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<TITLE>Linux 2.4 NAT HOWTO: Saying How To Mangle The Packets</TITLE>
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<H2><A NAME="s6">6.</A> <A HREF="NAT-HOWTO.html#toc6">Saying How To Mangle The Packets</A></H2>
<P>So now we know how to select the packets we want to mangle. To
complete our rule, we need to tell the kernel exactly what we want it
to do to the packets.</P>
<H2><A NAME="ss6.1">6.1</A> <A HREF="NAT-HOWTO.html#toc6.1">Source NAT</A>
</H2>
<P>You want to do Source NAT; change the source address of connections
to something different. This is done in the POSTROUTING chain, just
before it is finally sent out; this is an important detail, since it
means that anything else on the Linux box itself (routing, packet
filtering) will see the packet unchanged. It also means that the `-o'
(outgoing interface) option can be used.</P>
<P>Source NAT is specified using `-j SNAT', and the `--to-source'
option specifies an IP address, a range of IP addresses, and an
optional port or range of ports (for UDP and TCP protocols only).</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
## Change source addresses to 1.2.3.4.
# iptables -t nat -A POSTROUTING -o eth0 -j SNAT --to 1.2.3.4
## Change source addresses to 1.2.3.4, 1.2.3.5 or 1.2.3.6
# iptables -t nat -A POSTROUTING -o eth0 -j SNAT --to 1.2.3.4-1.2.3.6
## Change source addresses to 1.2.3.4, ports 1-1023
# iptables -t nat -A POSTROUTING -p tcp -o eth0 -j SNAT --to 1.2.3.4:1-1023
</PRE>
</CODE></BLOCKQUOTE>
</P>
<H3>Masquerading</H3>
<P>There is a specialized case of Source NAT called masquerading: it
should only be used for dynamically-assigned IP addresses, such as
standard dialups (for static IP addresses, use SNAT above).</P>
<P>You don't need to put in the source address explicitly with
masquerading: it will use the source address of the interface the
packet is going out from. But more importantly, if the link goes
down, the connections (which are now lost anyway) are forgotten,
meaning fewer glitches when connection comes back up with a new IP
address.</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
## Masquerade everything out ppp0.
# iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE
</PRE>
</CODE></BLOCKQUOTE>
</P>
<H2><A NAME="ss6.2">6.2</A> <A HREF="NAT-HOWTO.html#toc6.2">Destination NAT</A>
</H2>
<P>This is done in the PREROUTING chain, just as the packet comes in;
