<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>1997</year><year>2018</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</title>
    <prepared>bjorn@erix.ericsson.se</prepared>
    <docno></docno>
    <date>1998-02-04</date>
    <rev>A</rev>
  </header>
  <module>inet</module>
  <modulesummary>Access to TCP/IP protocols.</modulesummary>
  <description>
    <p>This module provides access to TCP/IP protocols.</p>
    <p>See also
      <seealso marker="erts:inet_cfg">ERTS User's Guide:
      Inet Configuration</seealso> for more information about how to
      configure an Erlang runtime system for IP communication.</p>
    <p>The following two Kernel configuration parameters affect the
      behavior of all sockets opened on an Erlang node:</p>
    <list type="bulleted">
      <item><p><c>inet_default_connect_options</c> can contain a list of
        default options used for all sockets returned when doing
        <c>connect</c>.</p></item>
      <item><p><c>inet_default_listen_options</c> can contain a list of
        default options used when issuing a <c>listen</c> call.</p></item>
    </list>
    <p>When <c>accept</c> is issued, the values of the listening socket options
      are inherited. No such application variable is therefore needed for
      <c>accept</c>.</p>
    <p>Using the Kernel configuration parameters above, one
      can set default options for all TCP sockets on a node, but use this
      with care. Options such as <c>{delay_send,true}</c> can be
      specified in this way. The following is an example of starting an Erlang
      node with all sockets using delayed send:</p>
    <pre>
$ <input>erl -sname test -kernel \</input>
<input>inet_default_connect_options '[{delay_send,true}]' \</input>
<input>inet_default_listen_options '[{delay_send,true}]'</input></pre>
    <p>Notice that default option <c>{active, true}</c>
      cannot be changed, for internal reasons.</p>
    <p>Addresses as inputs to functions can be either a string or a
      tuple. For example, the IP address 150.236.20.73 can be passed to
      <c>gethostbyaddr/1</c>, either as string <c>"150.236.20.73"</c>
      or as tuple <c>{150, 236, 20, 73}</c>.</p>
    <p><em>IPv4 address examples:</em></p>
    <code type="none">
Address          ip_address()
-------          ------------
127.0.0.1        {127,0,0,1}
192.168.42.2     {192,168,42,2}</code>
    <p><em>IPv6 address examples:</em></p>
    <code type="none">
Address          ip_address()
-------          ------------
::1             {0,0,0,0,0,0,0,1}
::192.168.42.2  {0,0,0,0,0,0,(192 bsl 8) bor 168,(42 bsl 8) bor 2}
::FFFF:192.168.42.2
                {0,0,0,0,0,16#FFFF,(192 bsl 8) bor 168,(42 bsl 8) bor 2}
3ffe:b80:1f8d:2:204:acff:fe17:bf38
                {16#3ffe,16#b80,16#1f8d,16#2,16#204,16#acff,16#fe17,16#bf38}
fe80::204:acff:fe17:bf38
                {16#fe80,0,0,0,0,16#204,16#acff,16#fe17,16#bf38}</code>
    <p>Function
      <seealso marker="#parse_address/1"><c>parse_address/1</c></seealso>
        can be useful:</p>
    <pre>
1> <input>inet:parse_address("192.168.42.2").</input>
{ok,{192,168,42,2}}
2> <input>inet:parse_address("::FFFF:192.168.42.2").</input>
{ok,{0,0,0,0,0,65535,49320,10754}}</pre>
  </description>

  <datatypes>
    <datatype>
      <name name="hostent"/>
      <desc>
        <p>The record is defined in the Kernel include file
          <c>"inet.hrl"</c>.</p>
        <p>Add the following directive to the module:</p>
        <code>
-include_lib("kernel/include/inet.hrl").</code>
      </desc>
    </datatype>
    <datatype>
      <name name="hostname"/>
    </datatype>
    <datatype>
      <name name="ip_address"/>
    </datatype>
    <datatype>
      <name name="ip4_address"/>
    </datatype>
    <datatype>
      <name name="ip6_address"/>
    </datatype>
    <datatype>
      <name name="port_number"/>
    </datatype>
    <datatype>
      <name name="local_address"/>
      <desc>
	<p>
	  This address family only works on Unix-like systems.
	</p>
	<p>
	  <c><anno>File</anno></c> is normally a file pathname
	  in a local filesystem.  It is limited in length by the
	  operating system, traditionally to 108 bytes.
	</p>
	<p>
	  A <c>binary()</c> is passed as is to the operating system,
	  but a <c>string()</c> is encoded according to the
	  <seealso marker="file#native_name_encoding/0">
	    system filename encoding mode.
	  </seealso>
	</p>
	<p>
	  Other addresses are possible, for example Linux implements
	  "Abstract Addresses".  See the documentation for
	  Unix Domain Sockets on your system,
	  normally <c>unix</c> in manual section 7.
	</p>
	<p>
	  In most API functions where you can use
	  this address family the port number must be <c>0</c>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="socket_address"/>
    </datatype>
    <datatype>
      <name name="socket_getopt"/>
    </datatype>
    <datatype>
      <name name="socket_setopt"/>
    </datatype>
    <datatype>
      <name name="returned_non_ip_address"/>
      <desc>
	<p>
	  Addresses besides
	  <seealso marker="#type-ip_address">
	    <c>ip_address()</c>
	  </seealso>
	  ones that are returned from socket API functions.
	  See in particular
	  <seealso marker="#type-local_address">
	    <c>local_address()</c>.
	  </seealso>
	  The <c>unspec</c> family corresponds to AF_UNSPEC and can
	  occur if the other side has no socket address.
	  The <c>undefined</c> family can only occur in the unlikely
	  event of an address family that the VM does not recognize.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="posix"/>
      <desc>
        <p>An atom that is named from the POSIX error codes used in Unix,
          and in the runtime libraries of most C compilers. See section
          <seealso marker="#error_codes">POSIX Error Codes</seealso>.</p>
      </desc>
    </datatype>
    <datatype>
      <name>socket()</name>
      <desc>
        <p>See
          <seealso marker="gen_tcp#type-socket"><c>gen_tcp:type-socket</c></seealso>
          and
          <seealso marker="gen_udp#type-socket"><c>gen_udp:type-socket</c></seealso>.
        </p>
      </desc>
    </datatype>
    <datatype>
      <name name="address_family"/>
    </datatype>
    <datatype>
      <name name="socket_protocol"/>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="close" arity="1"/>
      <fsummary>Close a socket of any type.</fsummary>
      <desc>
        <p>Closes a socket of any type.</p>
      </desc>
    </func>

