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 <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/inetdevice.h

···
 
 #define IN_DEV_FORWARD(in_dev)		IN_DEV_CONF_GET((in_dev), FORWARDING)
 #define IN_DEV_MFORWARD(in_dev)		IN_DEV_ANDCONF((in_dev), MC_FORWARDING)
+#define IN_DEV_BFORWARD(in_dev)		IN_DEV_ANDCONF((in_dev), BC_FORWARDING)
 #define IN_DEV_RPFILTER(in_dev)		IN_DEV_MAXCONF((in_dev), RP_FILTER)
 #define IN_DEV_SRC_VMARK(in_dev)    	IN_DEV_ORCONF((in_dev), SRC_VMARK)
 #define IN_DEV_SOURCE_ROUTE(in_dev)	IN_DEV_ANDCONF((in_dev), \

include/uapi/linux/ip.h

···
 	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
 };
 

include/uapi/linux/netconf.h

···
 	NETCONFA_PROXY_NEIGH,
 	NETCONFA_IGNORE_ROUTES_WITH_LINKDOWN,
 	NETCONFA_INPUT,
+	NETCONFA_BC_FORWARDING,
 	__NETCONFA_MAX
 };
 #define NETCONFA_MAX	(__NETCONFA_MAX - 1)

net/ipv4/devinet.c

···
 		size += nla_total_size(4);
 	if (all || type == NETCONFA_MC_FORWARDING)
 		size += nla_total_size(4);
+	if (all || type == NETCONFA_BC_FORWARDING)
+		size += nla_total_size(4);
 	if (all || type == NETCONFA_PROXY_NEIGH)
 		size += nla_total_size(4);
 	if (all || type == NETCONFA_IGNORE_ROUTES_WITH_LINKDOWN)
···
 	if ((all || type == NETCONFA_MC_FORWARDING) &&
 	    nla_put_s32(skb, NETCONFA_MC_FORWARDING,
 			IPV4_DEVCONF(*devconf, MC_FORWARDING)) < 0)
+		goto nla_put_failure;
+	if ((all || type == NETCONFA_BC_FORWARDING) &&
+	    nla_put_s32(skb, NETCONFA_BC_FORWARDING,
+			IPV4_DEVCONF(*devconf, BC_FORWARDING)) < 0)
 		goto nla_put_failure;
 	if ((all || type == NETCONFA_PROXY_NEIGH) &&
 	    nla_put_s32(skb, NETCONFA_PROXY_NEIGH,
···
 			if ((new_value == 0) && (old_value != 0))
 				rt_cache_flush(net);
 
+		if (i == IPV4_DEVCONF_BC_FORWARDING - 1 &&
+		    new_value != old_value)
+			rt_cache_flush(net);
+
 		if (i == IPV4_DEVCONF_RP_FILTER - 1 &&
 		    new_value != old_value) {
 			ifindex = devinet_conf_ifindex(net, cnf);
···
 		DEVINET_SYSCTL_COMPLEX_ENTRY(FORWARDING, "forwarding",
 					     devinet_sysctl_forward),
 		DEVINET_SYSCTL_RO_ENTRY(MC_FORWARDING, "mc_forwarding"),
+		DEVINET_SYSCTL_RW_ENTRY(BC_FORWARDING, "bc_forwarding"),
 
 		DEVINET_SYSCTL_RW_ENTRY(ACCEPT_REDIRECTS, "accept_redirects"),
 		DEVINET_SYSCTL_RW_ENTRY(SECURE_REDIRECTS, "secure_redirects"),

net/ipv4/route.c

···
 		goto no_route;
 	}
 
-	if (res->type == RTN_BROADCAST)
+	if (res->type == RTN_BROADCAST) {
+		if (IN_DEV_BFORWARD(in_dev))
+			goto make_route;
 		goto brd_input;
+	}
 
 	if (res->type == RTN_LOCAL) {
 		err = fib_validate_source(skb, saddr, daddr, tos,
···
 	if (res->type != RTN_UNICAST)
 		goto martian_destination;
 
+make_route:
 	err = ip_mkroute_input(skb, res, in_dev, daddr, saddr, tos, flkeys);
 out:	return err;