case sensitive.
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61       Either spaces or tabs may be used to separate the fields.
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63       Comments are started by the hash sign (#) and continue until the end of
64       the line.  Blank lines are skipped.
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66       The  service-name  should  begin in the first column of the file, since
67       leading spaces are not stripped.  service-names can  be  any  printable
68       characters  excluding space and tab.  However, a conservative choice of
69       characters should be used to minimize compatibility problems.  For  ex‐
70       ample, a-z, 0-9, and hyphen (-) would seem a sensible choice.
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72       Lines  not  matching  this  format  should  not be present in the file.
73       (Currently, they are  silently  skipped  by  getservent(3),  getservby‐
74       name(3),  and  getservbyport(3).   However, this behavior should not be
75       relied on.)
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77       This file might be distributed over a network using a network-wide nam‐
78       ing service like Yellow Pages/NIS or BIND/Hesiod.
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80       A sample services file might look like this:
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82           netstat         15/tcp
83           qotd            17/tcp          quote
84           msp             18/tcp          # message send protocol
85           msp             18/udp          # message send protocol
86           chargen         19/tcp          ttytst source
87           chargen         19/udp          ttytst source
88           ftp             21/tcp
89           # 22 - unassigned
90           telnet          23/tcp
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FILES

93       /etc/services
94              The Internet network services list
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96       <netdb.h>
97              Definition of _PATH_SERVICES
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COLOPHON

106       This page is part of release 5.10 of the Linux  man-pages  project.   A
107       description  of  the project, information about reporting bugs, and the
108       latest    version    of    this    page,    can     be     found     at
109       https://www.kernel.org/doc/man-pages/.
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113Linux                             2020-04-11                       SERVICES(5)

NAME

DESCRIPTION

FILES

SEE ALSO

COLOPHON