A minimal application using libnetfilter_queue

nfq_example.c

#include <iostream>
#include <cstdlib>
#include <netinet/in.h>
extern "C" {
	#include <linux/netfilter.h>
	#include <libnetfilter_queue/libnetfilter_queue.h>
}

using namespace std;

void printBytes(char *input, int len) {
    static const char* const lut = "0123456789ABCDEF";
    char output[len * 2 + 1];
    for (int i = 0; i < len ; ++i) {
        const unsigned char c = input[i];
        output[i * 2] = lut[c >> 4];
        output[i * 2 + 1] = lut[c & 15];
    }
    output[len * 2] = '\0';
    cout << output << endl;
}

static int handleNfq(nfq_q_handle *myQueue, struct nfgenmsg *msg, nfq_data *pkt, void *cbData) {
	uint32_t id = 0;
	nfqnl_msg_packet_hdr *header;
	if ((header = nfq_get_msg_packet_hdr(pkt))) {
		id = ntohl(header->packet_id);
	}
	char *payload;
	int len = nfq_get_payload(pkt, &payload);
	if (len) {
		printBytes(payload, len);
	}
	return nfq_set_verdict(myQueue, id, NF_ACCEPT, 0, NULL);
}
int main(int argc, char **argv) {
	struct nfq_handle *nfqHandle;
	struct nfq_q_handle *myQueue;
	int fd, res;
	char buf[4096];
	if (!(nfqHandle = nfq_open())) {
		cerr << "Error in nfq_open()" << endl;
		exit(-1);
	}
	if (nfq_unbind_pf(nfqHandle, AF_INET) < 0) {
		cerr << "Error in nfq_unbind_pf()" << endl;
		exit(1);
	}
	if (nfq_bind_pf(nfqHandle, AF_INET) < 0) {
		cerr << "Error in nfq_bind_pf()" << endl;
		exit(1);
	}
	if (!(myQueue = nfq_create_queue(nfqHandle, 0, &handleNfq, NULL))) {
		cerr << "Error in nfq_create_queue()" << endl;
		exit(1);
	}
	if (nfq_set_mode(myQueue, NFQNL_COPY_PACKET, 0xffff) < 0) {
		cerr << "Could not set packet copy mode" << endl;
		exit(1);
	}
	fd  = nfq_fd(nfqHandle);
	cerr << "fd: " << fd << endl;
	while ((res = recv(fd, buf, sizeof(buf), 0)) && res >= 0) {
		cerr << "res: " << res << endl;
		nfq_handle_packet(nfqHandle, buf, res);
		printBytes(buf, res);
	}
	nfq_destroy_queue(myQueue);
	nfq_close(nfqHandle);
	return 0;
}