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To understand firewalling in Linux, you need to know that it all is implemented in the

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kernel. In the kernel, we have a framework, and this framework is called Netfilter. And

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Netfilter is providing three different interfaces. The most important interfaces are the different

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chains. So the chains are input for incoming packages. We have forward, which is going

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to be if this is a router. And we have output, which is going to filter outgoing traffic.

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Now what are you going to do to manage your firewall? Well, you need to get them into

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Netfilter in some way. And there are different solutions in order to do so. In the old days,

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it was IP tables. Nowadays, NFT, the Netfilter tables, is the default. And using IP tables

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or NFT, you write rules into your input chain, or you write rules into your output chain.

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And these are always processed for every single packet that comes into your system. Now, IP

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tables and NF tables are a little bit complex, and that is why there are also front-end utilities.

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And these front-end utilities are firewalld and ufw. These write either to IF tables or

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NF tables, and the result is that you get your rules into the input chain or in the

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output chain to process the incoming and outgoing packets. In the next couple of videos, you'll

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learn how to work with these tools.