Let me tell you about RHEL firewalling.
It all starts with the kernel.
The Linux kernel provides the net filter framework,
and that takes care of firewall-related network
operations.
And these include packet filtering.
That is about allowing or denying packets
that you don't want to see on your network.
Or network address translation, which
allows you to publish a private IP
address to the external world so that it can communicate
with the external world.
Or port forwarding, which allows you
to expose a port on a host that is forwarded to a computer
on the private network.
Now, all of this is pretty interesting,
but network address translation and port forwarding
are mainly used on routers.
That's not something to worry about for RHCSA.
Packet filtering, that is something
that you might have to check.
So that's what we are going to focus on.
Net filter is a framework that forwards specific operations
to Linux kernel modules.
And NF tables is a framework that applies the firewalling.
Firewall D on RHEL 10 is used as the default firewalling
service.
It's managed by system D, and RHEL
uses it as a front end to manage NF tables firewalls.
Theoretically, you could create your firewall rules
in NF tables as well, but the syntax
is so complicated that nobody is doing that.
So what you need to learn about is a little bit of firewall D.