netdev - Re: Q: bad routing table cache entries

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Message-ID: <20160104010539.GC9087@oracle.com>
Date:	Sun, 3 Jan 2016 20:05:39 -0500
From:	Sowmini Varadhan <sowmini.varadhan@...cle.com>
To:	Stas Sergeev <stsp@...t.ru>
Cc:	netdev <netdev@...r.kernel.org>
Subject: Re: Q: bad routing table cache entries

On (12/30/15 15:42), Stas Sergeev wrote:
> 29.12.2015 18:22, Sowmini Varadhan пишет:
> > Do you have admin control over the ubuntu router?
> > If yes, you might want to check the shared_media [#] setting 
> > on that router for the interfaces with overlapping subnets.
> > (it is on by default, I would try turning it off).
> That didn't help, problem re-appears.

the code that sets things up for redirect is this:

  if (out_dev == in_dev && err && IN_DEV_TX_REDIRECTS(out_dev) &&
            skb->protocol == htons(ETH_P_IP) &&
            (IN_DEV_SHARED_MEDIA(out_dev) ||
             inet_addr_onlink(out_dev, saddr, FIB_RES_GW(*res))))
                IPCB(skb)->flags |= IPSKB_DOREDIRECT;

If you are still seeing the problematic redirect after disabling
shared_media, then you would need to trace through inet_addr_onlink()
to see why it was not returning false. As I said before, afaict from
reading the code, inet_addr_onlink looks right. So there may be something
unusual with your netmask config on in_dev/out_dev.

But even if the redirect is suppressed, sounds like the network/netmask
config is sub-optimal, since each packet gets (needlessly?) sent
up/down the router's in_dev/out_dev. That should be avoided, if possible.

--Sowmini
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