    <func>
      <name name="format_error" arity="1"/>
      <fsummary>Return a descriptive string for an error reason.</fsummary>
      <desc>
        <p>Returns a diagnostic error string. For possible POSIX values and
          corresponding strings, see section
          <seealso marker="#error_codes">POSIX Error Codes</seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="get_rc" arity="0"/>
      <fsummary>Return a list of IP configuration parameters.</fsummary>
      <desc>
        <p>
	  Returns the state of the <c>Inet</c> configuration database in
          form of a list of recorded configuration parameters. For more
          information, see <seealso marker="erts:inet_cfg">ERTS User's Guide:
          Inet Configuration</seealso>.
	</p>
        <p>
	  Only actual parameters with other than default values
	  are returned, for example not directives that specify
	  other sources for configuration parameters nor
	  directives that clear parameters.
	</p>
      </desc>
    </func>

    <func>
      <name name="getaddr" arity="2"/>
      <fsummary>Return the IP address for a host.</fsummary>
      <desc>
        <p>Returns the IP address for <c><anno>Host</anno></c> as a tuple of
          integers. <c><anno>Host</anno></c> can be an IP address, a single
          hostname, or a fully qualified hostname.</p>
      </desc>
    </func>

    <func>
      <name name="getaddrs" arity="2"/>
      <fsummary>Return the IP addresses for a host.</fsummary>
      <desc>
        <p>Returns a list of all IP addresses for <c><anno>Host</anno></c>.
          <c><anno>Host</anno></c> can be an IP address, a single hostname, or
          a fully qualified hostname.</p>
      </desc>
    </func>

    <func>
      <name name="gethostbyaddr" arity="1"/>
      <fsummary>Return a hostent record for the host with the specified
        address.</fsummary>
      <desc>
        <p>Returns a <c>hostent</c> record for the host with the specified
          address.</p></desc>
    </func>

    <func>
      <name name="gethostbyname" arity="1"/>
      <fsummary>Return a hostent record for the host with the specified name.
      </fsummary>
      <desc>
        <p>Returns a <c>hostent</c> record for the host with the specified
          hostname.</p>
        <p>If resolver option <c>inet6</c> is <c>true</c>,
          an IPv6 address is looked up.</p>
      </desc>
    </func>

    <func>
      <name name="gethostbyname" arity="2"/>
      <fsummary>Return a hostent record for the host with the specified name.
      </fsummary>
      <desc>
        <p>Returns a <c>hostent</c> record for the host with the specified
          name, restricted to the specified address family.</p>
      </desc>
    </func>

    <func>
      <name name="gethostname" arity="0"/>
      <fsummary>Return the local hostname.</fsummary>
      <desc>
        <p>Returns the local hostname. Never fails.</p>
      </desc>
    </func>

    <func>
      <name name="getifaddrs" arity="0"/>
      <fsummary>Return a list of interfaces and their addresses.</fsummary>
      <desc>
        <p>Returns a list of 2-tuples containing interface names and the
          interface addresses. <c><anno>Ifname</anno></c> is a Unicode string.
          <c><anno>Hwaddr</anno></c> is hardware dependent, for example, on
          Ethernet interfaces
          it is the 6-byte Ethernet address (MAC address (EUI-48 address)).</p>
        <p>The tuples <c>{addr,<anno>Addr</anno>}</c>, <c>{netmask,_}</c>, and
          <c>{broadaddr,_}</c> are repeated in the result list if the interface
          has multiple addresses. If you come across an interface with
          multiple <c>{flag,_}</c> or <c>{hwaddr,_}</c> tuples, you have
          a strange interface or possibly a bug in this function. The tuple
          <c>{flag,_}</c> is mandatory, all others are optional.</p>
        <p>Do not rely too much on the order of <c><anno>Flag</anno></c> atoms
          or <c><anno>Ifopt</anno></c> tuples. There are however some rules:</p>
        <list type="bulleted">
          <item><p>Immediately after
            <c>{addr,_}</c> follows <c>{netmask,_}</c>.</p></item>
          <item><p>Immediately thereafter follows <c>{broadaddr,_}</c> if flag
            <c>broadcast</c> is <em>not</em> set and flag
            <c>pointtopoint</c> <em>is</em> set.</p></item>
          <item><p>Any <c>{netmask,_}</c>, <c>{broadaddr,_}</c>, or
            <c>{dstaddr,_}</c> tuples that follow an <c>{addr,_}</c>
            tuple concerns that address.</p></item>
        </list>
        <p>The tuple <c>{hwaddr,_}</c> is not returned on Solaris, as the
          hardware address historically belongs to the link layer and only
          the superuser can read such addresses.</p>
	<warning>
	  <p>On Windows, the data is fetched from different OS API functions,
            so the <c><anno>Netmask</anno></c> and <c><anno>Broadaddr</anno></c>
            values can be calculated, just as some <c><anno>Flag</anno></c>
            values. Report flagrant bugs.</p>
	</warning>
      </desc>
    </func>

