TARPIT: add IPv6 support

This adds IPv6 support for the tarpit target. It performs the same
functionality as the v4 version, but with IPv6 connections.

Signed-off-by: Josh Hunt <johunt@akamai.com>

extensions/xt_TARPIT.c

@@ -42,10 +42,15 @@
 #include <linux/module.h>
 #include <linux/skbuff.h>
 #include <linux/version.h>
+#include <linux/netfilter_ipv6.h>
 #include <linux/netfilter/x_tables.h>
 #ifdef CONFIG_BRIDGE_NETFILTER
 #	include <linux/netfilter_bridge.h>
 #endif
+#include <net/addrconf.h>
+#include <net/ip6_checksum.h>
+#include <net/ip6_route.h>
+#include <net/ipv6.h>
 #include <net/route.h>
 #include <net/tcp.h>
 #include "compat_xtables.h"
@@ -162,7 +167,7 @@ static bool tarpit_generic(struct tcphdr *tcph, const struct tcphdr *oth,
 	return true;
 }
 
-static void tarpit_tcp(struct sk_buff *oldskb, unsigned int hook,
+static void tarpit_tcp4(struct sk_buff *oldskb, unsigned int hook,
     unsigned int mode)
 {
 	struct tcphdr _otcph, *tcph;
@@ -293,8 +298,130 @@ static void tarpit_tcp(struct sk_buff *oldskb, unsigned int hook,
 	kfree_skb(nskb);
 }
 
+static void tarpit_tcp6(struct sk_buff *oldskb, unsigned int hook,
+    unsigned int mode)
+{
+	struct sk_buff *nskb;
+	struct tcphdr *tcph, oth;
+	unsigned int otcplen;
+	int tcphoff;
+	const struct ipv6hdr *oip6h = ipv6_hdr(oldskb);
+	struct ipv6hdr *ip6h;
+	const uint8_t tclass = 0;
+	uint8_t proto;
+	uint16_t payload;
+
+	proto   = oip6h->nexthdr;
+	tcphoff = ipv6_skip_exthdr(oldskb,
+	          (uint8_t *)(oip6h + 1) - oldskb->data, &proto);
+
+	if (tcphoff < 0 || tcphoff > oldskb->len) {
+		pr_debug("Cannot get TCP header.\n");
+		return;
+	}
+
+	otcplen = oldskb->len - tcphoff;
+
+	/* IP header checks: fragment, too short. */
+	if (proto != IPPROTO_TCP || otcplen < sizeof(struct tcphdr)) {
+		pr_debug("proto(%d) != IPPROTO_TCP, "
+		         "or too short. otcplen = %d\n",
+		         proto, otcplen);
+		return;
+	}
+
+	if (skb_copy_bits(oldskb, tcphoff, &oth, sizeof(struct tcphdr))) {
+		WARN_ON(1);
+		return;
+	}
+
+	/* Check checksum. */
+	if (csum_ipv6_magic(&oip6h->saddr, &oip6h->daddr, otcplen, IPPROTO_TCP,
+	    skb_checksum(oldskb, tcphoff, otcplen, 0))) {
+		pr_debug("TCP checksum is invalid\n");
+		return;
+	}
+
+	nskb = skb_copy_expand(oldskb, LL_MAX_HEADER,
+	       skb_tailroom(oldskb), GFP_ATOMIC);
+	if (nskb == NULL) {
+		if (net_ratelimit())
+			pr_debug("cannot alloc skb\n");
+		return;
+	}
+
+	/* This packet will not be the same as the other: clear nf fields */
+	nf_reset(nskb);
+	skb_nfmark(nskb) = 0;
+	skb_init_secmark(nskb);
+
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(2, 6, 18)
+	skb_shinfo(nskb)->gso_size = 0;
+	skb_shinfo(nskb)->gso_segs = 0;
+	skb_shinfo(nskb)->gso_type = 0;
+#endif
+
+	skb_put(nskb, sizeof(struct ipv6hdr));
+	ip6h = ipv6_hdr(nskb);
+	*(__be32 *)ip6h =  htonl(0x60000000 | (tclass << 20));
+	ip6h->nexthdr = IPPROTO_TCP;
+	ipv6_addr_copy(&ip6h->saddr, &oip6h->daddr);
+	ipv6_addr_copy(&ip6h->daddr, &oip6h->saddr);
+
+	/* Adjust IP TTL */
+	if (mode == XTTARPIT_HONEYPOT)
+		ip6h->hop_limit = 128;
+	else
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(2, 6, 26)
+		ip6h->hop_limit = ip6_dst_hoplimit(skb_dst(nskb));
+#else
+		ip6h->hop_limit = dst_metric(dst, RTAX_HOPLIMIT);
+		if (ip6h->hop_limit < 0)
+			ip6h->hop_limit = ipv6_get_hoplimit((skb_dst(nskb))->dev).
+#endif
+
+	tcph = (struct tcphdr *)(skb_network_header(nskb) +
+	       sizeof(struct ipv6hdr));
+
+	/* Truncate to length (no data) */
+	skb_trim(nskb, sizeof(struct ipv6hdr) + sizeof(struct tcphdr));
+	tcph->doff    = sizeof(struct tcphdr)/4;
+	tcph->source  = oth.dest;
+	tcph->dest    = oth.source;
+	tcph->urg_ptr = 0;
+	/* Reset flags */
+	((uint8_t *)tcph)[13] = 0;
+
+	payload = nskb->len - sizeof(struct ipv6hdr) - sizeof(struct tcphdr);
+	if (!tarpit_generic(&oth, tcph, payload, mode))
+		return;
+
+	ip6h->payload_len = htons(sizeof(struct tcphdr));
+	tcph->check = 0;
+
+	/* Adjust TCP checksum */
+	tcph->check = csum_ipv6_magic(&ipv6_hdr(nskb)->saddr,
+	              &ipv6_hdr(nskb)->daddr, sizeof(struct tcphdr),
+	              IPPROTO_TCP,
+	              csum_partial(tcph, sizeof(struct tcphdr), 0));
+
+	if (ip6_route_me_harder(nskb))
+		goto free_nskb;
+
+	nskb->ip_summed = CHECKSUM_NONE;
+
+	nf_ct_attach(nskb, oldskb);
+
+	NF_HOOK(NFPROTO_IPV6, NF_INET_LOCAL_OUT, nskb, NULL,
+	        skb_dst(nskb)->dev, dst_output);
+	return;
+
+ free_nskb:
+	kfree_skb(nskb);
+}
+
 static unsigned int
-tarpit_tg(struct sk_buff **pskb, const struct xt_action_param *par)
+tarpit_tg4(struct sk_buff **pskb, const struct xt_action_param *par)
 {
 	const struct sk_buff *skb = *pskb;
 	const struct iphdr *iph = ip_hdr(skb);
@@ -325,29 +452,82 @@ tarpit_tg(struct sk_buff **pskb, const struct xt_action_param *par)
 	if (iph->frag_off & htons(IP_OFFSET))
 		return NF_DROP;
 
