This is a simple SYN scanner. It takes an IP address and a variable number of ports as command line arguments, and verifies if that host has those ports open or not.

This is done by sending TCP frames with the SYN flag on. If the host's answer has the RST flag on, then the port is closed, if it contains the SYN and ACK flag on, then it's open. This doesn't take into account filtered ports, so you won't see any output for any ports that are filtered by some firewall.

One thing to notice is how sniffing is stopped. libpcap, which is what the Sniffer class uses to achieve sniffing, can be stopped using the pcap_breakloop only from the thread that is actually sniffing. In most of the cases, this is completely useless, since you are probably sniffing on one thread, and sending packets or processing them in another.

That is why applications that use the Sniffer class usually use the same mechanism to stop sniffing: define a "special packet" that will indicate that sniffing has stopped. You can also use some flag as the return value of the sniffer functor, modify it when you're done and send a dummy packet so that the function checks the flag and ends. In this case, we use the first one, but generally the second one is simpler to implement.

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 <iostream>
#include <iomanip>
#include <vector>
#include <set>
#include <string>
#include <cstdlib>
#include <pthread.h>
#include <unistd.h>
#include <tins/ip.h>
#include <tins/tcp.h>
#include <tins/ip_address.h>
#include <tins/ethernetII.h>
#include <tins/network_interface.h>
#include <tins/sniffer.h>
#include <tins/utils.h>
#include <tins/packet_sender.h>

using std::cout;
using std::endl;
using std::vector;
using std::pair;
using std::setw;
using std::string;
using std::set;
using std::runtime_error;

using namespace Tins;

typedef pair<Sniffer*, string> sniffer_data;

class Scanner {
public:
Scanner(const NetworkInterface& interface,
const IPv4Address& address,
const vector<string>& ports);

void run();
private:
void send_syns(const NetworkInterface& iface, IPv4Address dest_ip);
bool callback(PDU& pdu);
static void* thread_proc(void* param);
void launch_sniffer();

NetworkInterface iface;
IPv4Address host_to_scan;
set<uint16_t> ports_to_scan;
Sniffer sniffer;
};

Scanner::Scanner(const NetworkInterface& interface,
const IPv4Address& address,
const vector<string>& ports)
: iface(interface), host_to_scan(address), sniffer(interface.name()) {
sniffer.set_filter(
"tcp and ip src " + address.to_string() + " and tcp[tcpflags] & (tcp-rst|tcp-syn) != 0"
);
for (size_t i = 0; i < ports.size(); ++i) {
ports_to_scan.insert(atoi(ports[i].c_str()));
}
}

void* Scanner::thread_proc(void* param) {
Scanner* data = (Scanner*)param;
data->launch_sniffer();
return 0;
}

void Scanner::launch_sniffer() {
sniffer.sniff_loop(make_sniffer_handler(this, &Scanner::callback));
}

/* Our scan handler. This will receive SYNs and RSTs and inform us
* the scanned port's status.
*/

bool Scanner::callback(PDU& pdu) {
// Find the layers we want.
const IP& ip = pdu.rfind_pdu<IP>();
const TCP& tcp = pdu.rfind_pdu<TCP>();
// Check if the host that we're scanning sent this packet and
// the source port is one of those that we scanned.
if(ip.src_addr() == host_to_scan && ports_to_scan.count(tcp.sport()) == 1) {
// Ok, it's a TCP PDU. Is RST flag on? Then port is closed.
if(tcp.get_flag(TCP::RST)) {
// This indicates we should stop sniffing.
if(tcp.get_flag(TCP::SYN))
return false;
cout << "Port: " << setw(5) << tcp.sport() << " closed\n";
}
// Is SYN flag on? Then port is open!
else if(tcp.flags() == (TCP::SYN | TCP::ACK)) {
cout << "Port: " << setw(5) << tcp.sport() << " open\n";
}
}
return true;
}

void Scanner::run() {
pthread_t thread;
// Launch our sniff thread.
pthread_create(&thread, 0, &Scanner::thread_proc, this);
// Start sending SYNs to port.
send_syns(iface, host_to_scan);

// Wait for our sniffer.
void* dummy;
pthread_join(thread, &dummy);
}

// Send syns to the given ip address, using the destination ports provided.
void Scanner::send_syns(const NetworkInterface& iface, IPv4Address dest_ip) {
// Retrieve the addresses.
NetworkInterface::Info info = iface.addresses();
PacketSender sender;
// Allocate the IP PDU
IP ip = IP(dest_ip, info.ip_addr) / TCP();
// Get the reference to the TCP PDU
TCP& tcp = ip.rfind_pdu<TCP>();
// Set the SYN flag on.
tcp.set_flag(TCP::SYN, 1);
// Just some random port.
tcp.sport(1337);
cout << "Sending SYNs..." << endl;
for (set<uint16_t>::const_iterator it = ports_to_scan.begin(); it != ports_to_scan.end(); ++it) {
// Set the new port and send the packet!
tcp.dport(*it);
sender.send(ip);
}
// Wait 1 second.
sleep(1);
/* Special packet to indicate that we're done. This will be sniffed
* by our function, which will in turn return false.
*/

tcp.set_flag(TCP::RST, 1);
tcp.sport(*ports_to_scan.begin());
// Pretend we're the scanned host...
ip.src_addr(dest_ip);
// We use an ethernet pdu, otherwise the kernel will drop it.
EthernetII eth = EthernetII(info.hw_addr, info.hw_addr) / ip;
sender.send(eth, iface);
}

void scan(int argc, char* argv[]) {
IPv4Address ip(argv[1]);
// Resolve the interface which will be our gateway
NetworkInterface iface(ip);
cout << "Sniffing on interface: " << iface.name() << endl;

// Consume arguments
argv += 2;
argc -= 2;
Scanner scanner(iface, ip, vector<string>(argv, argv + (argc)));
scanner.run();
}

int main(int argc, char* argv[]) {
if (argc < 3) {
cout << "Usage: " <<* argv << " <IPADDR> <port1> [port2] [port3]" << endl;
return 1;
}
try {
scan(argc, argv);
}
catch(runtime_error& ex) {
cout << "Error - " << ex.what() << endl;
}
}