Re: [patch] netfilter: implement TCPMSS target for IPv6

[Patrick McHardy <kaber@trash.net>]

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David Madore wrote:
> Implement TCPMSS target for IPv6 by shamelessly copying from
> Marc Boucher's IPv4 implementation.


This is an x_tables port of the TCPMSS target. Care to give it a try?
I believe Yasuyuki is currently working on proper x_tables support
for userspace, but that might still take a while, so If you send me
your userspace port I'll add it to SVN for the time being.


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[NETFILTER]: add IPv6-capable TCPMSS target

Signed-off-by: Patrick McHardy <kaber@trash.net>

---
commit 6acae256d49e86abf7c84c8a8129343edbca17e0
tree 8f0299bef85de4048509e04eab148b27fb224a4f
parent fd492ee593effa6505b8c8c593002d100440f95d
author Patrick McHardy <kaber@trash.net> Mon, 15 Jan 2007 15:54:23 +0100
committer Patrick McHardy <kaber@trash.net> Mon, 15 Jan 2007 15:54:23 +0100

 include/linux/netfilter/xt_TCPMSS.h       |   10 +
 include/linux/netfilter_ipv4/ipt_TCPMSS.h |    7 -
 net/ipv4/netfilter/Kconfig                |   26 ---
 net/ipv4/netfilter/Makefile               |    1 
 net/ipv4/netfilter/ipt_TCPMSS.c           |  207 ---------------------
 net/netfilter/Kconfig                     |   26 +++
 net/netfilter/Makefile                    |    1 
 net/netfilter/xt_TCPMSS.c                 |  292 +++++++++++++++++++++++++++++
 8 files changed, 332 insertions(+), 238 deletions(-)

diff --git a/include/linux/netfilter/xt_TCPMSS.h b/include/linux/netfilter/xt_TCPMSS.h
new file mode 100644
index 0000000..5016902
--- /dev/null
+++ b/include/linux/netfilter/xt_TCPMSS.h
@@ -0,0 +1,10 @@
+#ifndef _XT_TCPMSS_H
+#define _XT_TCPMSS_H
+
+struct xt_tcpmss_info {
+	u_int16_t mss;
+};
+
+#define XT_TCPMSS_CLAMP_PMTU 0xffff
+
+#endif /* _XT_TCPMSS_H*/
diff --git a/include/linux/netfilter_ipv4/ipt_TCPMSS.h b/include/linux/netfilter_ipv4/ipt_TCPMSS.h
index aadb395..7a850f9 100644
--- a/include/linux/netfilter_ipv4/ipt_TCPMSS.h
+++ b/include/linux/netfilter_ipv4/ipt_TCPMSS.h
@@ -1,10 +1,9 @@
 #ifndef _IPT_TCPMSS_H
 #define _IPT_TCPMSS_H
 
-struct ipt_tcpmss_info {
-	u_int16_t mss;
-};
+#include <linux/netfilter/xt_TCPMSS.h>
 
-#define IPT_TCPMSS_CLAMP_PMTU 0xffff
+#define ipt_tcpmss_info		xt_tcpmss_info
+#define IPT_TCPMSS_CLAMP_PMTU	XT_TCPMSS_CLAMP_PMTU
 
 #endif /*_IPT_TCPMSS_H*/
diff --git a/net/ipv4/netfilter/Kconfig b/net/ipv4/netfilter/Kconfig
index 47bd3ad..9b08e7a 100644
--- a/net/ipv4/netfilter/Kconfig
+++ b/net/ipv4/netfilter/Kconfig
@@ -361,32 +361,6 @@ config IP_NF_TARGET_ULOG
 
 	  To compile it as a module, choose M here.  If unsure, say N.
 
