path: root/lib/inets/doc/src/http_server.xml

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<chapter>
  <header>
    <copyright>
      <year>2004</year><year>2015</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>HTTP server</title>
    <file>http_server.xml</file>
  </header>

  <section>
    <title>Configuration</title>
    <marker id="config"></marker>
    <p>The HTTP server, also referred to as httpd, handles HTTP requests
      as described in 
      <url href="http://www.ietf.org/rfc/rfc2616.txt">RFC 2616</url>
      with a few exceptions, such as gateway
      and proxy functionality. The server supports IPv6 as long as the
      underlying mechanisms also do so.</p>

    <p>The server implements numerous features, such as:</p>
      <list type="bulleted">
       <item>Secure Sockets Layer (SSL)</item>
       <item>Erlang Scripting Interface (ESI)</item>
       <item>Common Gateway Interface (CGI)</item>
       <item>User Authentication (using <c>Mnesia</c>, 
       <c>Dets</c> or plain text database)</item>
       <item>Common Logfile Format (with or without disk_log(3) support)</item>
       <item>URL Aliasing</item>
       <item>Action Mappings</item>
       <item>Directory Listings</item>
      </list>

    <p>The configuration of the server is provided as an Erlang
      property list. For backwards compatibility, a configuration
      file using apache-style configuration directives is
      supported.</p>

    <p>As of <c>Inets</c> 5.0 the HTTP server is an easy to
      start/stop and customize web server providing the most basic
      web server functionality. Inets is designed for embedded systems
      and if you want a full-fledged web server there are exists other 
      erlang open source alternatives.</p>
    
    <p>Almost all server functionality has been implemented using an
      especially crafted server API, which is described in the Erlang Web
      Server API. This API can be used
      to enhance the core server functionality, for example with custom
      logging and authentication.</p>

    <p>The following is to be put in the Erlang node application configuration 
      file to start an HTTP server at application startup:</p>
    
    <code type="erl">
      [{inets, [{services, [{httpd, [{proplist_file,
                 "/var/tmp/server_root/conf/8888_props.conf"}]},
                {httpd, [{proplist_file,
                 "/var/tmp/server_root/conf/8080_props.conf"}]}]}]}].</code>

    <p>The server is configured using an Erlang property list.
      For the available properties, see
      <seealso marker="httpd">httpd(3)</seealso>.
      For backwards compatibility, apache-like configuration files
      are also supported.
    </p>
    
    <p>The available configuration properties are as follows:</p>
    <code type="none">
     httpd_service() -> {httpd, httpd()}
     httpd()         -> [httpd_config()] 
     httpd_config()  -> {file, file()} |
                        {proplist_file, file()}
                        {debug, debug()} |
                        {accept_timeout, integer()}
     debug()         -> disable | [debug_options()]
     debug_options() -> {all_functions, modules()} | 
                        {exported_functions, modules()} |
                        {disable, modules()}
     modules()       -> [atom()]</code>
     <p>Here:</p>
     <taglist>
       <tag><c>{file, file()}</c></tag>
       <item><p>If you use an old apace-like configuration file.</p></item>
       <tag><c>{proplist_file, file()}</c></tag>
       <item><p>File containing an Erlang property
      list, followed by a full stop, describing the HTTP server
      configuration.</p></item> 
       <tag><c>{debug, debug()}</c></tag>
       <item><p>Can enable trace on all functions or only exported functions 
       on chosen modules.</p></item>
       <tag><c>{accept_timeout, integer()}</c></tag>
       <item><p>Sets the wanted time-out value for
      the server to set up a request connection.</p></item>
     </taglist>
  </section>

  <section>
    <title>Getting Started</title>
    <marker id="using_http_server_api"></marker>
    <p>Start <c>Inets</c>:</p>
    <code type="none">
      1 > inets:start().
      ok</code>
    <p>Start an HTTP server with minimal required configuration.
       If you specify port <c>0</c>, an arbitrary available port is
       used, and you can use function <c>info</c> to find which port 
       number that was picked:</p>

