net: ipv4 sysctl option to ignore routes when nexthop link is down

This feature is only enabled with the new per-interface or ipv4 global
sysctls called 'ignore_routes_with_linkdown'.

net.ipv4.conf.all.ignore_routes_with_linkdown = 0
net.ipv4.conf.default.ignore_routes_with_linkdown = 0
net.ipv4.conf.lo.ignore_routes_with_linkdown = 0
...

When the above sysctls are set, will report to userspace that a route is
dead and will no longer resolve to this nexthop when performing a fib
lookup. This will signal to userspace that the route will not be
selected. The signalling of a RTNH_F_DEAD is only passed to userspace
if the sysctl is enabled and link is down. This was done as without it
the netlink listeners would have no idea whether or not a nexthop would
be selected. The kernel only sets RTNH_F_DEAD internally if the
interface has IFF_UP cleared.

With the new sysctl set, the following behavior can be observed
(interface p8p1 is link-down):

default via 10.0.5.2 dev p9p1
10.0.5.0/24 dev p9p1 proto kernel scope link src 10.0.5.15
70.0.0.0/24 dev p7p1 proto kernel scope link src 70.0.0.1
80.0.0.0/24 dev p8p1 proto kernel scope link src 80.0.0.1 dead linkdown
90.0.0.0/24 via 80.0.0.2 dev p8p1 metric 1 dead linkdown
90.0.0.0/24 via 70.0.0.2 dev p7p1 metric 2
90.0.0.1 via 70.0.0.2 dev p7p1 src 70.0.0.1
cache
local 80.0.0.1 dev lo src 80.0.0.1
cache <local>
80.0.0.2 via 10.0.5.2 dev p9p1 src 10.0.5.15
cache

While the route does remain in the table (so it can be modified if
needed rather than being wiped away as it would be if IFF_UP was
cleared), the proper next-hop is chosen automatically when the link is
down. Now interface p8p1 is linked-up:

default via 10.0.5.2 dev p9p1
10.0.5.0/24 dev p9p1 proto kernel scope link src 10.0.5.15
70.0.0.0/24 dev p7p1 proto kernel scope link src 70.0.0.1
80.0.0.0/24 dev p8p1 proto kernel scope link src 80.0.0.1
90.0.0.0/24 via 80.0.0.2 dev p8p1 metric 1
90.0.0.0/24 via 70.0.0.2 dev p7p1 metric 2
192.168.56.0/24 dev p2p1 proto kernel scope link src 192.168.56.2
90.0.0.1 via 80.0.0.2 dev p8p1 src 80.0.0.1
cache
local 80.0.0.1 dev lo src 80.0.0.1
cache <local>
80.0.0.2 dev p8p1 src 80.0.0.1
cache

and the output changes to what one would expect.

If the sysctl is not set, the following output would be expected when
p8p1 is down:

default via 10.0.5.2 dev p9p1
10.0.5.0/24 dev p9p1 proto kernel scope link src 10.0.5.15
70.0.0.0/24 dev p7p1 proto kernel scope link src 70.0.0.1
80.0.0.0/24 dev p8p1 proto kernel scope link src 80.0.0.1 linkdown
90.0.0.0/24 via 80.0.0.2 dev p8p1 metric 1 linkdown
90.0.0.0/24 via 70.0.0.2 dev p7p1 metric 2

Since the dead flag does not appear, there should be no expectation that
the kernel would skip using this route due to link being down.

v2: Split kernel changes into 2 patches, this actually makes a
behavioral change if the sysctl is set. Also took suggestion from Alex
to simplify code by only checking sysctl during fib lookup and
suggestion from Scott to add a per-interface sysctl.

v3: Code clean-ups to make it more readable and efficient as well as a
reverse path check fix.

v4: Drop binary sysctl

v5: Whitespace fixups from Dave

v6: Style changes from Dave and checkpatch suggestions

v7: One more checkpatch fixup

Signed-off-by: Andy Gospodarek <[e-mail]>
Signed-off-by: Dinesh Dutt <[e-mail]>
Acked-by: Scott Feldman <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/inetdevice.h

···
 	 || (!IN_DEV_FORWARD(in_dev) && \
 	  IN_DEV_ORCONF((in_dev), ACCEPT_REDIRECTS)))
 
