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<chapter>
  <header>
    <copyright>
      <year>2004</year><year>2022</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 section describes how the Erlang runtime system is configured
      for IP communication. It also explains how you can configure it
      for your needs by a configuration file.
      The information is primarily intended for users with special
      configuration needs or problems. There is normally no need
      for specific settings for Erlang to function properly on a correctly
      IP-configured platform.</p>

    <p>When Erlang starts up it reads the Kernel variable
      <c><![CDATA[inetrc]]></c>, which, if defined, is to specify the location
      and name of a user configuration file. Example:</p>

    <code type="none"><![CDATA[
% erl -kernel inetrc '"./cfg_files/erl_inetrc"']]></code>

    <p>Notice that the use of an <c><![CDATA[.inetrc]]></c> file, which was
      supported in earlier Erlang/OTP versions, is now obsolete.</p>

    <p>A second way to specify the configuration file is to set
      environment variable <c><![CDATA[ERL_INETRC]]></c> to the full name of
      the file. Example (bash):</p>

    <code type="none"><![CDATA[
% export ERL_INETRC=./cfg_files/erl_inetrc]]></code>

    <p>Notice 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 uses
      default configuration settings and a native lookup method that
      works correctly under most circumstances. Erlang reads no
      information from system <c>inet</c> configuration files (such as 
      <c>/etc/host.conf</c> and <c>/etc/nsswitch.conf</c>) in these modes,
      except for <c>/etc/resolv.conf</c> and <c>/etc/hosts</c> that is read and
      monitored for changes on Unix platforms for the internal DNS client
      <seeerl marker="kernel:inet_res"><c>inet_res(3)</c></seeerl>.</p>

    <p>If Erlang is started in long name distributed mode, it needs to
      get the domain name from somewhere and reads system <c>inet</c>
      configuration files for this information. Any hosts and resolver
      information found 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 OSE Delta where
      <c><![CDATA['file']]></c> or <c><![CDATA['dns']]></c> is used (in that
      priority order).</p>

    <p>On Windows platforms, Erlang searches the system registry rather than
      looks 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 
      <c>inet</c> configuration files (or system registry):</p>

   <list type="bulleted">
      <item>Hostnames and host addresses</item>
      <item>Domain name</item>
      <item>Nameservers</item>
      <item>Search domains</item>
      <item>Lookup method</item>
    </list>

    <p>This data can also be specified explicitly in the user
      configuration file. This file is to contain lines
      of configuration parameters (each terminated with a full stop).
      Some parameters add data to the configuration (such as host
      and nameserver), others overwrite any previous settings
      (such as 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
      <c>inet</c> configuration database.</p>