-config IP_NF_TARGET_TCPMSS
-	tristate "TCPMSS target support"
-	depends on IP_NF_IPTABLES
-	---help---
-	  This option adds a `TCPMSS' target, which allows you to alter the
-	  MSS value of TCP SYN packets, to control the maximum size for that
-	  connection (usually limiting it to your outgoing interface's MTU
-	  minus 40).
-
-	  This is used to overcome criminally braindead ISPs or servers which
-	  block ICMP Fragmentation Needed packets.  The symptoms of this
-	  problem are that everything works fine from your Linux
-	  firewall/router, but machines behind it can never exchange large
-	  packets:
-	  	1) Web browsers connect, then hang with no data received.
-	  	2) Small mail works fine, but large emails hang.
-	  	3) ssh works fine, but scp hangs after initial handshaking.
-
-	  Workaround: activate this option and add a rule to your firewall
-	  configuration like:
-
-	  iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN \
-	  		 -j TCPMSS --clamp-mss-to-pmtu
-
-	  To compile it as a module, choose M here.  If unsure, say N.
-
 # NAT + specific targets: ip_conntrack
 config IP_NF_NAT
 	tristate "Full NAT"
diff --git a/net/ipv4/netfilter/Makefile b/net/ipv4/netfilter/Makefile
index 15e741a..83cd6d2 100644
--- a/net/ipv4/netfilter/Makefile
+++ b/net/ipv4/netfilter/Makefile
@@ -93,7 +93,6 @@ obj-$(CONFIG_IP_NF_TARGET_SAME) += ipt_S
 obj-$(CONFIG_IP_NF_NAT_SNMP_BASIC) += ip_nat_snmp_basic.o
 obj-$(CONFIG_IP_NF_TARGET_LOG) += ipt_LOG.o
 obj-$(CONFIG_IP_NF_TARGET_ULOG) += ipt_ULOG.o
-obj-$(CONFIG_IP_NF_TARGET_TCPMSS) += ipt_TCPMSS.o
 obj-$(CONFIG_IP_NF_TARGET_CLUSTERIP) += ipt_CLUSTERIP.o
 obj-$(CONFIG_IP_NF_TARGET_TTL) += ipt_TTL.o
 
