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 2015-02-21 "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 resolv+ 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 resolv+ 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 resolv+ will attempt to reorder host addresses so that local addresses
  75 (i.e., on the same subnet) are listed first when a
  76 .BR gethostbyname (3)
  77 is performed.
  78 Reordering is done for all lookup methods.
  79 The default value is
  80 .IR off .
  81 .SH ENVIRONMENT
  82 The following environment variables can be used to allow users to
  83 override the behavior which is configured in
  84 .IR /etc/host.conf :
  85 .TP
  86 .B RESOLV_HOST_CONF
  87 If set, this variable points to a file that should be read instead of
  88 .IR /etc/host.conf .
  89 .TP
  90 .B RESOLV_MULTI
  91 Overrides the
  92 .I multi
  93 command.
  94 .TP
  95 .B RESOLV_REORDER
  96 Overrides the
  97 .I reorder
  98 command.
  99 .TP
 100 .B RESOLV_ADD_TRIM_DOMAINS
 101 A list of domains,  separated by colons (\(aq:\(aq), semicolons (\(aq;\(aq) or
 102 commas (\(aq,\(aq), with the leading dot, which will be added to the list of
 103 domains that should be trimmed.
 104 .TP
 105 .B RESOLV_OVERRIDE_TRIM_DOMAINS
 106 A list of domains,  separated by colons (\(aq:\(aq), semicolons (\(aq;\(aq) or
 107 commas (\(aq,\(aq), with the leading dot, which will replace the list of
 108 domains that should be trimmed.
 109 Overrides the
 110 .I trim
 111 command.
 112 .SH FILES
 113 .TP
 114 .I /etc/host.conf
 115 Resolver configuration file
 116 .TP
 117 .I /etc/resolv.conf
 118 Resolver configuration file
 119 .TP
 120 .I /etc/hosts
 121 Local hosts database
 122 .SH NOTES
 123 The following differences exist compared to the original implementation.
 124 A new command
 125 .I spoof
 126 and a new environment variable
 127 .B RESOLV_SPOOF_CHECK
 128 can take arguments like
 129 .IR off ", " nowarn " and " warn .
 130 Line comments can appear anywhere and not only at the beginning of a line.
 131 .SS Historical
 132 The
 133 .BR nsswitch.conf (5)
 134 file is the modern way of controlling the order of host lookups.
 135 .PP
 136 In glibc 2.4 and earlier, the following keyword is recognized:
 137 .TP
 138 .I order
 139 This keyword specifies how host lookups are to be performed.
 140 It should be followed by one or more lookup methods, separated by commas.
 141 Valid methods are
 142 .IR bind ", " hosts ", and " nis .
 143 .TP
 144 .B RESOLV_SERV_ORDER
 145 Overrides the order command.
 146 .PP
 147 Since glibc 2.0.7, the following keywords and environment variable have
 148 been recognized but never implemented:
 149 .TP
 150 .I nospoof
 151 Valid values are
 152 .IR on " and " off .
 153 If set to
 154 .IR on ,
 155 the resolv+ library will attempt to prevent hostname spoofing to
 156 enhance the security of
 157 .BR rlogin " and " rsh .
 158 It works as follows: after performing a host address lookup, resolv+
 159 will perform a hostname lookup for that address.
 160 If the two hostnames
 161 do not match, the query will fail.
 162 The default value is
 163 .IR off .
 164 .TP
 165 .I spoofalert
 166 Valid values are
 167 .IR on " and " off .
 168 If this option is set to
 169 .I on
 170 and the
 171 .I nospoof
 172 option is also set, resolv+ will log a warning of the error via the
 173 syslog facility.
 174 The default value is
 175 .IR off .
 176 .TP
 177 .I spoof
 178 Valid values are
 179 .IR off ", " nowarn " and " warn .
 180 If this option is set to
 181 .IR off ,
 182 spoofed addresses are permitted and no warnings will be emitted
 183 via the syslog facility.
 184 If this option is set to
 185 .IR warn ,
 186 resolv+ will attempt to prevent hostname spoofing to
 187 enhance the security and log a warning of the error via the syslog
 188 facility.
 189 If this option is set to
 190 .IR nowarn ,
 191 the resolv+ library will attempt to prevent hostname spoofing to
 192 enhance the security but not emit warnings via the syslog facility.
 193 Setting this option to anything else is equal to setting it to
 194 .IR nowarn .
 195 .TP
 196 .B RESOLV_SPOOF_CHECK
 197 Overrides the
 198 .IR nospoof ", " spoofalert " and " spoof
 199 commands in the same way as the
 200 .I spoof
 201 command is parsed.
 202 Valid values are
 203 .IR off ", " nowarn " and " warn .
 204 .SH SEE ALSO
 205 .BR gethostbyname (3),
 206 .BR hosts (5),
 207 .BR nsswitch.conf (5),
 208 .BR resolv.conf (5),
 209 .BR hostname (7),
 210 .BR named (8)