ipv4: add option to drop gratuitous ARP packets

In certain 802.11 wireless deployments, there will be ARP proxies
that use knowledge of the network to correctly answer requests.
To prevent gratuitous ARP frames on the shared medium from being
a problem, on such deployments wireless needs to drop them.

Enable this by providing an option called "drop_gratuitous_arp".

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

Documentation/networking/ip-sysctl.txt

···
 	1122, but is disabled by default for compatibility reasons.
 	Default: off (0)
 
+drop_gratuitous_arp - BOOLEAN
+	Drop all gratuitous ARP frames, for example if there's a known
+	good ARP proxy on the network and such frames need not be used
+	(or in the case of 802.11, must not be used to prevent attacks.)
+	Default: off (0)
+
 
 tag - INTEGER
 	Allows you to write a number, which can be used as required.

include/uapi/linux/ip.h

···
 	IPV4_DEVCONF_IGMPV3_UNSOLICITED_REPORT_INTERVAL,
 	IPV4_DEVCONF_IGNORE_ROUTES_WITH_LINKDOWN,
 	IPV4_DEVCONF_DROP_UNICAST_IN_L2_MULTICAST,
+	IPV4_DEVCONF_DROP_GRATUITOUS_ARP,
 	__IPV4_DEVCONF_MAX
 };
 

net/ipv4/arp.c

···
 	    (!IN_DEV_ROUTE_LOCALNET(in_dev) && ipv4_is_loopback(tip)))
 		goto out;
 
+ /*
+  *	For some 802.11 wireless deployments (and possibly other networks),
+  *	there will be an ARP proxy and gratuitous ARP frames are attacks
+  *	and thus should not be accepted.
+  */
+	if (sip == tip && IN_DEV_ORCONF(in_dev, DROP_GRATUITOUS_ARP))
+		goto out;
+
 /*
  *     Special case: We must set Frame Relay source Q.922 address
  */

net/ipv4/devinet.c

···
 					"igmpv3_unsolicited_report_interval"),
 		DEVINET_SYSCTL_RW_ENTRY(IGNORE_ROUTES_WITH_LINKDOWN,
 					"ignore_routes_with_linkdown"),
+		DEVINET_SYSCTL_RW_ENTRY(DROP_GRATUITOUS_ARP,
+					"drop_gratuitous_arp"),
 
 		DEVINET_SYSCTL_FLUSHING_ENTRY(NOXFRM, "disable_xfrm"),
 		DEVINET_SYSCTL_FLUSHING_ENTRY(NOPOLICY, "disable_policy"),