ipv6: sysctl to restrict candidate source addresses

Per RFC 6724, section 4, "Candidate Source Addresses":

    It is RECOMMENDED that the candidate source addresses be the set
    of unicast addresses assigned to the interface that will be used
    to send to the destination (the "outgoing" interface).

Add a sysctl to enable this behaviour.
Signed-off-by: Erik Kline <ek@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -1460,6 +1460,13 @@ router_solicitations - INTEGER
 	routers are present.
 	Default: 3
 
+use_oif_addrs_only - BOOLEAN
+	When enabled, the candidate source addresses for destinations
+	routed via this interface are restricted to the set of addresses
+	configured on this interface (vis. RFC 6724, section 4).
+
+	Default: false
+
 use_tempaddr - INTEGER
 	Preference for Privacy Extensions (RFC3041).
 	  <= 0 : disable Privacy Extensions

include/linux/ipv6.h

@@ -57,6 +57,7 @@ struct ipv6_devconf {
 		bool initialized;
 		struct in6_addr secret;
 	} stable_secret;
+	__s32		use_oif_addrs_only;
 	void		*sysctl;
 };
 

include/uapi/linux/ipv6.h

@@ -171,6 +171,7 @@ enum {
 	DEVCONF_USE_OPTIMISTIC,
 	DEVCONF_ACCEPT_RA_MTU,
 	DEVCONF_STABLE_SECRET,
+	DEVCONF_USE_OIF_ADDRS_ONLY,
 	DEVCONF_MAX
 };
 

net/ipv6/addrconf.c

@@ -211,7 +211,8 @@ static struct ipv6_devconf ipv6_devconf __read_mostly = {
 	.accept_ra_mtu		= 1,
 	.stable_secret		= {
 		.initialized = false,
-	}
+	},
+	.use_oif_addrs_only	= 0,
 };
 
 static struct ipv6_devconf ipv6_devconf_dflt __read_mostly = {
@@ -253,6 +254,7 @@ static struct ipv6_devconf ipv6_devconf_dflt __read_mostly = {
 	.stable_secret		= {
 		.initialized = false,
 	},
+	.use_oif_addrs_only	= 0,
 };
 
 /* Check if a valid qdisc is available */
@@ -1472,11 +1474,16 @@ int ipv6_dev_get_saddr(struct net *net, const struct net_device *dst_dev,
 	 *    include addresses assigned to interfaces
 	 *    belonging to the same site as the outgoing
 	 *    interface.)
+	 *  - "It is RECOMMENDED that the candidate source addresses
+	 *    be the set of unicast addresses assigned to the
+	 *    interface that will be used to send to the destination
+	 *    (the 'outgoing' interface)." (RFC 6724)
 	 */
 	if (dst_dev) {
+		idev = __in6_dev_get(dst_dev);
 		if ((dst_type & IPV6_ADDR_MULTICAST) ||
-		    dst.scope <= IPV6_ADDR_SCOPE_LINKLOCAL) {
-			idev = __in6_dev_get(dst_dev);
+		    dst.scope <= IPV6_ADDR_SCOPE_LINKLOCAL ||
+		    (idev && idev->cnf.use_oif_addrs_only)) {
 			use_oif_addr = true;
 		}
 	}
@@ -4607,6 +4614,7 @@ static inline void ipv6_store_devconf(struct ipv6_devconf *cnf,
 	array[DEVCONF_ACCEPT_RA_FROM_LOCAL] = cnf->accept_ra_from_local;
 	array[DEVCONF_ACCEPT_RA_MTU] = cnf->accept_ra_mtu;
 	/* we omit DEVCONF_STABLE_SECRET for now */
+	array[DEVCONF_USE_OIF_ADDRS_ONLY] = cnf->use_oif_addrs_only;
 }
 
 static inline size_t inet6_ifla6_size(void)
@@ -5605,6 +5613,14 @@ static struct addrconf_sysctl_table
 			.mode		= 0600,
 			.proc_handler	= addrconf_sysctl_stable_secret,
 		},
+		{
+			.procname       = "use_oif_addrs_only",
+			.data           = &ipv6_devconf.use_oif_addrs_only,
+			.maxlen         = sizeof(int),
+			.mode           = 0644,
+			.proc_handler   = proc_dointvec,
+
+		},
 		{
 			/* sentinel */
 		}