[NETFILTER]: add IPv6-capable TCPMSS target
Patrick McHardy [Wed, 7 Feb 2007 23:09:46 +0000 (15:09 -0800)]
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netfilter/Kbuild
include/linux/netfilter/xt_TCPMSS.h [new file with mode: 0644]
include/linux/netfilter_ipv4/ipt_TCPMSS.h
net/ipv4/netfilter/Kconfig
net/ipv4/netfilter/Makefile
net/ipv4/netfilter/ipt_TCPMSS.c [deleted file]
net/netfilter/Kconfig
net/netfilter/Makefile
net/netfilter/xt_TCPMSS.c [new file with mode: 0644]

index 6328175..43397a4 100644 (file)
@@ -33,6 +33,7 @@ header-y += xt_tcpmss.h
 header-y += xt_tcpudp.h
 header-y += xt_SECMARK.h
 header-y += xt_CONNSECMARK.h
+header-y += xt_TCPMSS.h
 
 unifdef-y += nf_conntrack_common.h
 unifdef-y += nf_conntrack_ftp.h
diff --git a/include/linux/netfilter/xt_TCPMSS.h b/include/linux/netfilter/xt_TCPMSS.h
new file mode 100644 (file)
index 0000000..53a292c
--- /dev/null
@@ -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 */
index aadb395..7a850f9 100644 (file)
@@ -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*/
index 47bd3ad..9b08e7a 100644 (file)
@@ -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"
index 16d177b..6625ec6 100644 (file)
@@ -103,7 +103,6 @@ obj-$(CONFIG_IP_NF_TARGET_SAME) += ipt_SAME.o
 obj-$(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 (file)
index 93eb5c3..0000000
+++ /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);
index 614c92c..748f7f0 100644 (file)
@@ -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 && (IPV6 || IPV6=n)
+       ---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
index 5054b0f..b2b5c75 100644 (file)
@@ -45,6 +45,7 @@ obj-$(CONFIG_NETFILTER_XT_TARGET_NFQUEUE) += xt_NFQUEUE.o
 obj-$(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 (file)
index 0000000..db7e38c
--- /dev/null
@@ -0,0 +1,296 @@
+/*
+ * 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);
+       if (tcphoff < 0) {
+               WARN_ON(1);
+               return NF_DROP;
+       }
+       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 */
+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);