netfilter: add CHECKSUM target

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

Typical expected use (helps old dhclient binary running in a VM):
iptables -A POSTROUTING -t mangle -p udp --dport bootpc \
	-j CHECKSUM --checksum-fill

Includes fixes by Jan Engelhardt <jengelh@medozas.de>

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>

1 files changed, 16 insertions, 0 deletions

diff --git a/net/netfilter/Kconfig b/net/netfilter/Kconfig
index aa2f106347e4..5fb8efa84df3 100644
--- a/net/netfilter/Kconfig
+++ b/net/netfilter/Kconfig
@@ -326,6 +326,22 @@ config NETFILTER_XT_CONNMARK
 
 comment "Xtables targets"
 
+config NETFILTER_XT_TARGET_CHECKSUM
+	tristate "CHECKSUM target support"
+	depends on IP_NF_MANGLE || IP6_NF_MANGLE
+	depends on NETFILTER_ADVANCED
+	---help---
+	  This option 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.
+
+	  To compile it as a module, choose M here.  If unsure, say N.
+
 config NETFILTER_XT_TARGET_CLASSIFY
 	tristate '"CLASSIFY" target support'
 	depends on NETFILTER_ADVANCED