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Re: [PATCH v2] net: neigh: disallow transition to NUD_STALE if lladdr is unchanged in neigh_update()

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On 26.07.2016 08:16, Chunhui He wrote:
> NUD_STALE is used when the caller(e.g. arp_process()) can't guarantee
> neighbour reachability. If the entry was NUD_VALID and lladdr is unchanged,
> the entry state should not be changed.
> 
> Currently the code puts an extra "NUD_CONNECTED" condition. So if old state
> was NUD_DELAY or NUD_PROBE (they are NUD_VALID but not NUD_CONNECTED), the
> state can be changed to NUD_STALE.
> 
> This may cause problem. Because NUD_STALE lladdr doesn't guarantee
> reachability, when we send traffic, the state will be changed to
> NUD_DELAY. In normal case, if we get no confirmation (by dst_confirm()),
> we will change the state to NUD_PROBE and send probe traffic. But now the
> state may be reset to NUD_STALE again(e.g. by broadcast ARP packets),
> so the probe traffic will not be sent. This situation may happen again and
> again, and packets will be sent to an non-reachable lladdr forever.
> 
> The fix is to remove the "NUD_CONNECTED" condition. After that the
> "NEIGH_UPDATE_F_WEAK_OVERRIDE" condition (used by IPv6) in that branch will
> be redundant, so remove it.
> 
> This change may increase probe traffic, but it's essential since NUD_STALE
> lladdr is unreliable. To ensure correctness, we prefer to resolve lladdr,
> when we can't get confirmation, even while remote packets try to set
> NUD_STALE state.
> 
> Signed-off-by: Chunhui He <hchunhui@mail.ustc.edu.cn>
> ---
> v2:
>  - change title from "net: neigh: disallow state transition DELAY->STALE in
>    neigh_update()"
>  - remove "NUD_CONNECTED" condition instead of "NUD_CONNECTED | NUD_DELAY"
>  - remove "NEIGH_UPDATE_F_WEAK_OVERRIDE" condition

Reviewed-by: Hannes Frederic Sowa <hannes@stressinduktion.org>

Thanks!

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[PATCH v2] net: neigh: disallow transition to NUD_STALE if lladdr is unchanged in neigh_update() Chunhui He <hchunhui@mail.ustc.edu.cn> - 2016-07-26 08:20 +0200
  Re: [PATCH v2] net: neigh: disallow transition to NUD_STALE if lladdr  is unchanged in neigh_update() Julian Anastasov <ja@ssi.bg> - 2016-07-26 09:20 +0200
  Re: [PATCH v2] net: neigh: disallow transition to NUD_STALE if lladdr  is unchanged in neigh_update() Hannes Frederic Sowa <hannes@stressinduktion.org> - 2016-07-26 11:10 +0200
  Re: [PATCH v2] net: neigh: disallow transition to NUD_STALE if  lladdr is unchanged in neigh_update() David Miller <davem@davemloft.net> - 2016-07-26 23:30 +0200

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