-	tarpit_tcp(*pskb, par->hooknum, info->variant);
+	tarpit_tcp4(*pskb, par->hooknum, info->variant);
 	return NF_DROP;
 }
 
-static struct xt_target tarpit_tg_reg __read_mostly = {
+static unsigned int
-	.name       = "TARPIT",
+tarpit_tg6(struct sk_buff **pskb, const struct xt_action_param *par)
-	.revision   = 0,
+{
-	.family     = NFPROTO_IPV4,
+	const struct sk_buff *skb = *pskb;
-	.hooks      = (1 << NF_INET_LOCAL_IN) | (1 << NF_INET_FORWARD),
+	const struct ipv6hdr *iph = ipv6_hdr(skb);
-	.proto      = IPPROTO_TCP,
+	const struct rt6_info *rt = (struct rt6_info *)skb_dst(skb);
-	.target     = tarpit_tg,
+	const struct xt_tarpit_tginfo *info = par->targinfo;
-	.targetsize = sizeof(struct xt_tarpit_tginfo),
+	uint8_t proto;
-	.me         = THIS_MODULE,
+
+	/* Do we have an input route cache entry? (Not in PREROUTING.) */
+	if (rt == NULL) {
+		pr_debug("Dropping no input route cache entry\n");
+		return NF_DROP;
+	}
+
+	/* No replies to physical multicast/broadcast */
+	/* skb != PACKET_OTHERHOST handled by ip_rcv() */
+	if (skb->pkt_type != PACKET_HOST) {
+		pr_debug("type != PACKET_HOST");
+		return NF_DROP;
+	}
+
+	/*
+	 * Our naive response construction does not deal with IP
+	 * options, and probably should not try.
+	 */
+	proto = iph->nexthdr;
+	if (ipv6_skip_exthdr(skb, skb_network_header_len(skb), &proto) !=
+	    sizeof(struct ipv6hdr))
+		return NF_DROP;
+
+	if ((!(ipv6_addr_type(&iph->saddr) & IPV6_ADDR_UNICAST)) ||
+	    (!(ipv6_addr_type(&iph->daddr) & IPV6_ADDR_UNICAST))) {
+		pr_debug("addr is not unicast.\n");
+		return NF_DROP;
+	}
+
+	tarpit_tcp6(*pskb, par->hooknum, info->variant);
+	return NF_DROP;
+}
+
+static struct xt_target tarpit_tg_reg[] __read_mostly = {
+	{
+		.name       = "TARPIT",
+		.revision   = 0,
+		.family     = NFPROTO_IPV4,
+		.hooks      = (1 << NF_INET_LOCAL_IN) | (1 << NF_INET_FORWARD),
+		.proto      = IPPROTO_TCP,
+		.target     = tarpit_tg4,
+		.targetsize = sizeof(struct xt_tarpit_tginfo),
+		.me         = THIS_MODULE,
+	},
+	{
+		.name       = "TARPIT",
+		.revision   = 0,
+		.family     = NFPROTO_IPV6,
+		.hooks      = (1 << NF_INET_LOCAL_IN) | (1 << NF_INET_FORWARD),
+		.proto      = IPPROTO_TCP,
+		.target     = tarpit_tg6,
+		.targetsize = sizeof(struct xt_tarpit_tginfo),
+		.me         = THIS_MODULE,
+	},
 };
 
 static int __init tarpit_tg_init(void)
 {
-	return xt_register_target(&tarpit_tg_reg);
+	return xt_register_targets(tarpit_tg_reg, ARRAY_SIZE(tarpit_tg_reg));
 }
 
 static void __exit tarpit_tg_exit(void)
 {
-	xt_unregister_target(&tarpit_tg_reg);
+	xt_unregister_targets(tarpit_tg_reg, ARRAY_SIZE(tarpit_tg_reg));
 }
 
 module_init(tarpit_tg_init);
@@ -356,3 +536,4 @@ MODULE_DESCRIPTION("Xtables: \"TARPIT\", capture and hold TCP connections");
 MODULE_AUTHOR("Jan Engelhardt <jengelh@medozas.de>");
 MODULE_LICENSE("GPL");
 MODULE_ALIAS("ipt_TARPIT");
+MODULE_ALIAS("ip6t_TARPIT");