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368 lines
15 KiB
XML
368 lines
15 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<article id="FTP">
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<!--$Id$-->
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<articleinfo>
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<title>Shorewall and FTP</title>
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<authorgroup>
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<author>
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<firstname>Tom</firstname>
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<surname>Eastep</surname>
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</author>
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</authorgroup>
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<pubdate>2006-01-27</pubdate>
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<copyright>
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<year>2003</year>
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<year>2004</year>
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<year>2005</year>
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<year>2006</year>
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<holder>Thomas M. Eastep</holder>
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</copyright>
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<legalnotice>
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<para>Permission is granted to copy, distribute and/or modify this
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document under the terms of the GNU Free Documentation License, Version
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1.2 or any later version published by the Free Software Foundation; with
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no Invariant Sections, with no Front-Cover, and with no Back-Cover
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Texts. A copy of the license is included in the section entitled
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<quote><ulink url="GnuCopyright.htm">GNU Free Documentation
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License</ulink></quote>.</para>
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</legalnotice>
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</articleinfo>
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<caution>
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<para><emphasis role="bold">This article applies to Shorewall 3.0 and
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later. If you are running a version of Shorewall earlier than Shorewall
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3.0.0 then please see the documentation for that
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release.</emphasis></para>
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</caution>
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<section>
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<title>FTP Protocol</title>
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<para>FTP transfers involve two TCP connections. The first <emphasis
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role="bold">control</emphasis> connection goes from the FTP client to port
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21 on the FTP server. This connection is used for logon and to send
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commands and responses between the endpoints. Data transfers (including
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the output of <quote>ls</quote> and <quote>dir</quote> commands) requires
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a second data connection. The <emphasis role="bold">data</emphasis>
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connection is dependent on the <emphasis role="bold">mode</emphasis> that
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the client is operating in:</para>
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<variablelist>
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<varlistentry>
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<term>Passive Mode</term>
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<listitem>
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<para>(often the default for web browsers) -- The client issues a
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PASV command. Upon receipt of this command, the server listens on a
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dynamically-allocated port then sends a PASV reply to the client.
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The PASV reply gives the IP address and port number that the server
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is listening on. The client then opens a second connection to that
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IP address and port number.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>Active Mode</term>
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<listitem>
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<para>(often the default for line-mode clients) -- The client
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listens on a dynamically-allocated port then sends a PORT command to
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the server. The PORT command gives the IP address and port number
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that the client is listening on. The server then opens a connection
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to that IP address and port number; the <emphasis role="bold">source
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port</emphasis> for this connection is 20 (ftp-data in
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/etc/services).</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>You can see these commands in action using your linux ftp
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command-line client in debugging mode. Note that my ftp client defaults to
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passive mode and that I can toggle between passive and active mode by
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issuing a <quote>passive</quote> command:</para>
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<programlisting>[teastep@wookie Shorewall]$ <emphasis role="bold">ftp ftp1.shorewall.net</emphasis>
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Connected to lists.shorewall.net.
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220-=(<*>)=-.:. (( Welcome to PureFTPd 1.0.12 )) .:.-=(<*>)=-
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220-You are user number 1 of 50 allowed.
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220-Local time is now 10:21 and the load is 0.14. Server port: 21.
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220 You will be disconnected after 15 minutes of inactivity.
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500 Security extensions not implemented
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500 Security extensions not implemented
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KERBEROS_V4 rejected as an authentication type
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Name (ftp1.shorewall.net:teastep): <command>ftp</command>
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331-Welcome to ftp.shorewall.net
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331-
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331 Any password will work
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Password:
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230 Any password will work
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Remote system type is UNIX.
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Using binary mode to transfer files.
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ftp> <emphasis role="bold">debug</emphasis>
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Debugging on (debug=1).
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ftp> <emphasis role="bold">ls</emphasis>
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---> <emphasis>PASV</emphasis>
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<emphasis>227 Entering Passive Mode (192,168,1,193,195,210)</emphasis>
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---> LIST
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150 Accepted data connection
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drwxr-xr-x 5 0 0 4096 Nov 9 2002 archives
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drwxr-xr-x 2 0 0 4096 Feb 12 2002 etc
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drwxr-sr-x 6 0 50 4096 Feb 19 15:24 pub
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226-Options: -l
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226 3 matches total
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ftp> <emphasis role="bold">passive</emphasis>
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Passive mode off.
