forked from extern/shorewall_code
5c9c9d97fd
git-svn-id: https://shorewall.svn.sourceforge.net/svnroot/shorewall/trunk@1058 fbd18981-670d-0410-9b5c-8dc0c1a9a2bb
349 lines
14 KiB
XML
349 lines
14 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
|
|
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
|
|
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
|
|
<article id="FTP">
|
|
<!--$Id$-->
|
|
|
|
<articleinfo>
|
|
<title>Shorewall and FTP</title>
|
|
|
|
<authorgroup>
|
|
<author>
|
|
<firstname>Tom</firstname>
|
|
|
|
<surname>Eastep</surname>
|
|
</author>
|
|
</authorgroup>
|
|
|
|
<pubdate>2003-12-01</pubdate>
|
|
|
|
<copyright>
|
|
<year>2003</year>
|
|
|
|
<holder>Thomas M. Eastep</holder>
|
|
</copyright>
|
|
|
|
<legalnotice>
|
|
<para>Permission is granted to copy, distribute and/or modify this
|
|
document under the terms of the GNU Free Documentation License, Version
|
|
1.2 or any later version published by the Free Software Foundation; with
|
|
no Invariant Sections, with no Front-Cover, and with no Back-Cover
|
|
Texts. A copy of the license is included in the section entitled
|
|
<quote><ulink url="GnuCopyright.htm">GNU Free Documentation License</ulink></quote>.</para>
|
|
</legalnotice>
|
|
</articleinfo>
|
|
|
|
<important>
|
|
<para>If you are running Mandrake 9.1 or 9.2 and are having problems with
|
|
FTP, you have three choices:</para>
|
|
|
|
<orderedlist>
|
|
<listitem>
|
|
<para>Edit /usr/share/shorewall/firewall and replace this line:</para>
|
|
|
|
<programlisting>for suffix in o gz ko ; do</programlisting>
|
|
|
|
<para>with</para>
|
|
|
|
<programlisting>for suffix in o gz ko o.gz ; do</programlisting>
|
|
|
|
<para>and at a root shell prompt:</para>
|
|
|
|
<programlisting><command>shorewall restart</command></programlisting>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>Install the Mandrake <quote>cooker</quote> version of Shorewall.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>Upgrade to Shorewall 1.4.7 or later.</para>
|
|
</listitem>
|
|
</orderedlist>
|
|
</important>
|
|
|
|
<section>
|
|
<title>FTP Protocol</title>
|
|
|
|
<para>FTP transfers involve two TCP connections. The first <emphasis
|
|
role="bold">control</emphasis> connection goes from the FTP client to port
|
|
21 on the FTP server. This connection is used for logon and to send
|
|
commands and responses between the endpoints. Data transfers (including
|
|
the output of <quote>ls</quote> and <quote>dir</quote> commands) requires
|
|
a second data connection. The <emphasis role="bold">data</emphasis>
|
|
connection is dependent on the <emphasis role="bold">mode</emphasis> that
|
|
the client is operating in:</para>
|
|
|
|
<variablelist>
|
|
<varlistentry>
|
|
<term>Passive Mode</term>
|
|
|
|
<listitem>
|
|
<para>(often the default for web browsers) -- The client issues a
|
|
PASV command. Upon receipt of this command, the server listens on a
|
|
dynamically-allocated port then sends a PASV reply to the client.
