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<div class="innertube">
<h1>7 Inet configuration</h1>
<h3><a name="id83274">7.1
Introduction</a></h3>
<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
<span class="code">inetrc</span> which, if defined, should specify the location and
name of a user configuration file. Example:</p>
<p><span class="code">% erl -kernel inetrc '"./cfg_files/erl_inetrc"'</span></p>
<p>Note that the usage of a <span class="code">.inetrc</span> 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 <span class="code">ERL_INETRC</span> to the full name of the file. Example (bash):</p>
<p><span class="code">% export ERL_INETRC=./cfg_files/erl_inetrc</span></p>
<p>Note that the kernel variable <span class="code">inetrc</span> 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
<span class="bold_code"><a href="javascript:erlhref('../../../doc/../','kernel','inet_res.html');">inet_res</a></span>.</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
<span class="code">'file'</span> or <span class="code">'dns'</span>, 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 <span class="code">'file'</span>
or <span class="code">'dns'</span> 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>
<h3><a name="id83378">7.2
Configuration Data</a></h3>
<p>Erlang records the following data in a local database if found in system
inet configuration files (or system registry):</p>
<ul>
<li>Host names and addresses</li>
<li>Domain name</li>
<li>Nameservers</li>
<li>Search domains</li>
<li>Lookup method</li>
</ul>
<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 <span class="code">inet:get_rc()</span> to view the state of the
inet configuration database.</p>
<p>These are the valid configuration parameters:</p>
<p></p>
<dl>
<dt><strong><strong><span class="code">{file, Format, File}.</span></strong></strong></dt>
<dd>
<p></p>
<p><span class="code">Format = atom()</span></p>
<p><span class="code">File = string()</span></p>
<p></p>
<p>Specify a system file that Erlang should read configuration
data from. <span class="code">Format</span> tells the parser how the file should be
interpreted: <span class="code">resolv</span> (Unix resolv.conf), <span class="code">host_conf_freebsd</span>
(FreeBSD host.conf), <span class="code">host_conf_bsdos</span> (BSDOS host.conf),
<span class="code">host_conf_linux</span> (Linux host.conf), <span class="code">nsswitch_conf</span>
(Unix nsswitch.conf) or <span class="code">hosts</span> (Unix hosts). <span class="code">File</span> should
specify the name of the file with full path.</p>
<p></p>
</dd>
<dt><strong><strong><span class="code">{resolv_conf, File}.</span></strong></strong></dt>
<dd>
<p></p>
<p><span class="code">File = string()</span></p>
<p></p>
<p>Specify a system file that Erlang should read resolver
configuration from for the internal DNS client
<span class="bold_code"><a href="javascript:erlhref('../../../doc/../','kernel','inet_res.html');">inet_res</a></span>,
and monitor for changes, even if it does not exist.
The path must be absolute.</p>
<p>This may override the configuration parameters
<span class="code">nameserver</span> and
<span class="code">search</span> 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
<span class="code">/etc/resolv.conf</span> unless the environment variable
<span class="code">ERL_INET_ETC_DIR</span> is set which defines
the directory for this file to some maybe other than
<span class="code">/etc</span>.</p>
<p></p>
</dd>
<dt><strong><strong><span class="code">{hosts_file, File}.</span></strong></strong></dt>
<dd>
<p></p>
<p><span class="code">File = string()</span></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
<span class="code">{file, hosts, File}</span> above or
<span class="code">{host, IP, Aliases}</span> below when the lookup option
<span class="code">file</span> 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
<span class="code">/etc/hosts</span> unless the environment variable
<span class="code">ERL_INET_ETC_DIR</span> is set which defines
the directory for this file to some maybe other than
