This is an ARP monitor. It keeps track of mappings between IP addresses and hardware addresses as informed by ARP replies. Everytime two different hardware addresses are mapped to the same IP address, the event is informed.

ARP Monitor output

You can download this code here.

/*
* Copyright (c) 2016, Matias Fontanini
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are
* met:
*
* * Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* * Redistributions in binary form must reproduce the above
* copyright notice, this list of conditions and the following disclaimer
* in the documentation and/or other materials provided with the
* distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
* OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
* LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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*/


#include <tins/tins.h>
#include <map>
#include <iostream>
#include <functional>

using std::cout;
using std::endl;
using std::map;
using std::bind;

using namespace Tins;

class arp_monitor {
public:
void run(Sniffer& sniffer);
private:
bool callback(const PDU& pdu);

map<IPv4Address, HWAddress<6>> addresses;
};

void arp_monitor::run(Sniffer& sniffer) {
sniffer.sniff_loop(
bind(
&arp_monitor::callback,
this,
std::placeholders::_1
)
);
}

bool arp_monitor::callback(const PDU& pdu) {
// Retrieve the ARP layer
const ARP& arp = pdu.rfind_pdu<ARP>();
// Is it an ARP reply?
if (arp.opcode() == ARP::REPLY) {
// Let's check if there's already an entry for this address
auto iter = addresses.find(arp.sender_ip_addr());
if (iter == addresses.end()) {
// We haven't seen this address. Save it.
addresses.insert({ arp.sender_ip_addr(), arp.sender_hw_addr()});
cout << "[INFO] " << arp.sender_ip_addr() << " is at "
<< arp.sender_hw_addr() << std::endl;
}
else {
// We've seen this address. If it's not the same HW address, inform it
if (arp.sender_hw_addr() != iter->second) {
cout << "[WARNING] " << arp.sender_ip_addr() << " is at "
<< iter->second << " but also at " << arp.sender_hw_addr()
<< endl;
}
}
}
return true;
}

int main(int argc, char* argv[]) {
if(argc != 2) {
cout << "Usage: " <<* argv << " <interface>" << endl;
return 1;
}
arp_monitor monitor;
// Sniffer configuration
SnifferConfiguration config;
config.set_promisc_mode(true);
config.set_filter("arp");

try {
// Sniff on the provided interface in promiscuous mode
Sniffer sniffer(argv[1], config);

// Only capture arp packets
monitor.run(sniffer);
}
catch (std::exception& ex) {
std::cerr << "Error: " << ex.what() << std::endl;
}
}