Subject: Passive OS fingerprint xtables match.

From: Evgeniy Polyakov <[email-masked]>
Date: Thu, 4 Jun 2009 20:22:12 +0400

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 netfilter netlink subsystem into the kernel via special
util found in archive.

Archive 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 version switches from printk to nf_log_packet() interface,
introduces separate add/remove commands for fingerprint processing,
drops unused variables and includes, adds more comments and contains
other misc cleanups.

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

Example usage:
# modrpobe xt_osf
# ./nfnl_osf -f ./pf.os
-d switch removes fingerprints
# iptables -I INPUT -j ACCEPT -p tcp -m osf --genre Linux --log 0 --ttl 2

Please consider for inclusion.
Thank you.

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

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

 include/linux/netfilter/nfnetlink.h |    3 +-
 include/linux/netfilter/xt_osf.h    |  133 ++++++++++
 net/netfilter/Kconfig               |   13 +
 net/netfilter/Makefile              |    1 +
 net/netfilter/xt_osf.c              |  474 +++++++++++++++++++++++++++++++++++
 5 files changed, 623 insertions(+), 1 deletions(-)

diff --git a/include/linux/netfilter/nfnetlink.h b/include/linux/netfilter/nfnetlink.h
index c600083..bb9c9ae 100644
--- a/include/linux/netfilter/nfnetlink.h
+++ b/include/linux/netfilter/nfnetlink.h
@@ -46,7 +46,8 @@ struct nfgenmsg {
 #define NFNL_SUBSYS_CTNETLINK_EXP	2
 #define NFNL_SUBSYS_QUEUE		3
 #define NFNL_SUBSYS_ULOG		4
-#define NFNL_SUBSYS_COUNT		5
+#define NFNL_SUBSYS_OSF			5
+#define NFNL_SUBSYS_COUNT		6
 
 #ifdef __KERNEL__
 
diff --git a/include/linux/netfilter/xt_osf.h b/include/linux/netfilter/xt_osf.h
new file mode 100644
index 0000000..fd2272e
--- /dev/null
+++ b/include/linux/netfilter/xt_osf.h
@@ -0,0 +1,133 @@
+/*
+ * 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 _XT_OSF_H
+#define _XT_OSF_H
+
+#define MAXGENRELEN		32
+
+#define XT_OSF_GENRE		(1<<0)
+#define	XT_OSF_TTL		(1<<1)
+#define XT_OSF_LOG		(1<<2)
+#define XT_OSF_INVERT		(1<<3)
+
+#define XT_OSF_LOGLEVEL_ALL	0	/* log all matched fingerprints */
+#define XT_OSF_LOGLEVEL_FIRST	1	/* log only the first matced fingerprint */
+#define XT_OSF_LOGLEVEL_ALL_KNOWN	2 /* do not log unknown packets */
+
+#define XT_OSF_TTL_TRUE		0	/* True ip and fingerprint TTL comparison */