+#define IN_DEV_IGNORE_ROUTES_WITH_LINKDOWN(in_dev) \
+	IN_DEV_CONF_GET((in_dev), IGNORE_ROUTES_WITH_LINKDOWN)
+
 #define IN_DEV_ARPFILTER(in_dev)	IN_DEV_ORCONF((in_dev), ARPFILTER)
 #define IN_DEV_ARP_ACCEPT(in_dev)	IN_DEV_ORCONF((in_dev), ARP_ACCEPT)
 #define IN_DEV_ARP_ANNOUNCE(in_dev)	IN_DEV_MAXCONF((in_dev), ARP_ANNOUNCE)

include/net/fib_rules.h

···
 	void			*result;
 	struct fib_rule		*rule;
 	int			flags;
-#define FIB_LOOKUP_NOREF	1
+#define FIB_LOOKUP_NOREF		1
+#define FIB_LOOKUP_IGNORE_LINKSTATE	2
 };
 
 struct fib_rules_ops {

include/net/ip_fib.h

···
 }
 
 static inline int fib_lookup(struct net *net, const struct flowi4 *flp,
-			     struct fib_result *res)
+			     struct fib_result *res, unsigned int flags)
 {
 	struct fib_table *tb;
 	int err = -ENETUNREACH;
···
 	rcu_read_lock();
 
 	tb = fib_get_table(net, RT_TABLE_MAIN);
-	if (tb && !fib_table_lookup(tb, flp, res, FIB_LOOKUP_NOREF))
+	if (tb && !fib_table_lookup(tb, flp, res, flags | FIB_LOOKUP_NOREF))
 		err = 0;
 
 	rcu_read_unlock();
···
 struct fib_table *fib_new_table(struct net *net, u32 id);
 struct fib_table *fib_get_table(struct net *net, u32 id);
 
-int __fib_lookup(struct net *net, struct flowi4 *flp, struct fib_result *res);
+int __fib_lookup(struct net *net, struct flowi4 *flp,
+		 struct fib_result *res, unsigned int flags);
 
 static inline int fib_lookup(struct net *net, struct flowi4 *flp,
-			     struct fib_result *res)
+			     struct fib_result *res, unsigned int flags)
 {
 	struct fib_table *tb;
 	int err;
 
+	flags |= FIB_LOOKUP_NOREF;
 	if (net->ipv4.fib_has_custom_rules)
-		return __fib_lookup(net, flp, res);
+		return __fib_lookup(net, flp, res, flags);
 
 	rcu_read_lock();
 
···
 
 	for (err = 0; !err; err = -ENETUNREACH) {
 		tb = rcu_dereference_rtnl(net->ipv4.fib_main);
-		if (tb && !fib_table_lookup(tb, flp, res, FIB_LOOKUP_NOREF))
+		if (tb && !fib_table_lookup(tb, flp, res, flags))
 			break;
 
 		tb = rcu_dereference_rtnl(net->ipv4.fib_default);
-		if (tb && !fib_table_lookup(tb, flp, res, FIB_LOOKUP_NOREF))
+		if (tb && !fib_table_lookup(tb, flp, res, flags))
 			break;
 	}
 

include/uapi/linux/ip.h

···
 	IPV4_DEVCONF_ROUTE_LOCALNET,
 	IPV4_DEVCONF_IGMPV2_UNSOLICITED_REPORT_INTERVAL,
 	IPV4_DEVCONF_IGMPV3_UNSOLICITED_REPORT_INTERVAL,
+	IPV4_DEVCONF_IGNORE_ROUTES_WITH_LINKDOWN,
 	__IPV4_DEVCONF_MAX
 };
 

net/ipv4/devinet.c

···
 					"igmpv2_unsolicited_report_interval"),
 		DEVINET_SYSCTL_RW_ENTRY(IGMPV3_UNSOLICITED_REPORT_INTERVAL,
 					"igmpv3_unsolicited_report_interval"),
+		DEVINET_SYSCTL_RW_ENTRY(IGNORE_ROUTES_WITH_LINKDOWN,
+					"ignore_routes_with_linkdown"),
 
