[NETFILTER]: Fix DHCP + MASQUERADE problem

In 2.6.13-rcX the MASQUERADE target was changed not to exclude local
packets for better source address consistency. This breaks DHCP clients
using UDP sockets when the DHCP requests are caught by a MASQUERADE rule
because the MASQUERADE target drops packets when no address is configured
on the outgoing interface. This patch makes it ignore packets with a
source address of 0.

Thanks to Rusty for this suggestion.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/netfilter/ipt_MASQUERADE.c

@@ -90,6 +90,12 @@ masquerade_target(struct sk_buff **pskb,
 	IP_NF_ASSERT(ct && (ctinfo == IP_CT_NEW || ctinfo == IP_CT_RELATED
 	                    || ctinfo == IP_CT_RELATED + IP_CT_IS_REPLY));
 
+	/* Source address is 0.0.0.0 - locally generated packet that is
+	 * probably not supposed to be masqueraded.
+	 */
+	if (ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple.src.ip == 0)
+		return NF_ACCEPT;
+
 	mr = targinfo;
 	rt = (struct rtable *)(*pskb)->dst;
 	newsrc = inet_select_addr(out, rt->rt_gateway, RT_SCOPE_UNIVERSE);