Embedded Linux Experts

 *      SNMP service broadcast connection tracking helper
 *      (c) 2011 Jiri Olsa <>
 *      This program is free software; you can redistribute it and/or
 *      modify it under the terms of the GNU General Public License
 *      as published by the Free Software Foundation; either version
 *      2 of the License, or (at your option) any later version.
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/in.h>

#include <net/netfilter/nf_conntrack.h>
#include <net/netfilter/nf_conntrack_helper.h>
#include <net/netfilter/nf_conntrack_expect.h>
#include <linux/netfilter/nf_conntrack_snmp.h>

#define SNMP_PORT	161

MODULE_AUTHOR("Jiri Olsa <>");
MODULE_DESCRIPTION("SNMP service broadcast connection tracking helper");

static unsigned int timeout __read_mostly = 30;
module_param(timeout, uint, S_IRUSR);
MODULE_PARM_DESC(timeout, "timeout for master connection/replies in seconds");

int (*nf_nat_snmp_hook)(struct sk_buff *skb,
			unsigned int protoff,
			struct nf_conn *ct,
			enum ip_conntrack_info ctinfo);

static int snmp_conntrack_help(struct sk_buff *skb, unsigned int protoff,
		struct nf_conn *ct, enum ip_conntrack_info ctinfo)
	typeof(nf_nat_snmp_hook) nf_nat_snmp;

	nf_conntrack_broadcast_help(skb, protoff, ct, ctinfo, timeout);

	nf_nat_snmp = rcu_dereference(nf_nat_snmp_hook);
	if (nf_nat_snmp && ct->status & IPS_NAT_MASK)
		return nf_nat_snmp(skb, protoff, ct, ctinfo);

	return NF_ACCEPT;

static struct nf_conntrack_expect_policy exp_policy = {
	.max_expected	= 1,

static struct nf_conntrack_helper helper __read_mostly = {
	.name			= "snmp",
	.tuple.src.l3num	= NFPROTO_IPV4,
	.tuple.src.u.udp.port	= cpu_to_be16(SNMP_PORT),
	.tuple.dst.protonum	= IPPROTO_UDP,
	.me			= THIS_MODULE,
	.help			= snmp_conntrack_help,
	.expect_policy		= &exp_policy,

static int __init nf_conntrack_snmp_init(void)
	exp_policy.timeout = timeout;
	return nf_conntrack_helper_register(&helper);

static void __exit nf_conntrack_snmp_fini(void)