 		DEVINET_SYSCTL_FLUSHING_ENTRY(NOXFRM, "disable_xfrm"),
 		DEVINET_SYSCTL_FLUSHING_ENTRY(NOPOLICY, "disable_policy"),

net/ipv4/fib_frontend.c

···
 		fl4.flowi4_tos = RT_TOS(ip_hdr(skb)->tos);
 		fl4.flowi4_scope = scope;
 		fl4.flowi4_mark = IN_DEV_SRC_VMARK(in_dev) ? skb->mark : 0;
-		if (!fib_lookup(net, &fl4, &res))
+		if (!fib_lookup(net, &fl4, &res, 0))
 			return FIB_RES_PREFSRC(net, res);
 	} else {
 		scope = RT_SCOPE_LINK;
···
 	fl4.flowi4_mark = IN_DEV_SRC_VMARK(idev) ? skb->mark : 0;
 
 	net = dev_net(dev);
-	if (fib_lookup(net, &fl4, &res))
+	if (fib_lookup(net, &fl4, &res, 0))
 		goto last_resort;
 	if (res.type != RTN_UNICAST &&
 	    (res.type != RTN_LOCAL || !IN_DEV_ACCEPT_LOCAL(idev)))
···
 	fl4.flowi4_oif = dev->ifindex;
 
 	ret = 0;
-	if (fib_lookup(net, &fl4, &res) == 0) {
+	if (fib_lookup(net, &fl4, &res, FIB_LOOKUP_IGNORE_LINKSTATE) == 0) {
 		if (res.type == RTN_UNICAST)
 			ret = FIB_RES_NH(res).nh_scope >= RT_SCOPE_HOST;
 	}

net/ipv4/fib_rules.c

···
 #endif
 };
 
-int __fib_lookup(struct net *net, struct flowi4 *flp, struct fib_result *res)
+int __fib_lookup(struct net *net, struct flowi4 *flp,
+		 struct fib_result *res, unsigned int flags)
 {
 	struct fib_lookup_arg arg = {
 		.result = res,
-		.flags = FIB_LOOKUP_NOREF,
+		.flags = flags,
 	};
 	int err;
 

net/ipv4/fib_semantics.c

···
 			/* It is not necessary, but requires a bit of thinking */
 			if (fl4.flowi4_scope < RT_SCOPE_LINK)
 				fl4.flowi4_scope = RT_SCOPE_LINK;
-			err = fib_lookup(net, &fl4, &res);
+			err = fib_lookup(net, &fl4, &res,
+					 FIB_LOOKUP_IGNORE_LINKSTATE);
 			if (err) {
 				rcu_read_unlock();
 				return err;
···
 	    nla_put_in_addr(skb, RTA_PREFSRC, fi->fib_prefsrc))
 		goto nla_put_failure;
 	if (fi->fib_nhs == 1) {
+		struct in_device *in_dev;
+
 		if (fi->fib_nh->nh_gw &&
 		    nla_put_in_addr(skb, RTA_GATEWAY, fi->fib_nh->nh_gw))
 			goto nla_put_failure;
 		if (fi->fib_nh->nh_oif &&
 		    nla_put_u32(skb, RTA_OIF, fi->fib_nh->nh_oif))
 			goto nla_put_failure;
+		if (fi->fib_nh->nh_flags & RTNH_F_LINKDOWN) {
+			in_dev = __in_dev_get_rcu(fi->fib_nh->nh_dev);
+			if (in_dev &&
+			    IN_DEV_IGNORE_ROUTES_WITH_LINKDOWN(in_dev))
+				rtm->rtm_flags |= RTNH_F_DEAD;
+		}
 #ifdef CONFIG_IP_ROUTE_CLASSID
 		if (fi->fib_nh[0].nh_tclassid &&
 		    nla_put_u32(skb, RTA_FLOW, fi->fib_nh[0].nh_tclassid))
···
 			goto nla_put_failure;
 
 		for_nexthops(fi) {
+			struct in_device *in_dev;
+
 			rtnh = nla_reserve_nohdr(skb, sizeof(*rtnh));
 			if (!rtnh)
 				goto nla_put_failure;
 
 			rtnh->rtnh_flags = nh->nh_flags & 0xFF;
+			if (nh->nh_flags & RTNH_F_LINKDOWN) {
+				in_dev = __in_dev_get_rcu(nh->nh_dev);
+				if (in_dev &&
+				    IN_DEV_IGNORE_ROUTES_WITH_LINKDOWN(in_dev))
+					rtnh->rtnh_flags |= RTNH_F_DEAD;
+			}
 			rtnh->rtnh_hops = nh->nh_weight - 1;
 			rtnh->rtnh_ifindex = nh->nh_oif;
 
