net/netfilter/xt_TCPMSS.c - linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0-only
 /*
  * This is a module which is used for setting the MSS option in TCP packets.
  *
  * Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
  * Copyright (C) 2007 Patrick McHardy <kaber@trash.net>
  */
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 #include <linux/module.h>
 #include <linux/skbuff.h>
 #include <linux/ip.h>
 #include <linux/gfp.h>
 #include <linux/ipv6.h>
 #include <linux/tcp.h>
 #include <net/dst.h>
 #include <net/flow.h>
 #include <net/ipv6.h>
 #include <net/route.h>
 #include <net/tcp.h>

 #include <linux/netfilter_ipv4/ip_tables.h>
 #include <linux/netfilter_ipv6/ip6_tables.h>
 #include <linux/netfilter/x_tables.h>
 #include <linux/netfilter/xt_tcpudp.h>
 #include <linux/netfilter/xt_TCPMSS.h>

 MODULE_LICENSE("GPL");
 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
 MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment");
 MODULE_ALIAS("ipt_TCPMSS");
 MODULE_ALIAS("ip6t_TCPMSS");

 static inline unsigned int
 optlen(const u_int8_t *opt, unsigned int offset)
 {
 	/* Beware zero-length options: make finite progress */
 	if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
 		return 1;
 	else
 		return opt[offset+1];
 }

 static u_int32_t tcpmss_reverse_mtu(struct net *net,
 				    const struct sk_buff *skb,
 				    unsigned int family)
 {
 	struct flowi fl;
 	struct rtable *rt = NULL;
 	u_int32_t mtu     = ~0U;

 	if (family == PF_INET) {
 		struct flowi4 *fl4 = &fl.u.ip4;
 		memset(fl4, 0, sizeof(*fl4));
 		fl4->daddr = ip_hdr(skb)->saddr;
 	} else {
 		struct flowi6 *fl6 = &fl.u.ip6;

 		memset(fl6, 0, sizeof(*fl6));
 		fl6->daddr = ipv6_hdr(skb)->saddr;
 	}

 	nf_route(net, (struct dst_entry **)&rt, &fl, false, family);
 	if (rt != NULL) {
 		mtu = dst_mtu(&rt->dst);
 		dst_release(&rt->dst);
 	}
 	return mtu;
 }

 static int
 tcpmss_mangle_packet(struct sk_buff *skb,
 		     const struct xt_action_param *par,
 		     unsigned int family,
 		     unsigned int tcphoff,
 		     unsigned int minlen)
 {
 	const struct xt_tcpmss_info *info = par->targinfo;
 	struct tcphdr *tcph;
 	int len, tcp_hdrlen;
 	unsigned int i;
 	__be16 oldval;
 	u16 newmss;
 	u8 *opt;

 	/* This is a fragment, no TCP header is available */
 	if (par->fragoff != 0)
 		return 0;

 	if (skb_ensure_writable(skb, skb->len))
 		return -1;

 	len = skb->len - tcphoff;
 	if (len < (int)sizeof(struct tcphdr))
 		return -1;

 	tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
 	tcp_hdrlen = tcph->doff * 4;

 	if (len < tcp_hdrlen || tcp_hdrlen < sizeof(struct tcphdr))
 		return -1;

 	if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
 		struct net *net = xt_net(par);
 		unsigned int in_mtu = tcpmss_reverse_mtu(net, skb, family);
 		unsigned int min_mtu = min(dst_mtu(skb_dst(skb)), in_mtu);

 		if (min_mtu <= minlen) {
 			net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
 					    min_mtu);
 			return -1;
 		}
 		newmss = min_mtu - minlen;
 	} else
 		newmss = info->mss;

 	opt = (u_int8_t *)tcph;
 	for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) {
 		if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) {
 			u_int16_t oldmss;

 			oldmss = (opt[i+2] << 8) | opt[i+3];

 			/* Never increase MSS, even when setting it, as
 			 * doing so results in problems for hosts that rely
 			 * on MSS being set correctly.
 			 */
 			if (oldmss <= newmss)
 				return 0;

 			opt[i+2] = (newmss & 0xff00) >> 8;
 			opt[i+3] = newmss & 0x00ff;

 			inet_proto_csum_replace2(&tcph->check, skb,
 						 htons(oldmss), htons(newmss),
 						 false);
 			return 0;
 		}
 	}

 	/* There is data after the header so the option can't be added
 	 * without moving it, and doing so may make the SYN packet
 	 * itself too large. Accept the packet unmodified instead.
 	 */
 	if (len > tcp_hdrlen)
 		return 0;

