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iptables NAT prerouting & postrouting

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Old 07-01-2012
iptables NAT prerouting & postrouting
Good morning,
I'm a newbie of iptables and as far as I've seen on tutorials on the Internet it seems that both prerouting and postrouting NAT chains are undergone both by a packet that goes from an internal LAN to the Internet and of a one that goes in the opposite direction (from the Internet to the LAN).
AS NAT prerouting is DNAT, i.e. changing of the destination IP address and NAT postrouting is SNAT, i.e. changing of the source IP address, I naturally think that for a packet that exits the internal LAN and goes to the Internet only NAT postrouting is necessary while for a packet that from the Internet goes into the internal LAN only NAT prerouting is necessary.
Can You please take me an example that demonstrates in each of the two cases that also the other NAT chain could be useful (NAT prerouting for a packet that goes from the LAN to the Internet, NAT postrouting for a packet that goes in the opposite direction)?
Please help me!!!

Thank You

Regards

Giacomo Odoardi
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LEARN ABOUT PHP

iptables-save

IPTABLES-SAVE(8)						  iptables 1.6.1						  IPTABLES-SAVE(8)

NAME

       iptables-save -- dump iptables rules to stdout

       ip6tables-save -- dump iptables rules to stdout

SYNOPSIS

       iptables-save [-M modprobe] [-c] [-t table]

       ip6tables-save [-M modprobe] [-c] [-t table]

DESCRIPTION

       iptables-save  and ip6tables-save are used to dump the contents of IP or IPv6 Table in easily parseable format to STDOUT. Use I/O-redirect-
       ion provided by your shell to write to a file.

       -M, --modprobe modprobe_program
	      Specify the path to the modprobe program. By default, iptables-save will inspect /proc/sys/kernel/modprobe  to  determine  the  exe-
	      cutable's path.

       -c, --counters
	      include the current values of all packet and byte counters in the output

       -t, --table tablename
	      restrict output to only one table. If not specified, output includes all available tables.

BUGS

       None known as of iptables-1.2.1 release

AUTHORS

       Harald Welte <laforge@gnumonks.org>
       Rusty Russell <rusty@rustcorp.com.au>
       Andras Kis-Szabo <kisza@sch.bme.hu> contributed ip6tables-save.

SEE ALSO

       iptables-apply(8),iptables-restore(8), iptables(8)

       The  iptables-HOWTO, which details more iptables usage, the NAT-HOWTO, which details NAT, and the netfilter-hacking-HOWTO which details the
       internals.

iptables 1.6.1															  IPTABLES-SAVE(8)