+#define XT_OSF_TTL_LESS		1	/* Check if ip TTL is less than fingerprint one */
+#define XT_OSF_TTL_NOCHECK	2	/* Do not compare ip and fingerprint TTL at all */
+
+struct xt_osf_info {
+	char			genre[MAXGENRELEN];
+	__u32			len;
+	__u32			flags;
+	__u32			loglevel;
+	__u32			ttl;
+};
+
+/*
+ * Wildcard MSS (kind of).
+ * It is used to implement a state machine for the different wildcard values
+ * of the MSS and window sizes.
+ */
+struct xt_osf_wc {
+	__u32			wc;
+	__u32			val;
+};
+
+/*
+ * This struct represents IANA options
+ * http://www.iana.org/assignments/tcp-parameters
+ */
+struct xt_osf_opt {
+	__u16			kind, length;
+	struct xt_osf_wc	wc;
+};
+
+struct xt_osf_user_finger {
+	struct xt_osf_wc	wss;
+
+	__u8			ttl, df;
+	__u16			ss, mss;
+	__u16			opt_num;
+
+	char			genre[MAXGENRELEN];
+	char			version[MAXGENRELEN];
+	char			subtype[MAXGENRELEN];
+
+	/* MAX_IPOPTLEN is maximum if all options are NOPs or EOLs */
+	struct xt_osf_opt	opt[MAX_IPOPTLEN];
+};
+
+struct xt_osf_nlmsg {
+	struct xt_osf_user_finger	f;
+	struct iphdr		ip;
+	struct tcphdr		tcp;
+};
+
+/* Defines for IANA option kinds */
+
+enum iana_options {
+	OSFOPT_EOL = 0,		/* End of options */
+	OSFOPT_NOP, 		/* NOP */
+	OSFOPT_MSS, 		/* Maximum segment size */
+	OSFOPT_WSO, 		/* Window scale option */
+	OSFOPT_SACKP,		/* SACK permitted */
+	OSFOPT_SACK,		/* SACK */
+	OSFOPT_ECHO,
+	OSFOPT_ECHOREPLY,
+	OSFOPT_TS,		/* Timestamp option */
+	OSFOPT_POCP,		/* Partial Order Connection Permitted */
+	OSFOPT_POSP,		/* Partial Order Service Profile */
+
+	/* Others are not used in the current OSF */
+	OSFOPT_EMPTY = 255,
+};
+
+/*
+ * Initial window size option state machine: multiple of mss, mtu or
+ * plain numeric value. Can also be made as plain numeric value which
+ * is not a multiple of specified value.
+ */
+enum xt_osf_window_size_options {
+	OSF_WSS_PLAIN	= 0,
+	OSF_WSS_MSS,
+	OSF_WSS_MTU,
+	OSF_WSS_MODULO,
+	OSF_WSS_MAX,
+};
+
+/*
+ * Add/remove fingerprint from the kernel.
+ */
+enum xt_osf_msg_types {
+	OSF_MSG_ADD,
+	OSF_MSG_REMOVE,
+	OSF_MSG_MAX,
+};
+
+enum xt_osf_attr_type {
+	OSF_ATTR_UNSPEC,
+	OSF_ATTR_FINGER,
+	OSF_ATTR_MAX,
+};
+
+#endif				/* _XT_OSF_H */
diff --git a/net/netfilter/Kconfig b/net/netfilter/Kconfig
index 2329c5f..3a6ecee 100644
--- a/net/netfilter/Kconfig
+++ b/net/netfilter/Kconfig
@@ -916,6 +916,19 @@ config NETFILTER_XT_MATCH_U32
 