diff --git a/net/ipv4/netfilter/ipt_TCPMSS.c b/net/ipv4/netfilter/ipt_TCPMSS.c
deleted file mode 100644
index 93eb5c3..0000000
--- a/net/ipv4/netfilter/ipt_TCPMSS.c
+++ /dev/null
@@ -1,207 +0,0 @@
-/*
- * This is a module which is used for setting the MSS option in TCP packets.
- *
- * Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
- *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License version 2 as
- * published by the Free Software Foundation.
- */
-
-#include <linux/module.h>
-#include <linux/skbuff.h>
-
-#include <linux/ip.h>
-#include <net/tcp.h>
-
-#include <linux/netfilter_ipv4/ip_tables.h>
-#include <linux/netfilter_ipv4/ipt_TCPMSS.h>
-
-MODULE_LICENSE("GPL");
-MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
-MODULE_DESCRIPTION("iptables TCP MSS modification module");
-
-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 unsigned int
-ipt_tcpmss_target(struct sk_buff **pskb,
-		  const struct net_device *in,
-		  const struct net_device *out,
-		  unsigned int hooknum,
-		  const struct xt_target *target,
-		  const void *targinfo)
-{
-	const struct ipt_tcpmss_info *tcpmssinfo = targinfo;
-	struct tcphdr *tcph;
-	struct iphdr *iph;
-	u_int16_t tcplen, newmss;
-	__be16 newtotlen, oldval;
-	unsigned int i;
-	u_int8_t *opt;
-
-	if (!skb_make_writable(pskb, (*pskb)->len))
-		return NF_DROP;
-
-	iph = (*pskb)->nh.iph;
-	tcplen = (*pskb)->len - iph->ihl*4;
-	tcph = (void *)iph + iph->ihl*4;
-
-	/* Since it passed flags test in tcp match, we know it is is
-	   not a fragment, and has data >= tcp header length.  SYN
-	   packets should not contain data: if they did, then we risk
-	   running over MTU, sending Frag Needed and breaking things
-	   badly. --RR */
-	if (tcplen != tcph->doff*4) {
-		if (net_ratelimit())
-			printk(KERN_ERR
-			       "ipt_tcpmss_target: bad length (%d bytes)\n",
-			       (*pskb)->len);
-		return NF_DROP;
-	}
-
-	if (tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU) {
-		if (dst_mtu((*pskb)->dst) <= sizeof(struct iphdr) +
-					     sizeof(struct tcphdr)) {
-			if (net_ratelimit())
-				printk(KERN_ERR "ipt_tcpmss_target: "
-				       "unknown or invalid path-MTU (%d)\n",
-				       dst_mtu((*pskb)->dst));
-			return NF_DROP; /* or IPT_CONTINUE ?? */
-		}
-
-		newmss = dst_mtu((*pskb)->dst) - sizeof(struct iphdr) -
-						 sizeof(struct tcphdr);
-	} else
-		newmss = tcpmssinfo->mss;
-
- 	opt = (u_int8_t *)tcph;
-	for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)) {
-		if (opt[i] == TCPOPT_MSS && tcph->doff*4 - i >= TCPOLEN_MSS &&
-		    opt[i+1] == TCPOLEN_MSS) {
-			u_int16_t oldmss;
-
-			oldmss = (opt[i+2] << 8) | opt[i+3];
-
-			if (tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU &&
-			    oldmss <= newmss)
-				return IPT_CONTINUE;
-
-			opt[i+2] = (newmss & 0xff00) >> 8;
-			opt[i+3] = (newmss & 0x00ff);
-
-			nf_proto_csum_replace2(&tcph->check, *pskb,
-						htons(oldmss), htons(newmss), 0);
-			return IPT_CONTINUE;
-		}
-	}
-
-	/*
-	 * MSS Option not found ?! add it..
-	 */
-	if (skb_tailroom((*pskb)) < TCPOLEN_MSS) {
-		struct sk_buff *newskb;
-
-		newskb = skb_copy_expand(*pskb, skb_headroom(*pskb),
-					 TCPOLEN_MSS, GFP_ATOMIC);
-		if (!newskb)
-			return NF_DROP;
-		kfree_skb(*pskb);
-		*pskb = newskb;
-		iph = (*pskb)->nh.iph;
-		tcph = (void *)iph + iph->ihl*4;
-	}
-
-	skb_put((*pskb), TCPOLEN_MSS);
-
- 	opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
-	memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
-
-	nf_proto_csum_replace2(&tcph->check, *pskb,
-				htons(tcplen), htons(tcplen + TCPOLEN_MSS), 1);
-	opt[0] = TCPOPT_MSS;
-	opt[1] = TCPOLEN_MSS;
-	opt[2] = (newmss & 0xff00) >> 8;
-	opt[3] = (newmss & 0x00ff);
-
-	nf_proto_csum_replace4(&tcph->check, *pskb, 0, *((__be32 *)opt), 0);
-
-	oldval = ((__be16 *)tcph)[6];
-	tcph->doff += TCPOLEN_MSS/4;
-	nf_proto_csum_replace2(&tcph->check, *pskb,
-				oldval, ((__be16 *)tcph)[6], 0);
-
-	newtotlen = htons(ntohs(iph->tot_len) + TCPOLEN_MSS);
-	nf_csum_replace2(&iph->check, iph->tot_len, newtotlen);
-	iph->tot_len = newtotlen;
-	return IPT_CONTINUE;
-}
-
-#define TH_SYN 0x02
-
-static inline int find_syn_match(const struct ipt_entry_match *m)
-{
-	const struct ipt_tcp *tcpinfo = (const struct ipt_tcp *)m->data;
-
-	if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
-	    tcpinfo->flg_cmp & TH_SYN &&
-	    !(tcpinfo->invflags & IPT_TCP_INV_FLAGS))
-		return 1;
-
-	return 0;
-}
-
-/* Must specify -p tcp --syn/--tcp-flags SYN */
-static int
-ipt_tcpmss_checkentry(const char *tablename,
-		      const void *e_void,
-		      const struct xt_target *target,
-		      void *targinfo,
-		      unsigned int hook_mask)
-{
-	const struct ipt_tcpmss_info *tcpmssinfo = targinfo;
-	const struct ipt_entry *e = e_void;
-
-	if (tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU &&
-	    (hook_mask & ~((1 << NF_IP_FORWARD) |
-			   (1 << NF_IP_LOCAL_OUT) |
-			   (1 << NF_IP_POST_ROUTING))) != 0) {
-		printk("TCPMSS: path-MTU clamping only supported in "
-		       "FORWARD, OUTPUT and POSTROUTING hooks\n");
-		return 0;
-	}
-
-	if (IPT_MATCH_ITERATE(e, find_syn_match))
-		return 1;
-	printk("TCPMSS: Only works on TCP SYN packets\n");
-	return 0;
-}
-
-static struct ipt_target ipt_tcpmss_reg = {
-	.name		= "TCPMSS",
-	.target		= ipt_tcpmss_target,
-	.targetsize	= sizeof(struct ipt_tcpmss_info),
-	.proto		= IPPROTO_TCP,
-	.checkentry	= ipt_tcpmss_checkentry,
-	.me		= THIS_MODULE,
-};
-
-static int __init ipt_tcpmss_init(void)
-{
-	return ipt_register_target(&ipt_tcpmss_reg);
-}
-
-static void __exit ipt_tcpmss_fini(void)
-{
-	ipt_unregister_target(&ipt_tcpmss_reg);
-}
-
-module_init(ipt_tcpmss_init);
-module_exit(ipt_tcpmss_fini);
diff --git a/net/netfilter/Kconfig b/net/netfilter/Kconfig
index 0c5e290..df4101c 100644
--- a/net/netfilter/Kconfig
+++ b/net/netfilter/Kconfig
@@ -395,6 +395,32 @@ config NETFILTER_XT_TARGET_CONNSECMARK
 