|
|
The PASV reply gives the IP address and port number that the server
|
|
is listening on. The client then opens a second connection to that
|
|
IP address and port number.</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term>Active Mode</term>
|
|
|
|
<listitem>
|
|
<para>(often the default for line-mode clients) -- The client
|
|
listens on a dynamically-allocated port then sends a PORT command to
|
|
the server. The PORT command gives the IP address and port number
|
|
that the client is listening on. The server then opens a connection
|
|
to that IP address and port number; the <emphasis role="bold">source
|
|
port</emphasis> for this connection is 20 (ftp-data in
|
|
/etc/services).</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
</variablelist>
|
|
|
|
<para>You can see these commands in action using your linux ftp
|
|
command-line client in debugging mode. Note that my ftp client defaults to
|
|
passive mode and that I can toggle between passive and active mode by
|
|
issuing a <quote>passive</quote> command:</para>
|
|
|
|
<programlisting>[teastep@wookie Shorewall]$ <emphasis role="bold">ftp ftp1.shorewall.net</emphasis>
|
|
Connected to lists.shorewall.net.
|
|
220-=(<*>)=-.:. (( Welcome to PureFTPd 1.0.12 )) .:.-=(<*>)=-
|
|
220-You are user number 1 of 50 allowed.
|
|
220-Local time is now 10:21 and the load is 0.14. Server port: 21.
|
|
220 You will be disconnected after 15 minutes of inactivity.
|
|
500 Security extensions not implemented
|
|
500 Security extensions not implemented
|
|
KERBEROS_V4 rejected as an authentication type
|
|
Name (ftp1.shorewall.net:teastep): ftp
|
|
331-Welcome to ftp.shorewall.net
|
|
331-
|
|
331 Any password will work
|
|
Password:
|
|
230 Any password will work
|
|
Remote system type is UNIX.
|
|
Using binary mode to transfer files.
|
|
ftp> <emphasis role="bold">debug</emphasis>
|
|
Debugging on (debug=1).
|
|
ftp> <emphasis role="bold">ls</emphasis>
|
|
---> <emphasis>PASV</emphasis>
|
|
<emphasis>227 Entering Passive Mode (192,168,1,193,195,210)</emphasis>
|
|
---> LIST
|
|
150 Accepted data connection
|
|
drwxr-xr-x 5 0 0 4096 Nov 9 2002 archives
|
|
drwxr-xr-x 2 0 0 4096 Feb 12 2002 etc
|
|
drwxr-sr-x 6 0 50 4096 Feb 19 15:24 pub
|
|
226-Options: -l
|
|
226 3 matches total
|
|
ftp> <emphasis role="bold">passive</emphasis>
|
|
Passive mode off.
|
|
ftp> <emphasis role="bold">ls</emphasis>
|
|
<emphasis>---> PORT 192,168,1,3,142,58</emphasis>
|
|
200 PORT command successful
|
|
---> LIST
|
|
150 Connecting to port 36410
|
|
drwxr-xr-x 5 0 0 4096 Nov 9 2002 archives
|
|
drwxr-xr-x 2 0 0 4096 Feb 12 2002 etc
|
|
drwxr-sr-x 6 0 50 4096 Feb 19 15:24 pub
|
|
226-Options: -l
|
|
226 3 matches total
|
|
ftp></programlisting>
|
|
|
|
<para>Things to notice:</para>
|
|
|
|
<orderedlist>
|
|
<listitem>
|
|
<para>The commands that I issued are <emphasis role="bold">strongly
|
|
emphasized</emphasis>.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>Commands sent by the client to the server are preceded by
|
|
---></para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>Command responses from the server over the control connection
|
|
are numbered.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>FTP uses a comma as a separator between the bytes of the IP
|
|
address; and</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>When sending a port number, FTP sends the MSB then the LSB and
|
|
