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    1 <?xml version='1.0' encoding='UTF-8' ?>
    2 <!DOCTYPE manualpage SYSTEM "../style/manualpage.dtd">
    3 <?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
    4 <!-- $LastChangedRevision: 420990 $ -->
    6 <!--
    7  Licensed to the Apache Software Foundation (ASF) under one or more
    8  contributor license agreements.  See the NOTICE file distributed with
    9  this work for additional information regarding copyright ownership.
   10  The ASF licenses this file to You under the Apache License, Version 2.0
   11  (the "License"); you may not use this file except in compliance with
   12  the License.  You may obtain a copy of the License at
   14      http://www.apache.org/licenses/LICENSE-2.0
   16  Unless required by applicable law or agreed to in writing, software
   17  distributed under the License is distributed on an "AS IS" BASIS,
   18  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   19  See the License for the specific language governing permissions and
   20  limitations under the License.
   21 -->
   23 <!-- 
   24  Original Copyright (c) 2005 Covalent Technologies
   26  FTP Protocol module for Apache 2.0
   27 -->
   29 <manualpage metafile="ftp_intro.xml.meta">
   30 <parentdocument href="../"/>
   32 <title>Introduction to the FTP Protocol</title>
   34 <summary>
   36 <p>The <strong>File Transfer Protocol (FTP)</strong> is a classic
   37 standard for transfer of files and records over a TCP/IP network.  It
   38 was defined by Jon Postel and Joyce Reynolds in <a
   39 href="http://www.rfc-archive.org/getrfc.php?rfc=959">RFC 959</a>,
   40 which was released in 1985.  This means that FTP precedes the HTTP
   41 protocol that is usually associated with the Apache server by more
   42 than half a decade.  The <code>mod_ftp</code> module brings support
   43 for FTP to the Apache server and includes several updates to the
   44 original protocol.  Most notably, <code>mod_ftp</code> implements FTP
   45 over Transport Layer Security (TLS) as described in <a
   46 href="http://www.rfc-archive.org/getrfc.php?rfc=4217">RFC
   47 4217</a>.</p>
   49 <p>On this manual page, a brief technical overview of the FTP protocol
   50 is provided, followed by a discussion of the FTP implementation by
   51 <code>mod_ftp</code>. </p>
   53 </summary>
   55 <seealso><a href="http://www.rfc-archive.org/getrfc.php?rfc=959">RFC
   56 959 &#8212; FILE TRANSFER PROTOCOL (FTP)</a></seealso>
   57 <seealso><a href="http://www.rfc-archive.org/getrfc.php?rfc=1579">RFC
   58 1579 &#8212; Firewall-Friendly FTP</a></seealso>
   59 <seealso><a href="http://www.rfc-archive.org/getrfc.php?rfc=4217">RFC
   60 4217 &#8212; Securing FTP with TLS</a></seealso> 
   61 <seealso><module>mod_ssl</module></seealso>
   62 <seealso><a href="../howto/auth.html">Authentication, Authorization
   63 and Access Control</a></seealso>
   66 <section id="protocol_overview">
   68 <title>Overview of the FTP Protocol</title> 
   70 <p>The File Transfer Protocol (FTP) is designed to facilitate
   71 bi-directional transfer of files and records between hosts on a TCP/IP
   72 network.  Unlike HTTP, the FTP protocol is stateful: the client
   73 establishes a <em>Control Connection</em> for the duration of an FTP
   74 session that typically spans multiple data transfers.</p>
   76 <p>FTP uses a separate TCP connection for data transfer.  Commands are
   77 issued and acknowledged over the <em>Control Connection</em>, a TCP
   78 connection to well-known port 21.  If the user issues a command that
   79 requires a response more elaborate than a one-line response code, a
   80 <em>Data Connection</em> is established between the client and the
   81 server.  The response data&#8212;the contents of a file or a
   82 directory listing&#8212;is sent over that data connection.</p>
   84 <p>Historically, the data connection was established from the server
   85 back to the client.  The client would bind to an arbitrary port, and
   86 then transmit its IP address and the port number to the server using
   87 the <em>PORT</em> command.  The server then set up a data connection
   88 to that port on the client host, whereupon the client issues the data
   89 transfer command.  This approach is referred to as <em>Active</em> FTP
   90 (since the server acts to set up the data connection).  Unfortunately,
   91 active FTP does not work well with firewalls and Network Address
   92 Translation (NAT) because incoming connections are often blocked.  In
   93 the case of NAT, the client only instructs the server to connect to
   94 its internal, non-routable IP address.  Some firewalls and NAT routers
   95 support the FTP protocol, but this support is not universal.  In cases
   96 where FTP is supported, these devices can rewrite the PORT command and
   97 establish ad-hoc access rules for FTP data connections.</p>
   99 <p>Because of these limitations, an alternative approach was developed
  100 in which the direction of the data connection is reversed.  This is
  101 known as <em>Passive</em> FTP. Before starting a data transfer, the
  102 client issues a <em>PASV</em> command.  The server binds to an
  103 arbitrary port number and transmits its IP address and that port
  104 number back to the client.  The client then sets up a data connection
  105 to this address and port on the server, and issues the data transfer
  106 command.  Passive FTP is more firewall-friendly than Active FTP,
  107 because client-side firewalls are typically more lenient on outgoing
  108 connections than inbound ones.</p>
  110 <p>While it is possible for FTP to support unauthenticated sessions,
  111 in practice all sessions are authenticated.  Typically, FTP servers
  112 authenticate against the user database of the server on which they
  113 run.  To facilitate downloads by the general public, FTP servers
  114 generally support a special username (by convention <em>"anonymous"</em>
  115 or <em>"ftp"</em>) to provide read-only access.  Users are asked (but
  116 often not required) to provide their e-mail address as response to the
  117 <code>Password</code> prompt.
  118 </p>
  120 <p>For more information on the basic functionality of the FTP protocol
  121 please refer to <a
  122 href="http://www.rfc-archive.org/getrfc.php?rfc=959">RFC 959</a> or <a
  123 href="http://en.wikipedia.org/wiki/FTP">Wikipedia</a>.</p>
  125 </section>
  127 <!--
  128 <section id="protocol_extensions">
  130 <title>FTP Protocol Extensions</title>
  134 </section>
  136 <section id="mod_ftp_implementation">
  137 <title>Implementation of the FTP Protocol by <code>mod_ftp</code></title>
  138 </section>
  139 -->
  141 </manualpage>