    <code type="none">
      2 > {ok, Pid} = inets:start(httpd, [{port, 0},
      {server_name,"httpd_test"}, {server_root,"/tmp"},
      {document_root,"/tmp/htdocs"}, {bind_address, "localhost"}]).
      {ok, 0.79.0}      </code>
    
      <p>Call <c>info</c>:</p>
    <code type="none">
      3 >  httpd:info(Pid).
      [{mime_types,[{"html","text/html"},{"htm","text/html"}]},
      {server_name,"httpd_test"},
      {bind_address, {127,0,0,1}},
      {server_root,"/tmp"},
      {port,59408},
      {document_root,"/tmp/htdocs"}]</code>

    <p>Reload the configuration without restarting the server:
      
    </p>
    <code type="none">
    4 > httpd:reload_config([{port, 59408},
      {server_name,"httpd_test"}, {server_root,"/tmp/www_test"},
      {document_root,"/tmp/www_test/htdocs"},
      {bind_address, "localhost"}], non_disturbing).
    ok.</code>

    <note><p><c>port</c> and <c>bind_address</c> cannot be changed. 
      Clients trying to access the server during the reload
      get a service temporary unavailable answer.</p></note>

    <code type="none">
      5 >  httpd:info(Pid, [server_root, document_root]).
      [{server_root,"/tmp/www_test"},{document_root,"/tmp/www_test/htdocs"}] </code>

    <code type="none">
      6 > ok = inets:stop(httpd, Pid).</code>
    
    <p>Alternative:</p>
    
    <code type="none">
      6 > ok = inets:stop(httpd, {{127,0,0,1}, 59408}).</code>

    <p>Notice that <c>bind_address</c> must be the IP address reported 
    by function <c>info</c> and cannot be the hostname that is allowed 
    when putting in <c>bind_address</c>.</p>
  </section>

  <section>
    <title>Htaccess - User Configurable Authentication</title>
    <marker id="htaccess"></marker>
    <p>Web server users without server administrative privileges
        that need to manage authentication of web pages that are local 
	to their user can use the per-directory runtime configurable 
	user-authentication scheme <c>htaccess</c>. 
        It works as follows:</p>
      <list type="bulleted">
        <item>Each directory in the path to the requested asset is
         searched for an access file (default is <c>.htaccess</c>), which 
	 restricts the web servers rights to respond to a request. 
	 If an access file is found, the rules in that file is applied to the
         request.</item>
        <item>The rules in an access file apply to files in the same
         directory and in subdirectories. If there exists more than one
         access file in the path to an asset, the rules in the
         access file nearest the requested asset is applied.</item>
        <item>To change the rules that restrict the use of 
         an asset, the user only needs write access 
         to the directory where the asset is.</item>
        <item>All access files in the path to a requested asset are read
         once per request. This means that the load on the server
         increases when <c>htaccess</c> is used.</item>
        <item>If a directory is limited both by authentication directives 
	 in the HTTP server configuration file and by the <c>htaccess</c> 
	 files, the user must be allowed to get access to the file by both 
	 methods for the request to succeed.</item>
      </list>

