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ARP cache poisoning / ARP spoofing (MIT) February 8, 2011

Posted by Tournas Dimitrios in Linux admin tools.

This article assumes that you are already familiar with the functionality of the arp protocol , as we already know it is the foundation for LAN addressing . Many people think that once they use a switch for connecting their local network they’re safe from network sniffing . Basically this is right because the traditional way of sniffing was to force a host to read all network packets ( the so called “promiscuous mode” ) .On a switched network this is not applicable because each computer has only access to traffic that is destined  to his own IP address .

arp-spoofing is also known as MIT (man in the middle attack)

However there are other means to achieve the same and because  some SysAdmins think they’re safe from sniffing thus designing their network a bit more open it’s even more dangerous .

The tool used here is called arpspoof  and is distributed in the dsniff package . This package is provided from the EPEL repository for a CentOs 5.x  distribution , just install it with :
yum install dsniff .

What we do is the following : we constantly send the victim computer ARP answers telling him that the MAC address belonging to the IP of the gateway machine (router) is our MAC address . After some time the victim computer will believe us and makes a wrong entry in his ARP cache. Next time the victim wants to send an IP packet to the router , it will use our MAC address , so actually we get the IP packets . Of course we do the same thing with the gateway (router) machine just the other way round . Yes it is a weak point of the ARP protocol , it works passively , so each computer that receives an ARP answer it will update it’s arp table even if no ARP request were made . Different jargon’s are used to describe this technique : arp spoofing , arp poisoning , MIT man in the middle .

The process is simple :

  1. In order to tell the victim host that now we (our MAC address) are the one belonging to the IP of the gateway enter the following command :
    arpspoof  -i eth0  -t  victimIP  gatewayIP
  2. In a separate shell we start the matching command to fool the gateway to believe we are victimIP .
    arpspoof -i eth0  -t   gatewayIP  victimIP
  3. Don’t forget to enable IP forwarding on your host so that the traffic goes through your host . Otherwise victim will loose connectivity .
    sysctl -w  net.ipv4.ip_forward=1
  4. Now watch all the traffic between the victim host and the outside network going throuhg your machine
    tcpdump  host  victimIP  and not arp

Step 2/3 can be combined on one terminal :
arpspoof -t victimIP  gatewayIP   2&>/dev/null
arpspoof -t gatewayIP  victimIP   2&>/dev/null
killall  arpspoof
The “& >/dev/null ” part is there to make it easier to run from one terminal but you may want to omit it for debugging purposes .

SysAdmins beware , as you see the process is easily to implement . Always use tools like arpwatch to monitor the changes of MAC / IP  address table ..

Reading :


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