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  <header>
    <copyright>
      <year>2009</year><year>2011</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.
    
      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.
    
    </legalnotice>

    <title>inet_res</title>
    <prepared>raimo@erix.ericsson.se</prepared>
    <docno></docno>
    <date>2009-09-11</date>
    <rev>A</rev>
  </header>
  <module>inet_res</module>
  <modulesummary>A Rudimentary DNS Client</modulesummary>
  <description>
    <p>Performs DNS name resolving towards recursive name servers</p>
    <p>See also 
      <seealso marker="erts:inet_cfg">
        ERTS User's Guide: Inet configuration
      </seealso> for more
      information on how to configure an Erlang runtime system for IP
      communication and how to enable this DNS client by defining
      <c><![CDATA['dns']]></c> as a lookup method. It then acts
      as a backend for the resolving functions in
      <seealso marker="kernel:inet">inet</seealso>.</p>
    <p>This DNS client can resolve DNS records even if it
      is not used for normal name resolving in the node.</p>
    <p>This is not a full-fledged resolver. It is just a
      DNS client that relies on asking trusted recursive nameservers.</p>
  </description>

  <section>
    <title>Name Resolving</title>
    <p>UDP queries are used unless resolver option
      <c>usevc</c> is <c>true</c>, which forces TCP queries.
      If the query is to large for UDP, TCP is used instead.
      For regular DNS queries 512 bytes is the size limit.
      When EDNS is enabled (resolver option
      <c>edns</c> is set to the EDNS version i.e <c>0</c>
      instead of <c>false</c>), resolver option
      <c>udp_payload_size</c> sets the limit. If a nameserver
      replies with the TC bit set (truncation), indicating
      the answer is incomplete, the query is retried
      to that nameserver using TCP. The resolver option
      <c>udp_payload_size</c> also sets the advertised
      size for the max allowed reply size, if EDNS is
      enabled, otherwise the nameserver uses the limit
      512 byte. If the reply is larger it gets truncated,
      forcing a TCP re-query.</p>
    <p>For UDP queries, the resolver options <c>timeout</c>
      and <c>retry</c> control retransmission.
      Each nameserver in the <c>nameservers</c> list is
      tried with a timeout of <c>timeout</c> / <c>retry</c>.
      Then all nameservers are tried again doubling the
      timeout, for a total of <c>retry</c> times.</p>
    <p>For queries that not use the <c>search</c> list,
      if the query to all <c>nameservers</c> results in
      <c>{error,nxdomain}</c>or an empty answer, the same
      query is tried for the <c>alt_nameservers</c>.</p>
  </section>

  <datatypes>
    <p>Resolver types:</p>
    <datatype>
      <name name="res_option"/>
    </datatype>
    <datatype>
      <name name="nameserver"/>
    </datatype>
    <datatype>
      <name name="res_error"/>
    </datatype>

    <p><marker id="dns_types"/>DNS types:</p>
    <datatype>
      <name name="dns_name"/>
      <desc><p>A string with no adjacent dots.</p></desc>
    </datatype>
    <datatype>
      <name name="rr_type"/>
    </datatype>
    <datatype>
      <name name="dns_class"/>
    </datatype>
    <datatype>
      <name name="dns_msg"/>
      <desc>
        <p>This is the start of a hiearchy of opaque data structures
          that can be examined with access functions in inet_dns that
          return lists of {Field,Value} tuples. The arity 2 functions
          just return the value for a given field.
<pre>
dns_msg() = DnsMsg
    inet_dns:msg(DnsMsg) ->
        [ {header, dns_header()}
        | {qdlist, dns_query()}
        | {anlist, dns_rr()}
        | {nslist, dns_rr()}
        | {arlist, dns_rr()} ]
    inet_dns:msg(DnsMsg, header) -> dns_header() % for example
    inet_dns:msg(DnsMsg, Field) -> Value

dns_header() = DnsHeader
    inet_dns:header(DnsHeader) ->
        [ {id, integer()}
        | {qr, boolean()}
        | {opcode, 'query' | iquery | status | integer()}
        | {aa, boolean()}
        | {tc, boolean()}
        | {rd, boolean()}
        | {ra, boolean()}
        | {pr, boolean()}
        | {rcode, integer(0..16)} ]
    inet_dns:header(DnsHeader, Field) -> Value

