net: ipv4: Consider failed nexthops in multipath routes

Multipath route lookups should consider knowledge about next hops and not
select a hop that is known to be failed.

Example:

                     [h2]                   [h3]   15.0.0.5
                      |                      |
                     3|                     3|
                    [SP1]                  [SP2]--+
                     1  2                   1     2
                     |  |     /-------------+     |
                     |   \   /                    |
                     |     X                      |
                     |    / \                     |
                     |   /   \---------------\    |
                     1  2                     1   2
         12.0.0.2  [TOR1] 3-----------------3 [TOR2] 12.0.0.3
                     4                         4
                      \                       /
                        \                    /
                         \                  /
                          -------|   |-----/
                                 1   2
                                [TOR3]
                                  3|
                                   |
                                  [h1]  12.0.0.1

host h1 with IP 12.0.0.1 has 2 paths to host h3 at 15.0.0.5:

    root@h1:~# ip ro ls
    ...
    12.0.0.0/24 dev swp1  proto kernel  scope link  src 12.0.0.1
    15.0.0.0/16
            nexthop via 12.0.0.2  dev swp1 weight 1
            nexthop via 12.0.0.3  dev swp1 weight 1
    ...

If the link between tor3 and tor1 is down and the link between tor1
and tor2 then tor1 is effectively cut-off from h1. Yet the route lookups
in h1 are alternating between the 2 routes: ping 15.0.0.5 gets one and
ssh 15.0.0.5 gets the other. Connections that attempt to use the
12.0.0.2 nexthop fail since that neighbor is not reachable:

    root@h1:~# ip neigh show
    ...
    12.0.0.3 dev swp1 lladdr 00:02:00:00:00:1b REACHABLE
    12.0.0.2 dev swp1  FAILED
    ...

The failed path can be avoided by considering known neighbor information
when selecting next hops. If the neighbor lookup fails we have no
knowledge about the nexthop, so give it a shot. If there is an entry
then only select the nexthop if the state is sane. This is similar to
what fib_detect_death does.

To maintain backward compatibility use of the neighbor information is
based on a new sysctl, fib_multipath_use_neigh.

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Reviewed-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index b183e2b606c8a8e39ea8ce9a95857b60224d139d..6c7f365b15157a42538a9fa65a43db06b1a08af9 100644 (file)
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -63,6 +63,16 @@ fwmark_reflect - BOOLEAN
        fwmark of the packet they are replying to.
        Default: 0
 
+fib_multipath_use_neigh - BOOLEAN
+       Use status of existing neighbor entry when determining nexthop for
+       multipath routes. If disabled, neighbor information is not used and
+       packets could be directed to a failed nexthop. Only valid for kernels
+       built with CONFIG_IP_ROUTE_MULTIPATH enabled.
+       Default: 0 (disabled)
+       Possible values:
+       0 - disabled
+       1 - enabled
+
 route/max_size - INTEGER
        Maximum number of routes allowed in the kernel.  Increase
        this when using large numbers of interfaces and/or routes.

diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
index a69cde3ce4608879ee3e4056d22ce512eb0c3ba5..d061ffeb1e71204c490abed907004804036030e0 100644 (file)
--- a/include/net/netns/ipv4.h
+++ b/include/net/netns/ipv4.h
@@ -132,6 +132,9 @@ struct netns_ipv4 {
        struct list_head        mr_tables;
        struct fib_rules_ops    *mr_rules_ops;
 #endif
+#endif
+#ifdef CONFIG_IP_ROUTE_MULTIPATH
+       int sysctl_fib_multipath_use_neigh;
 #endif
        atomic_t        rt_genid;
 };

diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index d97268e8ff103bd229de4fb99bbb40f51e61082d..ab64d9f2eef91dc22829a2eb442a6c887c9a8838 100644 (file)
--- a/net/ipv4/fib_semantics.c
+++ b/net/ipv4/fib_semantics.c
@@ -1559,21 +1559,45 @@ int fib_sync_up(struct net_device *dev, unsigned int nh_flags)
 }
 
 #ifdef CONFIG_IP_ROUTE_MULTIPATH
+static bool fib_good_nh(const struct fib_nh *nh)
+{
+       int state = NUD_REACHABLE;
+
+       if (nh->nh_scope == RT_SCOPE_LINK) {
+               struct neighbour *n;
+
+               rcu_read_lock_bh();
+
+               n = __ipv4_neigh_lookup_noref(nh->nh_dev, nh->nh_gw);
+               if (n)
+                       state = n->nud_state;
+
+               rcu_read_unlock_bh();
+       }
+
+       return !!(state & NUD_VALID);
+}
 
 void fib_select_multipath(struct fib_result *res, int hash)
 {
        struct fib_info *fi = res->fi;
+       struct net *net = fi->fib_net;
+       bool first = false;
 
        for_nexthops(fi) {
                if (hash > atomic_read(&nh->nh_upper_bound))
                        continue;
 
-               res->nh_sel = nhsel;
-               return;
+               if (!net->ipv4.sysctl_fib_multipath_use_neigh ||
+                   fib_good_nh(nh)) {
+                       res->nh_sel = nhsel;
+                       return;
+               }
+               if (!first) {
+                       res->nh_sel = nhsel;
+                       first = true;
+               }
        } endfor_nexthops(fi);
-
-       /* Race condition: route has just become dead. */
-       res->nh_sel = 0;
 }
 #endif
 

diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index 1e1fe6086dd912e5071524053581afa79578061b..bb0419582b8dfed626f04752307ddaf2671ed01c 100644 (file)
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -960,6 +960,17 @@ static struct ctl_table ipv4_net_table[] = {
                .mode           = 0644,
                .proc_handler   = proc_dointvec,
        },
+#ifdef CONFIG_IP_ROUTE_MULTIPATH
+       {
+               .procname       = "fib_multipath_use_neigh",
+               .data           = &init_net.ipv4.sysctl_fib_multipath_use_neigh,
+               .maxlen         = sizeof(int),
+               .mode           = 0644,
+               .proc_handler   = proc_dointvec_minmax,
+               .extra1         = &zero,
+               .extra2         = &one,
+       },
+#endif
        { }
 };