    <func>
      <name name="getopts" arity="2"/>
      <fsummary>Get one or more options for a socket.</fsummary>
      <desc>
        <p>Gets one or more options for a socket. For a list of available
          options, see
          <seealso marker="#setopts/2"><c>setopts/2</c></seealso>.</p>
        <p>The number of elements in the returned
          <c><anno>OptionValues</anno></c>
          list does not necessarily correspond to the number of options
          asked for. If the operating system fails to support an option,
          it is left out in the returned list. An error tuple is returned
          only when getting options for the socket is impossible (that is,
          the socket is closed or the buffer size in a raw request
          is too large). This behavior is kept for backward
          compatibility reasons.</p>
        <p>A raw option request
          <c>RawOptReq = {raw, Protocol, OptionNum, ValueSpec}</c>
          can be used to get information about
          socket options not (explicitly) supported by the emulator. The
          use of raw socket options makes the code non-portable, but
          allows the Erlang programmer to take advantage of unusual features
          present on the current platform.</p>
        <p><c>RawOptReq</c> consists of tag <c>raw</c> followed
          by the protocol level, the option number, and either a binary
          or the size, in bytes, of the
          buffer in which the option value is to be stored. A binary is to be
          used when the underlying <c>getsockopt</c> requires <em>input</em>
          in the argument field. In this case, the binary size
          is to correspond to the required buffer
          size of the return value. The supplied values in a <c>RawOptReq</c>
          correspond to the second, third, and fourth/fifth parameters to the
          <c>getsockopt</c> call in the C socket API. The value stored
          in the buffer is returned as a binary <c>ValueBin</c>,
          where all values are coded in the native endianess.</p>
        <p>Asking for and inspecting raw socket options require low-level
          information about the current operating system and TCP stack.</p>
        <p><em>Example:</em></p>
        <p>Consider a Linux machine where option
          <c>TCP_INFO</c> can be used to collect TCP statistics
          for a socket. Assume you are interested in field
          <c>tcpi_sacked</c> of <c>struct tcp_info</c>
          filled in when asking for <c>TCP_INFO</c>. To be able to access
          this information, you need to know the following:</p>
        <list type="bulleted">
          <item>The numeric value of protocol level <c>IPPROTO_TCP</c></item>
          <item>The numeric value of option <c>TCP_INFO</c></item>
          <item>The size of <c>struct tcp_info</c></item>
          <item>The size and offset of the specific field</item>
        </list>
        <p>By inspecting the headers or writing a small C program, it is found
          that <c>IPPROTO_TCP</c> is 6, <c>TCP_INFO</c> is 11, the structure
          size is 92 (bytes), the offset of <c>tcpi_sacked</c> is 28 bytes,
          and the value is a 32-bit integer. The following code can be used
          to retrieve the value:</p>
        <code type="none"><![CDATA[
get_tcpi_sacked(Sock) ->
    {ok,[{raw,_,_,Info}]} = inet:getopts(Sock,[{raw,6,11,92}]),
    <<_:28/binary,TcpiSacked:32/native,_/binary>> = Info,
    TcpiSacked.]]></code>
        <p>Preferably, you would check the machine type, the operating system,
          and the Kernel version before executing anything similar to
          this code.</p>
      </desc>
    </func>

    <func>
      <name name="getstat" arity="1"/>
      <name name="getstat" arity="2"/>
      <fsummary>Get one or more statistic options for a socket.</fsummary>
      <type name="stat_option"/>
      <desc>
        <p>Gets one or more statistic options for a socket.</p>
        <p><c>getstat(<anno>Socket</anno>)</c> is equivalent to
          <c>getstat(<anno>Socket</anno>, [recv_avg, recv_cnt, recv_dvi,
          recv_max, recv_oct, send_avg, send_cnt, send_dvi, send_max,
          send_oct])</c>.</p>
        <p>The following options are available:</p>
        <taglist>
          <tag><c>recv_avg</c></tag>
          <item>
            <p>Average size of packets, in bytes, received by the socket.</p>
          </item>
          <tag><c>recv_cnt</c></tag>
          <item>
            <p>Number of packets received by the socket.</p>
          </item>
          <tag><c>recv_dvi</c></tag>
          <item>
            <p>Average packet size deviation, in bytes, received by the socket.</p>
          </item>
          <tag><c>recv_max</c></tag>
          <item>
            <p>Size of the largest packet, in bytes, received by the socket.</p>
          </item>
          <tag><c>recv_oct</c></tag>
          <item>
            <p>Number of bytes received by the socket.</p>
          </item>
          <tag><c>send_avg</c></tag>
          <item>
            <p>Average size of packets, in bytes, sent from the socket.</p>
          </item>
          <tag><c>send_cnt</c></tag>
          <item>
            <p>Number of packets sent from the socket.</p>
          </item>
          <tag><c>send_dvi</c></tag>
          <item>
            <p>Average packet size deviation, in bytes, sent from the socket.</p>
          </item>
          <tag><c>send_max</c></tag>
          <item>
            <p>Size of the largest packet, in bytes, sent from the socket.</p>
          </item>
          <tag><c>send_oct</c></tag>
          <item>
            <p>Number of bytes sent from the socket.</p>
          </item>
        </taglist>
      </desc>
    </func>

    <func>
      <name name="i" arity="0" />
      <name name="i" arity="1" />
      <name name="i" arity="2" />
      <fsummary>Displays information and statistics about sockets on the terminal</fsummary>
      <desc>
        <p>
          Lists all TCP, UDP and SCTP sockets, including those that the Erlang runtime system uses as well as
          those created by the application.
        </p>
        <p>
          The following options are available:
        </p>

