Subject: Passive OS fingerprint xtables match.

From: Evgeniy Polyakov <[email-masked]>
Date: Thu, 29 Jan 2009 20:20:30 +0300

Hi.

Passive OS fingerprinting netfilter module allows to passively detect
remote OS and perform various netfilter actions based on that knowledge.
This module compares some data (WS, MSS, options and it's order, ttl, df
and others) from packets with SYN bit set with dynamically loaded OS
fingerprints.

Fingerprint matching rules can be downloaded from OpenBSD source tree
and loaded via netlink connector into the kernel via special util found
in archive. It will also listen for events about matching packets.

Archive also contains library file (also attached), which was shipped
with iptables extensions some time ago (at least when ipt_osf existed
in patch-o-matic).

This release moves all rules initialization to be handled over the
netlink and introduces lookup tables to speed-up RCU finger matching
a bit. Also fixed module unloading RCU completion race noticed by
Paul McKenney.

Fingerprints can be downloaded from
http://www.openbsd.org/cgi-bin/cvsweb/src/etc/pf.os

Example usage:
# modrpobe ipt_osf
# ./ucon_osf -f ./pf.os
^C Daemon will listen for incoming match events 
-d switch removes fingerprints
# iptables -I INPUT -j ACCEPT -p tcp -m osf --genre Linux --log 0 --ttl 2 --connector

You will find something like this in the syslog:
ipt_osf: Windows [2000:SP3:Windows XP Pro SP1, 2000 SP3]: 11.22.33.55:4024 -> 11.22.33.44:139

Passive OS fingerprint homepage (archives, examples):
http://www.ioremap.net/projects/osf

Signed-off-by: Evgeniy Polyakov <[email-masked]>

diff --git a/include/linux/connector.h b/include/linux/connector.h
index 5c7f946..b77e6fa 100644
--- a/include/linux/connector.h
+++ b/include/linux/connector.h
@@ -39,6 +39,8 @@
 #define CN_IDX_V86D			0x4
 #define CN_VAL_V86D_UVESAFB		0x1
 #define CN_IDX_BB			0x5	/* BlackBoard, from the TSP GPL sampling framework */
+#define CN_IDX_OSF			0x6	/* Passive OS fingerprint iptables module */
+#define CN_VAL_OSF			0x6
 