 	/* tcph->doff has 4 bits, do not wrap it to 0 */
 	if (tcp_hdrlen >= 15 * 4)
 		return 0;

 	/*
 	 * MSS Option not found ?! add it..
 	 */
 	if (skb_tailroom(skb) < TCPOLEN_MSS) {
 		if (pskb_expand_head(skb, 0,
 				     TCPOLEN_MSS - skb_tailroom(skb),
 				     GFP_ATOMIC))
 			return -1;
 		tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
 	}

 	skb_put(skb, TCPOLEN_MSS);

 	/*
 	 * IPv4: RFC 1122 states "If an MSS option is not received at
 	 * connection setup, TCP MUST assume a default send MSS of 536".
 	 * IPv6: RFC 2460 states IPv6 has a minimum MTU of 1280 and a minimum
 	 * length IPv6 header of 60, ergo the default MSS value is 1220
 	 * Since no MSS was provided, we must use the default values
 	 */
 	if (xt_family(par) == NFPROTO_IPV4)
 		newmss = min(newmss, (u16)536);
 	else
 		newmss = min(newmss, (u16)1220);

 	opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
 	memmove(opt + TCPOLEN_MSS, opt, len - sizeof(struct tcphdr));

 	inet_proto_csum_replace2(&tcph->check, skb,
 				 htons(len), htons(len + TCPOLEN_MSS), true);
 	opt[0] = TCPOPT_MSS;
 	opt[1] = TCPOLEN_MSS;
 	opt[2] = (newmss & 0xff00) >> 8;
 	opt[3] = newmss & 0x00ff;

 	inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), false);

 	oldval = ((__be16 *)tcph)[6];
 	tcph->doff += TCPOLEN_MSS/4;
 	inet_proto_csum_replace2(&tcph->check, skb,
 				 oldval, ((__be16 *)tcph)[6], false);
 	return TCPOLEN_MSS;
 }

 static unsigned int
 tcpmss_tg4(struct sk_buff *skb, const struct xt_action_param *par)
 {
 	struct iphdr *iph = ip_hdr(skb);
 	__be16 newlen;
 	int ret;

 	ret = tcpmss_mangle_packet(skb, par,
 				   PF_INET,
 				   iph->ihl * 4,
 				   sizeof(*iph) + sizeof(struct tcphdr));
 	if (ret < 0)
 		return NF_DROP;
 	if (ret > 0) {
 		iph = ip_hdr(skb);
 		newlen = htons(ntohs(iph->tot_len) + ret);
 		csum_replace2(&iph->check, iph->tot_len, newlen);
 		iph->tot_len = newlen;
 	}
 	return XT_CONTINUE;
 }

 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
 static unsigned int
 tcpmss_tg6(struct sk_buff *skb, const struct xt_action_param *par)
 {
 	struct ipv6hdr *ipv6h = ipv6_hdr(skb);
 	u8 nexthdr;
 	__be16 frag_off, oldlen, newlen;
 	int tcphoff;
 	int ret;

 	nexthdr = ipv6h->nexthdr;
 	tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
 	if (tcphoff < 0)
 		return NF_DROP;
 	ret = tcpmss_mangle_packet(skb, par,
 				   PF_INET6,
 				   tcphoff,
 				   sizeof(*ipv6h) + sizeof(struct tcphdr));
 	if (ret < 0)
 		return NF_DROP;
 	if (ret > 0) {
 		ipv6h = ipv6_hdr(skb);
 		oldlen = ipv6h->payload_len;
 		newlen = htons(ntohs(oldlen) + ret);
 		if (skb->ip_summed == CHECKSUM_COMPLETE)
 			skb->csum = csum_add(csum_sub(skb->csum, oldlen),
 					     newlen);
 		ipv6h->payload_len = newlen;
 	}
 	return XT_CONTINUE;
 }
 #endif

 /* Must specify -p tcp --syn */
 static inline bool find_syn_match(const struct xt_entry_match *m)
 {
 	const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;

 	if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
 	    tcpinfo->flg_cmp & TCPHDR_SYN &&
 	    !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
 		return true;

 	return false;
 }

 static int tcpmss_tg4_check(const struct xt_tgchk_param *par)
 {
 	const struct xt_tcpmss_info *info = par->targinfo;
 	const struct ipt_entry *e = par->entryinfo;
 	const struct xt_entry_match *ematch;

 	if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
 	    (par->hook_mask & ~((1 << NF_INET_FORWARD) |
 			   (1 << NF_INET_LOCAL_OUT) |
 			   (1 << NF_INET_POST_ROUTING))) != 0) {
 		pr_info_ratelimited("path-MTU clamping only supported in FORWARD, OUTPUT and POSTROUTING hooks\n");
 		return -EINVAL;
 	}
 	if (par->nft_compat)
 		return 0;

 	xt_ematch_foreach(ematch, e)
 		if (find_syn_match(ematch))
 			return 0;
 	pr_info_ratelimited("Only works on TCP SYN packets\n");
 	return -EINVAL;
 }

