Netfilter Queue Notes






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Netfilter Queue Notes

Netfilter Queue Notes

Netfilter Queue improves on IPQUEUE in several ways, most notably by providing for multiple queues (similar to ipqd).

Simple Example


This is a simple program to print information about intercepted packets, and mark them to be forwarded on. Note that the logic as to which packets to intercept is handled via iptables, explained below.

/* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 * Copyright (c) 2007 Joe Kopena, Drexel University
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation;
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * GNU General Public License for more details.
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 * Authors: Joe Kopena <tjkopena@cs.drexel.edu>

#include <iostream>
#include <iomanip>

#include <time.h>

#include <netinet/in.h>
extern "C" {
  #include <linux/netfilter.h>  /* Defines verdicts (NF_ACCEPT, etc) */
  #include <libnetfilter_queue/libnetfilter_queue.h>

using namespace std;

static int Callback(nfq_q_handle *myQueue, struct nfgenmsg *msg,
		    nfq_data *pkt, void *cbData) {
  uint32_t id = 0;
  nfqnl_msg_packet_hdr *header;

  cout << "pkt recvd: ";
  if ((header = nfq_get_msg_packet_hdr(pkt))) {
    id = ntohl(header->packet_id);
    cout << "id " << id << "; hw_protocol " << setfill('0') << setw(4) <<
      hex << ntohs(header->hw_protocol) << "; hook " << ('0'+header->hook)
	 << " ; ";

  // The HW address is only fetchable at certain hook points
  nfqnl_msg_packet_hw *macAddr = nfq_get_packet_hw(pkt);
  if (macAddr) {
    cout << "mac len " << ntohs(macAddr->hw_addrlen) << " addr ";
    for (int i = 0; i < 8; i++) {
      cout << setfill('0') << setw(2) << hex << macAddr->hw_addr;
    // end if macAddr
  } else {
    cout << "no MAC addr";

  timeval tv;
  if (!nfq_get_timestamp(pkt, &tv)) {
    cout << "; tstamp " << tv.tv_sec << "." << tv.tv_usec;
  } else {
    cout << "; no tstamp";

  cout << "; mark " << nfq_get_nfmark(pkt);

  // Note that you can also get the physical devices
  cout << "; indev " << nfq_get_indev(pkt);
  cout << "; outdev " << nfq_get_outdev(pkt);

  cout << endl;

  // Print the payload; in copy meta mode, only headers will be included;
  // in copy packet mode, whole packet will be returned.
  char *pktData;
  int len = nfq_get_payload(pkt, &pktData);
  if (len) {
    cout << "data[" << len << "]: '";
    for (int i = 0; i < len; i++) {
      if (isprint(pktData[i]))
	cout << pktData[i];
      else cout << " ";
    cout << "'" << endl;
    // end data found

  // For this program we'll always accept the packet...
  return nfq_set_verdict(myQueue, id, NF_ACCEPT, 0, NULL);

  // end Callback

int main(int argc, char **argv) {
  struct nfq_handle *nfqHandle;

  struct nfq_q_handle *myQueue;
  struct nfnl_handle *netlinkHandle;

  int fd, res;
  char buf[4096];

  // Get a queue connection handle from the module
  if (!(nfqHandle = nfq_open())) {
    cerr << "Error in nfq_open()" << endl;

  // Unbind the handler from processing any IP packets
  // Not totally sure why this is done, or if it's necessary...
  if (nfq_unbind_pf(nfqHandle, AF_INET) < 0) {
    cerr << "Error in nfq_unbind_pf()" << endl;

  // Bind this handler to process IP packets...
  if (nfq_bind_pf(nfqHandle, AF_INET) < 0) {
    cerr << "Error in nfq_bind_pf()" << endl;

  // Install a callback on queue 0
  if (!(myQueue = nfq_create_queue(nfqHandle,  0, &Callback, NULL))) {
    cerr << "Error in nfq_create_queue()" << endl;

  // Turn on packet copy mode
  if (nfq_set_mode(myQueue, NFQNL_COPY_PACKET, 0xffff) < 0) {
    cerr << "Could not set packet copy mode" << endl;

  netlinkHandle = nfq_nfnlh(nfqHandle);
  fd = nfnl_fd(netlinkHandle);

  while ((res = recv(fd, buf, sizeof(buf), 0)) && res >= 0) {
    // I am not totally sure why a callback mechanism is used
    // rather than just handling it directly here, but that
    // seems to be the convention...
    nfq_handle_packet(nfqHandle, buf, res);
    // end while receiving traffic



  return 0;

  // end main


Do something like this to compile the program:

  g++ -o nftest nftest.cc -lnetfilter_queue


The following command will pass all outgoing ICMP messages to the program; more correctly, it passes them to queue 0, on which the program installs itself:

  sudo iptables -A OUTPUT -p icmp -j NFQUEUE --queue-num 0

This installs a rule in the OUTPUT chain to direct ICMP traffic to NFQUEUE, and tells NFQUEUE to shunt them into queue 0. To run the program just execute as normal, but note that it'll require superuser privileges. The order of starting the program and starting the rule doesn't really matter. Note that if nothing is installed on the queue to set ACCEPT verdicts, the packets will be dropped. To remove the rule, do something like:

  sudo iptables -D OUTPUT -p icmp -j NFQUEUE --queue-num 0

You can also pass a rule number to iptables. If there are no rules, the default is usually to accept. You can see all the rules installed in all the chains using iptables -L.


There's not a lot, although the API is pretty small. The best is from Brad Fisher, posted here. I've also cached a copy here for safety's sake.

Note that the question in there about a possible bug in the HW address length was later answered in the negative; you simply need to run it through ntohs() on some platforms.

Also, Gregor Maier answered in this message the questions about verdict values, noting that they consist of NF_DROP, NF_ACCEPT, NF_REPEAT, NF_QUEUE.

Finally, note that Brad later noted here that the timestamp never seems to be set, regardless of the hook used.

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