netdev - Re: [PATCH net-next] net: change fib behavior based on interface link status

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Message-ID: <20150603134931.GD588@gospo.home.greyhouse.net>
Date:	Wed, 3 Jun 2015 09:49:32 -0400
From:	Andy Gospodarek <gospo@...ulusnetworks.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	netdev@...r.kernel.org, davem@...emloft.net,
	ddutt@...ulusnetworks.com
Subject: Re: [PATCH net-next] net: change fib behavior based on interface
 link status

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 <gospo@...ulusnetworks.com>
> > Suggested-by: Dinesh Dutt <ddutt@...ulusnetworks.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.

> Hannes
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