Let's briefly look at some virtual firewalls in Linux. This is Iptables, running in my Ubuntu Server. Remember that?
What is CHAIN in this context? My best guess is 'the route a packet takes'. Think "At this point in the process, what should happen to this packet?"  Here, it's saying "Forward everything!". At which point, you may as well not have a firewall at all.
So let's set some rules on this bad boy;
The command is going 'super user within iptables, append the INPUT chain regarding the protocol tcp'
Let's try it with the example at the second link;
lo = Loopback
j = target, which must be set with each new rule. After all, how will iptables know to ACCEPT or DROP, RETURN?
Today I was reminded that Cases Matter.
Now we're looking good! Let's save them with sudo /sbin/iptables-save. I added a few more rules to this, namely DROP this CERTAIN connection, without saving.
There is plenty more information at the above links, so check them out, and I'll be revisiting more Linux and open source firewalls in the future.
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