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.
  41 Every networking program should look into
  42 this file to get the port number (and protocol) for its service.
  43 The C library routines
  44 .BR getservent (3),
  45 .BR getservbyname (3),
  46 .BR getservbyport (3),
  47 .BR setservent (3),
  48 and
  49 .BR endservent (3)
  50 support querying this file from programs.
  51
  52 Port numbers are assigned by the IANA (Internet Assigned Numbers
  53 Authority), and their current policy is to assign both TCP and UDP
  54 protocols when assigning a port number.
  55 Therefore, most entries will
  56 have two entries, even for TCP only services.
  57
  58 Port numbers below 1024 (so-called 'low numbered' ports) can only be
  59 bound to by root (see
  60 .BR bind (2),
  61 .BR tcp (7),
  62 and
  63 .BR udp (7)).
  64 This is so clients connecting to low numbered ports can trust
  65 that the service running on the port is the standard implementation,
  66 and not a rogue service run by a user of the machine.
  67 Well-known port numbers specified by the IANA are normally
  68 located in this root-only space.
  69
  70 The presence of an entry for a service in the
  71 .B services
  72 file does not necessarily mean that the service is currently running
  73 on the machine.
  74 See
  75 .BR inetd.conf (5)
  76 for the configuration of Internet services offered.
  77 Note that not all
  78 networking services are started by
  79 .BR inetd (8),
  80 and so won't appear in
  81 .BR inetd.conf (5).
  82 In particular, news (NNTP) and mail (SMTP) servers are often
  83 initialized from the system boot scripts.
  84
  85 The location of the
  86 .B services
  87 file is defined by
  88 .B _PATH_SERVICES
  89 in
  90 .IR /usr/include/netdb.h "."
  91 This is usually set to
  92 .IR /etc/services "."
  93
  94 Each line describes one service, and is of the form:
  95 .IP
  96 \f2service-name\ \ \ port\f3/\f2protocol\ \ \ \f1[\f2aliases ...\f1]
  97 .TP
  98 where:
  99 .TP 10
 100 .I service-name
 101 is the friendly name the service is known by and looked up under.
 102 It is case sensitive.
 103 Often, the client program is named after the
 104 .IR service-name "."
 105 .TP
 106 .I port
 107 is the port number (in decimal) to use for this service.
 108 .TP
 109 .I protocol
 110 is the type of protocol to be used.
 111 This field should match an entry
 112 in the
 113 .BR protocols (5)
 114 file.
 115 Typical values include
 116 .B tcp
 117 and
 118 .BR udp .
 119 .TP
 120 .I aliases
 121 is an optional space or tab separated list of other names for this
 122 service (but see the BUGS section below).
 123 Again, the names are case
 124 sensitive.
 125 .PP
 126 Either spaces or tabs may be used to separate the fields.
 127
 128 Comments are started by the hash sign (#) and continue until the end
 129 of the line.
 130 Blank lines are skipped.
 131
 132 The
 133 .I service-name
 134 should begin in the first column of the file, since leading spaces are
 135 not stripped.
 136 .I service-names
 137 can be any printable characters excluding space and tab.
 138 However, a conservative choice of characters should be used to minimize
 139 inter-operability problems.
 140 E.g., a\-z, 0\-9, and hyphen (\-) would seem a
 141 sensible choice.
 142
 143 Lines not matching this format should not be present in the
 144 file.
 145 (Currently, they are silently skipped by
 146 .BR getservent (3),
 147 .BR getservbyname (3),
 148 and
 149 .BR getservbyport (3).
 150 However, this behavior should not be relied on.)
 151
 152 As a backwards compatibility feature, the slash (/) between the
 153 .I port
 154 number and
 155 .I protocol
 156 name can in fact be either a slash or a comma (,).
 157 Use of the comma in
 158 modern installations is depreciated.
 159
 160 This file might be distributed over a network using a network-wide
 161 naming service like Yellow Pages/NIS or BIND/Hesiod.
 162
 163 A sample
 164 .B services
 165 file might look like this:
 166 .RS
 167 .nf
 168 .sp
 169 .ta 3i
 170 netstat         15/tcp
 171 qotd            17/tcp          quote
 172 msp             18/tcp          # message send protocol
 173 msp             18/udp          # message send protocol
 174 chargen         19/tcp          ttytst source
 175 chargen         19/udp          ttytst source
 176 ftp             21/tcp
 177 # 22 \- unassigned
 178 telnet          23/tcp
 179 .fi
 180 .RE
 181 .SH FILES
 182 .TP
 183 .I /etc/services
 184 The Internet network services list
 185 .TP
 186 .I /usr/include/netdb.h
 187 Definition of
 188 .B _PATH_SERVICES
 189 .SH BUGS
 190 There is a maximum of 35 aliases, due to the way the
 191 .BR getservent (3)
 192 code is written.
 193
 194 Lines longer than
 195 .B BUFSIZ
 196 (currently 1024) characters will be ignored by
 197 .BR getservent (3),
 198 .BR getservbyname (3),
 199 and
 200 .BR getservbyport (3).
 201 However, this will also cause the next line to be mis-parsed.
 202 .SH "SEE ALSO"
 203 .BR listen (2),
 204 .BR endservent (3),
 205 .BR getservbyname (3),
 206 .BR getservbyport (3),
 207 .BR getservent (3),
 208 .BR setservent (3),
 209 .BR inetd.conf (5),
 210 .BR protocols (5),
 211 .BR inetd (8)
 212
 213 Assigned Numbers RFC, most recently RFC\ 1700, (AKA STD0002)
 214
 215 Guide to Yellow Pages Service
 216
 217 Guide to BIND/Hesiod Service