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<chapter>
  <header>
    <copyright>
      <year>2004</year><year>2016</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.

    </legalnotice>

    <title>Inet configuration</title>
    <prepared>Peter Andersson</prepared>
    <docno></docno>
    <date>2004-03-02</date>
    <rev>PA1</rev>
    <file>inet_cfg.xml</file>
  </header>

  <section>
    <title>Introduction</title>
    <p>This chapter tells you how the Erlang runtime system is configured
      for IP communication. It also explains how you may configure it 
      for your own particular needs by means of a configuration file.
      The information here is mainly intended for users with special 
      configuration needs or problems. There should normally be no need
      for specific settings for Erlang to function properly on a correctly 
      IP configured platform.      </p>
    <p>When Erlang starts up it will read the kernel variable 
      <c><![CDATA[inetrc]]></c> which, if defined, should specify the location and 
      name of a user configuration file. Example:</p>
    <p><c><![CDATA[% erl -kernel inetrc '"./cfg_files/erl_inetrc"']]></c></p>
    <p>Note that the usage of a <c><![CDATA[.inetrc]]></c> file, which was
      supported in earlier Erlang versions, is now obsolete.</p>
    <p>A second way to specify the configuration file is to set the
      environment variable <c><![CDATA[ERL_INETRC]]></c> to the full name of the file. Example (bash):</p>
    <p><c><![CDATA[% export ERL_INETRC=./cfg_files/erl_inetrc]]></c></p>
    <p>Note that the kernel variable <c><![CDATA[inetrc]]></c> overrides this environment variable.</p>
    <p>If no user configuration file is specified and Erlang is started
      in non-distributed or short name distributed mode, Erlang will use 
      default configuration settings and a native lookup method that should 
      work correctly under most circumstances. Erlang 
      will not read any information from system inet configuration files 
      (like /etc/host.conf, /etc/nsswitch.conf, etc) in these modes,
      except for /etc/resolv.conf and /etc/hosts that is read and monitored
      for changes on Unix platforms for the internal DNS client
      <seealso marker="kernel:inet_res">inet_res</seealso>.</p>
    <p>If Erlang is started in long name distributed mode, it needs to
      get the domain name from somewhere and will read system inet
      configuration files for this information. Any hosts and resolver
      information found then is also recorded, but not 
      used as long as Erlang is configured for native lookups. (The
      information becomes useful if the lookup method is changed to 
      <c><![CDATA['file']]></c> or <c><![CDATA['dns']]></c>, see below).</p>
    <p>Native lookup (system calls) is always the default resolver method. This
      is true for all platforms except VxWorks and OSE Delta where <c><![CDATA['file']]></c> 
      or <c><![CDATA['dns']]></c> is used (in that order of priority).</p>
    <p>On Windows platforms, Erlang will search the system registry rather than
      look for configuration files when started in long name distributed mode. </p>
  </section>

  <section>
    <title>Configuration Data</title>
    <p>Erlang records the following data in a local database if found in system 
      inet configuration files (or system registry):</p>
    <list type="bulleted">
      <item>Host names and addresses</item>
      <item>Domain name</item>
      <item>Nameservers</item>
      <item>Search domains</item>
      <item>Lookup method</item>
    </list>
    <p>This data may also be specified explicitly in the user
      configuration file. The configuration file should contain lines
      of configuration parameters (each terminated with a full
      stop). Some parameters add data to the configuration (e.g. host
      and nameserver), others overwrite any previous settings
      (e.g. domain and lookup). The user configuration file is always
      examined last in the configuration process, making it possible
      for the user to override any default values or previously made
      settings. Call <c><![CDATA[inet:get_rc()]]></c> to view the state of the
      inet configuration database.</p>
    <p>These are the valid configuration parameters:</p>
    <p></p>
    <taglist>
      <tag><em><c><![CDATA[{file, Format, File}.]]></c></em></tag>
      <item>
        <p></p>
        <p><c><![CDATA[Format = atom()]]></c></p>
        <p><c><![CDATA[File = string()]]></c></p>
        <p></p>
        <p>Specify a system file that Erlang should read configuration
          data from. <c><![CDATA[Format]]></c> tells the parser how the file should be
          interpreted: <c><![CDATA[resolv]]></c> (Unix resolv.conf), <c><![CDATA[host_conf_freebsd]]></c> 
          (FreeBSD host.conf), <c><![CDATA[host_conf_bsdos]]></c> (BSDOS host.conf), 
          <c><![CDATA[host_conf_linux]]></c> (Linux host.conf), <c><![CDATA[nsswitch_conf]]></c>
          (Unix nsswitch.conf) or <c><![CDATA[hosts]]></c> (Unix hosts). <c><![CDATA[File]]></c> should
          specify the name of the file with full path.</p>
        <p></p>
      </item>
      <tag><em><c><![CDATA[{resolv_conf, File}.]]></c></em></tag>
      <item>
        <p></p>
        <p><c><![CDATA[File = string()]]></c></p>
        <p></p>
        <p>Specify a system file that Erlang should read resolver
          configuration from for the internal DNS client
          <seealso marker="kernel:inet_res">inet_res</seealso>,
          and monitor for changes, even if it does not exist.
