Re: [PATCH] net: ipv6: fallback to full lookup if table lookup is unsuitable

On 9/16/16 6:55 AM, Vincent Bernat wrote:

Commit 8c14586fc320 ("net: ipv6: Use passed in table for nexthop
lookups") introduced a regression: insertion of an IPv6 route in a table
not containing the appropriate connected route for the gateway but which
contained a non-connected route (like a default gateway) fails while it
was previously working:

    $ ip link add eth0 type dummy
    $ ip link set up dev eth0
    $ ip addr add 2001:db8::1/64 dev eth0
    $ ip route add ::/0 via 2001:db8::5 dev eth0 table 20
    $ ip route add 2001:db8:cafe::1/128 via 2001:db8::6 dev eth0 table 20
    RTNETLINK answers: No route to host
    $ ip -6 route show table 20
    default via 2001:db8::5 dev eth0  metric 1024  pref medium

so your table 20 is not complete in that it lacks a connected route to resolve 2001:db8::6 as a nexthop, so you are relying on a fallback to other tables (main in this case).

-----8<-----

-----8<-----

-			if (!(grt->rt6i_flags & RTF_GATEWAY))
-				err = 0;

This is the check that is failing for your use case. ip6_nh_lookup_table is returning the default route and nexthops can not rely on a gateway. Given that a simpler and more direct change is (whitespace mangled on paste):

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index ad4a7ff301fc..48bae2ee2e18 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c

@@ -1991,9 +1991,19 @@ static struct rt6_info *ip6_route_info_create(struct fib6_config *cfg)
                        if (!(gwa_type & IPV6_ADDR_UNICAST))
                                goto out;

-                       if (cfg->fc_table)
+                       if (cfg->fc_table) {
                                grt = ip6_nh_lookup_table(net, cfg, gw_addr);

+                               /* a nexthop lookup can not go through a gw.
+                                * if this happens on a table based lookup
+                                * then fallback to a full lookup
+                                */
+                               if (grt && grt->rt6i_flags & RTF_GATEWAY) {
+                                       ip6_rt_put(grt);
+                                       grt = NULL;
+                               }
+                       }
+
                        if (!grt)
                                grt = rt6_lookup(net, gw_addr, NULL,
                                                 cfg->fc_ifindex, 1);