        <taglist>
          <tag><c>port</c></tag>
          <item>
            <p>The internal index of the port.</p>
          </item>
          <tag><c>module</c></tag>
          <item>
            <p>The callback module of the socket.</p>
          </item>
          <tag><c>recv</c></tag>
          <item>
            <p>Number of bytes received by the socket.</p>
          </item>
          <tag><c>sent</c></tag>
          <item>
            <p>Number of bytes sent from the socket.</p>
          </item>
          <tag><c>owner</c></tag>
          <item>
            <p>The socket owner process.</p>
          </item>
          <tag><c>local_address</c></tag>
          <item>
            <p>The local address of the socket.</p>
          </item>
          <tag><c>foreign_address</c></tag>
          <item>
            <p>The address and port of the other end of the connection.</p>
          </item>
          <tag><c>state</c></tag>
          <item>
            <p>The connection state.</p>
          </item>
          <tag><c>type</c></tag>
          <item>
            <p>STREAM or DGRAM or SEQPACKET.</p>
          </item>
        </taglist>
      </desc>
    </func>

    <func>
      <name name="ntoa" arity="1" />
      <fsummary>Convert IPv6/IPV4 address to ASCII.</fsummary>
      <desc>
	<p>Parses an
          <seealso marker="#type-ip_address"><c>ip_address()</c></seealso>
          and returns an IPv4 or IPv6 address string.</p>
      </desc>
    </func>

    <func>
      <name name="parse_address" arity="1" />
      <fsummary>Parse an IPv4 or IPv6 address.</fsummary>
      <desc>
	<p>Parses an IPv4 or IPv6 address string and returns an
          <seealso marker="#type-ip4_address"><c>ip4_address()</c></seealso> or
          <seealso marker="#type-ip6_address"><c>ip6_address()</c></seealso>.
          Accepts a shortened IPv4 address string.</p>
      </desc>
    </func>

    <func>
      <name name="parse_ipv4_address" arity="1" />
      <fsummary>Parse an IPv4 address.</fsummary>
      <desc>
	<p>Parses an IPv4 address string and returns an
          <seealso marker="#type-ip4_address"><c>ip4_address()</c></seealso>.
	  Accepts a shortened IPv4 address string.</p>
      </desc>
    </func>

    <func>
      <name name="parse_ipv4strict_address" arity="1" />
      <fsummary>Parse an IPv4 address strict.</fsummary>
      <desc>
	<p>Parses an IPv4 address string containing four fields, that is,
          <em>not</em> shortened, and returns an
          <seealso marker="#type-ip4_address"><c>ip4_address()</c></seealso>.
        </p>
      </desc>
    </func>

    <func>
      <name name="parse_ipv6_address" arity="1" />
      <fsummary>Parse an IPv6 address.</fsummary>
      <desc>
	<p>Parses an IPv6 address string and returns an
          <seealso marker="#type-ip6_address"><c>ip6_address()</c></seealso>.
          If an IPv4 address string is specified, an IPv4-mapped IPv6 address
          is returned.</p>
      </desc>
    </func>

    <func>
      <name name="parse_ipv6strict_address" arity="1" />
      <fsummary>Parse an IPv6 address strict.</fsummary>
      <desc>
	<p>Parses an IPv6 address string and returns an
          <seealso marker="#type-ip6_address"><c>ip6_address()</c></seealso>.
          Does <em>not</em> accept IPv4 addresses.</p>
      </desc>
    </func>

    <func>
      <name name="ipv4_mapped_ipv6_address" arity="1" />
      <fsummary>Convert to and from IPv4-mapped IPv6 address.</fsummary>
      <desc>
	<p>
	  Convert an IPv4 address to an IPv4-mapped IPv6 address
	  or the reverse.  When converting from an IPv6 address
	  all but the 2 low words are ignored so this function also
	  works on some other types of addresses than IPv4-mapped.
	</p>
      </desc>
    </func>

    <func>
      <name name="parse_strict_address" arity="1" />
      <fsummary>Parse an IPv4 or IPv6 address strict.</fsummary>
      <desc>
	<p>Parses an IPv4 or IPv6 address string and returns an
          <seealso marker="#type-ip4_address"><c>ip4_address()</c></seealso> or
          <seealso marker="#type-ip6_address"><c>ip6_address()</c></seealso>.
          Does <em>not</em> accept a shortened IPv4 address string.</p>
      </desc>
    </func>

    <func>
      <name name="peername" arity="1"/>
      <fsummary>Return the address and port for the other end of a connection.
      </fsummary>
      <desc>
        <p>Returns the address and port for the other end of a connection.</p>
	<p>Notice that for SCTP sockets, this function returns only
	  one of the peer addresses of the socket. Function
	  <seealso marker="#peernames/1"><c>peernames/1,2</c></seealso>
	  returns all.</p>
      </desc>
    </func>

    <func>
      <name name="peernames" arity="1"/>
      <fsummary>Return all address/port numbers for the other end of a
        connection.</fsummary>
      <desc>
	<p>Equivalent to
	  <seealso marker="#peernames/2"><c>peernames(<anno>Socket</anno>, 0)</c></seealso>.
        </p>
        <p>Notice that the behavior of this function for an SCTP
	  one-to-many style socket is not defined by the
	  <url href="http://tools.ietf.org/html/draft-ietf-tsvwg-sctpsocket-13">SCTP Sockets API Extensions</url>.</p>
      </desc>
    </func>

    <func>
      <name name="peernames" arity="2"/>
      <fsummary>Return all address/port numbers for the other end of a
        connection.</fsummary>
      <desc>
	<p>Returns a list of all address/port number pairs for the other end
	  of an association <c><anno>Assoc</anno></c> of a socket.</p>
	<p>This function can return multiple addresses for multihomed
	  sockets, such as SCTP sockets. For other sockets it
	  returns a one-element list.</p>
	<p>Notice that parameter <c><anno>Assoc</anno></c> is by the
	  <url href="http://tools.ietf.org/html/draft-ietf-tsvwg-sctpsocket-13">SCTP Sockets API Extensions</url>
	  defined to be ignored for
	  one-to-one style sockets. What the special value <c>0</c>
	  means, hence its behavior for one-to-many style sockets,
	  is unfortunately undefined.</p>
      </desc>
    </func>

