DHCP Starvation (DOS Attack - Penetration Testing) - Example Demonstration with Kali

DHCP Starvation is an attack that works by broadcasting vast numbers of DHCP requests with spoofed MAC addresses simultaneously.

I had showed my topology in the above snap, As you can see I have a router that acts as the DHCP server and a Kali linux 2.0 machine which is connected to the same network. The same steps can be followed to hack a Windows or a Linux DHCP server also. This is how my router's DHCP binding looks before the attempt,

Router (Before Attack)

R1#sh ip dhcp binding 
Bindings from all pools not associated with VRF:
IP address          Client-ID/              Lease expiration        Type
                    Hardware address/
                    User name
R1#

I used yersinia tool to perform the attack, this yersinia comes in built to your Kali operating system. Open terminal, type 'yersinia -G' and press enter.

Click on 'Launch Attack'. Select the tab 'DHCP' and check the second box 'sending DISCOVER packet' and press OK. Within seconds, hundreds of DHCP requests will be sent and the router will be busy handling all our requests and won't be able to handle IP addresses to genuine users. Below logs are taken after the attack,

Router (After Attack)

R1#sh ip dhcp binding 
% The DHCP database could not be locked. Please retry the command later.
R1#

Router couldn't respond, I had taken the below output after 10 mins.

R1#sh ip dhcp binding 
Bindings from all pools not associated with VRF:
IP address          Client-ID/              Lease expiration        Type
                    Hardware address/
                    User name
120.0.0.4           3669.9348.85d1          Mar 01 2002 12:20 AM    Automatic
120.0.0.5           7add.b556.f179          Mar 01 2002 12:20 AM    Automatic
120.0.0.6           347b.0c74.8013          Mar 01 2002 12:20 AM    Automatic
120.0.0.7           f976.c10d.205f          Mar 01 2002 12:20 AM    Automatic
120.0.0.8           b2de.b66b.5b30          Mar 01 2002 12:20 AM    Automatic
120.0.0.9           32cd.4b14.c1f7          Mar 01 2002 12:20 AM    Automatic
120.0.0.10          d0fd.bd17.a548          Mar 01 2002 12:20 AM    Automatic
120.0.0.11          e4be.180a.3fbd          Mar 01 2002 12:20 AM    Automatic
120.0.0.12          559b.e206.ffd0          Mar 01 2002 12:20 AM    Automatic
120.0.0.13          4e64.0231.a81d          Mar 01 2002 12:20 AM    Automatic
120.0.0.14          ac61.c36b.8931          Mar 01 2002 12:21 AM    Automatic
120.0.0.15          763d.5c2f.0d07          Mar 01 2002 12:21 AM    Automatic
120.0.0.16          d738.1831.3ffa          Mar 01 2002 12:21 AM    Automatic
120.0.0.17          0f64.dc31.3bfd          Mar 01 2002 12:21 AM    Automatic
120.0.0.18          beae.bf51.f15b          Mar 01 2002 12:21 AM    Automatic
120.0.0.19          b511.3b23.4732          Mar 01 2002 12:21 AM    Automatic
120.0.0.20          e682.902a.2069          Mar 01 2002 12:21 AM    Automatic
120.0.0.21          d253.6658.b71c          Mar 01 2002 12:21 AM    Automatic
120.0.0.22          11ed.8f0f.f330          Mar 01 2002 12:21 AM    Automatic
 --More-- 

As you can see, all the IPs are assigned to Kali's duplicate DHCP requests. To stop the attack, click on 'List attackts' and Cancel all attacks.

Mitigation

To secure our network from this attack, we have two options.

1) DHCP Snooping
2) Port Security

Cisco IOS Mitigation

To enable DHCP Snooping on a Cisco IOS switch, follow these steps:

switch(config)# ip dhcp snooping
!Enables DHCP Snooping globally!
switch(config)# ip dhcp snooping vlan <vlan_id> {,<vlan_id>}
!Enables DHCP Snooping for Specific VLANs!
switch(config-if)# ip dhcp snooping trust
!Sets the interface to trusted state; can then pass DHCP replies!
switch(config-if)# ip dhcp snooping limit rate <rate>
!Sets rate limit for DHCP Snooping!

To know more about DHCP snooping, visit http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-2SX/configuration/guide/book/snoodhcp.html

Cisco CatOS Mitigation

To mitigate DHCP Starvation attacks with port security on CatOS, use the following:

set port security 5/1 enable
set port security 5/1 port max 1
set port security 5/1 violation restrict
set port security 5/1 age 2
set port security 5/1 timer-type inactivity