ipv4: Add interface option to enable routing of 127.0.0.0/8

Routing of 127/8 is tradtionally forbidden, we consider
packets from that address block martian when routing and do
not process corresponding ARP requests.

This is a sane default but renders a huge address space
practically unuseable.

The RFC states that no address within the 127/8 block should
ever appear on any network anywhere but it does not forbid
the use of such addresses outside of the loopback device in
particular. For example to address a pool of virtual guests
behind a load balancer.

This patch adds a new interface option 'route_localnet'
enabling routing of the 127/8 address block and processing
of ARP requests on a specific interface.

Note that for the feature to work, the default local route
covering 127/8 dev lo needs to be removed.

Example:
  $ sysctl -w net.ipv4.conf.eth0.route_localnet=1
  $ ip route del 127.0.0.0/8 dev lo table local
  $ ip addr add 127.1.0.1/16 dev eth0
  $ ip route flush cache

V2: Fix invalid check to auto flush cache (thanks davem)

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/inetdevice.h b/include/linux/inetdevice.h
index 597f4a9..67f9dda 100644
--- a/include/linux/inetdevice.h
+++ b/include/linux/inetdevice.h
@@ -38,6 +38,7 @@ enum
 	IPV4_DEVCONF_ACCEPT_LOCAL,
 	IPV4_DEVCONF_SRC_VMARK,
 	IPV4_DEVCONF_PROXY_ARP_PVLAN,
+	IPV4_DEVCONF_ROUTE_LOCALNET,
 	__IPV4_DEVCONF_MAX
 };
 
@@ -131,6 +132,7 @@ static inline void ipv4_devconf_setall(struct in_device *in_dev)
 #define IN_DEV_PROMOTE_SECONDARIES(in_dev) \
 					IN_DEV_ORCONF((in_dev), \
 						      PROMOTE_SECONDARIES)
+#define IN_DEV_ROUTE_LOCALNET(in_dev)	IN_DEV_ORCONF(in_dev, ROUTE_LOCALNET)
 
 #define IN_DEV_RX_REDIRECTS(in_dev) \
 	((IN_DEV_FORWARD(in_dev) && \