mirror of
https://gitlab.com/shorewall/code.git
synced 2024-12-29 17:48:53 +01:00
435 lines
18 KiB
XML
435 lines
18 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
|
|
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
|
|
"http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd">
|
|
<article id="FTP">
|
|
<!--$Id$-->
|
|
|
|
<articleinfo>
|
|
<title>Shorewall and FTP</title>
|
|
|
|
<authorgroup>
|
|
<author>
|
|
<firstname>Tom</firstname>
|
|
|
|
<surname>Eastep</surname>
|
|
</author>
|
|
</authorgroup>
|
|
|
|
<pubdate><?dbtimestamp format="Y/m/d"?></pubdate>
|
|
|
|
<copyright>
|
|
<year>2003</year>
|
|
|
|
<year>2004</year>
|
|
|
|
<year>2005</year>
|
|
|
|
<year>2006</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>
|
|
|
|
<caution>
|
|
<para><emphasis role="bold">This article applies to Shorewall 4.0 and
|
|
later. If you are running a version of Shorewall earlier than Shorewall
|
|
4.0.0 then please see the documentation for that
|
|
release.</emphasis></para>
|
|
</caution>
|
|
|
|
<section id="Protocol">
|
|
<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): <command>ftp</command>
|
|
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.</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 id="Conntrack">
|
|
<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>nf_conntrack_ftp</quote> and
|
|
<quote>nf_nat_ftp</quote> need to be loaded. Shorewall automatically loads
|
|
these <quote>helper</quote> modules from
|
|
/lib/modules/<<emphasis>kernel-version</emphasis>>/kernel/net/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>
|
|
|
|
<important>
|
|
<para>Note: If you are running kernel 2.6.19 or earlier, then the module
|
|
names are <emphasis role="bold">ip_nat_ftp</emphasis> and <emphasis
|
|
role="bold">ip_conntrack_ftp</emphasis> and they are normally loaded
|
|
from
|
|
/lib/modules/<<emphasis>kernel-version</emphasis>>/kernel/net/ipv4/netfilter/.</para>
|
|
</important>
|
|
|
|
<important>
|
|
<para>Because the ftp helper modules must read and modify commands being
|
|
sent over the command channel, they won't work when the command channel
|
|
is encrypted through use of TLS/SSL.</para>
|
|
</important>
|
|
|
|
<example id="Example1">
|
|
<title>Example (Kernel 3.2.20)</title>
|
|
|
|
<programlisting>[root@lists etc]# lsmod
|
|
Module Size Used by Not tainted
|
|
iptable_filter 3072 1
|
|
iptable_mangle 2816 0
|
|
iptable_nat 7684 0
|
|
iptable_raw 2048 0
|
|
ip_tables 12232 4 iptable_raw,iptable_mangle,iptable_nat,iptable_filter
|
|
ipt_addrtype 1920 0
|
|
ipt_ah 2048 0
|
|
ipt_CLUSTERIP 8708 0
|
|
ipt_ecn 2304 0
|
|
ipt_ECN 3072 0