 	  To compile it as a module, choose M here.  If unsure, say N.
 
+config NETFILTER_XT_TARGET_TCPMSS
+	tristate '"TCPMSS" target support'
+	depends on NETFILTER_XTABLES
+	---help---
+	  This option adds a `TCPMSS' target, which allows you to alter the
+	  MSS value of TCP SYN packets, to control the maximum size for that
+	  connection (usually limiting it to your outgoing interface's MTU
+	  minus 40).
+
+	  This is used to overcome criminally braindead ISPs or servers which
+	  block ICMP Fragmentation Needed packets.  The symptoms of this
+	  problem are that everything works fine from your Linux
+	  firewall/router, but machines behind it can never exchange large
+	  packets:
+	        1) Web browsers connect, then hang with no data received.
+	        2) Small mail works fine, but large emails hang.
+	        3) ssh works fine, but scp hangs after initial handshaking.
+
+	  Workaround: activate this option and add a rule to your firewall
+	  configuration like:
+
+	  iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN \
+	                 -j TCPMSS --clamp-mss-to-pmtu
+
+	  To compile it as a module, choose M here.  If unsure, say N.
+
 config NETFILTER_XT_MATCH_COMMENT
 	tristate  '"comment" match support'
 	depends on NETFILTER_XTABLES
diff --git a/net/netfilter/Makefile b/net/netfilter/Makefile
index 5054b0f..b2b5c75 100644
--- a/net/netfilter/Makefile
+++ b/net/netfilter/Makefile
@@ -45,6 +45,7 @@ obj-$(CONFIG_NETFILTER_XT_TARGET_NFQUEUE
 obj-$(CONFIG_NETFILTER_XT_TARGET_NFLOG) += xt_NFLOG.o
 obj-$(CONFIG_NETFILTER_XT_TARGET_NOTRACK) += xt_NOTRACK.o
 obj-$(CONFIG_NETFILTER_XT_TARGET_SECMARK) += xt_SECMARK.o
+obj-$(CONFIG_NETFILTER_XT_TARGET_TCPMSS) += xt_TCPMSS.o
 obj-$(CONFIG_NETFILTER_XT_TARGET_CONNSECMARK) += xt_CONNSECMARK.o
 
