netfilter: SYNPROXY target: restrict to INPUT/FORWARD

Fix a crash in synproxy_send_tcp() when using the SYNPROXY target in the
PREROUTING chain caused by missing routing information.

Reported-by: Nicki P. <xastx@gmx.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
diff --git a/net/ipv4/netfilter/ipt_SYNPROXY.c b/net/ipv4/netfilter/ipt_SYNPROXY.c
index f13bd91..a313c3f 100644
--- a/net/ipv4/netfilter/ipt_SYNPROXY.c
+++ b/net/ipv4/netfilter/ipt_SYNPROXY.c
@@ -423,6 +423,7 @@
 static struct xt_target synproxy_tg4_reg __read_mostly = {
 	.name		= "SYNPROXY",
 	.family		= NFPROTO_IPV4,
+	.hooks		= (1 << NF_INET_LOCAL_IN) | (1 << NF_INET_FORWARD),
 	.target		= synproxy_tg4,
 	.targetsize	= sizeof(struct xt_synproxy_info),
 	.checkentry	= synproxy_tg4_check,
diff --git a/net/ipv6/netfilter/ip6t_SYNPROXY.c b/net/ipv6/netfilter/ip6t_SYNPROXY.c
index f78f41a..a0d1727 100644
--- a/net/ipv6/netfilter/ip6t_SYNPROXY.c
+++ b/net/ipv6/netfilter/ip6t_SYNPROXY.c
@@ -446,6 +446,7 @@
 static struct xt_target synproxy_tg6_reg __read_mostly = {
 	.name		= "SYNPROXY",
 	.family		= NFPROTO_IPV6,
+	.hooks		= (1 << NF_INET_LOCAL_IN) | (1 << NF_INET_FORWARD),
 	.target		= synproxy_tg6,
 	.targetsize	= sizeof(struct xt_synproxy_info),
 	.checkentry	= synproxy_tg6_check,