Route caches and multiple default routes

Discussion in 'Linux Networking' started by David Brown, Feb 14, 2014.

  1. David Brown

    David Brown Guest

    I am trying to understand a bit about the route caches in a failover
    networking setup.

    Roughly speaking, I have two network cards eth0 and eth1, on two
    different networks. Each has a default route, but with different metrics.

    So "route -n" gives something like this:

    estination Gateway Genmask Flags Metric Ref Use Iface U 1 0 0 eth0 U 1 0 0 eth1 UG 10 0 0 eth0 UG 20 0 0 eth0

    traceroute shows the routing uses eth0 by default, as the metric
    is lower, and "ip route get" confirms it:

    ip route get via dev eth0 src
    cache mtu 1500 advmss 1460 hoplimit 64

    If I now unplug eth0, but keep the interface up (alternatively,
    disconnect the router on eth0), then I can no longer access
    "ip route get" and "ip route show cache" confirm that the cached route
    is still via eth0. "ping" gives "Destination Host Unreachable".

    If I use "ip route flush cache", and then try "ip route get" again, the
    highest metric (lowest value) route is tried first, and fails, and then
    the next default route - via eth1 - succeeds. So the route via eth1
    gets put in the cache, and stays there.

    If I bring eth0 online again, the cached route via eth1 is still there -
    I need to flush the cache again to make it try eth0 (with the highest
    priority metric).

    I am trying to figure some way to make this all work automatically. If
    the highest priority default route fails, I would like the cached route
    to be dropped and the next default route tried - without having to
    manually issue a flush. And I would also like cached routes to have a
    timeout, so that after a while the higher priority default route will be
    tried again.

    Any ideas or hints would be much appreciated.

    David Brown, Feb 14, 2014
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  2. If I understand correctly what you're asking: they do. See

    /sbin/sysctl -a | fgrep net.ipv4.route.gc

    - Andrew
    Andrew Gideon, Feb 26, 2014
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