<span class="code">/etc</span>.</p>
<p></p>
</dd>
<dt><strong><strong><span class="code">{registry, Type}.</span></strong></strong></dt>
<dd>
<p></p>
<p><span class="code">Type = atom()</span></p>
<p></p>
<p>Specify a system registry that Erlang should read configuration
data from. Currently, <span class="code">win32</span> is the only valid option.</p>
<p></p>
</dd>
<dt><strong><strong><span class="code">{host, IP, Aliases}.</span></strong></strong></dt>
<dd>
<p></p>
<p><span class="code">IP = tuple()</span></p>
<p><span class="code">Aliases = [string()]</span></p>
<p></p>
<p>Add host entry to the hosts table.</p>
<p></p>
</dd>
<dt><strong><strong><span class="code">{domain, Domain}.</span></strong></strong></dt>
<dd>
<p></p>
<p><span class="code">Domain = string()</span></p>
<p></p>
<p>Set domain name.</p>
<p></p>
</dd>
<dt><strong><strong><span class="code">{nameserver, IP [,Port]}.</span></strong></strong></dt>
<dd>
<p></p>
<p><span class="code">IP = tuple()</span></p>
<p><span class="code">Port = integer()</span></p>
<p></p>
<p>Add address (and port, if other than default) of primary
nameserver to use for
<span class="bold_code"><a href="javascript:erlhref('../../../doc/../','kernel','inet_res.html');">inet_res</a></span>.</p>
<p></p>
</dd>
<dt><strong><strong><span class="code">{alt_nameserver, IP [,Port]}.</span></strong></strong></dt>
<dd>
<p></p>
<p><span class="code">IP = tuple()</span></p>
<p><span class="code">Port = integer()</span></p>
<p></p>
<p>Add address (and port, if other than default) of secondary
nameserver for
<span class="bold_code"><a href="javascript:erlhref('../../../doc/../','kernel','inet_res.html');">inet_res</a></span>.</p>
<p></p>
</dd>
<dt><strong><strong><span class="code">{search, Domains}.</span></strong></strong></dt>
<dd>
<p></p>
<p><span class="code">Domains = [string()]</span></p>
<p></p>
<p>Add search domains for
<span class="bold_code"><a href="javascript:erlhref('../../../doc/../','kernel','inet_res.html');">inet_res</a></span>.</p>
<p></p>
</dd>
<dt><strong><strong><span class="code">{lookup, Methods}.</span></strong></strong></dt>
<dd>
<p></p>
<p><span class="code">Methods = [atom()]</span></p>
<p></p>
<p>Specify lookup methods and in which order to try them.
The valid methods are: <span class="code">native</span> (use system calls),
<span class="code">file</span> (use host data retrieved from
system configuration files and/or
the user configuration file) or <span class="code">dns</span>
(use the Erlang DNS client
<span class="bold_code"><a href="javascript:erlhref('../../../doc/../','kernel','inet_res.html');">inet_res</a></span>
for nameserver queries).</p>
<p>The lookup method <span class="code">string</span> tries to
parse the hostname as a IPv4 or IPv6 string and return
the resulting IP address. It is automatically tried
first when <span class="code">native</span> is <strong>not</strong>
in the <span class="code">Methods</span> list. To skip it in this case
the pseudo lookup method <span class="code">nostring</span> can be
inserted anywhere in the <span class="code">Methods</span> list.</p>
<p></p>
</dd>
<dt><strong><strong><span class="code">{cache_size, Size}.</span></strong></strong></dt>
<dd>
<p></p>
<p><span class="code">Size = integer()</span></p>
<p></p>
<p>Set size of resolver cache. Default is 100 DNS records.</p>
<p></p>
</dd>
<dt><strong><strong><span class="code">{cache_refresh, Time}.</span></strong></strong></dt>
<dd>
<p></p>
<p><span class="code">Time = integer()</span></p>
<p></p>
<p>Set how often (in millisec)
the resolver cache for
<span class="bold_code"><a href="javascript:erlhref('../../../doc/../','kernel','inet_res.html');">inet_res</a></span>.
is refreshed (i.e. expired DNS records are deleted).
Default is 1 h.</p>
<p></p>
</dd>
<dt><strong><strong><span class="code">{timeout, Time}.</span></strong></strong></dt>
<dd>
<p></p>
<p><span class="code">Time = integer()</span></p>
<p></p>
<p>Set the time to wait until retry (in millisec) for DNS queries
made by
<span class="bold_code"><a href="javascript:erlhref('../../../doc/../','kernel','inet_res.html');">inet_res</a></span>.