          The path must be absolute.</p>
        <p>This may override the configuration parameters
          <c><![CDATA[nameserver]]></c> and
          <c><![CDATA[search]]></c> depending on the contents
          of the specified file. They may also change any time in the future
          reflecting the file contents.</p>
        <p>If the file is specified as an empty string "",
          no file is read nor monitored in the future. This emulates
          the old behaviour of not configuring the DNS client when
          the node is started in short name distributed mode.</p>
        <p>If this parameter is not specified it defaults to
          <c><![CDATA[/etc/resolv.conf]]></c> unless the environment variable
          <c><![CDATA[ERL_INET_ETC_DIR]]></c> is set which defines
          the directory for this file to some maybe other than
          <c><![CDATA[/etc]]></c>.</p>
        <p></p>
      </item>
      <tag><em><c><![CDATA[{hosts_file, File}.]]></c></em></tag>
      <item>
        <p></p>
        <p><c><![CDATA[File = string()]]></c></p>
        <p></p>
        <p>Specify a system file that Erlang should read resolver
          configuration from for the internal hosts file resolver
          and monitor for changes, even if it does not exist.
          The path must be absolute.</p>
        <p>These host entries are searched after all added with
          <c>{file, hosts, File}</c> above or
          <c>{host, IP, Aliases}</c> below when the lookup option
          <c>file</c> is used.</p>
        <p>If the file is specified as an empty string "",
          no file is read nor monitored in the future. This emulates
          the old behaviour of not configuring the DNS client when
          the node is started in short name distributed mode.</p>
        <p>If this parameter is not specified it defaults to
          <c><![CDATA[/etc/hosts]]></c> unless the environment variable
          <c><![CDATA[ERL_INET_ETC_DIR]]></c> is set which defines
          the directory for this file to some maybe other than
          <c><![CDATA[/etc]]></c>.</p>
        <p></p>
      </item>
      <tag><em><c><![CDATA[{registry, Type}.]]></c></em></tag>
      <item>
        <p></p>
        <p><c><![CDATA[Type = atom()]]></c></p>
        <p></p>
        <p>Specify a system registry that Erlang should read configuration
          data from. Currently, <c><![CDATA[win32]]></c> is the only valid option.</p>
        <p></p>
      </item>
      <tag><em><c><![CDATA[{host, IP, Aliases}.]]></c></em></tag>
      <item>
        <p></p>
        <p><c><![CDATA[IP = tuple()]]></c></p>
        <p><c><![CDATA[Aliases = [string()]]]></c></p>
        <p></p>
        <p>Add host entry to the hosts table.</p>
        <p></p>
      </item>
      <tag><em><c><![CDATA[{domain, Domain}.]]></c></em></tag>
      <item>
        <p></p>
        <p><c><![CDATA[Domain = string()]]></c></p>
        <p></p>
        <p>Set domain name.</p>
        <p></p>
      </item>
      <tag><em><c><![CDATA[{nameserver, IP [,Port]}.]]></c></em></tag>
      <item>
        <p></p>
        <p><c><![CDATA[IP = tuple()]]></c></p>
        <p><c><![CDATA[Port = integer()]]></c></p>
        <p></p>
        <p>Add address (and port, if other than default) of primary 
          nameserver to use for
         <seealso marker="kernel:inet_res">inet_res</seealso>.</p>
        <p></p>
      </item>
      <tag><em><c><![CDATA[{alt_nameserver, IP [,Port]}.]]></c></em></tag>
      <item>
        <p></p>
        <p><c><![CDATA[IP = tuple()]]></c></p>
        <p><c><![CDATA[Port = integer()]]></c></p>
        <p></p>
        <p>Add address (and port, if other than default) of secondary 
          nameserver for
         <seealso marker="kernel:inet_res">inet_res</seealso>.</p>
        <p></p>
      </item>
      <tag><em><c><![CDATA[{search, Domains}.]]></c></em></tag>
      <item>
        <p></p>
        <p><c><![CDATA[Domains = [string()]]]></c></p>
        <p></p>
        <p>Add search domains for
         <seealso marker="kernel:inet_res">inet_res</seealso>.</p>
        <p></p>
      </item>
      <tag><em><c><![CDATA[{lookup, Methods}.]]></c></em></tag>
      <item>
        <p></p>
        <p><c><![CDATA[Methods = [atom()]]]></c></p>
        <p></p>
        <p>Specify lookup methods and in which order to try them. 
