[NETFILTER]: Add new iptables TTL target

This new iptables target allows manipulation of the TTL of an IPv4 packet.
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netfilter_ipv4/ipt_TTL.h

+/* TTL modification module for IP tables
+ * (C) 2000 by Harald Welte <laforge@netfilter.org> */
+
+#ifndef _IPT_TTL_H
+#define _IPT_TTL_H
+
+enum {
+	IPT_TTL_SET = 0,
+	IPT_TTL_INC,
+	IPT_TTL_DEC
+};
+
+#define IPT_TTL_MAXMODE	IPT_TTL_DEC
+
+struct ipt_TTL_info {
+	u_int8_t	mode;
+	u_int8_t	ttl;
+};
+
+
+#endif

net/ipv4/netfilter/Kconfig

@@ -664,6 +664,20 @@ config IP_NF_TARGET_CLASSIFY
 
 	  To compile it as a module, choose M here.  If unsure, say N.
 
+config IP_NF_TARGET_TTL
+	tristate  'TTL target support'
+	depends on IP_NF_MANGLE
+	help
+	  This option adds a `TTL' target, which enables the user to modify
+	  the TTL value of the IP header.
+
+	  While it is safe to decrement/lower the TTL, this target also enables
+	  functionality to increment and set the TTL value of the IP header to
+	  arbitrary values.  This is EXTREMELY DANGEROUS since you can easily
+	  create immortal packets that loop forever on the network.
+
+	  To compile it as a module, choose M here.  If unsure, say N.
+
 config IP_NF_TARGET_CONNMARK
 	tristate  'CONNMARK target support'
 	depends on IP_NF_CONNTRACK_MARK && IP_NF_MANGLE

net/ipv4/netfilter/Makefile

@@ -85,6 +85,7 @@ obj-$(CONFIG_IP_NF_TARGET_ULOG) += ipt_ULOG.o
 obj-$(CONFIG_IP_NF_TARGET_TCPMSS) += ipt_TCPMSS.o
 obj-$(CONFIG_IP_NF_TARGET_NOTRACK) += ipt_NOTRACK.o
 obj-$(CONFIG_IP_NF_TARGET_CLUSTERIP) += ipt_CLUSTERIP.o
+obj-$(CONFIG_IP_NF_TARGET_TTL) += ipt_TTL.o
 
 # generic ARP tables
 obj-$(CONFIG_IP_NF_ARPTABLES) += arp_tables.o

net/ipv4/netfilter/ipt_TTL.c

+/* TTL modification target for IP tables
+ * (C) 2000,2005 by Harald Welte <laforge@netfilter.org>
+ *
+ * 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/checksum.h>
+
+#include <linux/netfilter_ipv4/ip_tables.h>
+#include <linux/netfilter_ipv4/ipt_TTL.h>
+
+MODULE_AUTHOR("Harald Welte <laforge@netfilter.org>");
+MODULE_DESCRIPTION("IP tables TTL modification module");
+MODULE_LICENSE("GPL");
+
+static unsigned int 
+ipt_ttl_target(struct sk_buff **pskb, const struct net_device *in, 
+		const struct net_device *out, unsigned int hooknum, 
+		const void *targinfo, void *userinfo)
+{
+	struct iphdr *iph;
+	const struct ipt_TTL_info *info = targinfo;
+	u_int16_t diffs[2];
+	int new_ttl;
+
+	if (!skb_make_writable(pskb, (*pskb)->len))
+		return NF_DROP;
+
+	iph = (*pskb)->nh.iph;
+
+	switch (info->mode) {
+		case IPT_TTL_SET:
+			new_ttl = info->ttl;
+			break;
+		case IPT_TTL_INC:
+			new_ttl = iph->ttl + info->ttl;
+			if (new_ttl > 255)
+				new_ttl = 255;
+			break;
+		case IPT_TTL_DEC:
+			new_ttl = iph->ttl - info->ttl;
+			if (new_ttl < 0)
+				new_ttl = 0;
+			break;
+		default:
+			new_ttl = iph->ttl;
+			break;
+	}
+
+	if (new_ttl != iph->ttl) {
+		diffs[0] = htons(((unsigned)iph->ttl) << 8) ^ 0xFFFF;
+		iph->ttl = new_ttl;
+		diffs[1] = htons(((unsigned)iph->ttl) << 8);
+		iph->check = csum_fold(csum_partial((char *)diffs,
+						    sizeof(diffs),
+						    iph->check^0xFFFF));
+	}
+
+	return IPT_CONTINUE;
+}
+
+static int ipt_ttl_checkentry(const char *tablename,
+		const struct ipt_entry *e,
+		void *targinfo,
+		unsigned int targinfosize,
+		unsigned int hook_mask)
+{
+	struct ipt_TTL_info *info = targinfo;
+
+	if (targinfosize != IPT_ALIGN(sizeof(struct ipt_TTL_info))) {
+		printk(KERN_WARNING "ipt_TTL: targinfosize %u != %Zu\n",
+				targinfosize,
+				IPT_ALIGN(sizeof(struct ipt_TTL_info)));
+		return 0;
+	}
+
+	if (strcmp(tablename, "mangle")) {
+		printk(KERN_WARNING "ipt_TTL: can only be called from "
+			"\"mangle\" table, not \"%s\"\n", tablename);
+		return 0;
+	}
+
+	if (info->mode > IPT_TTL_MAXMODE) {
+		printk(KERN_WARNING "ipt_TTL: invalid or unknown Mode %u\n", 
+			info->mode);
+		return 0;
+	}
+
+	if ((info->mode != IPT_TTL_SET) && (info->ttl == 0))
+		return 0;
+
+	return 1;
+}
+
+static struct ipt_target ipt_TTL = { 
+	.name 		= "TTL",
+	.target 	= ipt_ttl_target, 
+	.checkentry 	= ipt_ttl_checkentry, 
+	.me 		= THIS_MODULE,
+};
+
+static int __init init(void)
+{
+	return ipt_register_target(&ipt_TTL);
+}
+
+static void __exit fini(void)
+{
+	ipt_unregister_target(&ipt_TTL);
+}
+
+module_init(init);
+module_exit(fini);