 # matches
diff --git a/net/netfilter/xt_TCPMSS.c b/net/netfilter/xt_TCPMSS.c
new file mode 100644
index 0000000..5f6a23b
--- /dev/null
+++ b/net/netfilter/xt_TCPMSS.c
@@ -0,0 +1,292 @@
+/*
+ * This is a module which is used for setting the MSS option in TCP packets.
+ *
+ * Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+#include <linux/module.h>
+#include <linux/skbuff.h>
+#include <linux/ip.h>
+#include <linux/ipv6.h>
+#include <linux/tcp.h>
+#include <net/ipv6.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("x_tables TCP MSS modification module");
+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 int
+tcpmss_mangle_packet(struct sk_buff **pskb,
+		     const struct xt_tcpmss_info *info,
+		     unsigned int tcphoff,
+		     unsigned int minlen)
+{
+	struct tcphdr *tcph;
+	unsigned int tcplen, i;
+	__be16 oldval;
+	u16 newmss;
+	u8 *opt;
+
+	if (!skb_make_writable(pskb, (*pskb)->len))
+		return -1;
+
+	tcplen = (*pskb)->len - tcphoff;
+	tcph = (struct tcphdr *)((*pskb)->nh.raw + tcphoff);
+
+	/* Since it passed flags test in tcp match, we know it is is
+	   not a fragment, and has data >= tcp header length.  SYN
+	   packets should not contain data: if they did, then we risk
+	   running over MTU, sending Frag Needed and breaking things
+	   badly. --RR */
+	if (tcplen != tcph->doff*4) {
+		if (net_ratelimit())
+			printk(KERN_ERR "xt_TCPMSS: bad length (%u bytes)\n",
+			       (*pskb)->len);
+		return -1;
+	}
+
+	if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
+		if (dst_mtu((*pskb)->dst) <= minlen) {
+			if (net_ratelimit())
+				printk(KERN_ERR "xt_TCPMSS: "
+				       "unknown or invalid path-MTU (%u)\n",
+				       dst_mtu((*pskb)->dst));
+			return -1;
+		}
+		newmss = dst_mtu((*pskb)->dst) - minlen;
+	} else
+		newmss = info->mss;
+
+	opt = (u_int8_t *)tcph;
+	for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)) {
+		if (opt[i] == TCPOPT_MSS && tcph->doff*4 - i >= TCPOLEN_MSS &&
+		    opt[i+1] == TCPOLEN_MSS) {
+			u_int16_t oldmss;
+
+			oldmss = (opt[i+2] << 8) | opt[i+3];
+
+			if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
+			    oldmss <= newmss)
+			    	return 0;
+
+			opt[i+2] = (newmss & 0xff00) >> 8;
+			opt[i+3] = (newmss & 0x00ff);
+
+			nf_proto_csum_replace2(&tcph->check, *pskb,
+					       htons(oldmss), htons(newmss), 0);
+			return 0;
+		}
+	}
+
+	/*
+	 * MSS Option not found ?! add it..
+	 */
+	if (skb_tailroom((*pskb)) < TCPOLEN_MSS) {
+		struct sk_buff *newskb;
+
+		newskb = skb_copy_expand(*pskb, skb_headroom(*pskb),
+					 TCPOLEN_MSS, GFP_ATOMIC);
+		if (!newskb)
+			return -1;
+		kfree_skb(*pskb);
+		*pskb = newskb;
+		tcph = (struct tcphdr *)((*pskb)->nh.raw + tcphoff);
+	}
+
+	skb_put((*pskb), TCPOLEN_MSS);
+
+	opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
+	memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
+
+	nf_proto_csum_replace2(&tcph->check, *pskb,
+			       htons(tcplen), htons(tcplen + TCPOLEN_MSS), 1);
+	opt[0] = TCPOPT_MSS;
+	opt[1] = TCPOLEN_MSS;
+	opt[2] = (newmss & 0xff00) >> 8;
+	opt[3] = (newmss & 0x00ff);
+
+	nf_proto_csum_replace4(&tcph->check, *pskb, 0, *((__be32 *)opt), 0);
+
+	oldval = ((__be16 *)tcph)[6];
+	tcph->doff += TCPOLEN_MSS/4;
+	nf_proto_csum_replace2(&tcph->check, *pskb,
+				oldval, ((__be16 *)tcph)[6], 0);
+	return TCPOLEN_MSS;
+}
+
+static unsigned int
+xt_tcpmss_target4(struct sk_buff **pskb,
+		  const struct net_device *in,
+		  const struct net_device *out,
+		  unsigned int hooknum,
+		  const struct xt_target *target,
+		  const void *targinfo)
+{
+	struct iphdr *iph = (*pskb)->nh.iph;
+	__be16 newlen;
+	int ret;
+
+	ret = tcpmss_mangle_packet(pskb, targinfo, iph->ihl * 4,