 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
 static int tcpmss_tg6_check(const struct xt_tgchk_param *par)
 {
 	const struct xt_tcpmss_info *info = par->targinfo;
 	const struct ip6t_entry *e = par->entryinfo;
 	const struct xt_entry_match *ematch;

 	if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
 	    (par->hook_mask & ~((1 << NF_INET_FORWARD) |
 			   (1 << NF_INET_LOCAL_OUT) |
 			   (1 << NF_INET_POST_ROUTING))) != 0) {
 		pr_info_ratelimited("path-MTU clamping only supported in FORWARD, OUTPUT and POSTROUTING hooks\n");
 		return -EINVAL;
 	}
 	if (par->nft_compat)
 		return 0;

 	xt_ematch_foreach(ematch, e)
 		if (find_syn_match(ematch))
 			return 0;
 	pr_info_ratelimited("Only works on TCP SYN packets\n");
 	return -EINVAL;
 }
 #endif

 static struct xt_target tcpmss_tg_reg[] __read_mostly = {
 	{
 		.family		= NFPROTO_IPV4,
 		.name		= "TCPMSS",
 		.checkentry	= tcpmss_tg4_check,
 		.target		= tcpmss_tg4,
 		.targetsize	= sizeof(struct xt_tcpmss_info),
 		.proto		= IPPROTO_TCP,
 		.me		= THIS_MODULE,
 	},
 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
 	{
 		.family		= NFPROTO_IPV6,
 		.name		= "TCPMSS",
 		.checkentry	= tcpmss_tg6_check,
 		.target		= tcpmss_tg6,
 		.targetsize	= sizeof(struct xt_tcpmss_info),
 		.proto		= IPPROTO_TCP,
 		.me		= THIS_MODULE,
 	},
 #endif
 };

 static int __init tcpmss_tg_init(void)
 {
 	return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
 }

 static void __exit tcpmss_tg_exit(void)
 {
 	xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
 }

 module_init(tcpmss_tg_init);
 module_exit(tcpmss_tg_exit);
	// SPDX-License-Identifier: GPL-2.0-only
	/*
	* This is a module which is used for setting the MSS option in TCP packets.
	*
	* Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
	* Copyright (C) 2007 Patrick McHardy <kaber@trash.net>
	*/
	#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
	#include <linux/module.h>
	#include <linux/skbuff.h>
	#include <linux/ip.h>
	#include <linux/gfp.h>
	#include <linux/ipv6.h>
	#include <linux/tcp.h>
	#include <net/dst.h>
	#include <net/flow.h>
	#include <net/ipv6.h>
	#include <net/route.h>
	#include <net/tcp.h>

	#include <linux/netfilter_ipv4/ip_tables.h>
	#include <linux/netfilter_ipv6/ip6_tables.h>
	#include <linux/netfilter/x_tables.h>
	#include <linux/netfilter/xt_tcpudp.h>
	#include <linux/netfilter/xt_TCPMSS.h>

	MODULE_LICENSE("GPL");
	MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
	MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment");
	MODULE_ALIAS("ipt_TCPMSS");
	MODULE_ALIAS("ip6t_TCPMSS");

	static inline unsigned int
	optlen(const u_int8_t *opt, unsigned int offset)
	{
	/* Beware zero-length options: make finite progress */
	if (opt[offset] <= TCPOPT_NOP \|\| opt[offset+1] == 0)
	return 1;
	else
	return opt[offset+1];
	}

	static u_int32_t tcpmss_reverse_mtu(struct net *net,
	const struct sk_buff *skb,
	unsigned int family)
	{
	struct flowi fl;
	struct rtable *rt = NULL;
	u_int32_t mtu = ~0U;

	if (family == PF_INET) {
	struct flowi4 *fl4 = &fl.u.ip4;
	memset(fl4, 0, sizeof(*fl4));
	fl4->daddr = ip_hdr(skb)->saddr;
	} else {
	struct flowi6 *fl6 = &fl.u.ip6;

	memset(fl6, 0, sizeof(*fl6));
	fl6->daddr = ipv6_hdr(skb)->saddr;
	}

	nf_route(net, (struct dst_entry **)&rt, &fl, false, family);
	if (rt != NULL) {
	mtu = dst_mtu(&rt->dst);
	dst_release(&rt->dst);
	}
	return mtu;
	}

	static int
	tcpmss_mangle_packet(struct sk_buff *skb,
	const struct xt_action_param *par,
	unsigned int family,
	unsigned int tcphoff,
	unsigned int minlen)
	{
	const struct xt_tcpmss_info *info = par->targinfo;
	struct tcphdr *tcph;
	int len, tcp_hdrlen;
	unsigned int i;
	__be16 oldval;
	u16 newmss;
	u8 *opt;

