Add/improve use of iptables

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Add/improve use of iptables

Sources:

• https://unix.stackexchange.com/questions/11851/iptables-allow-certain-ips-and-block-all-other-connection
• https://github.com/TechBubbleTechnologies/IoT-JumpWay-RPI-Examples/blob/master/_DOCS/4-Securing-Your-Raspberry-Pi-With-IPTables.md

Nessus finding

• IP Forwarding Enabled
• Risk Factor: Medium
• CVSS Base Score: 5.8
• CVSS Vector: CVSS2#AV:A/AC:L/Au:N/C:P/I:P/A:P

The remote host has IP forwarding enabled. An attacker can exploit this to route packets through the host and potentially bypass some firewalls / routers / NAC filtering. Unless the remote host is a router, it is recommended that you disable IP forwarding.

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Added! It could perhaps use some refinement, but it is good enough for now.