 	  Details and examples are in the kernel module source.
 
+config NETFILTER_XT_MATCH_OSF
+	tristate '"osf" Passive OS fingerprint match'
+	depends on NETFILTER_ADVANCED && NETFILTER_NETLINK
+	help
+	  This option selects the Passive OS Fingerprinting match module
+	  that allows to passively match the remote operating system by
+	  analyzing incoming TCP SYN packets.
+
+	  Rules and loading software can be downloaded from
+	  http://www.ioremap.net/projects/osf
+
+	  To compile it as a module, choose M here.  If unsure, say N.
+
 endif # NETFILTER_XTABLES
 
 endmenu
diff --git a/net/netfilter/Makefile b/net/netfilter/Makefile
index 6282060..49f62ee 100644
--- a/net/netfilter/Makefile
+++ b/net/netfilter/Makefile
@@ -77,6 +77,7 @@ obj-$(CONFIG_NETFILTER_XT_MATCH_LIMIT) += xt_limit.o
 obj-$(CONFIG_NETFILTER_XT_MATCH_MAC) += xt_mac.o
 obj-$(CONFIG_NETFILTER_XT_MATCH_MARK) += xt_mark.o
 obj-$(CONFIG_NETFILTER_XT_MATCH_MULTIPORT) += xt_multiport.o
+obj-$(CONFIG_NETFILTER_XT_MATCH_OSF) += xt_osf.o
 obj-$(CONFIG_NETFILTER_XT_MATCH_OWNER) += xt_owner.o
 obj-$(CONFIG_NETFILTER_XT_MATCH_PHYSDEV) += xt_physdev.o
 obj-$(CONFIG_NETFILTER_XT_MATCH_PKTTYPE) += xt_pkttype.o
diff --git a/net/netfilter/xt_osf.c b/net/netfilter/xt_osf.c
new file mode 100644
index 0000000..55cabe4
--- /dev/null
+++ b/net/netfilter/xt_osf.c
@@ -0,0 +1,474 @@
+/*
+ * 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/if.h>
+#include <linux/inetdevice.h>
+#include <linux/ip.h>
+#include <linux/list.h>
+#include <linux/rculist.h>
+#include <linux/skbuff.h>
+#include <linux/slab.h>
+#include <linux/tcp.h>
+
+#include <net/ip.h>
+#include <net/tcp.h>
+
+#include <linux/netfilter/nfnetlink.h>
+#include <linux/netfilter/x_tables.h>
+#include <net/netfilter/nf_log.h>
+#include <linux/netfilter/xt_osf.h>
+
+struct xt_osf_finger {
+	struct rcu_head			rcu_head;
+	struct list_head		finger_entry;
+	struct xt_osf_user_finger	finger;
+};
+
+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 xt_osf_finger_storage {
+	struct list_head		finger_list;
+};
+
+/*
+ * Indexed by dont-fragment bit.
+ * It is the only constant value in the fingerprint.
+ */
+static struct xt_osf_finger_storage xt_osf_fingers[2];
+
+static const struct nla_policy xt_osf_policy[OSF_ATTR_MAX + 1] = {
+	[OSF_ATTR_FINGER]	= { .len = sizeof(struct xt_osf_user_finger) },
+};
+
+static void xt_osf_finger_free_rcu(struct rcu_head *rcu_head)
+{
+	struct xt_osf_finger *f = container_of(rcu_head, struct xt_osf_finger, rcu_head);
+
+	kfree(f);
+}
+
+static int xt_osf_add_callback(struct sock *ctnl, struct sk_buff *skb,
+			struct nlmsghdr *nlh, struct nlattr *osf_attrs[])
+{
+	struct xt_osf_user_finger *f;
+	struct xt_osf_finger *kf = NULL, *sf;
+	struct xt_osf_finger_storage *st;
+	int err = 0;
+
+	if (!osf_attrs[OSF_ATTR_FINGER])
+		return -EINVAL;
+
+	f = nla_data(osf_attrs[OSF_ATTR_FINGER]);
+	st = &xt_osf_fingers[!!f->df];
+
+	kf = kmalloc(sizeof(struct xt_osf_finger), GFP_KERNEL);
+	if (!kf)
+		return -ENOMEM;
+
+	memcpy(&kf->finger, f, sizeof(struct xt_osf_user_finger));
+
+	rcu_read_lock();
+	list_for_each_entry_rcu(sf, &st->finger_list, finger_entry) {
+		if (memcmp(&sf->finger, f, sizeof(struct xt_osf_user_finger)))
+			continue;
+
+		/*
+		 * We are protected by nfnl mutex.
+		 */
+		kfree(kf);
+		kf = NULL;
+
+		err = -EEXIST;
+		break;
+	}
+
+	if (kf)
+		list_add_tail_rcu(&kf->finger_entry, &st->finger_list);
+	rcu_read_unlock();
+
+	return err;
+}
+
+static int xt_osf_remove_callback(struct sock *ctnl, struct sk_buff *skb,