          The valid methods are: <c><![CDATA[native]]></c> (use system calls),
          <c><![CDATA[file]]></c> (use host data retrieved from
          system configuration files and/or 
          the user configuration file) or <c><![CDATA[dns]]></c>
          (use the Erlang DNS client
          <seealso marker="kernel:inet_res">inet_res</seealso>
          for nameserver queries).</p>
	<p>The lookup method <c><![CDATA[string]]></c> tries to
	  parse the hostname as a IPv4 or IPv6 string and return
	  the resulting IP address. It is automatically tried
	  first when <c><![CDATA[native]]></c> is <em>not</em>
	  in the <c><![CDATA[Methods]]></c> list. To skip it in this case
	  the pseudo lookup method <c><![CDATA[nostring]]></c> can be
	  inserted anywhere in the <c><![CDATA[Methods]]></c> list.</p>
        <p></p>
      </item>
      <tag><em><c><![CDATA[{cache_size, Size}.]]></c></em></tag>
      <item>
        <p></p>
        <p><c><![CDATA[Size = integer()]]></c></p>
        <p></p>
        <p>Set size of resolver cache. Default is 100 DNS records.</p>
        <p></p>
      </item>
      <tag><em><c><![CDATA[{cache_refresh, Time}.]]></c></em></tag>
      <item>
        <p></p>
        <p><c><![CDATA[Time = integer()]]></c></p>
        <p></p>
        <p>Set how often (in millisec)
          the resolver cache for
          <seealso marker="kernel:inet_res">inet_res</seealso>.
          is refreshed (i.e. expired DNS records are deleted).
          Default is 1 h.</p>
        <p></p>
      </item>
      <tag><em><c><![CDATA[{timeout, Time}.]]></c></em></tag>
      <item>
        <p></p>
        <p><c><![CDATA[Time = integer()]]></c></p>
        <p></p>
        <p>Set the time to wait until retry (in millisec) for DNS queries
          made by
          <seealso marker="kernel:inet_res">inet_res</seealso>.
          Default is 2 sec.</p>
        <p></p>
      </item>
      <tag><em><c><![CDATA[{retry, N}.]]></c></em></tag>
      <item>
        <p></p>
        <p><c><![CDATA[N = integer()]]></c></p>
        <p></p>
        <p>Set the number of DNS queries
          <seealso marker="kernel:inet_res">inet_res</seealso>
          will try before giving up.
