[NETFILTER]: nf_nat: fix random mode not to overwrite port rover

The port rover should not get overwritten when using random mode,
otherwise other rules will also use more or less random ports.
Signed-off-by: Patrick McHardy <kaber@trash.net>

net/ipv4/netfilter/nf_nat_proto_common.c

@@ -42,6 +42,7 @@ int nf_nat_proto_unique_tuple(struct nf_conntrack_tuple *tuple,
 {
 	unsigned int range_size, min, i;
 	__be16 *portptr;
+	u_int16_t off;
 
 	if (maniptype == IP_NAT_MANIP_SRC)
 		portptr = &tuple->src.u.all;
@@ -72,12 +73,16 @@ int nf_nat_proto_unique_tuple(struct nf_conntrack_tuple *tuple,
 		range_size = ntohs(range->max.all) - min + 1;
 	}
 
+	off = *rover;
 	if (range->flags & IP_NAT_RANGE_PROTO_RANDOM)
-		*rover = net_random();
+		off = net_random();
 
-	for (i = 0; i < range_size; i++, (*rover)++) {
-		*portptr = htons(min + *rover % range_size);
-		if (!nf_nat_used_tuple(tuple, ct))
+	for (i = 0; i < range_size; i++, off++) {
+		*portptr = htons(min + off % range_size);
+		if (nf_nat_used_tuple(tuple, ct))
+			continue;
+		if (!(range->flags & IP_NAT_RANGE_PROTO_RANDOM))
+			*rover = off;
 		return 1;
 	}
 	return 0;