    <p>The valid configuration parameters are as follows:</p>

    <taglist>
      <tag><c><![CDATA[{file, Format, File}.]]></c></tag>
      <item>
        <p><c><![CDATA[Format = atom()]]></c></p>
        <p><c><![CDATA[File = string()]]></c></p>
        <p>Specify a system file that Erlang is to read configuration data from.
          <c><![CDATA[Format]]></c> tells the parser how the file is to be
          interpreted:</p>
        <list type="bulleted">
          <item><c><![CDATA[resolv]]></c> (Unix resolv.conf)</item>
          <item><c><![CDATA[host_conf_freebsd]]></c> (FreeBSD host.conf)</item>
          <item><c><![CDATA[host_conf_bsdos]]></c> (BSDOS host.conf)</item>
          <item><c><![CDATA[host_conf_linux]]></c> (Linux host.conf)</item>
          <item><c><![CDATA[nsswitch_conf]]></c> (Unix nsswitch.conf)</item>
          <item><c><![CDATA[hosts]]></c> (Unix hosts)</item>
        </list>
        <p><c><![CDATA[File]]></c> is to specify the filename with full
          path.</p>
      </item>
      <tag><c><![CDATA[{resolv_conf, File}.]]></c></tag>
      <item>
        <p><c><![CDATA[File = string()]]></c></p>
        <p>Specify a system file that Erlang is to read resolver
          configuration from for the internal DNS client
          <seeerl marker="kernel:inet_res"><c>inet_res(3)</c></seeerl>,
          and monitor for changes, even if it does not exist.
          The path must be absolute.</p>
        <p>This can override the configuration parameters
          <c><![CDATA[nameserver]]></c> and
          <c><![CDATA[search]]></c> depending on the contents
          of the specified file. They can also change any time in the future
          reflecting the file contents.</p>
        <p>If the file is specified as an empty string <c>""</c>,
          no file is read or monitored in the future. This emulates
          the old behavior 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 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>
      </item>
      <tag><c><![CDATA[{hosts_file, File}.]]></c></tag>
      <item>
        <p><c><![CDATA[File = string()]]></c></p>
        <p>Specify a system file that Erlang is to 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 lookup option
          <c>file</c> is used.</p>
        <p>If the file is specified as an empty string <c>""</c>,
          no file is read or monitored in the future. This emulates
          the old behavior 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 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>
      </item>
      <tag><c><![CDATA[{registry, Type}.]]></c></tag>
      <item>
        <p><c><![CDATA[Type = atom()]]></c></p>
        <p>Specify a system registry that Erlang is to read configuration
          data from. <c><![CDATA[win32]]></c> is the only valid option.</p>
      </item>
      <tag><c><![CDATA[{host, IP, Aliases}.]]></c></tag>
      <item>
        <p><c><![CDATA[IP = tuple()]]></c></p>
        <p><c><![CDATA[Aliases = [string()]]]></c></p>
        <p>Add host entry to the hosts table.</p>
      </item>
      <tag><c><![CDATA[{domain, Domain}.]]></c></tag>
      <item>
        <p><c><![CDATA[Domain = string()]]></c></p>
        <p>Set domain name.</p>
      </item>
      <tag><c><![CDATA[{nameserver, IP [,Port]}.]]></c></tag>
      <item>
        <p><c><![CDATA[IP = tuple()]]></c></p>
        <p><c><![CDATA[Port = integer()]]></c></p>
        <p>Add address (and port, if other than default) of the primary
          nameserver to use for
          <seeerl marker="kernel:inet_res"><c>inet_res(3)</c></seeerl>.
        </p>
      </item>
      <tag><c><![CDATA[{alt_nameserver, IP [,Port]}.]]></c></tag>
      <item>
        <p><c><![CDATA[IP = tuple()]]></c></p>
        <p><c><![CDATA[Port = integer()]]></c></p>
        <p>Add address (and port, if other than default) of the secondary
          nameserver for
          <seeerl marker="kernel:inet_res"><c>inet_res(3)</c></seeerl>.
        </p>
      </item>
      <tag><c><![CDATA[{search, Domains}.]]></c></tag>
      <item>
        <p><c><![CDATA[Domains = [string()]]]></c></p>
        <p>Add search domains for
          <seeerl marker="kernel:inet_res"><c>inet_res(3)</c></seeerl>.
        </p>
      </item>
      <tag><c><![CDATA[{lookup, Methods}.]]></c></tag>
      <item>
        <p><c><![CDATA[Methods = [atom()]]]></c></p>
        <p>Specify lookup methods and in which order to try them.
          The valid methods are as follows:</p>
        <list type="bulleted">
          <item><c><![CDATA[native]]></c> (use system calls)</item>
          <item><c><![CDATA[file]]></c> (use host data retrieved from system
            configuration files and/or the user configuration file)</item>
          <item><c><![CDATA[dns]]></c> (use the Erlang DNS client
            <seeerl marker="kernel:inet_res"><c>inet_res(3)</c></seeerl>
            for nameserver queries)</item>
        </list>
	<p>The lookup method <c><![CDATA[string]]></c> tries to
	  parse the hostname as an 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>
      </item>
      <tag><c><![CDATA[{cache_size, Size}.]]></c></tag>
      <item>
        <p><c><![CDATA[Size = integer()]]></c></p>
	<p>Set the resolver cache size for <c>dns</c> lookups.
	  <c>native</c> lookups are not cached. Defaults to 100 DNS records.
	</p>
      </item>
      <tag><c><![CDATA[{cache_refresh, Time}.]]></c></tag>
        <item>
        <p><c><![CDATA[Time = integer()]]></c></p>
        <p>Set how often (in milliseconds) the resolver cache for
          <seeerl marker="kernel:inet_res"><c>inet_res(3)</c></seeerl>
          is refreshed (that is, expired DNS records are deleted).
          Defaults to 1 hour.</p>
      </item>
      <tag><c><![CDATA[{timeout, Time}.]]></c></tag>
      <item>
        <p><c><![CDATA[Time = integer()]]></c></p>
        <p>Set the time to wait until retry (in milliseconds) for DNS queries
          made by
          <seeerl marker="kernel:inet_res"><c>inet_res(3)</c></seeerl>.
          Defaults to 2 seconds.</p>
      </item>
      <tag><c><![CDATA[{retry, N}.]]></c></tag>
      <item>
        <p><c><![CDATA[N = integer()]]></c></p>
        <p>Set the number of DNS queries
          <seeerl marker="kernel:inet_res"><c>inet_res(3)</c></seeerl>
          will try before giving up. Defaults to 3.</p>
      </item>

