Re: Userland Netfilter

[Julien Vehent <julien@linuxwall.info>]

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On Thu, 30 Oct 2008 17:47:09 +0100, Sebastien Tricaud <stricaud@inl.fr>
wrote:
> On Thu, 2008-10-30 at 08:49 -0700, Gilad Benjamini wrote:
>> I need to create a userland simulation for filtering packets.
>> I remember running into a userland netfilter, but can't seem to find it.
>> Any
>> pointers or info would be appreciated.
>> Another option is to feed the packets into tun/tap devices, and let the
>> real
>> netfilter do the job. Performance, of course, is not a concern in my
>> case.
>> Does that sound reasonable ? Tun/tap seems to be an almost dead project.
>> Will it work in newer distributions ?
>> 
> 
> Hello Gilad,
> 
> I highly recommend you to use Pierre easy bindings, so that you can
> write your simulation in either Perl or Python :
> 
> http://software.inl.fr/trac/wiki/nfqueue-bindings
> 

Hi There,

Processing packets in perl or python ? Even if performances are not an
issue, you may want to be able to do something on the packet before the
user dies, no ?... :)

Just kidding, in fact, I found the regular netfilter queue library very
easy and convenient to use in C (see the code attached). And the
performances have nothing to be ashamed of...

http://netfilter.org/projects/libnetfilter_queue/downloads.html

Regards,
Julien

> 
> Regards,
> Sebastien Tricaud.
> 
> 
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[-- Attachment #2: netfilter_queue.c --]
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/*! netfilter_queue.c
 *
 \brief test file
 *
 \code gcc -o netfilter_queue netfilter_queue.c -lnetfilter_queue
 *
 */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <netinet/in.h>
#include <linux/netfilter.h>		/* for NF_ACCEPT */

#include <libnetfilter_queue/libnetfilter_queue.h>

/* returns packet id */
static u_int32_t print_pkt (struct nfq_data *tb)
{
	int id = 0;
	struct nfqnl_msg_packet_hdr *ph;
	u_int32_t mark,ifi;
	int ret;
	char *data;

	ph = nfq_get_msg_packet_hdr(tb);
	if (ph){
		id = ntohl(ph->packet_id);
		printf("hw_protocol=0x%04x hook=%u id=%u ",
			ntohs(ph->hw_protocol), ph->hook, id);
	}

	mark = nfq_get_nfmark(tb);
	if (mark)
		printf("mark=%u ", mark);

	ifi = nfq_get_indev(tb);
	if (ifi)
		printf("indev=%u ", ifi);

	ifi = nfq_get_outdev(tb);
	if (ifi)
		printf("outdev=%u ", ifi);

	ret = nfq_get_payload(tb, &data);
	if (ret >= 0)
		printf("payload_len=%d ", ret);

	fputc('\n', stdout);

	return id;
}


static int cb(struct nfq_q_handle *qh, struct nfgenmsg *nfmsg,
	      struct nfq_data *nfa, void *data)
{
	u_int32_t id = print_pkt(nfa);
	printf("entering callback\n");
	return nfq_set_verdict(qh, id, NF_ACCEPT, 0, NULL);
}

int main(int argc, char **argv)
{
	struct nfq_handle *h;
	struct nfq_q_handle *qh;
	struct nfnl_handle *nh;
	int fd;
	int rv;
	char buf[4096];

	printf("opening library handle\n");
	h = nfq_open();
	if (!h) {
		fprintf(stderr, "error during nfq_open()\n");
		exit(1);
	}

	printf("unbinding existing nf_queue handler for AF_INET (if any)\n");
	if (nfq_unbind_pf(h, AF_INET) < 0) {
		fprintf(stderr, "error during nfq_unbind_pf()\n");
		exit(1);
	}

	printf("binding nfnetlink_queue as nf_queue handler for AF_INET\n");
	if (nfq_bind_pf(h, AF_INET) < 0) {
		fprintf(stderr, "error during nfq_bind_pf()\n");
		exit(1);
	}

	printf("binding this socket to queue '0'\n");
	qh = nfq_create_queue(h,  0, &cb, NULL);
	if (!qh) {
		fprintf(stderr, "error during nfq_create_queue()\n");
		exit(1);
	}

	printf("setting copy_packet mode\n");
	if (nfq_set_mode(qh, NFQNL_COPY_PACKET, 0xffff) < 0) {
		fprintf(stderr, "can't set packet_copy mode\n");
		exit(1);
	}

	nh = nfq_nfnlh(h);
	fd = nfnl_fd(nh);

	while ((rv = recv(fd, buf, sizeof(buf), 0)) && rv >= 0) {
		printf("pkt received\n");
		nfq_handle_packet(h, buf, rv);
	}

	printf("unbinding from queue 0\n");
	nfq_destroy_queue(qh);

#ifdef INSANE
	/* normally, applications SHOULD NOT issue this command, since
	 * it detaches other programs/sockets from AF_INET, too ! */
	printf("unbinding from AF_INET\n");
	nfq_unbind_pf(h, AF_INET);
#endif

	printf("closing library handle\n");
	nfq_close(h);

	exit(0);
}