separates the two bytes by a comma. As shown in the PORT command, port
|
|
142,58 translates to 142*256+58 = 36410.</para>
|
|
</listitem>
|
|
</orderedlist>
|
|
</section>
|
|
|
|
<section>
|
|
<title>Linux FTP connection-tracking</title>
|
|
|
|
<para>Given the normal loc->net policy of ACCEPT, passive mode access
|
|
from local clients to remote servers will always work but active mode
|
|
requires the firewall to dynamically open a <quote>hole</quote> for the
|
|
server's connection back to the client. Similarly, if you are running
|
|
an FTP server in your local zone then active mode should always work but
|
|
passive mode requires the firewall to dynamically open a <quote>hole</quote>
|
|
for the client's second connection to the server. This is the role of
|
|
FTP connection-tracking support in the Linux kernel.</para>
|
|
|
|
<para>Where any form of NAT (SNAT, DNAT, Masquerading) on your firewall is
|
|
involved, the PORT commands and PASV responses may also need to be
|
|
modified by the firewall. This is the job of the FTP nat support kernel
|
|
function.</para>
|
|
|
|
<para>Including FTP connection-tracking and NAT support normally means
|
|
that the modules <quote>ip_conntrack_ftp</quote> and <quote>ip_nat_ftp</quote>
|
|
need to be loaded. Shorewall automatically loads these <quote>helper</quote>
|
|
modules from /lib/modules/<<emphasis>kernel-version</emphasis>>/kernel/net/ipv4/netfilter/
|
|
and you can determine if they are loaded using the <quote>lsmod</quote>
|
|
command. The <<emphasis>kernel-version</emphasis>> may be obtained
|
|
by typing</para>
|
|
|
|
<programlisting><command>uname -r</command></programlisting>
|
|
|
|
<example>
|
|
<title></title>
|
|
|
|
<programlisting>[root@lists etc]# lsmod
|
|
Module Size Used by Not tainted
|
|
autofs 12148 0 (autoclean) (unused)
|
|
ipt_TOS 1560 12 (autoclean)
|
|
ipt_LOG 4120 5 (autoclean)
|
|
ipt_REDIRECT 1304 1 (autoclean)
|
|
ipt_REJECT 3736 4 (autoclean)
|
|
ipt_state 1048 13 (autoclean)
|
|
ip_nat_irc 3152 0 (unused)
|
|
<emphasis role="bold">ip_nat_ftp 3888 0 (unused)</emphasis>
|
|
ip_conntrack_irc 3984 1
|
|
<emphasis role="bold">ip_conntrack_ftp 5008 1</emphasis>
|
|
ipt_multiport 1144 2 (autoclean)
|
|
ipt_conntrack 1592 0 (autoclean)
|
|
iptable_filter 2316 1 (autoclean)
|
|
iptable_mangle 2680 1 (autoclean)
|
|
iptable_nat 20568 3 (autoclean) [ipt_REDIRECT ip_nat_irc ip_nat_ftp]
|
|
ip_conntrack 26088 5 (autoclean) [ipt_REDIRECT ipt_state ip_nat_irc
|
|
ip_nat_ftp ip_conntrack_irc ip_conntrack_ftp
|
|
ipt_conntrack iptable_nat]
|
|
ip_tables 14488 12 [ipt_TOS ipt_LOG ipt_REDIRECT ipt_REJECT ipt_state
|
|
ipt_multiport ipt_conntrack iptable_filter
|
|
iptable_mangle iptable_nat]
|
|
tulip 42464 0 (unused)
|
|
e100 50596 1
|
|
keybdev 2752 0 (unused)
|
|
mousedev 5236 0 (unused)
|
|
hid 20868 0 (unused)
|
|
input 5632 0 [keybdev mousedev hid]
|
|
usb-uhci 24684 0 (unused)
|
|
usbcore 73280 1 [hid usb-uhci]
|
|
ext3 64704 2
|
|
jbd 47860 2 [ext3]
|
|
[root@lists etc]#</programlisting>
|
|
</example>
|
|
|
|
<para>If you want Shorewall to load these modules from an alternate
|
|
directory, you need to set the MODULESDIR variable in
|
|
/etc/shorewall/shorewall.conf to point to that directory.</para>
|
|
|
|
<para>If your FTP helper modules are compressed and have the names
|
|
<emphasis>ip_nat_ftp.o.gz and ip_conntrack_ftp.o.gz</emphasis> then you
|
|
will need Shorewall 1.4.7 or later if you want Shorewall to load them for