    <section>
      <title>Access Files Directives</title>
        <p>In every directory under <c>DocumentRoot</c> or under an
	<c>Alias</c> a user can place an access file. An access file
	is a plain text file that specifies the restrictions to
	consider before the web server answers to a
          request. If there are more than one access file in the path
          to the requested asset, the directives in the access file in
          the directory nearest the asset is used.</p>
        <taglist>
	  <tag><em>"allow"</em></tag>
          <item>
	    <p><em>Syntax:</em> <c>Allow</c> from subnet <c>subnet | from all</c></p>
	    <p><em>Default:</em> <c>from all</c></p>
            <p>Same as directive <c>allow</c> for the server configuration file.</p>
          </item>
	  <tag><em>"AllowOverRide"</em></tag>
          <item>  
            <p><em>Syntax:</em> <c>AllowOverRide</c> <c>all | none | Directives</c></p>
	    <p><em>Default:</em> <c>none</c></p>
	    <p><c>AllowOverRide</c> specifies the parameters that
              access files in subdirectories are not allowed to alter the value 
              for. If the parameter is set to <c>none</c>, no further 
              access files is parsed.
              </p>
            <p>If only one access file exists, setting this parameter to
              <c>none</c> can ease the burden on the server as the server
              then stops looking for access files.</p>
          </item>
	  <tag><em>"AuthGroupfile"</em></tag>
          <item>
            <p><em>Syntax:</em> <c>AuthGroupFile</c> Filename</p>
	    <p><em>Default:</em> <c>none</c></p>
            <p><c>AuthGroupFile</c> indicates which file that contains the list
              of groups. The filename must contain the absolute path to the
              file. The format of the file is one group per row and
              every row contains the name of the group and the members
              of the group, separated by a space, for example:</p>
            <pre>
GroupName: Member1 Member2 .... MemberN</pre>
          </item>
	  <tag><em>"AuthName"</em></tag>
          <item>
            <p><em>Syntax:</em> <c>AuthName</c> auth-domain</p>
	    <p><em>Default:</em> <c>none</c></p>
            <p>Same as directive <c>AuthName</c> for the server 
	    configuration file.</p>
          </item>
	  <tag><em>"AuthType"</em></tag>
          <item>
            <p><em>Syntax:</em> <c>AuthType</c> <c>Basic</c></p>
	    <p><em>Default:</em> <c>Basic</c></p>
            <p><c>AuthType</c> specifies which authentication scheme to
              be used. Only Basic Authenticating using UUEncoding of
              the password and user ID is implemented.</p>
          </item>
	  <tag><em>"AuthUserFile"</em></tag>
          <item>
            <p><em>Syntax:</em> <c>AuthUserFile</c> Filename</p>
	    <p><em>Default:</em><c>none</c></p>
            <p><c>AuthUserFile</c> indicates which file that contains the list
              of users. The filename must contain the absolute path to the
              file. The username and password are not encrypted so do not
              place the file with users in a directory that is accessible
              through the web server. The format of the file is one user per row.
              Every row contains <c>UserName</c> and <c>Password</c> separated 
	      by a colon, for example:</p>
            <pre>
UserName:Password
UserName:Password</pre>
          </item>
	  <tag><em>"deny"</em></tag>
          <item>
            <p><em>Syntax:</em> <c>deny</c> from subnet <c>subnet | from all</c></p>
	    <p><em>Context:</em> Limit</p>
            <p>Same as directive <c>deny</c> for the server configuration file.</p>
          </item>
	  <tag><em>"Limit"</em></tag>
          <item> 
            <p><em>Syntax:</em> <c><![CDATA[<Limit]]></c> RequestMethods<c>></c></p>
	    <p><em>Default:</em> <c>none</c></p>
            <p><c><![CDATA[<Limit>]]></c> and <c>&lt;/Limit&gt;</c> are used to enclose
              a group of directives applying only to requests using
              the specified methods. If no request method is specified,
              all request methods are verified against the restrictions.</p>
	      <p>Example:</p>
            <pre>
&lt;Limit POST GET HEAD&gt;
  order allow deny
  require group group1
  allow from 123.145.244.5
&lt;/Limit&gt;</pre>
          </item>
	  <tag><em>"order"</em></tag>
          <item>   
	    <p><em>Syntax:</em> <c>order</c> <c>allow deny | deny allow</c></p>
	    <p><em>Default:</em> <c>allow deny</c></p>
            <p><c>order</c> defines if the deny or allow control is to
              be performed first.</p>
            <p>If the order is set to <c>allow deny</c>, the users
              network address is first controlled to be in the allow subset. 
	      If the user network address is not in the allowed subset, the user
              is denied to get the asset. If the network address is in the
              allowed subset, a second control is performed. That is,
              the user network address is not in the subset of network
              addresses to be denied as specified by parameter <c>deny</c>.</p>
            <p>If the order is set to <c>deny allow</c>, only users from networks
              specified to be in the allowed subset succeeds to request  
              assets in the limited area.</p>
          </item>
	  <tag><em>"require"</em></tag>
          <item> 
            <p><em>Syntax:</em> <c>require</c>
              <c>group group1 group2... | user user1 user2...</c></p>
	      <p><em>Default:</em> <c>none</c></p>
	      <p><em>Context:</em> Limit</p>
            <p>For more information, see directive <c>require</c>  in
	    <seealso marker="mod_auth">mod_auth(3)</seealso>.</p>
          </item>
        </taglist>
      </section>
    </section>
  
