iptables NAT prerouting & postrouting


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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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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)

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