    <func>
      <name name="port" arity="1"/>
      <fsummary>Return the local port number for a socket.</fsummary>
      <desc>
        <p>Returns the local port number for a socket.</p>
      </desc>
    </func>

    <func>
      <name name="setopts" arity="2"/>
      <fsummary>Set one or more options for a socket.</fsummary>
      <desc>
        <p>Sets one or more options for a socket.</p>
        <p>The following options are available:</p>
        <taglist>
          <tag><c>{active, true | false | once | N}</c></tag>
          <item>
            <p>If the value is <c>true</c>, which is the default,
              everything received from the socket is sent as
              messages to the receiving process.</p>
            <p>If the value is <c>false</c> (passive mode), the process must
              explicitly receive incoming data by calling
              <seealso marker="gen_tcp#recv/2"><c>gen_tcp:recv/2,3</c></seealso>,
              <seealso marker="gen_udp#recv/2"><c>gen_udp:recv/2,3</c></seealso>,
              or <seealso marker="gen_sctp#recv/1"><c>gen_sctp:recv/1,2</c></seealso>
              (depending on the type of socket).</p>
            <p>If the value is <c>once</c> (<c>{active, once}</c>),
              <em>one</em> data message from the socket is sent
              to the process. To receive one more message,
              <c>setopts/2</c> must be called again with option
              <c>{active, once}</c>.</p>
            <p>If the value is an integer <c>N</c> in the range -32768 to 32767
              (inclusive), the value is added to the socket's count of data
              messages sent to the controlling process. A socket's default
              message count is <c>0</c>. If a negative value is specified, and
              its magnitude is equal to or greater than the socket's current
              message count, the socket's message count is set to <c>0</c>.
              Once the socket's message count reaches <c>0</c>, either because
              of sending
              received data messages to the process or by being explicitly set,
              the process is then notified by a special message, specific to
              the type of socket, that the socket has entered passive
              mode. Once the socket enters passive mode, to receive more
              messages <c>setopts/2</c> must be called again to set the
              socket back into an active mode.</p>
            <p>When using <c>{active, once}</c> or <c>{active, N}</c>, the
              socket changes behavior automatically when data is received.
              This can be confusing in combination with connection-oriented
              sockets (that is, <c>gen_tcp</c>), as a socket
              with <c>{active, false}</c> behavior reports closing
              differently than a socket with <c>{active, true}</c>
              behavior. To simplify programming, a socket where
              the peer closed, and this is detected while in
              <c>{active, false}</c> mode, still generates message
              <c>{tcp_closed,Socket}</c> when set to <c>{active, once}</c>,
              <c>{active, true}</c>, or <c>{active, N}</c> mode.
              It is therefore safe to assume that message
              <c>{tcp_closed,Socket}</c>, possibly followed by socket port
              termination (depending on option <c>exit_on_close</c>)
              eventually appears when a socket changes
              back and forth between <c>{active, true}</c> and
              <c>{active, false}</c> mode. However,
              <em>when</em> peer closing is detected it is all up to the
              underlying TCP/IP stack and protocol.</p>
            <p>Notice that <c>{active, true}</c> mode provides no flow
              control; a fast sender can easily overflow the
              receiver with incoming messages. The same is true for
              <c>{active, N}</c> mode, while the message count is greater
              than zero.</p>
            <p>Use active mode only if
              your high-level protocol provides its own flow control
              (for example, acknowledging received messages) or the
              amount of data exchanged is small. <c>{active, false}</c>
              mode, use of the <c>{active, once}</c> mode, or <c>{active, N}</c>
              mode with values of <c>N</c> appropriate for the application
              provides flow control. The other side cannot send
              faster than the receiver can read.</p>
          </item>
          <tag><c>{broadcast, Boolean}</c> (UDP sockets)</tag>
          <item>
            <p>Enables/disables permission to send broadcasts.</p>
            <marker id="option-buffer"></marker>
          </item>
          <tag><c>{buffer, Size}</c></tag>
          <item>
            <p>The size of the user-level software buffer used by
              the driver. 
              Not to be confused with options <c>sndbuf</c>
              and <c>recbuf</c>, which correspond to the
              Kernel socket buffers. It is recommended
              to have <c>val(buffer) &gt;= max(val(sndbuf),val(recbuf))</c> to
              avoid performance issues because of unnecessary copying.
              <c>val(buffer)</c> is automatically set to the above
              maximum when values <c>sndbuf</c> or <c>recbuf</c> are set.
              However, as the sizes set for <c>sndbuf</c> and <c>recbuf</c>
              usually become larger, you are encouraged to use
              <seealso marker="#getopts/2"><c>getopts/2</c></seealso>
              to analyze the behavior of your operating system.</p>
            <p>Note that this is also the maximum amount of data that can be
	       received from a single recv call. If you are using higher than 
	       normal MTU consider setting buffer higher.</p> 
          </item>
          <tag><c>{delay_send, Boolean}</c></tag>
          <item>
            <p>Normally, when an Erlang process sends to a socket,
              the driver tries to send the data immediately. If that
              fails, the driver uses any means available to queue
              up the message to be sent whenever the operating system
              says it can handle it. Setting <c>{delay_send, true}</c>
              makes <em>all</em> messages queue up. The messages sent
