route: add support for directed broadcast forwarding

This patch implements the feature described in rfc1812#section-5.3.5.2
and rfc2644. It allows the router to forward directed broadcast when
sysctl bc_forwarding is enabled.

Note that this feature could be done by iptables -j TEE, but it would
cause some problems:
  - target TEE's gateway param has to be set with a specific address,
    and it's not flexible especially when the route wants forward all
    directed broadcasts.
  - this duplicates the directed broadcasts so this may cause side
    effects to applications.

Besides, to keep consistent with other os router like BSD, it's also
necessary to implement it in the route rx path.

Note that route cache needs to be flushed when bc_forwarding is
changed.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 2 insertions, 0 deletions

diff --git a/include/uapi/linux/ip.h b/include/uapi/linux/ip.h
index b24a742beae5..e42d13b55cf3 100644
--- a/include/uapi/linux/ip.h
+++ b/include/uapi/linux/ip.h
@@ -168,6 +168,7 @@ enum
         IPV4_DEVCONF_IGNORE_ROUTES_WITH_LINKDOWN,
         IPV4_DEVCONF_DROP_UNICAST_IN_L2_MULTICAST,
         IPV4_DEVCONF_DROP_GRATUITOUS_ARP,
+        IPV4_DEVCONF_BC_FORWARDING,
         __IPV4_DEVCONF_MAX
 };
 
diff --git a/include/uapi/linux/netconf.h b/include/uapi/linux/netconf.h
index c84fcdfca862..fac4edd55379 100644
--- a/include/uapi/linux/netconf.h
+++ b/include/uapi/linux/netconf.h
@@ -18,6 +18,7 @@ enum {
         NETCONFA_PROXY_NEIGH,
         NETCONFA_IGNORE_ROUTES_WITH_LINKDOWN,
         NETCONFA_INPUT,
+        NETCONFA_BC_FORWARDING,
         __NETCONFA_MAX
 };
 #define NETCONFA_MAX    (__NETCONFA_MAX - 1)