 #define CN_NETLINK_USERS		6
 
diff --git a/include/linux/netfilter_ipv4/ipt_osf.h b/include/linux/netfilter_ipv4/ipt_osf.h
new file mode 100644
index 0000000..c2d2c7a
--- /dev/null
+++ b/include/linux/netfilter_ipv4/ipt_osf.h
@@ -0,0 +1,120 @@
+/*
+ * ipt_osf.h
+ *
+ * Copyright (c) 2003 Evgeniy Polyakov <[email-masked]>
+ *
+ *
+ * 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.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * 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
+ */
+
+#ifndef _IPT_OSF_H
+#define _IPT_OSF_H
+
+#define MAXGENRELEN		32
+#define MAXDETLEN		64
+
+#define IPT_OSF_GENRE		(1<<0)
+#define	IPT_OSF_TTL		(1<<1)
+#define IPT_OSF_LOG		(1<<2)
+#define IPT_OSF_UNUSED		(1<<3)
+#define IPT_OSF_CONNECTOR	(1<<4)
+#define IPT_OSF_INVERT		(1<<5)
+
+#define IPT_OSF_LOGLEVEL_ALL	0
+#define IPT_OSF_LOGLEVEL_FIRST	1
+#define IPT_OSF_LOGLEVEL_ALL_KNOWN	2
+
+#define IPT_OSF_TTL_TRUE	0	/* True ip and fingerprint TTL comparison */
+#define IPT_OSF_TTL_LESS	1	/* Check if ip TTL is less than fingerprint one */
+#define IPT_OSF_TTL_NOCHECK	2	/* Do not compare ip and fingerprint TTL at all */
+
+#define MAX_IPOPTLEN		40
+
+struct ipt_osf_info {
+	char			genre[MAXGENRELEN];
+	__u32			len;
+	__u32			flags;
+	__u32			loglevel;
+	__u32			ttl;
+};
+
+/*
+ * Wildcard MSS (kind of).
+ */
+struct ipt_osf_wc {
+	__u32			wc;
+	__u32			val;
+};
+
+/*
+ * This struct represents IANA options
+ * http://www.iana.org/assignments/tcp-parameters
+ */
+struct ipt_osf_opt {
+	__u16			kind, length;
+	struct ipt_osf_wc	wc;
+};
+
+struct ipt_osf_user_finger {
+	struct ipt_osf_wc wss;
+
+	__u8 ttl, df;
+	__u16 ss, mss;
+	int opt_num;
+
+	char genre[MAXGENRELEN];
+	char version[MAXGENRELEN];
+	char subtype[MAXGENRELEN];
+
+	/* MAX_IPOPTLEN is maximum if all options are NOPs or EOLs */
+	struct ipt_osf_opt opt[MAX_IPOPTLEN];
+};
+
+struct ipt_osf_nlmsg {
+	struct ipt_osf_user_finger	f;
+	struct iphdr		ip;
+	struct tcphdr		tcp;
+};
+
+/* Defines for IANA option kinds */
+
+#define OSFOPT_EOL		0	/* End of options */
+#define OSFOPT_NOP		1	/* NOP */
+#define OSFOPT_MSS		2	/* Maximum segment size */
+#define OSFOPT_WSO		3	/* Window scale option */
+#define OSFOPT_SACKP		4	/* SACK permitted */
+#define OSFOPT_SACK		5	/* SACK */
+#define OSFOPT_ECHO		6
+#define OSFOPT_ECHOREPLY	7
+#define OSFOPT_TS		8	/* Timestamp option */
+#define OSFOPT_POCP		9	/* Partial Order Connection Permitted */
+#define OSFOPT_POSP		10	/* Partial Order Service Profile */
+#define OSFOPT_EMPTY		255
+/* Others are not used in current OSF */
+
+#ifdef __KERNEL__
+
+#include <linux/list.h>
+#include <net/tcp.h>
+
+struct ipt_osf_finger {
+	struct rcu_head			rcu_head;
+	struct list_head		finger_entry;
+	struct ipt_osf_user_finger	finger;
+};
+
+#endif				/* __KERNEL__ */
+
+#endif				/* _IPT_OSF_H */
diff --git a/net/ipv4/netfilter/Kconfig b/net/ipv4/netfilter/Kconfig
index 3816e1d..280f779 100644
--- a/net/ipv4/netfilter/Kconfig
+++ b/net/ipv4/netfilter/Kconfig
@@ -101,6 +101,16 @@ config IP_NF_MATCH_TTL
 
 	  To compile it as a module, choose M here.  If unsure, say N.
 
+config IP_NF_MATCH_OSF
+	tristate '"osf" match support'
+	depends on NETFILTER_ADVANCED && CONNECTOR
+	help
+	  Passive OS fingerprint matching module.
+	  You should download and install rule loading software from
+	  http://www.ioremap.net/projects/osf
+
+	  To compile it as a module, choose M here.  If unsure, say N.
+
 # `filter', generic and specific targets
 config IP_NF_FILTER
 	tristate "Packet filtering"
diff --git a/net/ipv4/netfilter/Makefile b/net/ipv4/netfilter/Makefile
index 5f9b650..98daea9 100644
--- a/net/ipv4/netfilter/Makefile
+++ b/net/ipv4/netfilter/Makefile
@@ -52,6 +52,7 @@ obj-$(CONFIG_IP_NF_MATCH_ADDRTYPE) += ipt_addrtype.o
 obj-$(CONFIG_IP_NF_MATCH_AH) += ipt_ah.o
 obj-$(CONFIG_IP_NF_MATCH_ECN) += ipt_ecn.o
 obj-$(CONFIG_IP_NF_MATCH_TTL) += ipt_ttl.o
+obj-$(CONFIG_IP_NF_MATCH_OSF) += ipt_osf.o
 