+				   sizeof(*iph) + sizeof(struct tcphdr));
+	if (ret < 0)
+		return NF_DROP;
+	if (ret > 0) {
+		iph = (*pskb)->nh.iph;
+		newlen = htons(ntohs(iph->tot_len) + ret);
+		nf_csum_replace2(&iph->check, iph->tot_len, newlen);
+		iph->tot_len = newlen;
+	}
+	return XT_CONTINUE;
+}
+
+#if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
+static unsigned int
+xt_tcpmss_target6(struct sk_buff **pskb,
+		  const struct net_device *in,
+		  const struct net_device *out,
+		  unsigned int hooknum,
+		  const struct xt_target *target,
+		  const void *targinfo)
+{
+	struct ipv6hdr *ipv6h = (*pskb)->nh.ipv6h;
+	u8 nexthdr;
+	int tcphoff;
+	int ret;
+
+	nexthdr = ipv6h->nexthdr;
+	tcphoff = ipv6_skip_exthdr(*pskb, sizeof(*ipv6h), &nexthdr);
+	ret = tcpmss_mangle_packet(pskb, targinfo, tcphoff,
+				   sizeof(*ipv6h) + sizeof(struct tcphdr));
+	if (ret < 0)
+		return NF_DROP;
+	if (ret > 0) {
+		ipv6h = (*pskb)->nh.ipv6h;
+		ipv6h->payload_len = htons(ntohs(ipv6h->payload_len) + ret);
+	}
+	return XT_CONTINUE;
+}
+#endif
+
+#define TH_SYN 0x02
+
+/* Must specify -p tcp --syn/--tcp-flags SYN */
+static inline int 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 & TH_SYN &&
+	    !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
+		return 1;
+
+	return 0;
+}
+
+static int
+xt_tcpmss_checkentry4(const char *tablename,
+		      const void *entry,
+		      const struct xt_target *target,
+		      void *targinfo,
+		      unsigned int hook_mask)
+{
+	const struct xt_tcpmss_info *info = targinfo;
+	const struct ipt_entry *e = entry;
+
+	if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
+	    (hook_mask & ~((1 << NF_IP_FORWARD) |
+			   (1 << NF_IP_LOCAL_OUT) |
+			   (1 << NF_IP_POST_ROUTING))) != 0) {
+		printk("xt_TCPMSS: path-MTU clamping only supported in "
+		       "FORWARD, OUTPUT and POSTROUTING hooks\n");
+		return 0;
+	}
+	if (IPT_MATCH_ITERATE(e, find_syn_match))
+		return 1;
+	printk("xt_TCPMSS: Only works on TCP SYN packets\n");
+	return 0;
+}
+
+#if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
+static int
+xt_tcpmss_checkentry6(const char *tablename,
+		      const void *entry,
+		      const struct xt_target *target,
+		      void *targinfo,
+		      unsigned int hook_mask)
+{
+	const struct xt_tcpmss_info *info = targinfo;
+	const struct ip6t_entry *e = entry;
+
+	if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
+	    (hook_mask & ~((1 << NF_IP6_FORWARD) |
+			   (1 << NF_IP6_LOCAL_OUT) |
+			   (1 << NF_IP6_POST_ROUTING))) != 0) {
+		printk("xt_TCPMSS: path-MTU clamping only supported in "
+		       "FORWARD, OUTPUT and POSTROUTING hooks\n");
+		return 0;
+	}
+	if (IP6T_MATCH_ITERATE(e, find_syn_match))
+		return 1;
+	printk("xt_TCPMSS: Only works on TCP SYN packets\n");
+	return 0;
+}
+#endif
+
+static struct xt_target xt_tcpmss_reg[] = {
+	{
+		.family		= AF_INET,
+		.name		= "TCPMSS",
+		.checkentry	= xt_tcpmss_checkentry4,
+		.target		= xt_tcpmss_target4,
+		.targetsize	= sizeof(struct xt_tcpmss_info),
+		.proto		= IPPROTO_TCP,
+		.me		= THIS_MODULE,
+	},
+#if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
+	{
+		.family		= AF_INET6,
+		.name		= "TCPMSS",
+		.checkentry	= xt_tcpmss_checkentry6,
+		.target		= xt_tcpmss_target6,
+		.targetsize	= sizeof(struct xt_tcpmss_info),
+		.proto		= IPPROTO_TCP,
+		.me		= THIS_MODULE,
+	},
+#endif
+};
+
+static int __init xt_tcpmss_init(void)
+{
+	return xt_register_targets(xt_tcpmss_reg, ARRAY_SIZE(xt_tcpmss_reg));
+}
+
+static void __exit xt_tcpmss_fini(void)
+{
+	xt_unregister_targets(xt_tcpmss_reg, ARRAY_SIZE(xt_tcpmss_reg));
+}
+
+module_init(xt_tcpmss_init);
+module_exit(xt_tcpmss_fini);