[NETFILTER]: add IPv6-capable TCPMSS target

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

3 files changed, 323 insertions, 0 deletions

diff --git a/net/netfilter/Kconfig b/net/netfilter/Kconfig
index 614c92c17835..748f7f00909a 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 && (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
diff --git a/net/netfilter/Makefile b/net/netfilter/Makefile
index 5054b0ff8096..b2b5c7566b26 100644
--- a/net/netfilter/Makefile
+++ b/net/netfilter/Makefile
@@ -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
index 000000000000..db7e38c08de2
--- /dev/null
+++ b/net/netfilter/xt_TCPMSS.c
@@ -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);