 # targets
 obj-$(CONFIG_IP_NF_TARGET_CLUSTERIP) += ipt_CLUSTERIP.o
diff --git a/net/ipv4/netfilter/ipt_osf.c b/net/ipv4/netfilter/ipt_osf.c
new file mode 100644
index 0000000..9356a44
--- /dev/null
+++ b/net/ipv4/netfilter/ipt_osf.c
@@ -0,0 +1,476 @@
+/*
+ * Copyright (c) 2003+ Evgeniy Polyakov <[email-masked]>
+ *
+ *
+ * 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.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * 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
+ */
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+
+#include <linux/connector.h>
+#include <linux/if.h>
+#include <linux/inetdevice.h>
+#include <linux/ip.h>
+#include <linux/list.h>
+#include <linux/percpu.h>
+#include <linux/rculist.h>
+#include <linux/smp.h>
+#include <linux/skbuff.h>
+#include <linux/slab.h>
+#include <linux/spinlock.h>
+#include <linux/tcp.h>
+#include <linux/types.h>
+
+#include <net/ip.h>
+
+#include <linux/netfilter/x_tables.h>
+#include <linux/netfilter_ipv4/ip_tables.h>
+#include <linux/netfilter_ipv4/ipt_osf.h>
+
+enum osf_fmatch_states {
+	/* Packet does not match the fingerprint */
+	FMATCH_WRONG = 0,
+	/* Packet matches the fingerprint */
+	FMATCH_OK,
+	/* Options do not match the fingerprint, but header does */
+	FMATCH_OPT_WRONG,
+};
+
+struct ipt_osf_finger_storage
+{
+	struct list_head		finger_list;
+	spinlock_t			finger_lock;
+};
+
+/*
+ * Indexed by dont-fragment bit.
+ * It is the only constant value in the fingerprint.
+ */
+struct ipt_osf_finger_storage ipt_osf_fingers[2];
+
+struct ipt_osf_message {
+	struct cn_msg		cmsg;
+	struct ipt_osf_nlmsg	nlmsg;
+};
+
+static DEFINE_PER_CPU(struct ipt_osf_message, ipt_osf_mbuf);
+
+static struct cb_id cn_osf_id = { CN_IDX_OSF, CN_VAL_OSF };
+static u32 osf_seq;
+
+static void ipt_osf_send_connector(struct ipt_osf_user_finger *f,
+				   const struct sk_buff *skb)
+{
+	struct ipt_osf_message *msg = &per_cpu(ipt_osf_mbuf, smp_processor_id());
+	struct ipt_osf_nlmsg *data = &msg->nlmsg;
+	struct iphdr *iph = ip_hdr(skb);
+	struct tcphdr *tcph = tcp_hdr(skb);
+
+	memcpy(&msg->cmsg.id, &cn_osf_id, sizeof(struct cn_msg));
+	msg->cmsg.seq = osf_seq++;
+	msg->cmsg.ack = 0;
+	msg->cmsg.len = sizeof(struct ipt_osf_nlmsg);
+
+	memcpy(&data->f, f, sizeof(struct ipt_osf_user_finger));
+	memcpy(&data->ip, iph, sizeof(struct iphdr));
+	memcpy(&data->tcp, tcph, sizeof(struct tcphdr));
+
+	cn_netlink_send(&msg->cmsg, CN_IDX_OSF, GFP_ATOMIC);
+}
+
+static inline int ipt_osf_ttl(const struct sk_buff *skb, struct ipt_osf_info *info,
+			    unsigned char f_ttl)
+{
+	struct iphdr *ip = ip_hdr(skb);
+
+	if (info->flags & IPT_OSF_TTL) {
+		if (info->ttl == IPT_OSF_TTL_TRUE)
+			return (ip->ttl == f_ttl);
+		if (info->ttl == IPT_OSF_TTL_NOCHECK)
+			return 1;
+		else if (ip->ttl <= f_ttl)
+			return 1;
+		else {
+			struct in_device *in_dev = in_dev_get(skb->dev);
+			int ret = 0;
+
+			for_ifa(in_dev) {
+				if (inet_ifa_match(ip->saddr, ifa)) {
