Re: possible routing table cache bug for mtu/advmss attributes in Linux 3.0.4

["Yan, Zheng " <yanzheng@21cn.com>]

On Fri, Sep 2, 2011 at 5:46 AM, David Madore <david+ml@madore.org> wrote:
> Hi,
>
> I believe I've found a bug in the routing cache (perhaps only for
> IPv6) in Linux 3.0.4.  Since I'm uncertain about the diagnosis, I'm
> posting here for guidance as to how to proceed.
>
> In a nutshell, the problem is this: my (manually entered) routing
> tables specify an explicit MTU/MSS for certain IPv6 routes, and after
> upgrading from 2.6.38.7 to 3.0.4, I found that this MTU/MSS setting is
> sometimes ignored.  More precisely, it seems that connecting to a host
> through the route in question causes a routing cache entry to be
> created, and it is when this cache entry expires that the route
> forgets the MTU/MSS setting.  (It may also be relevant that in my case
> the route belongs to a routing table other than "main".)
>
> In more details: my routing tables contains the following:
>
> vega david ~ $ sudo ip -6 rule show
> 0:      from all lookup local
> 32766:  from all lookup main
> 40000:  from 2002::/16 lookup 2002
> 40100:  from all lookup 2001
> vega david ~ $ sudo ip -6 route show table 2001
> 2002::/16 via ::192.88.99.1 dev tun6to4  metric 512  mtu 1466 advmss 1406
> 2000::/3 dev ppp0  metric 64  mtu 1466 advmss 1406
>
> Initially, trying to get the route to 2001:41d0:1:a431::1 correctly
> returns route with the mtu and advmss values I entered:
>
> vega david ~ $ sudo ip route get 2001:41d0:1:a431::1
> 2001:41d0:1:a431::1 from :: via 2001:41d0:1:a431::1 dev ppp0  src 2001:7a8:78ae::1  metric 0
>    cache  mtu 1466 advmss 1406
>
> Now I start an ssh connection to 2001:41d0:1:a431::1, close it
> immediately, and fetch the route again:
>
> vega david ~ $ sudo ip route get 2001:41d0:1:a431::1
> 2001:41d0:1:a431::1 from :: via 2001:41d0:1:a431::1 dev ppp0  src 2001:7a8:78ae::1  metric 0
>    cache  mtu 1466 rtt 37ms rttvar 37ms cwnd 10 advmss 1406
>
> This is still correct.  But if I wait a few seconds (doing nothing)
> for these data to expire, they seems to take the mtu and advmss values
> away with them:
>
> vega david ~ $ sudo ip route get 2001:41d0:1:a431::1
> 2001:41d0:1:a431::1 from :: via 2001:41d0:1:a431::1 dev ppp0  src 2001:7a8:78ae::1  metric 0
>    cache
>
> (note the empty "cache" line and the absence of mtu/advmss settings).
>
> This is not just wrongly reported: the route really is wrong, and
> opening a TCP connection at this stage will use a wrong MSS (and
> freeze on large packets, because my Internet provider is worthless,
> but that's another story).  At some point the route seems to return to
> normal, but I couldn't figure out what causes this exactly.
>
> In case it's uesful, my kernel config is on <URL:
> http://www.madore.org/~david/.tmp/config-3.0.4-vega
>  >.
>

Would you please try patch http://marc.info/?l=linux-netdev&m=131529445904858

Thanks

> Any thoughts?
>
> --
>     David A. Madore
>   ( http://www.madore.org/~david/ )
> --
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