this means that anything else on the Linux box itself (routing, packet
filtering) will see the packet going to its `real' destination. It
also means that the `-i' (incoming interface) option can be used.</P>
<P>Destination NAT is specified using `-j DNAT', and the
`--to-destination' option specifies an IP address, a range of IP
addresses, and an optional port or range of ports (for UDP and TCP
protocols only).</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
## Change destination addresses to 5.6.7.8
# iptables -t nat -A PREROUTING -i eth0 -j DNAT --to 5.6.7.8
## Change destination addresses to 5.6.7.8, 5.6.7.9 or 5.6.7.10.
# iptables -t nat -A PREROUTING -i eth0 -j DNAT --to 5.6.7.8-5.6.7.10
## Change destination addresses of web traffic to 5.6.7.8, port 8080.
# iptables -t nat -A PREROUTING -p tcp --dport 80 -i eth0 \
-j DNAT --to 5.6.7.8:8080
</PRE>
</CODE></BLOCKQUOTE>
</P>
<H3>Redirection</H3>
<P>There is a specialized case of Destination NAT called redirection:
it is a simple convenience which is exactly equivalent to doing DNAT
to the address of the incoming interface.</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
## Send incoming port-80 web traffic to our squid (transparent) proxy
# iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 80 \
-j REDIRECT --to-port 3128
</PRE>
</CODE></BLOCKQUOTE>
</P>
<P>Note that squid needs to be configured to know it's a transparent proxy!</P>
<H2><A NAME="ss6.3">6.3</A> <A HREF="NAT-HOWTO.html#toc6.3">Mappings In Depth</A>
</H2>
<P>There are some subtleties to NAT which most people will never have
to deal with. They are documented here for the curious.</P>
<H3>Selection Of Multiple Addresses in a Range</H3>
<P>If a range of IP addresses is given, the IP address to use is
chosen based on the least currently used IP for connections the
machine knows about. This gives primitive load-balancing.</P>
<H3>Creating Null NAT Mappings</H3>
<P>You can use the `-j ACCEPT' target to let a connection through
without any NAT taking place.</P>
<H3>Standard NAT Behavior</H3>
<P>The default behavior is to alter the connection as little as
possible, within the constraints of the rule given by the user. This
means we won't remap ports unless we have to.</P>
<H3>Implicit Source Port Mapping</H3>
<P>Even when no NAT is requested for a connection, source port
translation may occur implicitly, if another connection has been
mapped over the new one. Consider the case of masquerading, which
is rather common:</P>
<P>
<OL>
<LI> A web connection is established by a box 192.1.1.1 from port
1024 to www.netscape.com port 80.
</LI>
<LI> This is masqueraded by the masquerading box to use its source
IP address (1.2.3.4).
</LI>
<LI> The masquerading box tries to make a web connection to
www.netscape.com port 80 from 1.2.3.4 (its external interface
address) port 1024.
</LI>
<LI> The NAT code will alter the source port of the second
connection to 1025, so that the two don't clash.</LI>
</OL>
</P>
<P>When this implicit source mapping occurs, ports are divided into
three classes:
<UL>
<LI> Ports below 512</LI>
<LI> Ports between 512 and 1023</LI>
<LI> Ports 1024 and above.</LI>
</UL>
</P>
<P>A port will never be implicitly mapped into a different class.</P>
<H3>What Happens When NAT Fails</H3>
<P>If there is no way to uniquely map a connection as the user
requests, it will be dropped. This also applies to packets which
could not be classified as part of any connection, because they are
malformed, or the box is out of memory, etc.</P>
<H3>Multiple Mappings, Overlap and Clashes</H3>
<P>You can have NAT rules which map packets onto the same range; the
NAT code is clever enough to avoid clashes. Hence having two rules
which map the source address 192.168.1.1 and 192.168.1.2 respectively
onto 1.2.3.4 is fine.</P>
<P>Furthermore, you can map over real, used IP addresses, as long as
those addresses pass through the mapping box as well. So if you have
an assigned network (1.2.3.0/24), but have one internal network using
those addresses and one using the Private Internet Addresses
192.168.1.0/24, you can simply NAT the 192.168.1.0/24 source addresses
onto the 1.2.3.0 network, without fear of clashing:</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
# iptables -t nat -A POSTROUTING -s 192.168.1.0/24 -o eth1 \
-j SNAT --to 1.2.3.0/24
</PRE>
</CODE></BLOCKQUOTE>
</P>
<P>The same logic applies to addresses used by the NAT box itself:
this is how masquerading works (by sharing the interface address
between masqueraded packets and `real' packets coming from the box
itself).</P>
<P>Moreover, you can map the same packets onto many different targets,
and they will be shared. For example, if you don't want to map
anything over 1.2.3.5, you could do:</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
# iptables -t nat -A POSTROUTING -s 192.168.1.0/24 -o eth1 \
-j SNAT --to 1.2.3.0-1.2.3.4 --to 1.2.3.6-1.2.3.254
</PRE>
</CODE></BLOCKQUOTE>
</P>
<H3>Altering the Destination of Locally-Generated Connections</H3>
<P>The NAT code allows you to insert DNAT rules in the OUTPUT chain,
but this is not fully supported in 2.4 (it can be, but it requires a
new configuration option, some testing, and a fair bit of coding, so
unless someone contracts Rusty to write it, I wouldn't expect it
soon).</P>
<P>The current limitation is that you can only change the destination
to the local machine (e.g. `j DNAT --to 127.0.0.1'), not to any other
machine, otherwise the replies won't be translated correctly.</P>
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