query_type() = axfr | mailb | maila | any | rr_type()

dns_query() = DnsQuery
    inet_dns:dns_query(DnsQuery) ->
        [ {domain, dns_name()}
        | {type, query_type()}
        | {class, dns_class()} ]
    inet_dns:dns_query(DnsQuery, Field) -> Value

dns_rr() = DnsRr
    inet_dns:rr(DnsRr) -> DnsRrFields | DnsRrOptFields
    DnsRrFields = [ {domain, dns_name()}
                  | {type, rr_type()}
                  | {class, dns_class()}
                  | {ttl, integer()}
                  | {data, dns_data()} ]
    DnsRrOptFields = [ {domain, dns_name()}
                     | {type, opt}
                     | {udp_payload_size, integer()}
                     | {ext_rcode, integer()}
                     | {version, integer()}
                     | {z, integer()}
                     | {data, dns_data()} ]
    inet_dns:rr(DnsRr, Field) -> Value

There is an info function for the types above:

inet_dns:record_type(dns_msg()) -> msg;
inet_dns:record_type(dns_header()) -> header;
inet_dns:record_type(dns_query()) -> dns_query;
inet_dns:record_type(dns_rr()) -> rr;
inet_dns:record_type(_) -> undefined.

So; inet_dns:(inet_dns:record_type(X))(X) will convert
any of these data structures into a {Field,Value} list.</pre></p>
      </desc>
    </datatype>
    <datatype>
      <name name="dns_data"/>
      <desc><p><c><anno>Regexp</anno></c> is a string with characters encoded in the
         UTF-8 coding standard.</p>
      </desc>
    </datatype>
  </datatypes>


  <funcs>

    <func>
      <name name="getbyname" arity="2"/>
      <name name="getbyname" arity="3"/>
      <fsummary>Resolve a DNS record of the given type for the given host
      </fsummary>
      <desc>
        <p>Resolve a DNS record of the given type for the given host,
          of class <c>in</c>. On success returns a <c>hostent()</c> record with
          <c>dns_data()</c> elements in the address list field.
        </p><p>
          This function uses the resolver option <c>search</c> that
          is a list of domain names. If the name to resolve contains
          no dots, it is prepended to each domain name in the
          search list, and they are tried in order. If the name
          contains dots, it is first tried as an absolute name
          and if that fails the search list is used. If the name
          has a trailing dot it is simply supposed to be
          an absolute name and the search list is not used.
        </p>
      </desc>
    </func>

    <func>
      <name name="gethostbyaddr" arity="1"/>
      <name name="gethostbyaddr" arity="2"/>
      <fsummary>Return a hostent record for the host with the given address
      </fsummary>
      <desc>
        <p>Backend functions used by
          <seealso marker="kernel:inet#gethostbyaddr/1">
            inet:gethostbyaddr/1
          </seealso>.
        </p>
      </desc>
    </func>

    <func>
      <name name="gethostbyname" arity="1"/>
      <name name="gethostbyname" arity="2"/>
      <name name="gethostbyname" arity="3"/>
      <fsummary>Return a hostent record for the host with the given name
      </fsummary>
      <desc>
        <p>Backend functions used by
          <seealso marker="kernel:inet#gethostbyname/1">
            inet:gethostbyname/1,2
          </seealso>.
        </p><p>
          This function uses the resolver option <c>search</c> just like 
          <seealso marker="#getbyname/2">getbyname/2,3</seealso>.
        </p><p>
          If the resolver option <c>inet6</c> is <c>true</c>,
          an IPv6 address is looked up, and if that fails
          the IPv4 address is looked up and returned on
          IPv6 mapped IPv4 format.
        </p>
      </desc>
    </func>

    <func>
      <name name="lookup" arity="3"/>
      <name name="lookup" arity="4"/>
      <name name="lookup" arity="5"/>
      <fsummary>Resolve the DNS data for the record of the given type and class
        for the given name
      </fsummary>
      <desc>
        <p>Resolve the DNS data for the record of the given type and class
          for the given name. On success filters out the answer records
          with the correct <c><anno>Class</anno></c> and <c><anno>Type</anno></c> and returns
          a list of their data fields. So a lookup for type <c>any</c>
          will give an empty answer since the answer records have
          specific types that are not <c>any</c>. An empty answer
          as well as a failed lookup returns an empty list.
        </p><p>
          Calls <seealso marker="#resolve/3">resolve/2..4</seealso>
          with the same arguments and filters the result, so
          <c><anno>Opts</anno></c> is explained there.
        </p>
      </desc>      
    </func>