|
|
ipt_iprange 1920 0
|
|
ipt_LOG 6528 0
|
|
ipt_MASQUERADE 3456 0
|
|
ipt_NETMAP 2048 0
|
|
ipt_owner 2048 0
|
|
ipt_recent 9496 0
|
|
ipt_REDIRECT 2048 0
|
|
ipt_REJECT 4608 0
|
|
ipt_SAME 2432 0
|
|
ipt_TCPMSS 4096 0
|
|
ipt_tos 1664 0
|
|
ipt_TOS 2304 0
|
|
ipt_ttl 1920 0
|
|
ipt_TTL 2432 0
|
|
ipt_ULOG 8068 0
|
|
nf_conntrack 59864 28 ipt_MASQUERADE,ipt_CLUSTERIP,nf_nat_tftp,nf_nat_snmp_basic,nf_nat_sip,nf_nat_pptp,nf_nat_irc,nf_nat_h323,nf_nat_ftp,nf_nat_amanda,nf_conntrack_ama
|
|
nda,nf_conntrack_tftp,nf_conntrack_sip,nf_conntrack_proto_sctp,nf_conntrack_pptp,nf_conntrack_proto_gre,nf_conntrack_netlink,nf_conntrack_netbios_ns,nf_conntrack_irc,nf_conntrack_
|
|
h323,nf_conntrack_ftp,xt_helper,xt_state,xt_connmark,xt_conntrack,iptable_nat,nf_nat,nf_conntrack_ipv4
|
|
nf_conntrack_amanda 5248 1 nf_nat_amanda
|
|
<emphasis role="bold">nf_conntrack_ftp</emphasis> 9728 1 nf_nat_ftp
|
|
nf_conntrack_h323 50396 1 nf_nat_h323
|
|
nf_conntrack_ipv4 17932 2 iptable_nat
|
|
nf_conntrack_irc 7064 1 nf_nat_irc
|
|
nf_conntrack_netbios_ns 3072 0
|
|
nf_conntrack_netlink 26240 0
|
|
nf_conntrack_pptp 6912 1 nf_nat_pptp
|
|
nf_conntrack_proto_gre 5632 1 nf_conntrack_pptp
|
|
nf_conntrack_proto_sctp 8328 0
|
|
nf_conntrack_sip 9748 1 nf_nat_sip
|
|
nf_conntrack_tftp 5780 1 nf_nat_tftp
|
|
nf_nat 17964 14 ipt_SAME,ipt_REDIRECT,ipt_NETMAP,ipt_MASQUERADE,nf_nat_tftp,nf_nat_sip,nf_nat_pptp,nf_nat_proto_gre,nf_nat_irc,nf_nat_h323,nf_nat_ftp,nf_nat_amand
|
|
a,nf_conntrack_netlink,iptable_nat
|
|
nf_nat_amanda 2432 0
|
|
<emphasis role="bold">nf_nat_ftp</emphasis> 3584 0
|
|
nf_nat_h323 7808 0
|
|
nf_nat_irc 2816 0
|
|
nf_nat_pptp 3840 0
|
|
nf_nat_proto_gre 3204 1 nf_nat_pptp
|
|
nf_nat_sip 4608 0
|
|
nf_nat_snmp_basic 10372 0
|
|
nf_nat_tftp 1920 0
|
|
xt_CLASSIFY 1920 0
|
|
xt_comment 1920 0
|
|
xt_connmark 2432 0
|
|
xt_conntrack 2944 0
|
|
xt_dccp 3588 0
|
|
xt_hashlimit 10252 0
|
|
xt_helper 2688 0
|
|
xt_length 1920 0
|
|
xt_limit 2688 0
|
|
xt_mac 1920 0
|
|
xt_mark 1920 0
|
|
xt_MARK 2304 0
|
|
xt_multiport 3328 1
|
|
xt_NFLOG 2176 0
|
|
xt_NFQUEUE 2048 0
|
|
xt_physdev 2704 2
|
|
xt_pkttype 1920 0
|
|
xt_policy 3840 0
|
|
xt_state 2560 0
|
|
xt_tcpmss 2304 0
|
|
xt_tcpudp 3328 0
|
|
[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>
|
|
</section>
|
|
|
|
<section id="Ports">
|
|
<title>FTP on Non-standard Ports</title>
|
|
|
|
<note>
|
|
<para>If you are running <emphasis role="bold">kernel 2.6.19 or
|
|
earlier</emphasis>, replace <emphasis
|
|
role="bold">nf_conntrack_ftp</emphasis> with <emphasis
|
|
role="bold">ip_conntrack_ftp</emphasis> in the following instructions.