              to the network are then larger but fewer.
              The option affects the scheduling of send
              requests versus Erlang processes instead of changing any
              real property of the socket. The option is
              implementation-specific. Defaults to <c>false</c>.</p>
          </item>
          <tag><c>{deliver, port | term}</c></tag>
          <item>
            <p>When <c>{active, true}</c>, data is delivered on the form
              <c>port</c> : <c>{S, {data, [H1,..Hsz | Data]}}</c> or
              <c>term</c> : <c>{tcp, S, [H1..Hsz | Data]}</c>.</p>
          </item>
          <tag><c>{dontroute, Boolean}</c></tag>
          <item>
            <p>Enables/disables routing bypass for outgoing messages.</p>
          </item>
          <tag><c>{exit_on_close, Boolean}</c></tag>
          <item>
            <p>This option is set to <c>true</c> by default.</p>
            <p>The only reason to set it to <c>false</c> is if you want
              to continue sending data to the socket after a close is
              detected, for example, if the peer uses
              <seealso marker="gen_tcp#shutdown/2"><c>gen_tcp:shutdown/2</c></seealso>
              to shut down the write side.</p>
          </item>
          <tag><c>{header, Size}</c></tag>
          <item>
            <p>This option is only meaningful if option <c>binary</c>
              was specified when the socket was created. If option
              <c>header</c> is specified, the first
              <c>Size</c> number bytes of data received from the socket
              are elements of a list, and the remaining data is
              a binary specified as the tail of the same list. For example,
              if <c>Size == 2</c>, the data received matches
              <c>[Byte1,Byte2|Binary]</c>.</p>
          </item>
          <tag><c>{high_msgq_watermark, Size}</c></tag>
          <item>
	    <p>The socket message queue is set to a busy
	      state when the amount of data on the message
	      queue reaches this limit. Notice that this limit only
	      concerns data that has not yet reached the ERTS internal
	      socket implementation. Defaults to 8 kB.</p>
	    <p>Senders of data to the socket are suspended if
	      either the socket message queue is busy or the socket
	      itself is busy.</p>
	    <p>For more information, see options <c>low_msgq_watermark</c>,
	      <c>high_watermark</c>, and <c>low_watermark</c>.</p>
	    <p>Notice that distribution sockets disable the use of
	      <c>high_msgq_watermark</c> and <c>low_msgq_watermark</c>.
	      Instead use the
	      <seealso marker="erts:erlang#system_info_dist_buf_busy_limit">distribution buffer busy limit</seealso>,
              which is a similar feature.</p>
          </item>
          <tag><c>{high_watermark, Size}</c> (TCP/IP sockets)</tag>
          <item>
	    <p>The socket is set to a busy state when the amount
	      of data queued internally by the ERTS socket implementation
	      reaches this limit. Defaults to 8 kB.</p>
	    <p>Senders of data to the socket are suspended if
	      either the socket message queue is busy or the socket
	      itself is busy.</p>
	    <p>For more information, see options <c>low_watermark</c>,
	      <c>high_msgq_watermark</c>, and <c>low_msqg_watermark</c>.</p>
          </item>
	  <tag><c>{ipv6_v6only, Boolean}</c></tag>
	  <item>
	    <p>Restricts the socket to use only IPv6, prohibiting any
	      IPv4 connections. This is only applicable for
	      IPv6 sockets (option <c>inet6</c>).</p>
            <p>On most platforms this option must be set on the socket
	      before associating it to an address. It is therefore only
	      reasonable to specify it when creating the socket and not
	      to use it when calling function
	      (<seealso marker="#setopts/2"><c>setopts/2</c></seealso>)
	      containing this description.</p>
	    <p>The behavior of a socket with this option set to
	      <c>true</c> is the only portable one. The original
	      idea when IPv6 was new of using IPv6 for all traffic
	      is now not recommended by FreeBSD (you can use
	      <c>{ipv6_v6only,false}</c> to override the recommended
	      system default value),
	      forbidden by OpenBSD (the supported GENERIC kernel),
	      and impossible on Windows (which has separate
	      IPv4 and IPv6 protocol stacks). Most Linux distros
	      still have a system default value of <c>false</c>.
	      This policy shift among operating systems to
	      separate IPv6 from IPv4 traffic has evolved, as
	      it gradually proved hard and complicated to get
	      a dual stack implementation correct and secure.</p>
	    <p>On some platforms, the only allowed value for this option
	      is <c>true</c>, for example, OpenBSD and Windows. Trying to set
	      this option to <c>false</c>, when creating the socket, fails
	      in this case.</p>
	    <p>Setting this option on platforms where it does not exist
	      is ignored. Getting this option with
	      <seealso marker="#getopts/2"><c>getopts/2</c></seealso>
	      returns no value, that is, the returned list does not contain an
	      <c>{ipv6_v6only,_}</c> tuple. On Windows, the option
	      does not exist, but it is emulated as a
	      read-only option with value <c>true</c>.</p>
	    <p>Therefore, setting this option to <c>true</c>
	      when creating a socket never fails, except possibly on a
              platform where you
	      have customized the kernel to only allow <c>false</c>,
	      which can be doable (but awkward) on, for example, OpenBSD.</p>
	    <p>If you read back the option value using
	      <seealso marker="#getopts/2"><c>getopts/2</c></seealso>
	      and get no value, the option does not exist in the host
              operating system. The behavior of both an IPv6 and an IPv4
	      socket listening on the same port, and for an IPv6 socket
	      getting IPv4 traffic is then no longer predictable.</p>
	  </item>
          <tag><c>{keepalive, Boolean}</c>(TCP/IP sockets)</tag>
          <item>
            <p>Enables/disables periodic transmission on a connected
              socket when no other data is exchanged. If
              the other end does not respond, the connection is
              considered broken and an error message is sent to