    <section>
      <title>Dynamic Web Pages</title>
      <marker id="dynamic_we_pages"></marker>
      <p><c>Inets</c> HTTP server provides two ways of creating dynamic web
        pages, each with its own advantages and disadvantages:</p>
	<taglist>
       <tag><em>CGI scripts</em></tag>
       <item><p>Common Gateway Interface (CGI) scripts can be written 
       in any programming language. CGI scripts are standardized and 
       supported by most web servers. The drawback with CGI scripts is that 
       they are resource-intensive because of their design. CGI requires the 
       server to fork a new OS process for each executable it needs to start.
       </p></item>
       <tag><em>ESI-functions</em></tag>
       <item><p>Erlang Server Interface (ESI) functions provide a tight and efficient
        interface to the execution of Erlang functions. This interface,
        on the other hand, is <c>Inets</c> specific.</p></item>
     </taglist>
      
      <section>
        <title>CGI Version 1.1, RFC 3875</title>
        <p>The module <c>mod_cgi</c> enables execution of 
	<url href="http://www.ietf.org/rfc/rfc3875.txt">CGI scripts</url>
          on the server. A file matching the definition of a
          ScriptAlias config directive is treated as a CGI script. A CGI
          script is executed by the server and its output is returned to
          the client.</p>
        <p>The CGI script response comprises a message header and a
          message body, separated by a blank line. The message header
          contains one or more header fields. The body can be
          empty.</p>
	 <p>Example:</p>
      
      <code>"Content-Type:text/plain\nAccept-Ranges:none\n\nsome very
	plain text"</code>
      
        <p>The server interprets the message headers and most of them
          are transformed into HTTP headers and sent back to the
          client together with the message-body.</p>
        <p>Support for CGI-1.1 is implemented in accordance with 
	<url href="http://www.ietf.org/rfc/rfc3875.txt">RFC 3875</url>.</p>
      </section>

      <section>
        <title>ESI</title>
        <p>The Erlang server interface is implemented by
          module <c>mod_esi</c>.</p>

        <section>
          <title>ERL Scheme</title>
          <p>The erl scheme is designed to mimic plain CGI, but without
            the extra overhead. An URL that calls an Erlang <c>erl</c> function
            has the following syntax (regular expression): </p>
          <code type="none">
http://your.server.org/***/Module[:/]Function(?QueryString|/PathInfo)</code>
          <p>*** depends on how the ErlScriptAlias config
            directive has been used.</p>
          <p>The module <c>Module</c> referred to must be found in the code
            path, and it must define a function <c>Function</c> with an arity
            of two or three. It is preferable to implement a function
            with arity three, as it permits to send chunks of the
            web page to the client during the generation
            phase instead of first generating the whole web page and
            then sending it to the client. The option to implement a
            function with arity two is only kept for
            backwards compatibility reasons.
            For implementation details of the ESI callback function, 
	    see <seealso marker="mod_esi">mod_esi(3)</seealso>.</p>
        </section>

        <section>
          <title>EVAL Scheme</title>
          <p>The eval scheme is straight-forward and does not mimic the
            behavior of plain CGI. An URL that calls an Erlang <c>eval</c>
            function has the following syntax:</p>
          <code type="none">
http://your.server.org/***/Mod:Func(Arg1,...,ArgN)</code>
          <p>*** depends on how the ErlScriptAlias config
            directive has been used.</p>
          <p>The module <c>Mod</c> referred to must be found in the code
            path and data returned by the function <c>Func</c> is passed
            back to the client. Data returned from the
            function must take the form as specified in
            the CGI specification. For implementation details of the ESI 
	    callback function, 
	    see <seealso marker="mod_esi">mod_esi(3)</seealso>.</p>
          <note>
            <p>The eval scheme can seriously threaten the
              integrity of the Erlang node housing a web server, for
              example:</p>
            <code type="none">
http://your.server.org/eval?httpd_example:print(atom_to_list(apply(erlang,halt,[])))</code>
            <p>This effectively closes down the Erlang node.
              Therefore, use the erl scheme instead, until this
              security breach is fixed.</p>
            <p>Today there are no good ways of solving this problem
              and therefore the eval scheme can be removed in future
              release of <c>Inets</c>.</p>
          </note>
        </section>
      </section>
    </section>

