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- So let me give you an
overview of Linux firewalling.

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It all starts with the kernel.

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In the kernel, there is this functionality

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called net filter.

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And net filter is the packet filtering

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that makes sure that
packets are going to pass

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through the kernel.

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Now, in order to do so,

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there are the different flows of packages.

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We have input.

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We have forward for routing.

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And we have output.

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These are the different directions

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that the kernel is going to track.

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Now, you need to define what
you want to allow to come in,

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what you want to allow in this forward,

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and what you want to allow

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in this output phase of the firewall.

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So if you look at the essential utilities,

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there was always iptables.

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And nowadays, that has been
replaced with nftables.

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These are low end utilities.

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They are pretty complex.

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And because they are so complex,

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they are not used a lot.

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In order to write these firewall rules

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in a much easier way,

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nowadays, distributions
are providing firewalld

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or ufw,

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where firewalld is the default in Red Hat

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and ufw is the default in Ubuntu.

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So what is happening?

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Well, firewalld or ufw writes
to iptables or nftables

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depending on the Linux
distribution that you are using,

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and these on their turn

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are defining these input,
output, and forward rules.

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And that's how firewalling
in Linux is organized.