|
|
you.</para>
|
|
|
|
<para>Server configuration is covered in the <ulink type=""
|
|
url="Documentation.htm#Rules">/etc/shorewall/rules documentation</ulink>,</para>
|
|
|
|
<para>For a client, you must open outbound TCP port 21.</para>
|
|
|
|
<para>The above discussion about commands and responses makes it clear
|
|
that the FTP connection-tracking and NAT helpers must scan the traffic on
|
|
the control connection looking for PASV and PORT commands as well as PASV
|
|
responses. If you run an FTP server on a nonstandard port or you need to
|
|
access such a server, you must therefore let the helpers know by
|
|
specifying the port in /etc/shorewall/modules entries for the helpers. For
|
|
example, if you run an FTP server that listens on port 49 or you need to
|
|
access a server on the internet that listens on that port then you would
|
|
have:</para>
|
|
|
|
<example>
|
|
<title>if you run an FTP server that listens on port 49 or you need to
|
|
access a server on the internet that listens on that port then you would
|
|
have:</title>
|
|
|
|
<programlisting>loadmodule ip_conntrack_ftp ports=21,49
|
|
loadmodule ip_nat_ftp ports=21,49</programlisting>
|
|
|
|
<para><note><para>you MUST include port 21 in the ports list or you may
|
|
have problems accessing regular FTP servers.</para></note></para>
|
|
|
|
<para>If there is a possibility that these modules might be loaded
|
|
before Shorewall starts, then you should include the port list in
|
|
/etc/modules.conf:</para>
|
|
|
|
<programlisting>options ip_conntrack_ftp ports=21,49
|
|
options ip_nat_ftp ports=21,49</programlisting>
|
|
|
|
<para><important><para>Once you have made these changes to
|
|
/etc/shorewall/modules and/or /etc/modules.conf, you must either:</para><orderedlist><listitem><para>Unload
|
|
the modules and restart shorewall:</para><programlisting><command>rmmod ip_nat_ftp; rmmod ip_conntrack_ftp; shorewall restart</command></programlisting></listitem><listitem><para>Reboot</para></listitem></orderedlist></important></para>
|
|
</example>
|
|
|
|
<para>One problem that I see occasionally involves active mode and the FTP
|
|
server in my DMZ. I see the active data connection to <emphasis
|
|
role="bold">certain client IP addresses</emphasis> being continuously
|
|
rejected by my firewall. It is my conjecture that there is some broken
|
|
client out there that is sending a PORT command that is being either
|
|
missed or mis-interpreted by the FTP connection tracking helper yet it is
|
|
being accepted by my FTP server. My solution is to add the following rule:</para>
|
|
|
|
<informaltable>
|
|
<tgroup cols="7">
|
|
<thead>
|
|
<row>
|
|
<entry align="center">ACTION</entry>
|
|
|
|
<entry align="center">SOURCE</entry>
|
|
|
|
<entry align="center">DESTINATION</entry>
|
|
|
|
<entry align="center">PROTOCOL</entry>
|
|
|
|
<entry align="center">PORT(S)</entry>
|
|
|
|
<entry align="center">SOURCE PORT(S)</entry>
|
|
|
|
<entry align="center">ORIGINAL DESTINATION</entry>
|
|
</row>
|
|
</thead>
|
|
|
|
<tbody>
|
|
<row>
|
|
<entry>ACCEPT:info</entry>
|
|
|
|
<entry>dmz</entry>
|
|
|
|
<entry>net</entry>
|
|
|
|
<entry>tcp</entry>
|
|
|
|
<entry>-</entry>
|
|
|
|
<entry>20</entry>
|
|
|
|
<entry></entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</informaltable>
|
|
|
|
<para>The above rule accepts and logs all active mode connections from my
|
|
DMZ to the net.</para>
|
|
</section>
|
|
</article> |