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ftp> <emphasis role="bold">ls</emphasis>
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<emphasis>---> PORT 192,168,1,3,142,58</emphasis>
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200 PORT command successful
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---> LIST
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150 Connecting to port 36410
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drwxr-xr-x 5 0 0 4096 Nov 9 2002 archives
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drwxr-xr-x 2 0 0 4096 Feb 12 2002 etc
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drwxr-sr-x 6 0 50 4096 Feb 19 15:24 pub
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226-Options: -l
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226 3 matches total
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ftp></programlisting>
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<para>Things to notice:</para>
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<orderedlist>
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<listitem>
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<para>The commands that I issued are <emphasis role="bold">strongly
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emphasized</emphasis>.</para>
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</listitem>
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<listitem>
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<para>Commands sent by the client to the server are preceded by
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---></para>
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</listitem>
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<listitem>
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<para>Command responses from the server over the control connection
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are numbered.</para>
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</listitem>
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<listitem>
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<para>FTP uses a comma as a separator between the bytes of the IP
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address; and</para>
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</listitem>
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<listitem>
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<para>When sending a port number, FTP sends the MSB then the LSB and
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separates the two bytes by a comma. As shown in the PORT command, port
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142,58 translates to 142*256+58 = 36410.</para>
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</listitem>
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</orderedlist>
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</section>
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<section>
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<title>Linux FTP connection-tracking</title>
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<para>Given the normal loc->net policy of ACCEPT, passive mode access
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from local clients to remote servers will always work but active mode
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requires the firewall to dynamically open a <quote>hole</quote> for the
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server's connection back to the client. Similarly, if you are running an
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FTP server in your local zone then active mode should always work but
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passive mode requires the firewall to dynamically open a
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<quote>hole</quote> for the client's second connection to the server. This
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is the role of FTP connection-tracking support in the Linux kernel.</para>
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<para>Where any form of NAT (SNAT, DNAT, Masquerading) on your firewall is
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involved, the PORT commands and PASV responses may also need to be
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modified by the firewall. This is the job of the FTP nat support kernel
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function.</para>
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<para>Including FTP connection-tracking and NAT support normally means
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that the modules <quote>ip_conntrack_ftp</quote> and
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<quote>ip_nat_ftp</quote> need to be loaded. Shorewall automatically loads
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these <quote>helper</quote> modules from
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/lib/modules/<<emphasis>kernel-version</emphasis>>/kernel/net/ipv4/netfilter/
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and you can determine if they are loaded using the <quote>lsmod</quote>
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command. The <<emphasis>kernel-version</emphasis>> may be obtained
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by typing</para>
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<programlisting><command>uname -r</command></programlisting>
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<example>
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<title></title>
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<programlisting>[root@lists etc]# lsmod
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Module Size Used by Not tainted
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autofs 12148 0 (autoclean) (unused)
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ipt_TOS 1560 12 (autoclean)
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ipt_LOG 4120 5 (autoclean)
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ipt_REDIRECT 1304 1 (autoclean)
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ipt_REJECT 3736 4 (autoclean)
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ipt_state 1048 13 (autoclean)
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ip_nat_irc 3152 0 (unused)
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<emphasis role="bold">ip_nat_ftp 3888 0 (unused)</emphasis>
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ip_conntrack_irc 3984 1
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<emphasis role="bold">ip_conntrack_ftp 5008 1</emphasis>
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ipt_multiport 1144 2 (autoclean)
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ipt_conntrack 1592 0 (autoclean)
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iptable_filter 2316 1 (autoclean)
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iptable_mangle 2680 1 (autoclean)
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iptable_nat 20568 3 (autoclean) [ipt_REDIRECT ip_nat_irc ip_nat_ftp]
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ip_conntrack 26088 5 (autoclean) [ipt_REDIRECT ipt_state ip_nat_irc
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ip_nat_ftp ip_conntrack_irc ip_conntrack_ftp
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ipt_conntrack iptable_nat]
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ip_tables 14488 12 [ipt_TOS ipt_LOG ipt_REDIRECT ipt_REJECT ipt_state
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ipt_multiport ipt_conntrack iptable_filter
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iptable_mangle iptable_nat]
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tulip 42464 0 (unused)
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e100 50596 1
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keybdev 2752 0 (unused)
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mousedev 5236 0 (unused)
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hid 20868 0 (unused)
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input 5632 0 [keybdev mousedev hid]
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usb-uhci 24684 0 (unused)
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usbcore 73280 1 [hid usb-uhci]
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ext3 64704 2
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jbd 47860 2 [ext3]
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[root@lists etc]#</programlisting>
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</example>
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<para>If you want Shorewall to load these modules from an alternate
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directory, you need to set the MODULESDIR variable in
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/etc/shorewall/shorewall.conf to point to that directory.</para>
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</section>
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<section>
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<title>FTP on Non-standard Ports</title>
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<para>The above discussion about commands and responses makes it clear
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that the FTP connection-tracking and NAT helpers must scan the traffic on
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the control connection looking for PASV and PORT commands as well as PASV
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responses. If you run an FTP server on a nonstandard port or you need to
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access such a server, you must therefore let the helpers know by
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specifying the port in /etc/shorewall/modules entries for the helpers.
