On Wed, Jun 03, 2015 at 11:35:09AM +0200, Hannes Frederic Sowa wrote: > On Wed, Jun 3, 2015, at 05:07, Andy Gospodarek wrote: > > This patch adds the ability to have the Linux kernel track whether or > > not a particular route should be used based on the link-status of the > > interface associated with the next-hop. > > > > Before this patch any link-failure on an interface that was serving as a > > gateway for some systems could result in those systems being isolated > > from the rest of the network as the stack would continue to attempt to > > send frames out of an interface that is actually linked-down. When the > > kernel is responsible for all forwarding, it should also be responsible > > for taking action when the traffic can no longer be forwarded -- there > > is no real need to outsource link-monitoring to userspace anymore. > > > > This feature is only enabled with the new sysctl set (default is off): > > net.core.kill_routes_on_linkdown = 1 > > > > When this is set, the following behavior can be observed (interface p8p1 > > is link-down): > > > > # ip route show > > 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 > > 90.0.0.0/24 via 80.0.0.2 dev p8p1 metric 1 dead > > 90.0.0.0/24 via 70.0.0.2 dev p7p1 metric 2 > > # ip route get 90.0.0.1 > > 90.0.0.1 via 70.0.0.2 dev p7p1 src 70.0.0.1 > > cache > > # ip route get 80.0.0.1 > > local 80.0.0.1 dev lo src 80.0.0.1 > > cache <local> > > # ip route get 80.0.0.2 > > 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: > > > > # ip route show > > 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 > > # ip route get 90.0.0.1 > > 90.0.0.1 via 80.0.0.2 dev p8p1 src 80.0.0.1 > > cache > > # ip route get 80.0.0.1 > > local 80.0.0.1 dev lo src 80.0.0.1 > > cache <local> > > # ip route get 80.0.0.2 > > 80.0.0.2 dev p8p1 src 80.0.0.1 > > cache > > > > and the output changes to what one would expect. > > > > Signed-off-by: Andy Gospodarek <go...@cumulusnetworks.com> > > Suggested-by: Dinesh Dutt <dd...@cumulusnetworks.com> > > > > --- > > Though there were some that preferred not to have a configuration option > > and to make this behavior the default when it was discussed in Ottawa > > earlier this year since "it was time to do this." I wanted to propose > > the config option to preserve the current behavior for those that desire > > it. I'll happily remove it if Dave and Linus approve. > > I raised the concern that in case we don't have any other fallback route > and the kernel decides to send back ICMP errors to the end host, we > could kill TCP connections with those error messages. The current > behavior is that the packet gets silently dropped and TCP will retry, no > ICMP error message is send by immediate routers. This is especially > important if only a short link loss event happens on a default route. If you do not have any default route configured (or your default route is the one that went down!), then you could see this happening. > [...] > > This is a great feature, thanks! Glad you like it.
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