+					ret = (ip->ttl == f_ttl);
+					break;
+				}
+			}
+			endfor_ifa(in_dev);
+
+			in_dev_put(in_dev);
+			return ret;
+		}
+	}
+	
+	return (ip->ttl == f_ttl);
+}
+
+static bool ipt_osf_match_packet(const struct sk_buff *skb,
+		const struct xt_match_param *p)
+{
+	struct ipt_osf_info *info = (struct ipt_osf_info *)p->matchinfo;
+	struct iphdr *ip;
+	struct tcphdr _tcph, *tcp;
+	int fmatch = FMATCH_WRONG, fcount = 0;
+	unsigned int optsize = 0, check_WSS = 0;
+	u16 window, totlen, mss = 0;
+	unsigned char df, *optp = NULL, *_optp = NULL;
+	unsigned char opts[MAX_IPOPTLEN];
+	struct ipt_osf_finger *kf;
+	struct ipt_osf_user_finger *f;
+	struct ipt_osf_finger_storage *st;
+
+	if (!info)
+		return 0;
+
+	ip = ip_hdr(skb);
+	if (!ip)
+		return 0;
+
+	tcp = skb_header_pointer(skb, ip->ihl * 4, sizeof(struct tcphdr), &_tcph);
+	if (!tcp)
+		return 0;
+
+	if (!tcp->syn)
+		return 0;
+
+	totlen = ntohs(ip->tot_len);
+	df = ((ntohs(ip->frag_off) & IP_DF) ? 1 : 0);
+	window = ntohs(tcp->window);
+
+	if (tcp->doff * 4 > sizeof(struct tcphdr)) {
+		optsize = tcp->doff * 4 - sizeof(struct tcphdr);
+
+		if (optsize > sizeof(opts))
+			optsize = sizeof(opts);
+
+		_optp = optp = skb_header_pointer(skb, ip->ihl * 4 + sizeof(struct tcphdr), 
+				optsize, opts);
+	}
+
+	st = &ipt_osf_fingers[!!df];
+	
+	rcu_read_lock();
+	list_for_each_entry_rcu(kf, &st->finger_list, finger_entry) {
+		f = &kf->finger;
+
+		if (!(info->flags & IPT_OSF_LOG) && strcmp(info->genre, f->genre))
+			continue;
+
+		optp = _optp;
+		fmatch = FMATCH_WRONG;
+
+		if (totlen == f->ss && df == f->df && ipt_osf_ttl(skb, info, f->ttl)) {
+			int foptsize, optnum;
+
+			check_WSS = 0;
+
+			switch (f->wss.wc) {
+			case 0:
+				check_WSS = 0;
+				break;
+			case 'S':
+				check_WSS = 1;
+				break;
+			case 'T':
+				check_WSS = 2;
+				break;
+			case '%':
+				check_WSS = 3;
+				break;
+			default:
+				check_WSS = 4;
+				break;
+			}
+			if (check_WSS == 4)
+				continue;
+
+			/* Check options */
+
+			foptsize = 0;
+			for (optnum = 0; optnum < f->opt_num; ++optnum)
+				foptsize += f->opt[optnum].length;
+
+			if (foptsize > MAX_IPOPTLEN || optsize > MAX_IPOPTLEN || optsize != foptsize)
+				continue;
+
+			for (optnum = 0; optnum < f->opt_num; ++optnum) {
+				if (f->opt[optnum].kind == (*optp)) {
+					__u32 len = f->opt[optnum].length;
+					__u8 *optend = optp + len;
+					int loop_cont = 0;
+					
+					fmatch = FMATCH_OK;
+
+					switch (*optp) {
+					case OSFOPT_MSS:
+						mss = ntohs(*(u16 *)(optp + 2));
+						break;
+					case OSFOPT_TS:
+						loop_cont = 1;
+						break;
+					}
+
+					optp = optend;
+				} else
+					fmatch = FMATCH_OPT_WRONG;
+
+				if (fmatch != FMATCH_OK)
+					break;
+			}
+
+			if (fmatch != FMATCH_OPT_WRONG) {
+				fmatch = FMATCH_WRONG;
+
+				switch (check_WSS) {
+				case 0:
+					if (f->wss.val == 0 || window == f->wss.val)
+						fmatch = FMATCH_OK;
+					break;
+				case 1:	/* MSS */