	/* This is a fragment, no TCP header is available */
	if (par->fragoff != 0)
	return 0;

	if (skb_ensure_writable(skb, skb->len))
	return -1;

	len = skb->len - tcphoff;
	if (len < (int)sizeof(struct tcphdr))
	return -1;

	tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
	tcp_hdrlen = tcph->doff * 4;

	if (len < tcp_hdrlen \|\| tcp_hdrlen < sizeof(struct tcphdr))
	return -1;

	if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
	struct net *net = xt_net(par);
	unsigned int in_mtu = tcpmss_reverse_mtu(net, skb, family);
	unsigned int min_mtu = min(dst_mtu(skb_dst(skb)), in_mtu);

	if (min_mtu <= minlen) {
	net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
	min_mtu);
	return -1;
	}
	newmss = min_mtu - minlen;
	} else
	newmss = info->mss;

	opt = (u_int8_t *)tcph;
	for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) {
	if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) {
	u_int16_t oldmss;

	oldmss = (opt[i+2] << 8) \| opt[i+3];

	/* Never increase MSS, even when setting it, as
	* doing so results in problems for hosts that rely
	* on MSS being set correctly.
	*/
	if (oldmss <= newmss)
	return 0;

	opt[i+2] = (newmss & 0xff00) >> 8;
	opt[i+3] = newmss & 0x00ff;

	inet_proto_csum_replace2(&tcph->check, skb,
	htons(oldmss), htons(newmss),
	false);
	return 0;
	}
	}

	/* There is data after the header so the option can't be added
	* without moving it, and doing so may make the SYN packet
	* itself too large. Accept the packet unmodified instead.
	*/
	if (len > tcp_hdrlen)
	return 0;

	/* tcph->doff has 4 bits, do not wrap it to 0 */
	if (tcp_hdrlen >= 15 * 4)
	return 0;

	/*
	* MSS Option not found ?! add it..
	*/
	if (skb_tailroom(skb) < TCPOLEN_MSS) {
	if (pskb_expand_head(skb, 0,
	TCPOLEN_MSS - skb_tailroom(skb),
	GFP_ATOMIC))
	return -1;
	tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
	}

	skb_put(skb, TCPOLEN_MSS);

	/*
	* IPv4: RFC 1122 states "If an MSS option is not received at
	* connection setup, TCP MUST assume a default send MSS of 536".
	* IPv6: RFC 2460 states IPv6 has a minimum MTU of 1280 and a minimum
	* length IPv6 header of 60, ergo the default MSS value is 1220
	* Since no MSS was provided, we must use the default values
	*/
	if (xt_family(par) == NFPROTO_IPV4)
	newmss = min(newmss, (u16)536);
	else
	newmss = min(newmss, (u16)1220);

	opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
	memmove(opt + TCPOLEN_MSS, opt, len - sizeof(struct tcphdr));

	inet_proto_csum_replace2(&tcph->check, skb,
	htons(len), htons(len + TCPOLEN_MSS), true);
	opt[0] = TCPOPT_MSS;
	opt[1] = TCPOLEN_MSS;
	opt[2] = (newmss & 0xff00) >> 8;
	opt[3] = newmss & 0x00ff;

	inet_proto_csum_replace4(&tcph->check, skb, 0, ((__be32 )opt), false);

	oldval = ((__be16 *)tcph)[6];
	tcph->doff += TCPOLEN_MSS/4;
	inet_proto_csum_replace2(&tcph->check, skb,
	oldval, ((__be16 *)tcph)[6], false);
	return TCPOLEN_MSS;
	}

	static unsigned int
	tcpmss_tg4(struct sk_buff skb, const struct xt_action_param par)
	{
	struct iphdr *iph = ip_hdr(skb);
	__be16 newlen;
	int ret;

	ret = tcpmss_mangle_packet(skb, par,
	PF_INET,
	iph->ihl * 4,
	sizeof(*iph) + sizeof(struct tcphdr));
	if (ret < 0)
	return NF_DROP;
	if (ret > 0) {
	iph = ip_hdr(skb);
	newlen = htons(ntohs(iph->tot_len) + ret);
	csum_replace2(&iph->check, iph->tot_len, newlen);
	iph->tot_len = newlen;
	}
	return XT_CONTINUE;
	}

