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Date: Thu, 8 Sep 2011 22:42:20 +0800 From: "Yan, Zheng " <yanzheng@...n.com> To: David Madore <david+ml@...ore.org> Cc: Linux Netdev Mailing-List <netdev@...r.kernel.org> Subject: Re: possible routing table cache bug for mtu/advmss attributes in Linux 3.0.4 On Fri, Sep 2, 2011 at 5:46 AM, David Madore <david+ml@...ore.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/ ) > -- > To unsubscribe from this list: send the line "unsubscribe netdev" in > the body of a message to majordomo@...r.kernel.org > More majordomo info at http://vger.kernel. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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