xt_CHECKSUM: initial import

This adds a "CHECKSUM" target, which can be used in the iptables mangle
table.

You can use this target to compute and fill in the checksum in a packet
that lacks a checksum. This is particularly useful, if you need to work
around old applications such as dhcp clients, that do not work well with
checksum offloads, but don't want to disable checksum offload in your
device.

The problem happens in the field with virtualized applications. For
reference, see Red Hat bz 605555, as well as
http://www.spinics.net/lists/kvm/msg37660.html

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

extensions/xt_CHECKSUM.h

@@ -0,0 +1,18 @@
+/* Header file for iptables ipt_CHECKSUM target
+ *
+ * (C) 2002 by Harald Welte <laforge@gnumonks.org>
+ * (C) 2010 Red Hat Inc
+ * Author: Michael S. Tsirkin <mst@redhat.com>
+ *
+ * This software is distributed under GNU GPL v2, 1991
+*/
+#ifndef _IPT_CHECKSUM_TARGET_H
+#define _IPT_CHECKSUM_TARGET_H
+
+#define XT_CHECKSUM_OP_FILL	0x01	/* fill in checksum in IP header */
+
+struct xt_CHECKSUM_info {
+	u_int8_t operation;	/* bitset of operations */
+};
+
+#endif /* _IPT_CHECKSUM_TARGET_H */