      <tag><c><![CDATA[{servfail_retry_timeout, Time}.]]></c></tag>
      <item>
        <p><c><![CDATA[Time = non_neg_integer()]]></c></p>
	<p>After all name servers have been tried, there is a timeout
	before the name servers are tried again. This is to prevent
	the server from answering the query with what's in the servfail cache,
	<seeerl marker="kernel:inet_res#servfail_retry_timeout"><c>inet_res(3)</c></seeerl>.
	Defaults to 1500 milli seconds .</p>
      </item>

      <tag><c><![CDATA[{inet6, Bool}.]]></c></tag>
      <item>
        <p><c><![CDATA[Bool = true | false]]></c></p>
        <p>Tells the DNS client
          <seeerl marker="kernel:inet_res"><c>inet_res(3)</c></seeerl>
          to look up IPv6 addresses. Defaults to <c>false</c>.</p>
      </item>
      <tag><c><![CDATA[{usevc, Bool}.]]></c></tag>
      <item>
        <p><c><![CDATA[Bool = true | false]]></c></p>
        <p>Tells the DNS client
          <seeerl marker="kernel:inet_res"><c>inet_res(3)</c></seeerl>
          to use TCP (Virtual Circuit) instead of UDP. Defaults to
          <c>false</c>.</p>
      </item>
      <tag><c><![CDATA[{edns, Version}.]]></c></tag>
      <item>
        <p><c><![CDATA[Version = false | 0]]></c></p>
        <p>Sets the EDNS version that
          <seeerl marker="kernel:inet_res"><c>inet_res(3)</c></seeerl>
          will use. The only allowed version is zero. Defaults to <c>false</c>,
          which means not to use EDNS.</p>
      </item>
      <tag><c><![CDATA[{udp_payload_size, Size}.]]></c></tag>
      <item>
        <p><c><![CDATA[N = integer()]]></c></p>
        <p>Sets the allowed UDP payload size
          <seeerl marker="kernel:inet_res"><c>inet_res(3)</c></seeerl>
          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; this  is not entirely
          correct, as the advertised UDP payload size of the
          individual nameserver is what is to be used,
          but this simple strategy will do until a more intelligent
          (probing, caching) algorithm needs to be implemented.
          Default to 1280, which stems from the standard Ethernet MTU size.</p>
      </item>
      <tag><c><![CDATA[{udp, Module}.]]></c></tag>
      <item>
        <p><c><![CDATA[Module = atom()]]></c></p>
        <p>Tell Erlang to use another primitive UDP module than
          <c>inet_udp</c>.</p>
      </item>
      <tag><c><![CDATA[{tcp, Module}.]]></c></tag>
      <item>
        <p><c><![CDATA[Module = atom()]]></c></p>
        <p>Tell Erlang to use another primitive TCP module than
          <c>inet_tcp</c>.</p>
      </item>
      <tag><c><![CDATA[clear_hosts.]]></c></tag>
      <item>
        <p>Clear the hosts table.</p>
      </item>
      <tag><c><![CDATA[clear_ns.]]></c></tag>
      <item>
        <p>Clear the list of recorded nameservers (primary and secondary).</p>
      </item>
      <tag><c><![CDATA[clear_search.]]></c></tag>
      <item>
        <p>Clear the list of search domains.</p>
      </item>
    </taglist>
  </section>

  <section>
    <title>User Configuration Example</title>
    <p>Assume that 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. If lookup fails, Erlang
      is to request the data from a nameserver (using the Erlang
      DNS client, set to use EDNS allowing larger responses).
      The resolver configuration is updated when
      its configuration file changes. Also, DNS records
      are never to be cached. The user configuration file
      (in this example named <c><![CDATA[erl_inetrc]]></c>, stored
      in directory <c><![CDATA[./cfg_files]]></c>) can then look as follows
      (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 can, for example, be started as follows:</p>

    <code type="none"><![CDATA[
% erl -sname my_node -kernel inetrc '"./cfg_files/erl_inetrc"']]></code>
  </section>
</chapter>

