This small example shows how to sniff packets from wireless interfaces, capture 802.11 beacon frames, and display the SSID associated with each BSSID.

Note that depending on the interface and driver used, 802.11 packets could contain RadioTap encapsulation. This means that the first PDU in the PDU chain is not necessarily some subclass of Dot11. That is why you should always use the PDU::find_pdu method template to find the layer you're looking for.

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
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#include <iostream>
#include <set>
#include <string>
#include <tins/tins.h>

using std::set;
using std::cout;
using std::endl;
using std::string;
using std::runtime_error;

using namespace Tins;

class BeaconSniffer {
public:
void run(const string& iface);
private:
typedef Dot11::address_type address_type;
typedef set<address_type> ssids_type;

bool callback(PDU& pdu);

ssids_type ssids;
};

void BeaconSniffer::run(const std::string& iface) {
SnifferConfiguration config;
config.set_promisc_mode(true);
config.set_filter("type mgt subtype beacon");
config.set_rfmon(true);
Sniffer sniffer(iface, config);
sniffer.sniff_loop(make_sniffer_handler(this, &BeaconSniffer::callback));
}

bool BeaconSniffer::callback(PDU& pdu) {
// Get the Dot11 layer
const Dot11Beacon& beacon = pdu.rfind_pdu<Dot11Beacon>();
// All beacons must have from_ds == to_ds == 0
if (!beacon.from_ds() && !beacon.to_ds()) {
// Get the AP address
address_type addr = beacon.addr2();
// Look it up in our set
ssids_type::iterator it = ssids.find(addr);
if (it == ssids.end()) {
// First time we encounter this BSSID.
try {
/* If no ssid option is set, then Dot11::ssid will throw
* a std::runtime_error.
*/

string ssid = beacon.ssid();
// Save it so we don't show it again.
ssids.insert(addr);
// Display the tuple "address - ssid".
cout << addr << " - " << ssid << endl;
}
catch (runtime_error&) {
// No ssid, just ignore it.
}
}
}
return true;
}

int main(int argc, char* argv[]) {
if (argc != 2) {
cout << "Usage: " <<* argv << " <interface>" << endl;
return 1;
}
string interface = argv[1];
BeaconSniffer sniffer;
sniffer.run(interface);
}