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Edit: /usr/share/doc/iptables/html/NAT-HOWTO-10.html (2525B)

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
<HTML>
<HEAD>
 <META NAME="GENERATOR" CONTENT="LinuxDoc-Tools 0.9.82">
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 <TITLE>Linux 2.4 NAT HOWTO: Destination NAT Onto the Same Network</TITLE>
 <LINK HREF="NAT-HOWTO-11.html" REL=next>
 <LINK HREF="NAT-HOWTO-9.html" REL=previous>
 <LINK HREF="NAT-HOWTO.html#toc10" REL=contents>
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<HR>
<H2><A NAME="s10">10.</A> <A HREF="NAT-HOWTO.html#toc10">Destination NAT Onto the Same Network</A></H2>

If you are doing port forwarding back onto the same network, you
need to make sure that both future packets and reply packets pass
through the NAT box (so they can be altered). The NAT code will now
(since 2.4.0-test6), block the outgoing ICMP redirect which is
produced when the NAT'ed packet heads out the same interface it came
in on, but the receiving server will still try to reply directly to
the client (which won't recognize the reply).

The classic case is that internal staff try to access your `public'
web server, which is actually DNAT'ed from the public address
(1.2.3.4) to an internal machine (192.168.1.1), like so:

<BLOCKQUOTE><CODE>
<PRE>
# iptables -t nat -A PREROUTING -d 1.2.3.4 \
 -p tcp --dport 80 -j DNAT --to 192.168.1.1
</PRE>
</CODE></BLOCKQUOTE>

One way is to run an internal DNS server which knows the real
(internal) IP address of your public web site, and forward all other
requests to an external DNS server. This means that the logging on
your web server will show the internal IP addresses correctly.

The other way is to have the NAT box also map the source IP address
to its own for these connections, fooling the server into replying
through it. In this example, we would do the following (assuming the
internal IP address of the NAT box is 192.168.1.250):

<BLOCKQUOTE><CODE>
<PRE>
# iptables -t nat -A POSTROUTING -d 192.168.1.1 -s 192.168.1.0/24 \
 -p tcp --dport 80 -j SNAT --to 192.168.1.250
</PRE>
</CODE></BLOCKQUOTE>

Because the PREROUTING rule gets run first, the packets will
already be destined for the internal web server: we can tell which
ones are internally sourced by the source IP addresses.

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