|
|
Similarly, replace <emphasis role="bold">nf_nat_ftp</emphasis> with
|
|
<emphasis role="bold">ip_nat_ftp</emphasis>.</para>
|
|
</note>
|
|
|
|
<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 <filename>/etc/shorewall/modules</filename> entries
|
|
for the helpers. You should create<filename>
|
|
/etc/shorewall/modules</filename> by copying
|
|
<filename>/usr/share/shorewall/modules</filename>.<caution>
|
|
<para>You must have modularized FTP connection tracking support in
|
|
order to use FTP on a non-standard port.</para>
|
|
</caution></para>
|
|
|
|
<example id="Example2">
|
|
<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 nf_conntrack_ftp ports=21,49
|
|
loadmodule nf_nat_ftp # NOTE: With kernels prior to 2.6.11, you must specify the ports on this line also</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 nf_conntrack_ftp ports=21,49
|
|
options nf_nat_ftp</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 nf_nat_ftp; rmmod nf_conntrack_ftp; shorewall restart</command></programlisting>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>Reboot</para>
|
|
</listitem>
|
|
</orderedlist>
|
|
</important></para>
|
|
</example>
|
|
</section>
|
|
|
|
<section id="Rules">
|
|
<title>Rules</title>
|
|
|
|
<warning>
|
|
<para>If you run an FTP server behind your firewall and your server
|
|
offers a method of specifying the external IP address of your firewall,
|
|
DON'T USE THAT FEATURE OF YOUR SERVER. Using that option will defeat the
|
|
purpose of the ftp helper modules and can result in a server that
|
|
doesn't work.</para>
|
|
</warning>
|
|
|
|
<para>If the policy from the source zone to the destination zone is ACCEPT
|
|
and you don't need DNAT (see <ulink url="FAQ.htm#faq30">FAQ 30</ulink>)
|
|
then <emphasis role="bold">you need no rule</emphasis>.</para>
|
|
|
|
<para>Otherwise, for FTP you need exactly <emphasis
|
|
role="bold">one</emphasis> rule:</para>
|
|
|
|
<programlisting>#ACTION SOURCE DESTINATION PROTO DEST SOURCE ORIGINAL
|
|
# PORT(S) PORT(S) DESTINATION
|
|
ACCEPT or <<emphasis>source</emphasis>> <<emphasis>destination</emphasis>> tcp 21 - <external IP addr> if
|
|
DNAT ACTION = DNAT</programlisting>
|
|
|
|
<para>You need an entry in the ORIGINAL DESTINATION column only if the
|
|
ACTION is DNAT, you have multiple external IP addresses and you want a
|
|
specific IP address to be forwarded to your server.</para>
|
|
|
|
<para>Note that you do <emphasis role="bold">NOT </emphasis>need a rule
|
|
with 20 (ftp-data) in the DEST PORT(S) column. If you post your rules on
|
|
the mailing list and they show 20 in the DEST PORT(S) column, we will know
|
|
that you haven't read this article and will either ignore your post or
|
|
tell you to RTFM.</para>
|
|
|
|
<para>Shorewall includes an FTP macro that simplifies creation of FTP
|
|
rules. The macro source is in
|
|
<filename>/usr/share/shorewall/macro.FTP</filename>. Using the macro is
|
|
the preferred way to generate the rules described above. Here are a couple
|
|
of examples.</para>
|
|
|
|
<para><example id="Example3">
|
|
<title>Server running behind a Masquerading Gateway</title>
|
|
|
|
<para>Suppose that you run an FTP server on 192.168.1.5 in your local
|
|
zone using the standard port (21). You need this rule:</para>
|
|
|
|
<programlisting>#ACTION SOURCE DESTINATION PROTO DEST SOURCE ORIGINAL
|
|
# PORT(S) PORT(S) DESTINATION
|
|
FTP(DNAT) net loc:192.168.1.5</programlisting>
|
|
</example><example id="Example4">
|
|
<title>Allow your DMZ FTP access to the Internet</title>
|
|
|
|
<programlisting>#ACTION SOURCE DESTINATION PROTO DEST SOURCE ORIGINAL
|
|
# PORT(S) PORT(S) DESTINATION
|
|
FTP(ACCEPT) dmz net</programlisting>
|
|
</example></para>
|
|
|
|
<para>Note that the FTP connection tracking in the kernel cannot handle
|
|
cases where a PORT command (or PASV reply) is broken across two packets or
|
|
is missing the ending <cr>/<lf>. When such cases occur, you
|
|
will see a console message similar to this one:</para>
|
|
|
|
<programlisting>Apr 28 23:55:09 gateway kernel: conntrack_ftp: partial PORT 715014972+1</programlisting>
|
|
|
|
<para>I see this problem occasionally with the FTP server in my DMZ. My
|
|
solution is to add the following rule:</para>
|
|
|
|
<programlisting>#ACTION SOURCE DESTINATION PROTO DEST SOURCE ORIGINAL
|
|
# PORT(S) PORT(S) DESTINATION
|
|
ACCEPT:info dmz net tcp - 20</programlisting>
|
|
|
|
<para>The above rule accepts and logs all active mode connections from my
|
|
DMZ to the net.</para>
|
|
</section>
|
|
</article>
|