    <func>
      <name name="resolve" arity="3"/>
      <name name="resolve" arity="4"/>
      <name name="resolve" arity="5"/>
      <fsummary>Resolve a DNS record of the given type and class
        for the given name
      </fsummary>
      <desc>
        <p>Resolve a DNS record of the given type and class for the given name.
          The returned <c>dns_msg()</c> can be examined using
          access functions in <c>inet_db</c> as described 
          in <seealso marker="#dns_types">DNS types</seealso>.
        </p><p>
          If <c><anno>Name</anno></c> is an <c>ip_address()</c>, the domain name
          to query for is generated as the standard reverse
          ".IN-ADDR.ARPA." name for an IPv4 address, or the
          ".IP6.ARPA." name for an IPv6 address.
          In this case you most probably want to use
          <c><anno>Class</anno> = in</c> and <c><anno>Type</anno> = ptr</c> but it
          is not done automatically.
        </p><p>
          <c><anno>Opts</anno></c> override the corresponding resolver options.
          If the option <c>nameservers</c> is given, it is
          also assumed that it is the complete list of nameserves,
          so the resolver option <c>alt_nameserves</c> is ignored.
          Of course, if that option is also given to this function,
          it is used.
        </p><p>
          The <c>verbose</c> option (or rather <c>{verbose,true}</c>),
          causes diagnostics printout through
          <seealso marker="stdlib:io#format/3">io:format/2</seealso>
          of queries, replies retransmissions, etc, similar
          to from utilities like <c>dig</c>, <c>nslookup</c> et.al.
        </p><p>
          If <c><anno>Opt</anno></c> is an arbitrary atom it is interpreted
          as <c>{<anno>Opt</anno>,true}</c> unless the atom string starts with
          <c>"no"</c> making the interpretation <c>{<anno>Opt</anno>,false}</c>.
          For example: <c>usevc</c> is an alias for <c>{usevc,true}</c>,
          and <c>nousevc</c> an alias for <c>{usevc,false}</c>.
        </p><p>
          The <c>inet6</c> option currently has no effect on this function.
          You probably want to use <c><anno>Type</anno> = a | aaaa</c> instead.
        </p>
      </desc>      
    </func>

  </funcs>



  <section>
    <title>Examples</title>
    <p>Access functions example: how
      <seealso marker="#lookup/3">lookup/3</seealso>
      could have been implemented using
      <seealso marker="#resolve/3">resolve/3</seealso>
      from outside the module.
    </p><code type="none">
    example_lookup(Name, Class, Type) ->
        case inet_res:resolve(Name, Class, Type) of
            {ok,Msg} ->
                [inet_dns:rr(RR, data)
                 || RR &lt;- inet_dns:msg(Msg, anlist),
                    inet_dns:rr(RR, type) =:= Type,
                    inet_dns:rr(RR, class) =:= Class];
            {error,_} ->
                []
        end.</code>
  </section>



  <section>
    <title>Legacy Functions</title>
    <p>These have been deprecated due to the annoying double
      meaning of the nameservers/timeout argument, and
      because they had no decent place for a resolver options list.</p>
  </section>

  <funcs>

    <func>
      <name name="nslookup" arity="3"/>
      <name name="nslookup" arity="4" clause_i="1"/>
      <name name="nslookup" arity="4" clause_i="2"/>
      <fsummary>Resolve a DNS record of the given type and class
        for the given name
      </fsummary>
      <type variable="Name"/>
      <type variable="Class"/>
      <type variable="Type"/>
      <type variable="Timeout" name_i="2"/>
      <type variable="Nameservers"/>
      <type variable="Reason"/>
      <desc>
        <p>Resolve a DNS record of the given type and class for the given name.
        </p>
      </desc>      
    </func>

    <func>
      <name name="nnslookup" arity="4"/>
      <name name="nnslookup" arity="5"/>
      <fsummary>Resolve a DNS record of the given type and class
        for the given name
      </fsummary>
      <desc>
        <p>Resolve a DNS record of the given type and class for the given name.
        </p>
      </desc>      
    </func>

  </funcs>

</erlref>
