In this video I want to talk to you about Firewalls and just a couple of concepts that you may see on the exam. And I'm going to start off with a ridiculously simple animation, but just hang on with me here. Firewalls sound very simple, but there's a lot more going on under the hood than what you may have realized if you haven't worked with them very much and the concept of a Firewall is we have a server, or any kind of machine that we need to contact by a remote client, and this remote client's out on a public network, or an unsecured network. And the server, or the resource, is in our private network and we have protected or walled off our private network behind a Firewall. Now the first thing we want to do is, any unsolicited traffic or traffic types that we don't trust, we want to simply bounce off the Firewall and not come into our network. Ports that we have open, say Port 80 for Internet traffic, for Web traffic, any other ports, mail ports, any kind of information that we're looking for or listening for, we want to just pass through the Firewall and come in. Now there's a lot of things that can happen with this process here. The main purpose of the Firewall is to protect the network from outside traffic. Now this can be a hardware, Firewall, Black box, device. It can be a router, it can be software running on a server. There's a lot of different forms of Firewalls but they're all going to do basically the same type thing, and then the more you spend, obviously, you can get some more features. So let's talk about those. First thing it does, is it just looks at the packets that are hitting it, that are trying to come into your network, and it's applying some sort of rules to those packets to determine whether it should let that packet or that traffic come through and get into the private network. Now you're the one, or the administrator, builds the rules that lays out the plan for which particular packets are allowed to come in, are blocked at the Firewall. Now there are three types of Firewalls that you need to be familiar with on this exam. Packet Level, Application Level, and Circuit Level, and I'm going to give you the basics on each one of those. So Packet Level Firewall blocks incoming or outgoing traffic. We can block it either way based on the source address, the destination address, and of course, we're talking about IP addresses here, the Protocol type that's in the frame, or the datagram, or the packet, if you will, the source port number, or the destination port number. So we can block by any of those things, and of course, rules are created to perform that filtering. Now, you've got to be careful here because in most Firewall situations, and if you think about this, the first rule that applies to the packet is the one that happens, and so if you have multiple rules you have to make sure that you get them in the right order so that you cover all the various scenarios that you're concerned about. Now an Application Level Firewall operates at the Application, Presentation, and Session layers of the OSI Model. And really what that means in English is that it's looking at the actual application. What is the packet trying to do? And based on what it's trying to do, we'll block it or allow it in. Now an Application level type Firewall can act as a proxy for the applications that are requesting some sort of service, or the applications that live on the server behind the Firewall. So when someone hits the Firewall asking for a server to do something, this basically captures that request, forwards it to the machine, handles the request, brings the result back, and passes it back. So it performs all the interactions for the outside requester, and it keeps your private sources back behind the Firewall totally private and this is really cool. Now, Application Level Firewalls block system calls, and this is neat because when an application from the outside makes a request that normally goes to the OS kernel such as Open, Read, Write something, Close something, Wait, Execute, Kill something - it can determine whether I'm going to let that happen or not. Now the Application Level Firewall is generally regarded as the most secure type of Firewall that you can use. Now the last type that you need to be aware of is a Circuit Level Firewall. Now this one operates a lot differently from the Packet Level-based, or the Application, in that it's going to validate the sessions that are created between TCP Host or UDP Host, depending on whether you're using which of those Protocols, TCP or UDP. And if you remember from previous videos, TCP is reliable connection, UDP is unreliable, but it's going to make sure before we ever open a connection that it's going to make sure everybody's cool on both ends. We won't go into exactly how it's doing that, but that's what you need to know for now. So connections on both ends have to meet rules that we've set up and this can to be based on the IP address, source and destination, the time of day, the Protocol that's being used for the connection and for the data that's going to flow. And then you can take a look at the user and-or the user's password, and then once all this has happened and the rule says OK, you can come through, it's going to recheck this, or validate this data with every session of data exchange, OK? So, Circuit Level Firewalls operate just at a little higher level. In other words it says, wait a minute. You can't even connect up here, Dude, until I know who you are and what's going on. So anyway, that's the information you need on Firewalls. These are fascinating devices, and again, if you really want to nerd out, go out there and do some reading on Firewalls, some really cool stuff.
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