[NETFILTER6]: Add new ip6tables HOPLIMIT target

This target allows users to modify the hoplimit header field of the
IPv6 header.
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netfilter_ipv6/ip6t_HL.h

+/* Hop Limit modification module for ip6tables
+ * Maciej Soltysiak <solt@dns.toxicfilms.tv>
+ * Based on HW's TTL module */
+
+#ifndef _IP6T_HL_H
+#define _IP6T_HL_H
+
+enum {
+	IP6T_HL_SET = 0,
+	IP6T_HL_INC,
+	IP6T_HL_DEC
+};
+
+#define IP6T_HL_MAXMODE	IP6T_HL_DEC
+
+struct ip6t_HL_info {
+	u_int8_t	mode;
+	u_int8_t	hop_limit;
+};
+
+
+#endif

net/ipv6/netfilter/Kconfig

@@ -239,6 +239,22 @@ config IP6_NF_TARGET_MARK
 
 	  To compile it as a module, choose M here.  If unsure, say N.
 
+config IP6_NF_TARGET_HL
+	tristate  'HL (hoplimit) target support'
+	depends on IP6_NF_MANGLE
+	help
+	  This option adds a `HL' target, which enables the user to decrement
+	  the hoplimit value of the IPv6 header or set it to a given (lower)
+	  value.
+	
+	  While it is safe to decrement the hoplimit value, this option also
+	  enables functionality to increment and set the hoplimit value of the
+	  IPv6 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.
+
 #dep_tristate '  LOG target support' CONFIG_IP6_NF_TARGET_LOG $CONFIG_IP6_NF_IPTABLES
 config IP6_NF_RAW
 	tristate  'raw table support (required for TRACE)'

net/ipv6/netfilter/Makefile

@@ -20,6 +20,7 @@ obj-$(CONFIG_IP6_NF_MATCH_PHYSDEV) += ip6t_physdev.o
 obj-$(CONFIG_IP6_NF_FILTER) += ip6table_filter.o
 obj-$(CONFIG_IP6_NF_MANGLE) += ip6table_mangle.o
 obj-$(CONFIG_IP6_NF_TARGET_MARK) += ip6t_MARK.o
+obj-$(CONFIG_IP6_NF_TARGET_HL) += ip6t_HL.o
 obj-$(CONFIG_IP6_NF_QUEUE) += ip6_queue.o
 obj-$(CONFIG_IP6_NF_TARGET_LOG) += ip6t_LOG.o
 obj-$(CONFIG_IP6_NF_RAW) += ip6table_raw.o

net/ipv6/netfilter/ip6t_HL.c

+/* 
+ * Hop Limit modification target for ip6tables
+ * Maciej Soltysiak <solt@dns.toxicfilms.tv>
+ * Based on HW's TTL module
+ *
+ * This software is distributed under the terms of GNU GPL
+ */
+
+#include <linux/module.h>
+#include <linux/skbuff.h>
+#include <linux/ip.h>
+
+#include <linux/netfilter_ipv6/ip6_tables.h>
+#include <linux/netfilter_ipv6/ip6t_HL.h>
+
+MODULE_AUTHOR("Maciej Soltysiak <solt@dns.toxicfilms.tv>");
+MODULE_DESCRIPTION("IP tables Hop Limit modification module");
+MODULE_LICENSE("GPL");
+
+static unsigned int ip6t_hl_target(struct sk_buff **pskb, 
+				   const struct net_device *in,
+				   const struct net_device *out,
+				   unsigned int hooknum,
+				   const void *targinfo, void *userinfo)
+{
+	struct ipv6hdr *ip6h;
+	const struct ip6t_HL_info *info = targinfo;
+	u_int16_t diffs[2];
+	int new_hl;
+
+	if (!skb_make_writable(pskb, (*pskb)->len))
+		return NF_DROP;
+
+	ip6h = (*pskb)->nh.ipv6h;
+
+	switch (info->mode) {
+		case IP6T_HL_SET:
+			new_hl = info->hop_limit;
+			break;
+		case IP6T_HL_INC:
+			new_hl = ip6h->hop_limit + info->hop_limit;
+			if (new_hl > 255)
+				new_hl = 255;
+			break;
+		case IP6T_HL_DEC:
+			new_hl = ip6h->hop_limit - info->hop_limit;
+			if (new_hl < 0)
+				new_hl = 0;
+			break;
+		default:
+			new_hl = ip6h->hop_limit;
+			break;
+	}
+
+	if (new_hl != ip6h->hop_limit) {
+		diffs[0] = htons(((unsigned)ip6h->hop_limit) << 8) ^ 0xFFFF;
+		ip6h->hop_limit = new_hl;
+		diffs[1] = htons(((unsigned)ip6h->hop_limit) << 8);
+	}
+
+	return IP6T_CONTINUE;
+}
+
+static int ip6t_hl_checkentry(const char *tablename,
+		const struct ip6t_entry *e,
+		void *targinfo,
+		unsigned int targinfosize,
+		unsigned int hook_mask)
+{
+	struct ip6t_HL_info *info = targinfo;
+
+	if (targinfosize != IP6T_ALIGN(sizeof(struct ip6t_HL_info))) {
+		printk(KERN_WARNING "ip6t_HL: targinfosize %u != %Zu\n",
+				targinfosize,
+				IP6T_ALIGN(sizeof(struct ip6t_HL_info)));
+		return 0;	
+	}	
+
+	if (strcmp(tablename, "mangle")) {
+		printk(KERN_WARNING "ip6t_HL: can only be called from "
+			"\"mangle\" table, not \"%s\"\n", tablename);
+		return 0;
+	}
+
+	if (info->mode > IP6T_HL_MAXMODE) {
+		printk(KERN_WARNING "ip6t_HL: invalid or unknown Mode %u\n", 
+			info->mode);
+		return 0;
+	}
+
+	if ((info->mode != IP6T_HL_SET) && (info->hop_limit == 0)) {
+		printk(KERN_WARNING "ip6t_HL: increment/decrement doesn't "
+			"make sense with value 0\n");
+		return 0;
+	}
+	
+	return 1;
+}
+
+static struct ip6t_target ip6t_HL = { 
+	.name 		= "HL", 
+	.target		= ip6t_hl_target, 
+	.checkentry	= ip6t_hl_checkentry, 
+	.me		= THIS_MODULE
+};
+
+static int __init init(void)
+{
+	return ip6t_register_target(&ip6t_HL);
+}
+
+static void __exit fini(void)
+{
+	ip6t_unregister_target(&ip6t_HL);
+}
+
+module_init(init);
+module_exit(fini);