man5/services.5

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   3 .\" This manpage is Copyright (C) 1996 Austin Donnelly <and1000@cam.ac.uk>,
   4 .\"  with additional material (c) 1995 Martin Schulze <joey@infodrom.north.de>
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  26 .\"   This manpage was made by merging two independently written manpages,
  27 .\"   one written by Martin Schulze (18 Oct 95), the other written by
  28 .\"   Austin Donnelly, (9 Jan 96).
  29 .\"
  30 .\" Thu Jan 11 12:14:41 1996 Austin Donnelly  <and1000@cam.ac.uk>
  31 .\"   * Merged two services(5) manpages
  32 .\"
  33 .TH SERVICES 5 1996-01-11 "Linux" "Linux Programmer's Manual"
  34 .SH NAME
  35 services \- Internet network services list
  36 .SH DESCRIPTION
  37 .B services
  38 is a plain ASCII file providing a mapping between friendly textual
  39 names for internet services, and their underlying assigned port
  40 numbers and protocol types. Every networking program should look into
  41 this file to get the port number (and protocol) for its service.
  42 The C library routines
  43 .BR getservent (3),
  44 .BR getservbyname (3),
  45 .BR getservbyport (3),
  46 .BR setservent (3),
  47 and
  48 .BR endservent (3)
  49 support querying this file from programs.
  50
  51 Port numbers are assigned by the IANA (Internet Assigned Numbers
  52 Authority), and their current policy is to assign both TCP and UDP
  53 protocols when assigning a port number. Therefore, most entries will
  54 have two entries, even for TCP only services.
  55
  56 Port numbers below 1024 (so-called 'low numbered' ports) can only be
  57 bound to by root (see
  58 .BR bind (2),
  59 .BR tcp (7),
  60 and
  61 .BR udp (7)).
  62 This is so clients connecting to low numbered ports can trust
  63 that the service running on the port is the standard implementation,
  64 and not a rogue service run by a user of the machine.  Well-known port
  65 numbers specified by the IANA are normally located in this root-only
  66 space.
  67
  68 The presence of an entry for a service in the
  69 .B services
  70 file does not necessarily mean that the service is currently running
  71 on the machine. See
  72 .BR inetd.conf (5)
  73 for the configuration of Internet services offered. Note that not all
  74 networking services are started by
  75 .BR inetd (8),
  76 and so won't appear in
  77 .BR inetd.conf (5).
  78 In particular, news (NNTP) and mail (SMTP) servers are often
  79 initialized from the system boot scripts.
  80
  81 The location of the
  82 .B services
  83 file is defined by
  84 .B _PATH_SERVICES
  85 in
  86 .IR /usr/include/netdb.h "."
  87 This is usually set to
  88 .IR /etc/services "."
  89
  90 Each line describes one service, and is of the form:
  91 .IP
  92 \f2service-name\ \ \ port\f3/\f2protocol\ \ \ \f1[\f2aliases ...\f1]
  93 .TP
  94 where:
  95 .TP 10
  96 .I service-name
  97 is the friendly name the service is known by and looked up under. It
  98 is case sensitive. Often, the client program is named after the
  99 .IR service-name "."
 100 .TP
 101 .I port
 102 is the port number (in decimal) to use for this service.
 103 .TP
 104 .I protocol
 105 is the type of protocol to be used. This field should match an entry
 106 in the
 107 .BR protocols (5)
 108 file. Typical values include
 109 .B tcp
 110 and
 111 .BR udp .
 112 .TP
 113 .I aliases
 114 is an optional space or tab separated list of other names for this
 115 service (but see the BUGS section below). Again, the names are case
 116 sensitive.
 117 .PP
 118
 119 Either spaces or tabs may be used to separate the fields.
 120
 121 Comments are started by the hash sign (#) and continue until the end
 122 of the line. Blank lines are skipped.
 123
 124 The
 125 .I service-name
 126 should begin in the first column of the file, since leading spaces are
 127 not stripped.
 128 .I service-names
 129 can be any printable characters excluding space and tab. However,
 130 a conservative choice of characters should be used to minimize
 131 inter-operability problems. E.g., a\-z, 0\-9, and hyphen (\-) would seem a
 132 sensible choice.
 133
 134 Lines not matching this format should not be present in the
 135 file. (Currently, they are silently skipped by
 136 .BR getservent (3),
 137 .BR getservbyname (3),
 138 and
 139 .BR getservbyport (3).
 140 However, this behaviour should not be relied on.)
 141
 142 As a backwards compatibility feature, the slash (/) between the
 143 .I port
 144 number and
 145 .I protocol
 146 name can in fact be either a slash or a comma (,). Use of the comma in
 147 modern installations is depreciated.
 148
 149 This file might be distributed over a network using a network-wide
 150 naming service like Yellow Pages/NIS or BIND/Hesiod.
 151
 152 A sample
 153 .B services
 154 file might look like this:
 155 .RS
 156 .nf
 157 .sp
 158 .ta 3i
 159 netstat         15/tcp
 160 qotd            17/tcp          quote
 161 msp             18/tcp          # message send protocol
 162 msp             18/udp          # message send protocol
 163 chargen         19/tcp          ttytst source
 164 chargen         19/udp          ttytst source
 165 ftp             21/tcp
 166 # 22 \- unassigned
 167 telnet          23/tcp
 168 .sp
 169 .fi
 170 .RE
 171 .SH BUGS
 172 There is a maximum of 35 aliases, due to the way the
 173 .BR getservent (3)
 174 code is written.
 175
 176 Lines longer than
 177 .B BUFSIZ
 178 (currently 1024) characters will be ignored by
 179 .BR getservent (3),
 180 .BR getservbyname (3),
 181 and
 182 .BR getservbyport (3).
 183 However, this will also cause the next line to be mis-parsed.
 184 .SH FILES
 185 .TP
 186 .I /etc/services
 187 The Internet network services list
 188 .TP
 189 .I /usr/include/netdb.h
 190 Definition of
 191 .B _PATH_SERVICES
 192 .SH "SEE ALSO"
 193 .BR listen (2),
 194 .BR endservent (3),
 195 .BR getservbyname (3),
 196 .BR getservbyport (3),
 197 .BR getservent (3),
 198 .BR setservent (3),
 199 .BR inetd.conf (5),
 200 .BR protocols (5),
 201 .BR inetd (8)
 202
 203 Assigned Numbers RFC, most recently RFC 1700, (AKA STD0002)
 204
 205 Guide to Yellow Pages Service
 206
 207 Guide to BIND/Hesiod Service