	#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
	static unsigned int
	tcpmss_tg6(struct sk_buff skb, const struct xt_action_param par)
	{
	struct ipv6hdr *ipv6h = ipv6_hdr(skb);
	u8 nexthdr;
	__be16 frag_off, oldlen, newlen;
	int tcphoff;
	int ret;

	nexthdr = ipv6h->nexthdr;
	tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
	if (tcphoff < 0)
	return NF_DROP;
	ret = tcpmss_mangle_packet(skb, par,
	PF_INET6,
	tcphoff,
	sizeof(*ipv6h) + sizeof(struct tcphdr));
	if (ret < 0)
	return NF_DROP;
	if (ret > 0) {
	ipv6h = ipv6_hdr(skb);
	oldlen = ipv6h->payload_len;
	newlen = htons(ntohs(oldlen) + ret);
	if (skb->ip_summed == CHECKSUM_COMPLETE)
	skb->csum = csum_add(csum_sub(skb->csum, oldlen),
	newlen);
	ipv6h->payload_len = newlen;
	}
	return XT_CONTINUE;
	}
	#endif

	/* Must specify -p tcp --syn */
	static inline bool find_syn_match(const struct xt_entry_match *m)
	{
	const struct xt_tcp tcpinfo = (const struct xt_tcp )m->data;

	if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
	tcpinfo->flg_cmp & TCPHDR_SYN &&
	!(tcpinfo->invflags & XT_TCP_INV_FLAGS))
	return true;

	return false;
	}

	static int tcpmss_tg4_check(const struct xt_tgchk_param *par)
	{
	const struct xt_tcpmss_info *info = par->targinfo;
	const struct ipt_entry *e = par->entryinfo;
	const struct xt_entry_match *ematch;

	if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
	(par->hook_mask & ~((1 << NF_INET_FORWARD) \|
	(1 << NF_INET_LOCAL_OUT) \|
	(1 << NF_INET_POST_ROUTING))) != 0) {
	pr_info_ratelimited("path-MTU clamping only supported in FORWARD, OUTPUT and POSTROUTING hooks\n");
	return -EINVAL;
	}
	if (par->nft_compat)
	return 0;

	xt_ematch_foreach(ematch, e)
	if (find_syn_match(ematch))
	return 0;
	pr_info_ratelimited("Only works on TCP SYN packets\n");
	return -EINVAL;
	}

	#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
	static int tcpmss_tg6_check(const struct xt_tgchk_param *par)
	{
	const struct xt_tcpmss_info *info = par->targinfo;
	const struct ip6t_entry *e = par->entryinfo;
	const struct xt_entry_match *ematch;

	if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
	(par->hook_mask & ~((1 << NF_INET_FORWARD) \|
	(1 << NF_INET_LOCAL_OUT) \|
	(1 << NF_INET_POST_ROUTING))) != 0) {
	pr_info_ratelimited("path-MTU clamping only supported in FORWARD, OUTPUT and POSTROUTING hooks\n");
	return -EINVAL;
	}
	if (par->nft_compat)
	return 0;

	xt_ematch_foreach(ematch, e)
	if (find_syn_match(ematch))
	return 0;
	pr_info_ratelimited("Only works on TCP SYN packets\n");
	return -EINVAL;
	}
	#endif

	static struct xt_target tcpmss_tg_reg[] __read_mostly = {
	{
	.family = NFPROTO_IPV4,
	.name = "TCPMSS",
	.checkentry = tcpmss_tg4_check,
	.target = tcpmss_tg4,
	.targetsize = sizeof(struct xt_tcpmss_info),
	.proto = IPPROTO_TCP,
	.me = THIS_MODULE,
	},
	#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
	{
	.family = NFPROTO_IPV6,
	.name = "TCPMSS",
	.checkentry = tcpmss_tg6_check,
	.target = tcpmss_tg6,
	.targetsize = sizeof(struct xt_tcpmss_info),
	.proto = IPPROTO_TCP,
	.me = THIS_MODULE,
	},
	#endif
	};

	static int __init tcpmss_tg_init(void)
	{
	return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
	}

	static void __exit tcpmss_tg_exit(void)
	{
	xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
	}

	module_init(tcpmss_tg_init);
	module_exit(tcpmss_tg_exit);