+			struct nlmsghdr *nlh, struct nlattr *osf_attrs[])
+{
+	struct xt_osf_user_finger *f;
+	struct xt_osf_finger *sf;
+	struct xt_osf_finger_storage *st;
+	int err = ENOENT;
+
+	if (!osf_attrs[OSF_ATTR_FINGER])
+		return -EINVAL;
+
+	f = nla_data(osf_attrs[OSF_ATTR_FINGER]);
+	st = &xt_osf_fingers[!!f->df];
+
+	rcu_read_lock();
+	list_for_each_entry_rcu(sf, &st->finger_list, finger_entry) {
+		if (memcmp(&sf->finger, f, sizeof(struct xt_osf_user_finger)))
+			continue;
+
+		/*
+		 * We are protected by nfnl mutex.
+		 */
+		list_del_rcu(&sf->finger_entry);
+		call_rcu(&sf->rcu_head, xt_osf_finger_free_rcu);
+
+		err = 0;
+		break;
+	}
+	rcu_read_unlock();
+
+	return err;
+}
+
+static const struct nfnl_callback xt_osf_nfnetlink_callbacks[OSF_MSG_MAX] = {
+	[OSF_MSG_ADD]	= {
+		.call		= xt_osf_add_callback,
+		.attr_count	= OSF_ATTR_MAX,
+		.policy		= xt_osf_policy,
+	},
+	[OSF_MSG_REMOVE]	= {
+		.call		= xt_osf_remove_callback,
+		.attr_count	= OSF_ATTR_MAX,
+		.policy		= xt_osf_policy,
+	},
+};
+
+static const struct nfnetlink_subsystem xt_osf_nfnetlink = {
+	.name			= "osf",
+	.subsys_id		= NFNL_SUBSYS_OSF,
+	.cb_count		= OSF_MSG_MAX,
+	.cb			= xt_osf_nfnetlink_callbacks,
+};
+
+static inline int xt_osf_ttl(const struct sk_buff *skb, const struct xt_osf_info *info,
+			    unsigned char f_ttl)
+{
+	const struct iphdr *ip = ip_hdr(skb);
+
+	if (info->flags & XT_OSF_TTL) {
+		if (info->ttl == XT_OSF_TTL_TRUE)
+			return ip->ttl == f_ttl;
+		if (info->ttl == XT_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 xt_osf_match_packet(const struct sk_buff *skb,
+		const struct xt_match_param *p)
+{
+	const struct xt_osf_info *info = p->matchinfo;
+	const struct iphdr *ip = ip_hdr(skb);
+	const struct tcphdr *tcp;
+	struct tcphdr _tcph;
+	int fmatch = FMATCH_WRONG, fcount = 0;
+	unsigned int optsize = 0, check_WSS = 0;
+	u16 window, totlen, mss = 0;
+	bool df;
+	const unsigned char *optp = NULL, *_optp = NULL;
+	unsigned char opts[MAX_IPOPTLEN];
+	const struct xt_osf_finger *kf;
+	const struct xt_osf_user_finger *f;
+	const struct xt_osf_finger_storage *st;
+
+	if (!info)
+		return false;
+
+	tcp = skb_header_pointer(skb, ip_hdrlen(skb), sizeof(struct tcphdr), &_tcph);
+	if (!tcp)
+		return false;
+
+	if (!tcp->syn)
+		return false;
+
+	totlen = ntohs(ip->tot_len);
+	df = ntohs(ip->frag_off) & IP_DF;
+	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_hdrlen(skb) +
+				sizeof(struct tcphdr), optsize, opts);
+	}
+
+	st = &xt_osf_fingers[df];
+
+	rcu_read_lock();
+	list_for_each_entry_rcu(kf, &st->finger_list, finger_entry) {
+		f = &kf->finger;
+
+		if (!(info->flags & XT_OSF_LOG) && strcmp(info->genre, f->genre))
+			continue;
+
+		optp = _optp;
+		fmatch = FMATCH_WRONG;
+
+		if (totlen == f->ss && xt_osf_ttl(skb, info, f->ttl)) {
+			int foptsize, optnum;
+
+			/*
+			 * Should not happen if userspace parser was written correctly.
+			 */
+			if (f->wss.wc >= OSF_WSS_MAX)
+				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;
+
+			check_WSS = f->wss.wc;
+
+			for (optnum = 0; optnum < f->opt_num; ++optnum) {
+				if (f->opt[optnum].kind == (*optp)) {
+					__u32 len = f->opt[optnum].length;
+					const __u8 *optend = optp + len;
+					int loop_cont = 0;
+
+					fmatch = FMATCH_OK;
+
+					switch (*optp) {
+					case OSFOPT_MSS:
+						mss = optp[3];
+						mss <<= 8;
+						mss |= optp[2];
+
+						mss = ntohs(mss);
+						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 OSF_WSS_PLAIN:
+					if (f->wss.val == 0 || window == f->wss.val)
+						fmatch = FMATCH_OK;
+					break;