  <section>
    <title>Logging</title>
    <marker id="logging"></marker>
    <p>Three types of logs are supported: transfer logs,
      security logs, and error logs. The de-facto standard Common
        Logfile Format is used for the transfer and security logging.
      There are numerous statistics programs available to analyze Common
      Logfile Format. The Common Logfile Format looks as follows:
        </p>
    <p><em>remotehost rfc931 authuser [date] "request" status bytes</em></p>
    <p>Here:</p>
    <taglist>
      <tag><em>remotehost</em></tag>
      <item>Remote hostname.</item>
      <tag><em>rfc931</em></tag>
        <item>The client remote username (<url href="http://www.ietf.org/rfc/rfc931.txt">RFC 931</url>).</item>
      <tag><em>authuser</em></tag>
      <item>The username used for authentication.</item>
      <tag><em>[date]</em></tag>
        <item>Date and time of the request (<url href="http://www.ietf.org/rfc/rfc1123.txt">RFC 1123</url>).</item>
      <tag><em>"request"</em></tag>
        <item>The request line exactly as it came from the client (<url href="http://www.ietf.org/rfc/rfc1945.txt">RFC 1945</url>).</item>
        <tag><em>status</em></tag>
        <item>The HTTP status code returned to the client  (<url href="http://www.ietf.org/rfc/rfc1945.txt">RFC 1945</url>).</item>
        <tag><em>bytes</em></tag>
        <item>The content-length of the document transferred. </item>
      </taglist>
      <p>Internal server errors are recorded in the error log file. The
        format of this file is a more unplanned format than the logs using
      Common Logfile Format, but conforms to the following syntax:
        </p>
      <p><em>[date]</em> access to <em>path</em> failed for
        <em>remotehost</em>, reason: <em>reason</em></p>
  </section>
  
  <section>
      <title>Erlang Web Server API</title>
      <p>The process of handling an HTTP request involves several steps,
        such as:</p>
      <list type="bulleted">
        <item>Setting up connections, sending and receiving data.</item>
        <item>URI to filename translation.</item>
        <item>Authentication/access checks.</item>
        <item>Retrieving/generating the response.</item>
        <item>Logging.</item>
      </list>
      <p>To provide customization and extensibility of the request 
         handling of the HTTP servers, most of these steps are handled by 
	 one or more modules. These modules can be replaced or removed at 
	 runtime and new ones can be added. For each request, all modules are
         traversed in the order specified by the module directive in the
         server configuration file. Some parts, mainly the communication-
         related steps, are considered server core functionality and are
         not implemented using the Erlang web server API. A description of
         functionality implemented by the Erlang webserver API is described
         in <seealso marker="#Inets_Web_Server_Modules">Section 
	 Inets Web Server Modules</seealso>.</p>

      <p>A module can use data generated by previous modules in the
        Erlang webserver API module sequence or generate data to be used
        by consecutive Erlang Web Server API modules. This is
        possible owing to an internal list of key-value tuples, referred to
        as interaction data.</p>
      <note>
        <p>Interaction data enforces module dependencies and
          is to be avoided if possible. This means that the order
          of modules in the modules property is significant.</p>
      </note>

      <section>
        <title>API Description</title>
        <p>Each module that implements server functionality
          using the Erlang web server API is to implement the following
          call back functions:</p>
        <list type="bulleted">
          <item><c>do/1</c> (mandatory) - the function called when
           a request is to be handled</item>
          <item><c>load/2</c></item>
          <item><c>store/2</c></item>
          <item><c>remove/1</c></item>
        </list>
        <p>The latter functions are needed only when new config
          directives are to be introduced. For details, see
          <seealso marker="httpd">httpd(3)</seealso>.</p>
      </section>
    </section>

