man5/host.conf.5

   1 .\" Copyright (c) 1997 Martin Schulze (joey@infodrom.north.de)
   2 .\" Much of the text is copied from the manpage of resolv+(8).
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  25 .\" 2003-08-23 Martin Schulze <joey@infodrom.org> Updated according to glibc 2.3.2
  26 .TH HOST.CONF 5 2015-07-23 "Linux" "Linux System Administration"
  27 .SH NAME
  28 host.conf \- resolver configuration file
  29 .SH DESCRIPTION
  30 The file
  31 .I /etc/host.conf
  32 contains configuration information specific to the resolver library.
  33 It should contain one configuration keyword per line, followed by
  34 appropriate configuration information.
  35 The following keywords are recognized:
  36 .TP
  37 .I trim
  38 This keyword may be listed more than once.
  39 Each time it should be
  40 followed by a list of domains, separated by colons (\(aq:\(aq), semicolons
  41 (\(aq;\(aq) or commas (\(aq,\(aq), with the leading dot.
  42 When set, the
  43 resolver library will automatically trim the given domain name from the
  44 end of any hostname resolved via DNS.
  45 This is intended for use with
  46 local hosts and domains.
  47 (Related note: trim will not affect hostnames
  48 gathered via NIS or the hosts file.
  49 Care should be taken to
  50 ensure that the first hostname for each entry in the hosts file is
  51 fully qualified or unqualified, as appropriate for the local
  52 installation.)
  53 .TP
  54 .I multi
  55 Valid values are
  56 .IR on " and " off .
  57 If set to
  58 .IR on ,
  59 the resolver library will return all valid addresses for a host that
  60 appears in the
  61 .I /etc/hosts
  62 file,
  63 instead of only the first.
  64 This is
  65 .I off
  66 by default, as it may cause a substantial performance loss at sites
  67 with large hosts files.
  68 .TP
  69 .I reorder
  70 Valid values are
  71 .IR on " and " off .
  72 If set to
  73 .IR on ,
  74 the resolver library
  75 will attempt to reorder host addresses so that local addresses
  76 (i.e., on the same subnet) are listed first when a
  77 .BR gethostbyname (3)
  78 is performed.
  79 Reordering is done for all lookup methods.
  80 The default value is
  81 .IR off .
  82 .SH ENVIRONMENT
  83 The following environment variables can be used to allow users to
  84 override the behavior which is configured in
  85 .IR /etc/host.conf :
  86 .TP
  87 .B RESOLV_HOST_CONF
  88 If set, this variable points to a file that should be read instead of
  89 .IR /etc/host.conf .
  90 .TP
  91 .B RESOLV_MULTI
  92 Overrides the
  93 .I multi
  94 command.
  95 .TP
  96 .B RESOLV_REORDER
  97 Overrides the
  98 .I reorder
  99 command.
 100 .TP
 101 .B RESOLV_ADD_TRIM_DOMAINS
 102 A list of domains,  separated by colons (\(aq:\(aq), semicolons (\(aq;\(aq) or
 103 commas (\(aq,\(aq), with the leading dot, which will be added to the list of
 104 domains that should be trimmed.
 105 .TP
 106 .B RESOLV_OVERRIDE_TRIM_DOMAINS
 107 A list of domains,  separated by colons (\(aq:\(aq), semicolons (\(aq;\(aq) or
 108 commas (\(aq,\(aq), with the leading dot, which will replace the list of
 109 domains that should be trimmed.
 110 Overrides the
 111 .I trim
 112 command.
 113 .SH FILES
 114 .TP
 115 .I /etc/host.conf
 116 Resolver configuration file
 117 .TP
 118 .I /etc/resolv.conf
 119 Resolver configuration file
 120 .TP
 121 .I /etc/hosts
 122 Local hosts database
 123 .SH NOTES
 124 The following differences exist compared to the original implementation.
 125 A new command
 126 .I spoof
 127 and a new environment variable
 128 .B RESOLV_SPOOF_CHECK
 129 can take arguments like
 130 .IR off ", " nowarn " and " warn .
 131 Line comments can appear anywhere and not only at the beginning of a line.
 132 .SS Historical
 133 The
 134 .BR nsswitch.conf (5)
 135 file is the modern way of controlling the order of host lookups.
 136 .PP
 137 In glibc 2.4 and earlier, the following keyword is recognized:
 138 .TP
 139 .I order
 140 This keyword specifies how host lookups are to be performed.
 141 It should be followed by one or more lookup methods, separated by commas.
 142 Valid methods are
 143 .IR bind ", " hosts ", and " nis .
 144 .TP
 145 .B RESOLV_SERV_ORDER
 146 Overrides the order command.
 147 .PP
 148 Since glibc 2.0.7, the following keywords and environment variable have
 149 been recognized but never implemented:
 150 .TP
 151 .I nospoof
 152 Valid values are
 153 .IR on " and " off .
 154 If set to
 155 .IR on ,
 156 the resolver library will attempt to prevent hostname spoofing to
 157 enhance the security of
 158 .BR rlogin " and " rsh .
 159 It works as follows: after performing a host address lookup,
 160 the resolver library will perform a hostname lookup for that address.
 161 If the two hostnames
 162 do not match, the query will fail.
 163 The default value is
 164 .IR off .
 165 .TP
 166 .I spoofalert
 167 Valid values are
 168 .IR on " and " off .
 169 If this option is set to
 170 .I on
 171 and the
 172 .I nospoof
 173 option is also set,
 174 the resolver library will log a warning of the error via the
 175 syslog facility.
 176 The default value is
 177 .IR off .
 178 .TP
 179 .I spoof
 180 Valid values are
 181 .IR off ", " nowarn ", and " warn .
 182 If this option is set to
 183 .IR off ,
 184 spoofed addresses are permitted and no warnings will be emitted
 185 via the syslog facility.
 186 If this option is set to
 187 .IR warn ,
 188 the resolver library will attempt to prevent hostname spoofing to
 189 enhance the security and log a warning of the error via the syslog
 190 facility.
 191 If this option is set to
 192 .IR nowarn ,
 193 the resolver library will attempt to prevent hostname spoofing to
 194 enhance the security but not emit warnings via the syslog facility.
 195 Setting this option to anything else is equal to setting it to
 196 .IR nowarn .
 197 .TP
 198 .B RESOLV_SPOOF_CHECK
 199 Overrides the
 200 .IR nospoof ", " spoofalert, " and " spoof
 201 commands in the same way as the
 202 .I spoof
 203 command is parsed.
 204 Valid values are
 205 .IR off ", " nowarn, " and " warn .
 206 .SH SEE ALSO
 207 .BR gethostbyname (3),
 208 .BR hosts (5),
 209 .BR nsswitch.conf (5),
 210 .BR resolv.conf (5),
 211 .BR hostname (7),
 212 .BR named (8)