+#define SMART_MSS_1	1460
+#define SMART_MSS_2	1448
+					if (window == f->wss.val * mss ||
+					    window == f->wss.val * SMART_MSS_1 ||
+					    window == f->wss.val * SMART_MSS_2)
+						fmatch = FMATCH_OK;
+					break;
+				case 2:	/* MTU */
+					if (window == f->wss.val * (mss + 40) ||
+					    window == f->wss.val * (SMART_MSS_1 + 40) ||
+					    window == f->wss.val * (SMART_MSS_2 + 40))
+						fmatch = FMATCH_OK;
+					break;
+				case 3:	/* MOD */
+					if ((window % f->wss.val) == 0)
+						fmatch = FMATCH_OK;
+					break;
+				}
+			}
+			
+			if (fmatch != FMATCH_OK)
+				continue;
+
+			fcount++;
+			if (info->flags & IPT_OSF_LOG)
+				printk(KERN_INFO "%s [%s:%s] : "
+					"%u.%u.%u.%u:%u -> %u.%u.%u.%u:%u hops=%d\n",
+					f->genre, f->version, f->subtype, 
+					NIPQUAD(ip->saddr), ntohs(tcp->source), 
+					NIPQUAD(ip->daddr), ntohs(tcp->dest), 
+					f->ttl - ip->ttl);
+
+			if (info->flags & IPT_OSF_CONNECTOR)
+				ipt_osf_send_connector(f, skb);
+
+			if ((info->flags & IPT_OSF_LOG) &&
+			    info->loglevel == IPT_OSF_LOGLEVEL_FIRST)
+				break;
+		}
+	}
+	rcu_read_unlock();
+
+	if (!fcount && (info->flags & (IPT_OSF_LOG | IPT_OSF_CONNECTOR))) {
+		unsigned char opt[4 * 15 - sizeof(struct tcphdr)];
+		unsigned int i, optsize;
+		struct ipt_osf_user_finger fg;
+
+		memset(&fg, 0, sizeof(fg));
+#if 1
+		if (info->flags & IPT_OSF_LOG) {
+			if (info->loglevel != IPT_OSF_LOGLEVEL_ALL_KNOWN)
+				printk(KERN_INFO "Unknown: win: %u, mss: %u, "
+					"totlen: %u, df: %d, ttl: %u : ",
+					window, mss, totlen, df, ip->ttl);
+			if (optp) {
+				optsize = tcp->doff * 4 - sizeof(struct tcphdr);
+				if (skb_copy_bits(skb, ip->ihl * 4 + sizeof(struct tcphdr),
+						opt, optsize) < 0)
+					printk("TRUNCATED");
+				else 
+					for (i = 0; i < optsize; i++)
+						printk("%02X ", opt[i]);
+			}
+
+			printk(" %u.%u.%u.%u:%u -> %u.%u.%u.%u:%u\n",
+			     NIPQUAD(ip->saddr), ntohs(tcp->source),
+			     NIPQUAD(ip->daddr), ntohs(tcp->dest));
+		}
+#endif
+		if (info->flags & IPT_OSF_CONNECTOR) {
+			fg.wss.val = window;
+			fg.ttl = ip->ttl;
+			fg.df = df;
+			fg.ss = totlen;
+			fg.mss = mss;
+			strncpy(fg.genre, "Unknown", MAXGENRELEN);
+
+			ipt_osf_send_connector(&fg, skb);
+		}
+	}
+
+	if (fcount)
+		fmatch = FMATCH_OK;
+
+	return (fmatch == FMATCH_OK) ? 1 : 0;
+}
+
+static bool
+ipt_osf_checkentry(const struct xt_mtchk_param *m)
+{
+	struct ipt_ip *ip = (struct ipt_ip *)m->entryinfo;
+
+	if (ip->proto != IPPROTO_TCP)
+		return false;
+
+	return true;
+}
+
+static struct xt_match ipt_osf_match = {
+	.name 		= "osf",
+	.revision	= 0,
+	.family		= AF_INET,
+	.hooks      	= (1 << NF_INET_LOCAL_IN) | (1 << NF_INET_PRE_ROUTING),
+	.match 		= ipt_osf_match_packet,
+	.checkentry	= ipt_osf_checkentry,
+	.matchsize	= sizeof(struct ipt_osf_info),
+	.me		= THIS_MODULE,
+};
+
+static void ipt_osf_finger_free_rcu(struct rcu_head *rcu_head)
+{