  <section>
    <title>Inets Web Server Modules</title>
    <marker id="Inets_Web_Server_Modules"></marker>
    <p>The convention is that
      all modules implementing some web server functionality has the
      name <c>mod_*</c>. When configuring the web server, an appropriate
      selection of these modules is to be present in the module
      directive. Notice that there are some interaction dependencies
      to take into account, so the order of the modules cannot be
      random.</p>

    <section>
      <title>mod_action - Filetype/Method-Based Script Execution</title>
      <p>This module runs CGI scripts whenever a file of a
	certain type or HTTP method (see 
	<url href="http://tools.ietf.org/html/rfc1945">RFC 1945</url>)
	is requested.
      </p>
      <p>Uses the following Erlang Web Server API interaction data:
      </p>
      <list type="bulleted">
	<item><c>real_name</c> - from <seealso marker="mod_alias">mod_alias</seealso>.</item>
      </list>
      <p>Exports the following Erlang Web Server API interaction data, if possible:
      </p>
      <taglist>
	<tag><c>{new_request_uri, RequestURI}</c></tag>
	<item>An alternative <c>RequestURI</c> has been generated.</item>
      </taglist>
    </section>

    <section>
      <title>mod_alias - URL Aliasing</title>
      <p>The <seealso marker="mod_alias">mod_alias</seealso>
      module makes it possible to map different parts of the
	host file system into the document tree, that is, creates aliases and
	redirections.</p>
        <p>Exports the following Erlang Web Server API interaction data, if possible:
      </p>
      <taglist>
	<tag><c>{real_name, PathData}</c></tag>
	<item><c>PathData</c> is the argument used for API function 
	<seealso marker="mod_alias#path/3">mod_alias:path/3</seealso>.</item>
      </taglist>
    </section>
    
    <section>
      <title>mod_auth - User Authentication</title>
        <p>The <seealso marker="mod_auth">mod_auth(3)</seealso> 
	module provides for basic user authentication using
	textual files, <c>Dets</c> databases as well as <c>Mnesia</c> databases.</p>
      <p>Uses the following Erlang Web Server API interaction data:
      </p>
      <list type="bulleted">
	<item><c>real_name</c> - from <seealso marker="mod_alias">mod_alias</seealso></item>
        </list>
      <p>Exports the following Erlang Web Server API interaction data:
      </p>
      <taglist>
	<tag><c>{remote_user, User}</c></tag>
          <item>The username used for authentication.</item>
      </taglist>
      
      
      <section>
	<title>Mnesia As Authentication Database</title>
	
	<p>If <c>Mnesia</c> is used as storage method, <c>Mnesia</c> must be
	  started before the HTTP server. The first time <c>Mnesia</c> is
	  started, the schema and the tables must be created before
	  <c>Mnesia</c> is started. A simple example of a module with two
	  functions that creates and start <c>Mnesia</c> is provided
	  here. Function <c>first_start/0</c> is to be used the first
	  time. It creates the schema and the tables.
	  <c>start/0</c> is to be used in consecutive startups. 
	  <c>start/0</c> starts <c>Mnesia</c> and waits for the tables to
	  be initiated. This function must only be used when the
	  schema and the tables are already created.</p>
	
	<code>
-module(mnesia_test).
-export([start/0,load_data/0]).
-include_lib("mod_auth.hrl").

first_start() ->
    mnesia:create_schema([node()]),
    mnesia:start(),
    mnesia:create_table(httpd_user,
                        [{type, bag},
                         {disc_copies, [node()]},
                         {attributes, record_info(fields, 
                                                  httpd_user)}]),
    mnesia:create_table(httpd_group,
                        [{type, bag},
                         {disc_copies, [node()]},          
                         {attributes, record_info(fields, 
                                                  httpd_group)}]),
    mnesia:wait_for_tables([httpd_user, httpd_group], 60000).