          Default is 3.</p>
        <p></p>
      </item>
      <tag><em><c><![CDATA[{inet6, Bool}.]]></c></em></tag>
      <item>
        <p></p>
        <p><c><![CDATA[Bool = true | false]]></c></p>
        <p></p>
        <p>Tells the DNS client 
          <seealso marker="kernel:inet_res">inet_res</seealso>
          to look up IPv6 addresses. Default is false.</p>
        <p></p>
      </item>
      <tag><em><c><![CDATA[{usevc, Bool}.]]></c></em></tag>
      <item>
        <p></p>
        <p><c><![CDATA[Bool = true | false]]></c></p>
        <p></p>
        <p>Tells the DNS client 
          <seealso marker="kernel:inet_res">inet_res</seealso>
          to use TCP (Virtual Circuit) instead of UDP. Default is false.</p>
        <p></p>
      </item>
      <tag><em><c><![CDATA[{edns, Version}.]]></c></em></tag>
      <item>
        <p></p>
        <p><c><![CDATA[Version = false | 0]]></c></p>
        <p></p>
        <p>Sets the EDNS version that
          <seealso marker="kernel:inet_res">inet_res</seealso>
          will use. The only allowed is zero. Default is false
          which means to not use EDNS.</p>
        <p></p>
      </item>
      <tag><em><c><![CDATA[{udp_payload_size, Size}.]]></c></em></tag>
      <item>
        <p></p>
        <p><c><![CDATA[N = integer()]]></c></p>
        <p></p>
        <p>Sets the allowed UDP payload size
          <seealso marker="kernel:inet_res">inet_res</seealso>
          will advertise in EDNS queries. Also sets the limit
          when the DNS query will be deemed too large for UDP
          forcing a TCP query instead, which is not entirely
          correct since the advertised UDP payload size of the
          individual nameserver is what should be used,
          but this simple strategy will do until a more intelligent
          (probing, caching) algorithm need be implemented.
          The default is 1280 which stems from the
          standard Ethernet MTU size.</p>
        <p></p>
      </item>
      <tag><em><c><![CDATA[{udp, Module}.]]></c></em></tag>
      <item>
        <p></p>
        <p><c><![CDATA[Module = atom()]]></c></p>
        <p></p>
        <p>Tell Erlang to use other primitive UDP module than inet_udp.</p>
        <p></p>
      </item>
      <tag><em><c><![CDATA[{tcp, Module}.]]></c></em></tag>
      <item>
        <p></p>
        <p><c><![CDATA[Module = atom()]]></c></p>
        <p></p>
        <p>Tell Erlang to use other primitive TCP module than inet_tcp.</p>
        <p></p>
      </item>
      <tag><em><c><![CDATA[clear_hosts.]]></c></em></tag>
      <item>
        <p></p>
        <p>Clear the hosts table.</p>
        <p></p>
      </item>
      <tag><em><c><![CDATA[clear_ns.]]></c></em></tag>
      <item>
        <p></p>
        <p>Clear the list of recorded nameservers (primary and secondary).</p>
        <p></p>
      </item>
      <tag><em><c><![CDATA[clear_search.]]></c></em></tag>
      <item>
        <p></p>
        <p>Clear the list of search domains.</p>
        <p></p>
      </item>
    </taglist>
  </section>

  <section>
    <title>User Configuration Example</title>
    <p>Here follows a user configuration example.</p>
    <p>Assume a user does not want Erlang to use the native lookup method, 
      but wants Erlang to read all information necessary from start and use 
      that for resolving names and addresses. In case lookup fails, Erlang 
      should request the data from a nameserver (using the Erlang 
      DNS client, set to use EDNS allowing larger responses).
      The resolver configuration will be updated when
      its configuration file changes, furthermore, DNS records
      should never be cached. The user configuration file
      (in this example named <c><![CDATA[erl_inetrc]]></c>, stored
      in directory <c><![CDATA[./cfg_files]]></c>) could then look like this 
      (Unix):</p>
    <pre>
      %% -- ERLANG INET CONFIGURATION FILE --
      %% read the hosts file
      {file, hosts, "/etc/hosts"}.
      %% add a particular host
      {host, {134,138,177,105}, ["finwe"]}.
      %% do not monitor the hosts file
      {hosts_file, ""}.
      %% read and monitor nameserver config from here
      {resolv_conf, "/usr/local/etc/resolv.conf"}.
      %% enable EDNS
      {edns,0}.
      %% disable caching
      {cache_size, 0}.
      %% specify lookup method
      {lookup, [file, dns]}.</pre>
    <p>And Erlang could, for example, be started like this:</p>
    <p><c><![CDATA[% erl -sname my_node -kernel inetrc '"./cfg_files/erl_inetrc"']]></c></p>
  </section>
</chapter>