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<caution>
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<para>You must have modularized FTP connection tracking support in
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order to use FTP on a non-standard port.</para>
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</caution></para>
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<example>
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<title>if you run an FTP server that listens on port 49 or you need to
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access a server on the internet that listens on that port then you would
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have:</title>
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<programlisting>loadmodule ip_conntrack_ftp ports=21,49
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loadmodule ip_nat_ftp ports=21,49 # NOTE: This is not necessary with kernel 2.6.11 and later!</programlisting>
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<para><note>
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<para>you MUST include port 21 in the ports list or you may have
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problems accessing regular FTP servers.</para>
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</note></para>
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<para>If there is a possibility that these modules might be loaded
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before Shorewall starts, then you should include the port list in
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/etc/modules.conf:</para>
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<programlisting>options ip_conntrack_ftp ports=21,49
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options ip_nat_ftp ports=21,49 # NOTE: This is not necessary with kernel 2.6.11 and later!</programlisting>
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<para><important>
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<para>Once you have made these changes to /etc/shorewall/modules
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and/or /etc/modules.conf, you must either:</para>
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<orderedlist>
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<listitem>
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<para>Unload the modules and restart shorewall:</para>
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<programlisting><command>rmmod ip_nat_ftp; rmmod ip_conntrack_ftp; shorewall restart</command></programlisting>
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</listitem>
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<listitem>
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<para>Reboot</para>
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</listitem>
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</orderedlist>
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</important></para>
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</example>
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</section>
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<section id="Rules">
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<title>Rules</title>
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<warning>
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<para>If you run an FTP server behind your firewall and your server
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offers a method of specifying the external IP address of your firewall,
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DON'T USE THAT FEATURE OF YOUR SERVER. Using that option will defeat the
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purpose of the ftp helper modules and can result in a server that
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doesn't work.</para>
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</warning>
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<para>If the policy from the source zone to the destination zone is ACCEPT
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and you don't need DNAT (see <ulink url="FAQ.htm#faq30">FAQ 30</ulink>)
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then <emphasis role="bold">you need no rule</emphasis>.</para>
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<para>Otherwise, for FTP you need exactly <emphasis
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role="bold">one</emphasis> rule:</para>
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<programlisting>#ACTION SOURCE DESTINATION PROTO PORT(S) SOURCE ORIGINAL
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# PORT(S) DESTINATION
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ACCEPT or <<emphasis>source</emphasis>> <<emphasis>destination</emphasis>> tcp 21 - <external IP addr> if
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DNAT ACTION = DNAT</programlisting>
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<para>You need an entry in the ORIGINAL DESTINATION column only if the
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ACTION is DNAT, you have multiple external IP addresses and you want a
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specific IP address to be forwarded to your server.</para>
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<para>Note that you do <emphasis role="bold">NOT </emphasis>need a rule
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with 20 (ftp-data) in the PORT(S) column. If you post your rules on the
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mailing list and they show 20 in the PORT(S) column, I will know that you
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haven't read this article and I will either ignore your post or tell you
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to RTFM.</para>
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<para>Shorewall includes an FTP macro that simplifies creation of FTP
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rules. The macro source is in
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<filename>/usr/share/shorewall/macro.FTP</filename>. Using the macro is
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the preferred way to generate the rules described above. Here are a couple
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of examples.</para>
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<para><example>
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<title>Server running behind a Masquerading Gateway</title>
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<para>Suppose that you run an FTP server on 192.168.1.5 in your local
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zone using the standard port (21). You need this rule:</para>
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<programlisting>#ACTION SOURCE DESTINATION PROTO PORT(S) SOURCE ORIGINAL
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# PORT(S) DESTINATION
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FTP/DNAT net loc:192.168.1.5</programlisting>
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</example><example>
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<title>Allow your DMZ FTP access to the Internet</title>
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<programlisting>#ACTION SOURCE DESTINATION PROTO PORT(S) SOURCE ORIGINAL
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# PORT(S) DESTINATION
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FTP/ACCEPT dmz net</programlisting>
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</example></para>
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<para>Note that the FTP connection tracking in the kernel cannot handle
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cases where a PORT command (or PASV reply) is broken across two packets or
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is misssing the ending <cr>/<lf>. When such cases occur, you
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will see a console message similar to this one:</para>
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<programlisting>Apr 28 23:55:09 gateway kernel: conntrack_ftp: partial PORT 715014972+1</programlisting>
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<para>I see this problem occasionally with the FTP server in my DMZ. My
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solution is to add the following rule:</para>
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<programlisting>#ACTION SOURCE DESTINATION PROTO PORT(S) SOURCE ORIGINAL
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# PORT(S) DESTINATION
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ACCEPT:info dmz net tcp - 20</programlisting>
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<para>The above rule accepts and logs all active mode connections from my
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DMZ to the net.</para>
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</section>
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</article> |