start() ->
    mnesia:start(),
    mnesia:wait_for_tables([httpd_user, httpd_group], 60000).  </code>
	
	<p>To create the <c>Mnesia</c> tables, we use two records defined in
	  <c>mod_auth.hrl</c>, so that file must be included. <c>first_start/0</c> 
	  creates a schema that specifies on which nodes the database is to reside. 
	  Then it starts <c>Mnesia</c> and creates the tables. The first argument 
	  is the name of the tables, the second argument is a list of options of 
	  how to create the table, see 
	  <seealso marker="mnesia:mnesia"><c>mnesia</c></seealso>, documentation for 
	  more information. As the implementation of the <c>mod_auth_mnesia</c> 
	  saves one row for each user, the type must be <c>bag</c>.
	  When the schema and the tables are created, function
	  <seealso marker="mnesia:mnesia#start-0">mnesia:start/0</seealso>
	  is used to start <c>Mnesia</c>  and 
	  waits for the tables to be loaded. <c>Mnesia</c> uses the
	  directory specified as <c>mnesia_dir</c> at startup if specified,
	  otherwise <c>Mnesia</c> uses the current directory. For security
	  reasons, ensure that the <c>Mnesia</c> tables are stored outside
	  the document tree of the HTTP server. If they are placed in the
	  directory which it protects, clients can download the tables. 
	  Only the <c>Dets</c> and <c>Mnesia</c> storage
	  methods allow writing of dynamic user data to disk. <c>plain</c> is
	  a read only method.</p>
      </section>

    </section>
    
    <section>
      <title>mod_cgi - CGI Scripts</title>
        <p>This module handles invoking of CGI scripts.</p>
    </section>
    
    <section>
      <title>mod_dir - Directories</title>
      <p>This module generates an HTML directory listing
	(Apache-style) if a client sends a request for a directory
	instead of a file. This module must be removed from the
	Modules config directive if directory listings is unwanted.</p>
      <p>Uses the following Erlang Web Server API interaction data:
      </p>
      <list type="bulleted">
	<item><c>real_name</c> - from <seealso marker="mod_alias">mod_alias</seealso></item>
      </list>
      <p>Exports the following Erlang Web Server API interaction data:
      </p>
      <taglist>
	<tag><c>{mime_type, MimeType}</c></tag>
	<item>The file suffix of the incoming URL mapped into a
          <c>MimeType</c>.</item>
        </taglist>
      </section>

    <section>
      <title>mod_disk_log - Logging Using Disk_Log.</title>
        <p>Standard logging using the "Common Logfile Format" and
	<seealso marker="kernel:disk_log">kernel:disk_log(3)</seealso>.</p>
        <p>Uses the following Erlang Web Server API interaction data:
      </p>
        <list type="bulleted">
	<item><c>remote_user</c> - from <c>mod_auth</c></item>
      </list>
    </section>

    <section>
      <title>mod_esi - Erlang Server Interface</title>
      <p>The <seealso marker="mod_esi">mod_esi(3)</seealso>
      module implements the Erlang Server Interface (ESI) providing a 
      tight and efficient interface to the execution of Erlang functions.</p>
      <p>Uses the following Erlang web server API interaction data:
      </p>
      <list type="bulleted">
	<item><c>remote_user</c> - from <c>mod_auth</c></item>
      </list>
      <p>Exports the following Erlang web server API interaction data:
          </p>
      <taglist>
	<tag><c>{mime_type, MimeType}</c></tag>
	<item>The file suffix of the incoming URL mapped into a
          <c>MimeType</c></item>
        </taglist>
      </section>
    
      <section>
      <title>mod_get - Regular GET Requests</title>
      <p>This module is responsible for handling GET requests to regular 
	files. GET requests for parts of files is handled by <c>mod_range</c>.</p>
        <p>Uses the following Erlang web server API interaction data:
      </p>
      <list type="bulleted">
          <item><c>real_name</c> - from <seealso marker="mod_alias">mod_alias</seealso></item>
      </list>
      </section>
    
