Linux IP Masquerade HOWTO
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# 7.18. ( PORTFW ) - IP Masquerading with PORTFWing seems to break when my line is idle for long periods

If you have a DSL or Cablemodem, this behavior is unfortunately quite common. Basically your ISP is putting your connection into a very low priority queue to better service non-idle connections. The problem is that some enduser's connections will actually be taken OFFLINE until some traffic from the user's DSL/Cablemodem connection awakens the ISP's hardware.

- Some DSL installations can take an idle connection OFFLINE and only be checked for activity once every 30 seconds or so.

- Some Cablemodem setups can set an idle connection into a low priority queue and only be checked for activity every minute or so.

What do I recommend to do? Ping your default gateway every 30 seconds. To do this, edit the /etc/rc.d/rc.local file and add the following to the bottom of the file:

             ping -i 30 100.200.212.121 > /dev/null &

Replace the 100.200.212.121 with your default router (upstream router).

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