              the controlling process. Defaults to <c>disabled</c>.</p>
            <marker id="option-linger"></marker>
          </item>
          <tag><c>{linger, {true|false, Seconds}}</c></tag>
          <item>
            <p>Determines the time-out, in seconds, for flushing unsent data
              in the <c>close/1</c> socket call. If the first component of
              the value tuple is <c>false</c>, the second is ignored. This
              means that <c>close/1</c> returns immediately, not waiting
              for data to be flushed. Otherwise, the second component is
              the flushing time-out, in seconds.</p>
          </item>
          <tag><c>{low_msgq_watermark, Size}</c></tag>
          <item>
	    <p>If the socket message queue is in a busy state, the
	      socket message queue is set in a not busy state when
	      the amount of data queued in the message queue falls
	      below this limit. Notice that this limit only concerns data
	      that has not yet reached the ERTS internal socket
	      implementation. Defaults to 4 kB.</p>
	    <p>Senders that are suspended because of either a
	      busy message queue or a busy socket are resumed
	      when the socket message queue and the socket
	      are not busy.</p>
	    <p>For more information, see options <c>high_msgq_watermark</c>,
	      <c>high_watermark</c>, and <c>low_watermark</c>.</p>
	    <p>Notice that distribution sockets disable the use of
	      <c>high_msgq_watermark</c> and <c>low_msgq_watermark</c>.
	      Instead they use the
	      <seealso marker="erts:erlang#system_info_dist_buf_busy_limit">distribution
	      buffer busy limit</seealso>, which is a similar feature.</p>
          </item>
          <tag><c>{low_watermark, Size}</c> (TCP/IP sockets)</tag>
          <item>
	    <p>If the socket is in a busy state, the socket is
	      set in a not busy state when the amount of data
	      queued internally by the ERTS socket implementation
	      falls below this limit. Defaults to 4 kB.</p>
	    <p>Senders that are suspended because of a
	      busy message queue or a busy socket are resumed
	      when the socket message queue and the socket are not busy.</p>
	    <p>For more information, see options <c>high_watermark</c>,
	      <c>high_msgq_watermark</c>, and <c>low_msgq_watermark</c>.</p>
          </item>
          <tag><c>{mode, Mode :: binary | list}</c></tag>
          <item>
            <p>Received <c>Packet</c> is delivered as defined by <c>Mode</c>.
            </p>
          </item>
	  <tag><c>{netns, Namespace :: file:filename_all()}</c></tag>
	  <item>
	    <p>Sets a network namespace for the socket. Parameter
	      <c>Namespace</c> is a filename defining the namespace, for
	      example, <c>"/var/run/netns/example"</c>, typically created by
	      command <c>ip netns add example</c>. This option must be used in
	      a function call that creates a socket, that is,
	      <seealso marker="gen_tcp#connect/3"><c>gen_tcp:connect/3,4</c></seealso>,
	      <seealso marker="gen_tcp#listen/2"><c>gen_tcp:listen/2</c></seealso>,
	      <seealso marker="gen_udp#open/1"><c>gen_udp:open/1,2</c></seealso>, or
	    <seealso marker="gen_sctp#open/0"><c>gen_sctp:open/0,1,2</c></seealso>.</p>
	    <p>This option uses the Linux-specific syscall
	      <c>setns()</c>, such as in Linux kernel 3.0 or later,
	      and therefore only exists when the runtime system
	      is compiled for such an operating system.</p>
	    <p>The virtual machine also needs elevated privileges, either
	      running as superuser or (for Linux) having capability
	      <c>CAP_SYS_ADMIN</c> according to the documentation for
	      <c>setns(2)</c>.
	      However, during testing also <c>CAP_SYS_PTRACE</c>
	      and <c>CAP_DAC_READ_SEARCH</c> have proven to be necessary.</p>
            <p><em>Example:</em></p>
	    <code>
setcap cap_sys_admin,cap_sys_ptrace,cap_dac_read_search+epi beam.smp</code>
            <p>Notice that the filesystem containing the virtual machine
	      executable (<c>beam.smp</c> in the example) must be local,
	      mounted without flag <c>nosetuid</c>,
	      support extended attributes, and
	      the kernel must support file capabilities.
	      All this runs out of the box on at least Ubuntu 12.04 LTS,
	      except that SCTP sockets appear to not support
	      network namespaces.</p>
	    <p><c>Namespace</c> is a filename and is encoded
	      and decoded as discussed in module
	      <seealso marker="file">file</seealso>, with the
              following exceptions:</p>
	    <list type="bulleted">
	      <item><p>Emulator flag <c>+fnu</c> is ignored.</p></item>
	      <item><p><seealso marker="#getopts/2"><c>getopts/2</c></seealso>
	      for this option returns a binary for the filename if the stored
	      filename cannot be decoded. This is only to occur if you set the
	      option using a binary that cannot be decoded with the emulator's
	      filename encoding:
	      <seealso marker="file#native_name_encoding/0"><c>file:native_name_encoding/0</c></seealso>.</p></item>
	    </list>
          </item>
	  <tag><c>{bind_to_device, Ifname :: binary()}</c></tag>
	  <item>
	    <p>Binds a socket to a specific network interface. This option
	      must be used in a function call that creates a socket, that is,
	      <seealso marker="gen_tcp#connect/3"><c>gen_tcp:connect/3,4</c></seealso>,
	      <seealso marker="gen_tcp#listen/2"><c>gen_tcp:listen/2</c></seealso>,
	      <seealso marker="gen_udp#open/1"><c>gen_udp:open/1,2</c></seealso>, or
	      <seealso marker="gen_sctp#open/0"><c>gen_sctp:open/0,1,2</c></seealso>.</p>
	    <p>Unlike <seealso marker="#getifaddrs/0"><c>getifaddrs/0</c></seealso>, Ifname
	      is encoded a binary. In the unlikely case that a system is using
	      non-7-bit-ASCII characters in network device names, special care
	      has to be taken when encoding this argument.</p>
	    <p>This option uses the Linux-specific socket option
	      <c>SO_BINDTODEVICE</c>, such as in Linux kernel 2.0.30 or later,
	      and therefore only exists when the runtime system
	      is compiled for such an operating system.</p>
	    <p>Before Linux 3.8, this socket option could be set, but could not retrieved
	      with <seealso marker="#getopts/2"><c>getopts/2</c></seealso>. Since Linux 3.8,
	      it is readable.</p>
