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This turns your iptables packet filter to a "Allow any from any to any" filter, so you can rule out any filtering issues when you have a problem to enable a connection from or to your host.
To re-enable it, run /etc/init.d/iptables restart
This is just one method of checking to see if an IP is blocked via IP tables or CSF. Simple and to the point. Replace xx.xx.xx.xx with the IP you wish to check.
Replace 10.0.0.0/8 with your largest local subnet. replace 10.1.1.123:3128 with your proxy information.. Note this only works with a proxy server configured for passive setup..
Now your firefox transparently proxy's stuff destined outside your network.. and Doesn't proxy stuff inside your network. as well as all your other favorite web applications. curl, wget, aria2 ect.
This appends (-A) a new rule to the INPUT chain, which specifies to drop all packets from a source (-s) IP address.