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In the last lesson, I introduced the concepts

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of zones and I started talking about screened subnets.

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In this lesson, I want to dig a little bit further into that,

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and then talk about how we can manage them.

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So the first thing I had mentioned

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was if you have an internet-facing host.

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Now, I didn't really go into what that was,

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so let me take a step here

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and go ahead and define it for you.

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When I'm talking about something that's internet-facing,

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like an internet-facing host or an internet-facing server,

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I'm talking about a host or server that accepts

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inbound connections from the internet.

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So if I have a web server in my screened subnet,

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that is an internet-facing host.

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So you can see here, again, if I bring up my diagram,

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I have my inside zone, my screened subnet,

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and my outside zone.

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In that screened subnet, I have two internet-facing hosts,

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I have an email server and a web server.

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Now, only the email and web servers

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that are in the screened subnet are going to be able

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to get traffic from the outside,

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even though they haven't requested it.

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So if you want to connect to my web server,

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you're going to go to diontraining.com,

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and it's going to go through my screened subnet

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into my web server, and then give you back your response.

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Now, if you wanted to get to PC2 or PC3

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in my inside network, you couldn't do that

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because the firewall would block you.

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Those are not internet-facing.

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They have access to the internet,

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but they're not facing the internet,

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meaning they are not open and waiting for a connection.

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And that's the difference

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when you're dealing with internet-facing hosts.

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Now, anytime you have internet-facing hosts,

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you want to place them into someplace secure,

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like your screened subnet.

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Now, your screened subnet is actually a segment

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that is isolated from the rest of the private network

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by one or more firewalls, and it's set up to accept

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connections from the internet over designated ports.

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Now, the reason we do this is we want to keep

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all those forward facing servers

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out of our internal network.

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We don't want people from the internet

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touching our internal network,

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we only want them in our screened subnet,

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this place that is kind of this semi-trusted zone.

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And we know that anything that's behind the screened subnet,

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such as my inside zone,

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is actually invisible to the outside network.

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So if you start scanning my network from the outside,

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you're not going to see all those PCs

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inside of the inside zone inside.

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Instead, you're only going to see the web server

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and the email server, 'cause those are forward facing

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and they're internet-facing.

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Now, the next thing we need to talk about

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in terms of the screened subnet is,

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what kind of stuff should you put in the screened subnet?

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You can see here that I have my email and my web server

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in the screened subnet, but any other kind

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of communication servers, proxy servers,

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or remote access servers

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should also be in the screened subnet.

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Anything that somebody from the internet

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needs access to should be placed in your screened subnet.

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This is essentially anything that provides public services,

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or even extra net capabilities.

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Any of your hosts that are in the screened subnet,

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we don't fully trust those, even though they're our devices.

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So we want to make sure that we harden them as best as we can.

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And we have to remember that those devices,

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because they're forward facing,

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they could be touched by an attacker,

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they could be compromised by an attacker.

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So that's why they're not fully trusted

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to our internal network.

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And that's why we actually have it go through the firewall,

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anything that's going from the screened subnet

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to the inside, and from the inside

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back to the screened subnet.

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That's another good place

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to put intrusion detection systems, to make sure

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that you're catching anything that may be going

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from your screened subnet, because a common technique

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for an attacker is to compromise something

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in the screened subnet and then use that

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to pivot into your network.

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So you want to protect yourself against that.

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Now, any kind of host you put in the screened subnet

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should really be what we consider a bastion host.

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This is a host or server

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that we put into the screened subnet,

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which is not configured with any services

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that run on the local network.

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So I don't want to run something like active directory

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inside the screened subnet,

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that's an internal network service.

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Instead, I only want to run things

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that should be in the internet.

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Things like email, things like web,

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things like remote access, those things can be hardened

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and put into the screened subnet, because we know

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that they're going to be more vulnerable to attack.

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Now, when we want to configure our devices

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inside the screened subnet, what are we going to do?

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Well, we're going to use something known as a jumpbox.

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Now, a jumpbox is a hardened server that provides access

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to other hosts within the screened subnet.

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So essentially, we have this one server,

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and it is what can talk to the screened subnet.

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And we configure all the access control to make sure

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that only the jumpbox can communicate

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from the internal network to the screened subnet.

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Now, because of that, that jumpbox

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has to be heavily hardened, it needs to be protected.

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And what ends up happening is the administrator will connect

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to the jumpbox and then the jumpbox will connect

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to the host and the screened subnet.

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Then I call a jump box,

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'cause we're almost pivoting off of it.

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We're going to connect from me to the jumpbox,

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and the jumpbox to the server I want to configure,

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and that's why we call it a jumpbox.

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Now, this jumpbox can be a physical PC

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or it can be a virtual machine, either one is fine.

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A lot of people use virtual machines as a jumpbox

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because you can have it hardened and secured.

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You can use it for the time you need,

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and then destroy it and rebuild a new one,

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because it's very quick to rebuild an image

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from a virtual machine if you already have

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a known good image.

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And so, a lot of people will do it that way.

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Now, the jumpbox and the management workstation

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that you're using to connect to that jumpbox

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should have only the minimum required software

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to perform their job, and they should be well hardened.

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Again, this is the one box that has the permissions

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to go through the firewall and touch the screened subnet

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from your internal network.

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So you want to make sure it is well protected.

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This is why you want to make sure that management workstation

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and the jumpbox are fully hardened,

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and they have the least amount of software on them

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to make sure they're fully hardened and fully secured.