	    <p>The virtual machine also needs elevated privileges, either
	      running as superuser or (for Linux) having capability
	    <c>CAP_NET_RAW</c>.</p>
	    <p>The primary use case for this option is to bind sockets into
	      <url href="http://www.kernel.org/doc/Documentation/networking/vrf.txt">Linux VRF instances</url>.
	    </p>
	  </item>
          <tag><c>list</c></tag>
          <item>
            <p>Received <c>Packet</c> is delivered as a list.</p>
          </item>
          <tag><c>binary</c></tag>
          <item>
            <p>Received <c>Packet</c> is delivered as a binary.</p>
          </item>
          <tag><c>{nodelay, Boolean}</c>(TCP/IP sockets)</tag>
          <item>
            <p>If <c>Boolean == true</c>, option <c>TCP_NODELAY</c>
              is turned on for the socket, which means that also small
              amounts of data are sent immediately.</p>
          </item>
          <tag><c>{packet, PacketType}</c>(TCP/IP sockets)</tag>
          <item>
            <p><marker id="packet"/>Defines the type of packets to use for a socket.
	      Possible values:</p>
            <taglist>
              <tag><c>raw | 0</c></tag>
              <item>
                <p>No packaging is done.</p>
              </item>
              <tag><c>1 | 2 | 4</c></tag>
              <item>
                <p>Packets consist of a header specifying the number of
                  bytes in the packet, followed by that number of bytes.
                  The header length can be one, two, or four bytes, and
                  containing an unsigned integer in big-endian byte order.
                  Each send operation generates the header, and the header
                  is stripped off on each receive operation.</p>
                <p>The 4-byte header is limited to 2Gb.</p>
              </item>
              <tag><c>asn1 | cdr | sunrm | fcgi | tpkt | line</c></tag>
              <item>
                <p>These packet types only have effect on receiving.
                  When sending a packet, it is the responsibility of
                  the application to supply a correct header. On
                  receiving, however, one message is sent to
                  the controlling process for each complete packet
                  received, and, similarly, each call to
                  <c>gen_tcp:recv/2,3</c> returns one complete packet.
                  The header is <em>not</em> stripped off.</p>
                <p>The meanings of the packet types are as follows:</p>
		<list type="bulleted">
		  <item><c>asn1</c> - ASN.1 BER</item>
		  <item><c>sunrm</c> - Sun's RPC encoding</item>
		  <item><c>cdr</c> - CORBA (GIOP 1.1)</item>
		  <item><c>fcgi</c> - Fast CGI</item>
		  <item><c>tpkt</c> - TPKT format [RFC1006]</item>
		  <item><c>line</c> - Line mode, a packet is a line-terminated
		    with newline, lines longer than the receive buffer are
		    truncated</item>
		</list>
	      </item>
              <tag><c>http | http_bin</c></tag>
              <item>
                <p>The Hypertext Transfer Protocol. The packets
                 are returned with the format according to <c>HttpPacket</c>
                 described in
                 <seealso marker="erts:erlang#decode_packet/3">
                 <c>erlang:decode_packet/3</c></seealso> in ERTS.
                 A socket in passive
                 mode returns <c>{ok, HttpPacket}</c> from <c>gen_tcp:recv</c>
                 while an active socket sends messages like
		 <c>{http, Socket, HttpPacket}</c>.</p>
              </item>
              <tag><c>httph | httph_bin</c></tag>
              <item>
                <p>These two types are often not needed, as the socket
		automatically switches from <c>http</c>/<c>http_bin</c> to
		<c>httph</c>/<c>httph_bin</c> internally after the first line
		is read. However, there can be occasions when they are
		useful, such as parsing trailers from chunked encoding.</p>
              </item>
            </taglist>
          </item>
          <tag><c>{packet_size, Integer}</c>(TCP/IP sockets)</tag>
          <item>
            <p>Sets the maximum allowed length of the packet body. If
              the packet header indicates that the length of the packet
              is longer than the maximum allowed length, the packet is
              considered invalid. The same occurs if the packet header
              is too large for the socket receive buffer.</p>
	    <p>For line-oriented protocols (<c>line</c>, <c>http*</c>),
	      option <c>packet_size</c> also guarantees that lines up to the
	      indicated length are accepted and not considered invalid
	      because of internal buffer limitations.</p>
          </item>
          <tag><c>{line_delimiter, Char}</c>(TCP/IP sockets)</tag>
          <item>
            <p>Sets the line delimiting character for line-oriented protocols
              (<c>line</c>). Defaults to <c>$\n</c>.</p>
          </item>
          <tag><c>{raw, Protocol, OptionNum, ValueBin}</c></tag>
          <item>
            <p>See below.</p>
          </item>
          <tag><c>{read_packets, Integer}</c>(UDP sockets)</tag>
          <item>
            <p>Sets the maximum number of UDP packets to read without
              intervention from the socket when data is available.
              When this many packets have been read and delivered
              to the destination process, new packets are not read
              until a new notification of available data has arrived.
              Defaults to <c>5</c>. If this parameter is set too
              high, the system can become unresponsive because of
              UDP packet flooding.</p>
          </item>
          <tag><c>{recbuf, Size}</c></tag>
          <item>
            <p>The minimum size of the receive buffer to use for
              the socket. You are encouraged to use
              <seealso marker="#getopts/2"><c>getopts/2</c></seealso>
              to retrieve the size set by your operating system.</p>
          </item>
          <tag><c>{reuseaddr, Boolean}</c></tag>
          <item>
            <p>Allows or disallows local reuse of port numbers. By
              default, reuse is disallowed.</p>
          </item>
          <tag><c>{send_timeout, Integer}</c></tag>
          <item>
            <p>Only allowed for connection-oriented sockets.</p>…