    <section>
      <title>mod_head - Regular HEAD Requests</title>
        <p>This module is responsible for handling HEAD requests to regular 
	files. HEAD requests for dynamic content is handled by each module 
	responsible for dynamic content.</p>
        <p>Uses the following Erlang Web Server API interaction data:
      </p>
      <list type="bulleted">
	<item><c>real_name</c> - from <seealso marker="mod_alias">mod_alias</seealso></item>
        </list>
    </section>
    
    <section>
      <title>mod_htaccess - User Configurable Access</title>
      <p>This module provides per-directory user configurable access
          control.</p>
      <p>Uses the following Erlang Web Server API interaction data:
      </p>
      <list type="bulleted">
          <item><c>real_name</c> - from <seealso marker="mod_alias">mod_alias</seealso></item>
      </list>
      <p>Exports the following Erlang Web Server API interaction data:
      </p>
      <taglist>
	<tag><c>{remote_user_name, User}</c></tag>
	<item>The username used for authentication.</item>
        </taglist>
    </section>
    
    <section>
      <title>mod_log - Logging Using Text Files.</title>
      <p>Standard logging using the "Common Logfile Format" and text
	files.</p>
        <p>Uses the following Erlang Web Server API interaction data:
      </p>
      <list type="bulleted">
	<item><c>remote_user</c> - from <c>mod_auth</c></item>
        </list>
    </section>
    
      <section>
      <title>mod_range - Requests with Range Headers</title>
      <p>This module responses to requests for one or many ranges of a
	file. This is especially useful when downloading large files,
	as a broken download can be resumed.</p>
      <p>Notice that request for multiple parts of a document report a
	size of zero to the log file.</p>
        <p>Uses the following Erlang Web Server API interaction data:
      </p>
      <list type="bulleted">
	<item><c>real_name</c> - from <seealso marker="mod_alias">mod_alias</seealso></item>
      </list>
    </section>

    <section>
      <title>mod_response_control - Requests with If* Headers</title>
      <p>This module controls that the conditions in the requests are
	fulfilled. For example, a request can specify that the answer
	only is of interest if the content is unchanged since the last
	retrieval. If the content is changed, the range request is to
	be converted to a request for the whole file instead.</p> 
	<p>If a client sends more than one of the header fields that 
	restricts the servers right to respond, the standard does not 
	specify how this is to be handled.
	<seealso marker="httpd">httpd(3)</seealso> controls each 
	field in the following order and if one of the fields does not 
	match the current state, the request is rejected with a proper 
	response:</p>
	<p><c>If-modified</c></p>
	<p><c>If-Unmodified</c></p>
	<p><c>If-Match</c></p>
	<p><c>If-Nomatch</c></p>
	
      <p>Uses the following Erlang Web Server API interaction data:
      </p>
      <list type="bulleted">
	<item><c>real_name</c> - from  <seealso marker="mod_alias">mod_alias</seealso></item>
      </list>
      <p>Exports the following Erlang Web Server API interaction data:
      </p>
      <taglist>
	<tag><c>{if_range, send_file}</c></tag>
	<item>The conditions for the range request are not fulfilled.
	  The response must not be treated as a range request, instead it
	  must be treated as an ordinary get request.</item>
        </taglist>
    </section>
    
    <section>
      <title>mod_security - Security Filter</title>
        <p>The <seealso marker="mod_security">mod_security</seealso> 
	module serves as a filter for authenticated requests
	handled in <seealso marker="mod_auth">mod_auth(3)</seealso>. 
	It provides a possibility to restrict users from 
	access for a specified amount of time if they fail to
	authenticate several times. It logs failed authentication as
	well as blocking of users, and it calls a configurable
	callback module when the events occur.</p>
      <p>There is also an
	API to block or unblock users manually. This API can also list 
	blocked users or users who have been authenticated within a 
	configurable amount of time.</p>
    </section>
    
    <section>
      <title>mod_trace - TRACE Request</title>
      <p><c>mod_trace</c> is responsible for handling of TRACE requests.
	Trace is a new request method in HTTP/1.1. The intended use of
	trace requests is for testing. The body of the trace response is
	the request message that the responding web server or proxy
	received.</p>
    </section>
  </section>
</chapter>