+				case OSF_WSS_MSS:
+					/*
+					 * Some smart modems decrease mangle MSS to 
+					 * SMART_MSS_2, so we check standard, decreased
+					 * and the one provided in the fingerprint MSS
+					 * values.
+					 */
+#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 OSF_WSS_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 OSF_WSS_MODULO:
+					if ((window % f->wss.val) == 0)
+						fmatch = FMATCH_OK;
+					break;
+				}
+			}
+
+			if (fmatch != FMATCH_OK)
+				continue;
+
+			fcount++;
+			/*
+			 * Everyone uses NULL loginfo (well, everyone uses
+			 * NULL for all but format and message fields), so
+			 * I did not set it up either.
+			 */
+			if (info->flags & XT_OSF_LOG)
+				nf_log_packet(p->hooknum, 0, skb, p->in, p->out, NULL,
+					"%s [%s:%s] : %pi4:%d -> %pi4:%d hops=%d\n",
+					f->genre, f->version, f->subtype,
+					&ip->saddr, ntohs(tcp->source),
+					&ip->daddr, ntohs(tcp->dest),
+					f->ttl - ip->ttl);
+
+			if ((info->flags & XT_OSF_LOG) &&
+			    info->loglevel == XT_OSF_LOGLEVEL_FIRST)
+				break;
+		}
+	}
+	rcu_read_unlock();
+
+	if (!fcount && (info->flags & XT_OSF_LOG)) {
+		unsigned int i;
+		struct xt_osf_user_finger fg;
+
+		memset(&fg, 0, sizeof(fg));
+
+		if (info->flags & XT_OSF_LOG) {
+			if (info->loglevel != XT_OSF_LOGLEVEL_ALL_KNOWN)
+				nf_log_packet(p->hooknum, 0, skb, p->in, p->out, NULL,
+					"window: %u, mss: %u, totlen: %u, df: %d, ttl: %u : ",
+					window, mss, totlen, df, ip->ttl);
+			if (_optp) {
+				optp = _optp;
+				for (i = 0; i < optsize; i++)
+					nf_log_packet(p->hooknum, 0, skb, p->in, p->out, NULL,
+						"%02X ", optp[i]);
+			}
+
+			nf_log_packet(p->hooknum, 0, skb, p->in, p->out, NULL,
+					"%pi4:%u -> %pi4:%u\n",
+			     &ip->saddr, ntohs(tcp->source),
+			     &ip->daddr, ntohs(tcp->dest));
+		}
+	}
+
+	if (fcount)
+		fmatch = FMATCH_OK;
+
+	return fmatch == FMATCH_OK;
+}
+
+static struct xt_match xt_osf_match = {
+	.name 		= "osf",
+	.revision	= 0,
+	.family		= NFPROTO_IPV4,
+	.proto		= IPPROTO_TCP,
+	.hooks      	= (1 << NF_INET_LOCAL_IN) |
+				(1 << NF_INET_PRE_ROUTING) |
+				(1 << NF_INET_FORWARD),
+	.match 		= xt_osf_match_packet,
+	.matchsize	= sizeof(struct xt_osf_info),
+	.me		= THIS_MODULE,
+};
+
+static int __init xt_osf_init(void)
+{
+	int err = -EINVAL;
+	int i;
+
+	for (i=0; i<ARRAY_SIZE(xt_osf_fingers); ++i) {
+		struct xt_osf_finger_storage *st = &xt_osf_fingers[i];
+
+		INIT_LIST_HEAD(&st->finger_list);
+	}
+
+	err = nfnetlink_subsys_register(&xt_osf_nfnetlink);
+	if (err < 0) {
+		printk(KERN_ERR "Failed (%d) to register OSF nsfnetlink helper.\n", err);
+		goto err_out_exit;
+	}
+
+	err = xt_register_match(&xt_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:
+	nfnetlink_subsys_unregister(&xt_osf_nfnetlink);
+err_out_exit:
+	return err;
+}
+
+static void __exit xt_osf_fini(void)
+{
+	struct xt_osf_finger *f;
+	int i;
+
+	nfnetlink_subsys_unregister(&xt_osf_nfnetlink);
+	xt_unregister_match(&xt_osf_match);
+
+	rcu_read_lock();
+	for (i=0; i<ARRAY_SIZE(xt_osf_fingers); ++i) {
+		struct xt_osf_finger_storage *st = &xt_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, xt_osf_finger_free_rcu);
+		}
+	}
+	rcu_read_unlock();
+
+	rcu_barrier();
+
+	printk(KERN_INFO "Passive OS fingerprint matching module finished.\n");
+}
+
+module_init(xt_osf_init);
+module_exit(xt_osf_fini);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Evgeniy Polyakov <[email-masked]>");
+MODULE_DESCRIPTION("Passive OS fingerprint matching.");
+MODULE_ALIAS_NFNL_SUBSYS(NFNL_SUBSYS_OSF);

-- 
	Evgeniy Polyakov