Default is 2 sec.</p>
<p></p>
</dd>
<dt><strong><strong><span class="code">{retry, N}.</span></strong></strong></dt>
<dd>
<p></p>
<p><span class="code">N = integer()</span></p>
<p></p>
<p>Set the number of DNS queries
<span class="bold_code"><a href="javascript:erlhref('../../../doc/../','kernel','inet_res.html');">inet_res</a></span>
will try before giving up.
Default is 3.</p>
<p></p>
</dd>
<dt><strong><strong><span class="code">{inet6, Bool}.</span></strong></strong></dt>
<dd>
<p></p>
<p><span class="code">Bool = true | false</span></p>
<p></p>
<p>Tells the DNS client
<span class="bold_code"><a href="javascript:erlhref('../../../doc/../','kernel','inet_res.html');">inet_res</a></span>
to look up IPv6 addresses. Default is false.</p>
<p></p>
</dd>
<dt><strong><strong><span class="code">{usevc, Bool}.</span></strong></strong></dt>
<dd>
<p></p>
<p><span class="code">Bool = true | false</span></p>
<p></p>
<p>Tells the DNS client
<span class="bold_code"><a href="javascript:erlhref('../../../doc/../','kernel','inet_res.html');">inet_res</a></span>
to use TCP (Virtual Circuit) instead of UDP. Default is false.</p>
<p></p>
</dd>
<dt><strong><strong><span class="code">{edns, Version}.</span></strong></strong></dt>
<dd>
<p></p>
<p><span class="code">Version = false | 0</span></p>
<p></p>
<p>Sets the EDNS version that
<span class="bold_code"><a href="javascript:erlhref('../../../doc/../','kernel','inet_res.html');">inet_res</a></span>
will use. The only allowed is zero. Default is false
which means to not use EDNS.</p>
<p></p>
</dd>
<dt><strong><strong><span class="code">{udp_payload_size, Size}.</span></strong></strong></dt>
<dd>
<p></p>
<p><span class="code">N = integer()</span></p>
<p></p>
<p>Sets the allowed UDP payload size
<span class="bold_code"><a href="javascript:erlhref('../../../doc/../','kernel','inet_res.html');">inet_res</a></span>
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>
</dd>
<dt><strong><strong><span class="code">{udp, Module}.</span></strong></strong></dt>
<dd>
<p></p>
<p><span class="code">Module = atom()</span></p>
<p></p>
<p>Tell Erlang to use other primitive UDP module than inet_udp.</p>
<p></p>
</dd>
<dt><strong><strong><span class="code">{tcp, Module}.</span></strong></strong></dt>
<dd>
<p></p>
<p><span class="code">Module = atom()</span></p>
<p></p>
<p>Tell Erlang to use other primitive TCP module than inet_tcp.</p>
<p></p>
</dd>
<dt><strong><strong><span class="code">clear_hosts.</span></strong></strong></dt>
<dd>
<p></p>
<p>Clear the hosts table.</p>
<p></p>
</dd>
<dt><strong><strong><span class="code">clear_ns.</span></strong></strong></dt>
<dd>
<p></p>
<p>Clear the list of recorded nameservers (primary and secondary).</p>
<p></p>
</dd>
<dt><strong><strong><span class="code">clear_search.</span></strong></strong></dt>
<dd>
<p></p>
<p>Clear the list of search domains.</p>
<p></p>
</dd>
</dl>
<h3><a name="id84255">7.3
User Configuration Example</a></h3>
<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 <span class="code">erl_inetrc</span>, stored
in directory <span class="code">./cfg_files</span>) could then look like this
(Unix):</p>
<div class="example"><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></div>
<p>And Erlang could, for example, be started like this:</p>
<p><span class="code">% erl -sname my_node -kernel inetrc '"./cfg_files/erl_inetrc"'</span></p>
</div>
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