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git-svn-id: https://shorewall.svn.sourceforge.net/svnroot/shorewall/trunk@8283 fbd18981-670d-0410-9b5c-8dc0c1a9a2bb
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teastep 2008-03-12 23:19:31 +00:00
parent 10e2cb3751
commit 07c46bce79
2 changed files with 90 additions and 3 deletions
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91
docs/FAQ.xml
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@ -219,8 +219,9 @@ DNAT net:<emphasis>address</emphasis> loc:<emphasis>local-IP-address</empha
<listitem>
<para>You have a more basic problem with your local system (the
one that you are trying to forward to) such as an incorrect
default gateway (it should be set to the IP address of your
firewall's internal interface).</para>
default gateway (it must be set to the IP address of your
firewall's internal interface; if that isn't possible for some
reason, see <link linkend="faq1f">FAQ 1f</link>).</para>
</listitem>
<listitem>
@ -406,6 +407,92 @@ DNAT loc dmz:192.168.2.4 tcp 80 - $ETH0
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
DNAT net fw:192.168.1.1:22 tcp 4104</programlisting>
</section>
<section id="faq1f">
<title>(FAQ 1f) Why must the server that I port forward to have it's
default gateway set to my Shorewall system's IP address?</title>
<para><emphasis role="bold">Answer</emphasis>: Let's take an example.
Suppose that</para>
<itemizedlist>
<listitem>
<para>Your Shorewall firewall's external IP address is
206.124.146.176 (eth0) and internal IP address 192.168.1.1
(eth1).</para>
</listitem>
<listitem>
<para>You have another gateway router with external IP address
130.252.100.109 and internal IP address 192.168.1.254.</para>
</listitem>
<listitem>
<para>You have an FTP server behind both routers with IP address
192.168.1.4</para>
</listitem>
<listitem>
<para>The FTP server's default gateway is through the second
router (192.168.1.254).</para>
</listitem>
<listitem>
<para>You have this rule on the Shorewall system:<programlisting>#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL
# PORT DEST.
DNAT net loc:192.168.1.4 tcp 21 - 206.124.146.176</programlisting></para>
</listitem>
<listitem>
<para>Internet host 16.105.221.4 issues the command <command>ftp
206.124.146.176</command></para>
</listitem>
</itemizedlist>
<para>This results in the following set of events:</para>
<orderedlist>
<listitem>
<para>16.105.221.4 sends a TCP syn packet to 206.124.146.176
specifying destination port 21.</para>
</listitem>
<listitem>
<para>The Shorewall box rewrites the destination IP address to
192.168.1.4 and forwards the packet.</para>
</listitem>
<listitem>
<para>The FTP server receives the packet and accepts the
connection, generating a SYN,ACK packet back to 16.105.221.4.
Because the server's default gateway is through the second router,
it sends the packet to that router.</para>
</listitem>
</orderedlist>
<para>At this point, one of two things can happen.either the second
router discards or rejects the packet; or, it rewrites the source IP
address to 130.252.100.109 and forwards the packet back to
16.105.221.4. Regardless of which happens, the connection is doomed.
Clearly if the packet is rejected or dropped, the connection will not
be successful. But even if the packet reaches 16.105.221.4, that host
will reject it since it's SOURCE IP address (130.252.100.109) doesn't
match the DESTINATION IP ADDRESS (206.124.146.176) of the original SYN
packet.</para>
<para>The best way to work around this problem is to change the
default gateway on the FTP server to the Shorewall system's internal
IP address (192.168.1.1). But if that isn't possible, you can work
around the problem with the following ugly hack in
<filename>/etc/shorewall/masq</filename>:<programlisting>#INTERFACE SOURCE ADDRESS PROTO PORT
eth1:192.168.1.4 0.0.0.0/0 192.168.1.1 tcp 21</programlisting></para>
<para>This rule has the undesirable side effect that it makes all FTP
connections from the net appear to the FTP server as if they
originated on the Shorewall system. But it will force the FTP server
to reply back through the Shorewall system who can then rewrite the
SOURCE IP address in the responses properly.</para>
</section>
</section>
<section id="faq30">

2
docs/support.xml
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@ -396,7 +396,7 @@ State:Stopped (Thu Mar 30 14:08:11 PDT 2006)</programlisting>
</listitem>
<listitem>
<para>Please do not include Shorewall configuration file unless you
<para>Please do not include Shorewall configuration files unless you
have been specifically asked to do so. The output of
<command>shorewall dump</command> collected as described above is much
more useful.</para>