This is a tool that uses ARP poisoning  to  have  a  scenario
like this: we have a LAN and we want offer connectivity to every-
one coming here with his laptop for example. It could happen that
our  customer  has  his  network parameters already configured to
work correctly in his own LAN, but not working here. We can  have
then this scenario:
Customer's  host (10.0.0.2/8 and default gateway set to 10.0.0.1)
Our LAN (192.168.0.0/24 with real gateway 192.168.0.254).
All that we want is that our customer plugs his laptop  and  joins
the  internet without changing nothing of his network parameters.
Here comes this tool installed in my real gw(192.168.0.254)  It's
a  sort  of sniffer, because it sniffs broadcast ARP requests for
the gateway and answers that the gateway is itself In our example
our  customer's  laptop  sends  this request: arp who-has 10.0.0.1
tell 10.0.0.2 Now our gateway does the following: 1)  Sends  back
this  reply to 10.0.0.2: arp reply 10.0.0.1 is-at his_mac_address
2)Create the alias 10.0.0.254 (ARP is not routable so we need one
alias  for each subnet that is not our one) 3)Sends itself an ARP
reply to refresh his ARP cache
It is different from proxy arp for two reasons: first it runs  in
user  space,  then in this case we can plug machines belonging to
whatever subnet, while proxy arp is used in the case of only  two
different ones.