The first one sends the packets to squid-box from iptables-box. The second makes sure that the reply gets sent back through iptables-box, instead of directly to the client (this is very important!). The last one makes sure the iptables-box will forward the appropriate …
iptables INPUT vs. OUTPUT vs. FORWARD - YouTube Jul 16, 2019 iptables: A Basic Router - YouTube Sep 20, 2017 iptables rules to forward tftp via NAT - Unix & Linux
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Here is the chapter about FORWARD and NAT Rules. As it states: For example, if you want to forward incoming HTTP requests to your dedicated Apache HTTP Server at 172.31.0.23, use the following command as the root user: ~]# iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j DNAT --to 172.31.0.23:80 Here is what happens: Iptables Tutorial: Ultimate Guide to Linux Firewall Jan 28, 2020