+	struct ipt_osf_finger *f = container_of(rcu_head, struct ipt_osf_finger, rcu_head);
+
+	kfree(f);
+}
+
+static void osf_cn_callback(void *data)
+{
+	struct cn_msg *msg = data;
+	struct ipt_osf_user_finger *f = (struct ipt_osf_user_finger *)(msg + 1);
+	struct ipt_osf_finger *kf = NULL, *sf;
+	struct ipt_osf_finger_storage *st = &ipt_osf_fingers[!!f->df];
+
+	/*
+	 * If msg->ack is set to 0 then we add attached fingerprint,
+	 * otherwise remove, and in this case we do not need to allocate data.
+	 */
+	if (!msg->ack) {
+		kf = kmalloc(sizeof(struct ipt_osf_finger), GFP_KERNEL);
+		if (!kf)
+			return;
+
+		memcpy(&kf->finger, f, sizeof(struct ipt_osf_user_finger));
+	}
+
+	rcu_read_lock();
+	list_for_each_entry_rcu(sf, &st->finger_list, finger_entry) {
+		if (memcmp(&sf->finger, f, sizeof(struct ipt_osf_user_finger)))
+			continue;
+
+		if (msg->ack) {
+			spin_lock_bh(&st->finger_lock);
+			list_del_rcu(&sf->finger_entry);
+			spin_unlock_bh(&st->finger_lock);
+			call_rcu(&sf->rcu_head, ipt_osf_finger_free_rcu);
+		} else {
+			kfree(kf);
+			kf = NULL;
+		}
+
+		break;
+	}
+
+	if (kf) {
+		spin_lock_bh(&st->finger_lock);
+		list_add_tail_rcu(&kf->finger_entry, &st->finger_list);
+		spin_unlock_bh(&st->finger_lock);
+
+		printk("%s: added rule for %s:%s:%s.\n", __func__, kf->finger.genre, kf->finger.version, kf->finger.subtype);
+	}
+	rcu_read_unlock();
+}
+
+static int __devinit ipt_osf_init(void)
+{
+	int err = -EINVAL;
+	int i;
+
+	for (i=0; i<ARRAY_SIZE(ipt_osf_fingers); ++i) {
+		struct ipt_osf_finger_storage *st = &ipt_osf_fingers[i];
+
+		INIT_LIST_HEAD(&st->finger_list);
+		spin_lock_init(&st->finger_lock);
+	}
+
+	err = cn_add_callback(&cn_osf_id, "osf", osf_cn_callback);
+	if (err) {
+		printk(KERN_ERR "Failed (%d) to register OSF connector.\n", err);
+		goto err_out_exit;
+	}
+
+	err = xt_register_match(&ipt_osf_match);
+	if (err) {
+		printk(KERN_ERR "Failed (%d) to register OS fingerprint "
+				"matching module.\n", err);
+		goto err_out_remove;
+	}
+
+	printk(KERN_INFO "Started passive OS fingerprint matching module.\n");
+
+	return 0;
+
+err_out_remove:
+	cn_del_callback(&cn_osf_id);
+err_out_exit:
+	return err;
+}
+
+static void __devexit ipt_osf_fini(void)
+{
+	struct ipt_osf_finger *f;
+	int i;
+
+	cn_del_callback(&cn_osf_id);
+	xt_unregister_match(&ipt_osf_match);
+
+	rcu_read_lock();
+	for (i=0; i<ARRAY_SIZE(ipt_osf_fingers); ++i) {
+		struct ipt_osf_finger_storage *st = &ipt_osf_fingers[i];
+
+		list_for_each_entry_rcu(f, &st->finger_list, finger_entry) {
+			list_del_rcu(&f->finger_entry);
+			call_rcu(&f->rcu_head, ipt_osf_finger_free_rcu);
+		}
+	}
+	rcu_read_unlock();
+
+	rcu_barrier();
+
+	printk(KERN_INFO "Passive OS fingerprint matching module finished.\n");
+}
+
+module_init(ipt_osf_init);
+module_exit(ipt_osf_fini);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Evgeniy Polyakov <[email-masked]>");
+MODULE_DESCRIPTION("Passive OS fingerprint matching.");

-- 
	Evgeniy Polyakov