···
 void fib_select_multipath(struct fib_result *res)
 {
 	struct fib_info *fi = res->fi;
+	struct in_device *in_dev;
 	int w;
 
 	spin_lock_bh(&fib_multipath_lock);
 	if (fi->fib_power <= 0) {
 		int power = 0;
 		change_nexthops(fi) {
-			if (!(nexthop_nh->nh_flags & RTNH_F_DEAD)) {
-				power += nexthop_nh->nh_weight;
-				nexthop_nh->nh_power = nexthop_nh->nh_weight;
-			}
+			in_dev = __in_dev_get_rcu(nexthop_nh->nh_dev);
+			if (nexthop_nh->nh_flags & RTNH_F_DEAD)
+				continue;
+			if (in_dev &&
+			    IN_DEV_IGNORE_ROUTES_WITH_LINKDOWN(in_dev) &&
+			    nexthop_nh->nh_flags & RTNH_F_LINKDOWN)
+				continue;
+			power += nexthop_nh->nh_weight;
+			nexthop_nh->nh_power = nexthop_nh->nh_weight;
 		} endfor_nexthops(fi);
 		fi->fib_power = power;
 		if (power <= 0) {

net/ipv4/fib_trie.c

···
 			continue;
 		for (nhsel = 0; nhsel < fi->fib_nhs; nhsel++) {
 			const struct fib_nh *nh = &fi->fib_nh[nhsel];
+			struct in_device *in_dev = __in_dev_get_rcu(nh->nh_dev);
 
 			if (nh->nh_flags & RTNH_F_DEAD)
+				continue;
+			if (in_dev &&
+			    IN_DEV_IGNORE_ROUTES_WITH_LINKDOWN(in_dev) &&
+			    nh->nh_flags & RTNH_F_LINKDOWN &&
+			    !(fib_flags & FIB_LOOKUP_IGNORE_LINKSTATE))
 				continue;
 			if (flp->flowi4_oif && flp->flowi4_oif != nh->nh_oif)
 				continue;

net/ipv4/netfilter/ipt_rpfilter.c

···
 	struct net *net = dev_net(dev);
 	int ret __maybe_unused;
 
-	if (fib_lookup(net, fl4, &res))
+	if (fib_lookup(net, fl4, &res, FIB_LOOKUP_IGNORE_LINKSTATE))
 		return false;
 
 	if (res.type != RTN_UNICAST) {

net/ipv4/route.c

···
 		if (!(n->nud_state & NUD_VALID)) {
 			neigh_event_send(n, NULL);
 		} else {
-			if (fib_lookup(net, fl4, &res) == 0) {
+			if (fib_lookup(net, fl4, &res, 0) == 0) {
 				struct fib_nh *nh = &FIB_RES_NH(res);
 
 				update_or_create_fnhe(nh, fl4->daddr, new_gw,
···
 		return;
 
 	rcu_read_lock();
-	if (fib_lookup(dev_net(dst->dev), fl4, &res) == 0) {
+	if (fib_lookup(dev_net(dst->dev), fl4, &res, 0) == 0) {
 		struct fib_nh *nh = &FIB_RES_NH(res);
 
 		update_or_create_fnhe(nh, fl4->daddr, 0, mtu,
···
 		fl4.flowi4_mark = skb->mark;
 
 		rcu_read_lock();
-		if (fib_lookup(dev_net(rt->dst.dev), &fl4, &res) == 0)
+		if (fib_lookup(dev_net(rt->dst.dev), &fl4, &res, 0) == 0)
 			src = FIB_RES_PREFSRC(dev_net(rt->dst.dev), res);
 		else
 			src = inet_select_addr(rt->dst.dev,
···
 	fl4.flowi4_scope = RT_SCOPE_UNIVERSE;
 	fl4.daddr = daddr;
 	fl4.saddr = saddr;
-	err = fib_lookup(net, &fl4, &res);
+	err = fib_lookup(net, &fl4, &res, 0);
 	if (err != 0) {
 		if (!IN_DEV_FORWARD(in_dev))
 			err = -EHOSTUNREACH;
···
 		goto make_route;
 	}
 
-	if (fib_lookup(net, fl4, &res)) {
+	if (fib_lookup(net, fl4, &res, 0)) {
 		res.fi = NULL;
 		res.table = NULL;
 		if (fl4->flowi4_oif) {