patch-2.4.3 linux/net/ipv4/netfilter/ipt_TCPMSS.c

• Lines: 246
• Date: Tue Mar 6 22:44:16 2001
• Orig file: v2.4.2/linux/net/ipv4/netfilter/ipt_TCPMSS.c
• Orig date: Wed Dec 31 16:00:00 1969

diff -u --recursive --new-file v2.4.2/linux/net/ipv4/netfilter/ipt_TCPMSS.c linux/net/ipv4/netfilter/ipt_TCPMSS.c
@@ -0,0 +1,245 @@
+/*
+ * This is a module which is used for setting the MSS option in TCP packets.
+ *
+ * Copyright (c) 2000 Marc Boucher
+ */
+#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>
+
+#if 0
+#define DEBUGP printk
+#else
+#define DEBUGP(format, args...)
+#endif
+
+static u_int16_t
+cheat_check(u_int32_t oldvalinv, u_int32_t newval, u_int16_t oldcheck)
+{
+	u_int32_t diffs[] = { oldvalinv, newval };
+	return csum_fold(csum_partial((char *)diffs, sizeof(diffs),
+                                      oldcheck^0xFFFF));
+}
+
+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,
+		  unsigned int hooknum,
+		  const struct net_device *in,
+		  const struct net_device *out,
+		  const void *targinfo,
+		  void *userinfo)
+{
+	const struct ipt_tcpmss_info *tcpmssinfo = targinfo;
+	struct tcphdr *tcph;
+	struct iphdr *iph = (*pskb)->nh.iph;
+	u_int16_t tcplen, newtotlen, oldval, newmss;
+	unsigned int i;
+	u_int8_t *opt;
+
+	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(!(*pskb)->dst) {
+			if (net_ratelimit())
+				printk(KERN_ERR
+			       		"ipt_tcpmss_target: no dst?! can't determine path-MTU\n");
+			return NF_DROP; /* or IPT_CONTINUE ?? */
+		}
+
+		if((*pskb)->dst->pmtu <= (sizeof(struct iphdr) + sizeof(struct tcphdr))) {
+			if (net_ratelimit())
+				printk(KERN_ERR
+		       			"ipt_tcpmss_target: unknown or invalid path-MTU (%d)\n", (*pskb)->dst->pmtu);
+			return NF_DROP; /* or IPT_CONTINUE ?? */
+		}
+
+		newmss = (*pskb)->dst->pmtu - 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);
+
+			tcph->check = cheat_check(htons(oldmss)^0xFFFF,
+						  htons(newmss),
+						  tcph->check);
+
+			DEBUGP(KERN_INFO "ipt_tcpmss_target: %u.%u.%u.%u:%hu"
+			       "->%u.%u.%u.%u:%hu changed TCP MSS option"
+			       " (from %u to %u)\n", 
+			       NIPQUAD((*pskb)->nh.iph->saddr),
+			       ntohs(tcph->source),
+			       NIPQUAD((*pskb)->nh.iph->daddr),
+			       ntohs(tcph->dest),
+			       oldmss, newmss);
+			goto retmodified;
+		}
+	}
+
+	/*
+	 * 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) {
+			if (net_ratelimit())
+				printk(KERN_ERR "ipt_tcpmss_target:"
+				       " unable to allocate larger skb\n");
+			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));
+
+	tcph->check = cheat_check(htons(tcplen) ^ 0xFFFF,
+				  htons(tcplen + TCPOLEN_MSS), tcph->check);
+	tcplen += TCPOLEN_MSS;
+
+	opt[0] = TCPOPT_MSS;
+	opt[1] = TCPOLEN_MSS;
+	opt[2] = (newmss & 0xff00) >> 8;
+	opt[3] = (newmss & 0x00ff);
+
+	tcph->check = cheat_check(~0, *((u_int32_t *)opt), tcph->check);
+
+	oldval = ((u_int16_t *)tcph)[6];
+	tcph->doff += TCPOLEN_MSS/4;
+	tcph->check = cheat_check(oldval ^ 0xFFFF,
+				  ((u_int16_t *)tcph)[6], tcph->check);
+
+	newtotlen = htons(ntohs(iph->tot_len) + TCPOLEN_MSS);
+	iph->check = cheat_check(iph->tot_len ^ 0xFFFF,
+				 newtotlen, iph->check);
+	iph->tot_len = newtotlen;
+
+	DEBUGP(KERN_INFO "ipt_tcpmss_target: %u.%u.%u.%u:%hu"
+	       "->%u.%u.%u.%u:%hu added TCP MSS option (%u)\n",
+	       NIPQUAD((*pskb)->nh.iph->saddr),
+	       ntohs(tcph->source),
+	       NIPQUAD((*pskb)->nh.iph->daddr),
+	       ntohs(tcph->dest),
+	       newmss);
+
+ retmodified:
+	/* If we had a hardware checksum before, it's now invalid */
+	(*pskb)->ip_summed = CHECKSUM_NONE;
+	(*pskb)->nfcache |= NFC_UNKNOWN | NFC_ALTERED;
+	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 struct ipt_entry *e,
+		      void *targinfo,
+		      unsigned int targinfosize,
+		      unsigned int hook_mask)
+{
+	const struct ipt_tcpmss_info *tcpmssinfo = targinfo;
+
+	if (targinfosize != IPT_ALIGN(sizeof(struct ipt_tcpmss_info))) {
+		DEBUGP("ipt_tcpmss_checkentry: targinfosize %u != %u\n",
+		       targinfosize, IPT_ALIGN(sizeof(struct ipt_tcpmss_info)));
+		return 0;
+	}
+
+
+	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 (e->ip.proto == IPPROTO_TCP
+	    && !(e->ip.invflags & IPT_INV_PROTO)
+	    && 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
+= { { NULL, NULL }, "TCPMSS",
+    ipt_tcpmss_target, ipt_tcpmss_checkentry, NULL, THIS_MODULE };
+
+static int __init init(void)
+{
+	return ipt_register_target(&ipt_tcpmss_reg);
+}
+
+static void __exit fini(void)
+{
+	ipt_unregister_target(&ipt_tcpmss_reg);
+}
+
+module_init(init);
+module_exit(fini);