All language subtitles for [English (United States)] Free CCNA _ Kali Linux Demo _ Day 48 Lab _ CCNA 200-301 Complete Course [DownSub.com]

Welcome to Jeremy’s IT Lab. This is a free,
complete course for the CCNA. If you like

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This video, as you have probably noticed,
will be a bit different than my other lab

videos in the course. We didn’t actually
cover any configurations in the lecture video,

so there won’t be a packet tracer practice
lab file for you to practice with. Instead,

I will just give you a quick demo in EVE-NG,
a network simulator, of one kind of network

attack that I mentioned in the lecture video.
Then, in the next lecture video we will see

how to configure a Cisco switch to prevent
this kind of attack.

The attack I will show you is the DHCP exhaustion
attack, also called DHCP starvation attack.

In this attack, the attacker sends countless
DHCP Discover messages, and then any DHCP

servers on the network will send Offer messages.
After the server sends the DHCP offer, the

IP address it offers is reserved while it
waits for a response from the client. So,

by sending tons of Discover messages, the
attacker uses up all of the IP addresses in

the DHCP pool, and other hosts on the network
will not be able to get an IP address. To

demonstrate that I will connect PC1 to the
network, and we’ll see if it can get an

address via DHCP.

In this network, R1 is the DHCP server. The
attacker is this PC here, running a Linux

distribution called Kali Linux. Kali Linux
is popular in the world of cyber security,

penetration testing, and such. It comes with
various pre-installed tools for digital forensics

and penetration testing, and we will be using
one called Yersinia for the DHCP exhaustion

attack.

First let’s open the CLI of R1 to take a
look at the DHCP settings. ENABLE. SHOW RUN

| SECTION DHCP. Okay, so here’s the DHCP
pool. 192.168.1.0/24, and it will tell hosts

to use R1’s own IP address, 192.168.1.1,
as the default gateway. The DNS server is

8.8.8.8, Google’s DNS server. This IP ADDRESS
DHCP at the bottom isn’t actually part of

the DHCP pool, it’s from R1’s G0/1 interface
settings, but it was included in the output

of SHOW RUN | SECTION DHCP,. Now let’s use
some other show commands. SHOW IP DHCP POOL.

As you can see, in POOL1 there are 254 available
addresses. I didn’t configure any excluded

addresses. Note that R1 won’t assign it’s
own IP address, 192.168.1.1, to another host,

so really there are 253 available addresses
that can be assigned. However, currently there

are 0 leased addresses. And if I use the command
SHOW IP DHCP BINDING, we can see that there

are currently no entries.

So, let’s go into the Kali Linux box. I
already have the terminal open, so let’s

open Yersinia. The command is YERSINIA hyphen
G. So, this opens the GUI, graphical user

interface, of Yersinia. To launch our attack
I’ll click on ‘launch attack’. Then

select DHCP, and there are a few DHCP attacks
we can do. In this case I want to select ‘sending

DISCOVER packet’. As the check mark indicates
here, this will cause a DoS, Denial of Service.

Then I simply click OK, and the attack has
started. On the right you can see each DHCP

DISCOVER message being sent, and here on the
left you can see the DHCP packet count rapidly

rising. Unless I tell the attacker to stop,
it will continue sending these DISCOVER messages.

So, let’s see what’s happening on R1.

First let’s check the pool again. SHOW IP
DHCP POOL. Okay, of the 254 total addresses

253 are being leased, and as I said one is
used by R1 itself so that means the entire

pool is being used. Let’s check the binding
table. SHOW IP DHCP BINDING. Okay, I’m just

going to keep pressing the space bar as R1
goes through the DHCP binding table. You’ll

see that every address, from 192.168.1.2 to
192.168.1.254, is currently taken. Also notice

that each of these IP addresses has a unique
MAC address. That’s because the attacker

is spoofing the source MAC address used for
each DISCOVER message, so R1 thinks that each

of the messages is coming from a different
host.

Okay, let’s see if PC1 is able to get an
IP address. Before connecting it to the network,

I’ll check some settings on PC1. Here in
the command prompt let’s check IPCONFIG

/ALL. Okay, here under the Ethernet adapter
we can see media disconnected, because it’s

currently not connected to the network. And
we can see DHCP Enabled, Yes. So, once we

connect PC1 to SW1, it should try to get an
IP address via DHCP. Let’s see if it works.

So, I’ll click here on PC1 and connect it
to SW1. PC1’s interface is e0 and SW1’s

is G0/1. So I’ll just click on save.

Now let’s check on PC1 again. From the commad
prompt, IPCONFIG /ALL. Okay, under the Ethernet

adapter we can once again see DHCP Enabled,
Yes, It has a link-local IPv6 address, automatically

generated. And below that we see an IPv4 address.
So, did DHCP work? Actually, no it didn’t.

As it says here, this is an autoconfigured
IPv4 address, PC1 generated it automatically.

It has the prefix 169.254/16, the subnet mask
is 255.255.0.0. 169.254.0.0/16 is the IPv4

link-local address range, so it functions
like the IPv6 link-local address above. So,

PC1 auto-generated IPv4 and IPv6 link-local
addresses, but it wasn’t able to get a regular

IPv4 address from the DHCP server, R1. So,
PC1 won’t be able to communicate with any

other devices on the network. If I try to
ping R1 for example, PING 192.168.1.1, as

you can see it doesn’t work. What if I try
to renew the IP address? IPCONFIG /RENEW.

So, PC1 will once again try to get an IPv4
address via DHCP, but once again it won’t

work. We could wait here for a while, so I’ll
just use control C to stop it.

Next, I’ll delete the connection from the
attacker to SW1 by right clicking here and

selecting delete. Next I’ll go on to R1
and clear the DHCP bindings. I taught you

this command in the DHCP video, the command
is CLEAR IP DHCP BINDING, followed by an asterisk.

Okay, now I’ll go back on PC1 and try to
get an IP address once more. IPCONFIG /RENEW.

This time, R1 should have an available IP
address for PC1 now that the attack has stopped,

and indeed PC1 was able to get an IP address.
So let’s test if it can use the Internet.

I’ll open firefox and go to google.com.
Okay, PC1 is now able to access the Internet.

So, in this video we took a look at one kind
of network attack, the DHCP exhaustion attack.

Although there is no packet tracer lab file
for you to practice with this time, I hope

it was at least interesting to see. In the
next video we’ll look at a Cisco switch

feature called port security, which can be
used to prevent this kind of attack, as well

as others. Okay, that’s all for this lab.

Before finishing today’s video I want to
thank my JCNP-level channel members. To join,

please click the ‘Join’ button under the
video. Thank you to C Mohd, Scott, Martin,

Khoa, Justin, Christopher, Tebogo, Anand,
Pavel, Abraham, Serge, Njoku, Viktor, Roger,

Raj, Kenneth, Seamus, Brandon, Marcel, Kone,
Donald, Gustavo, Prakaash, Nasir, Erlison,

Marko, Daming, Ed, John, Funnydart, Velvijaykum,
Mark, Yousif, Boson Software, Devin, Yonatan,

and Vance. Sorry if I pronounced your name
incorrectly, but thank you so much for your

support. This is the list of JCNP-level members
at the time of recording by the way, May 13th

2021. If you signed up recently and your name
isn’t on here don’t worry, you’ll be

in future videos.

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