Messages in this thread | | | Date | Mon, 14 Sep 2009 09:31:28 -0700 | From | Stephen Hemminger <> | Subject | Re: ipv4 regression in 2.6.31 ? |
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On Mon, 14 Sep 2009 17:55:05 +0200 Stephan von Krawczynski <skraw@ithnet.com> wrote:
> On Mon, 14 Sep 2009 15:57:03 +0200 > Eric Dumazet <eric.dumazet@gmail.com> wrote: > > > Stephan von Krawczynski a écrit : > > > Hello all, > > > > > > today we experienced some sort of regression in 2.6.31 ipv4 implementation, or > > > at least some incompatibility with former 2.6.30.X kernels. > > > > > > We have the following situation: > > > > > > ---------- vlan1@eth0 192.168.2.1/24 > > > / > > > host A 192.168.1.1/24 eth0 -------<router> host B > > > \ > > > ---------- eth1 192.168.3.1/24 > > > > > > > > > Now, if you route 192.168.1.0/24 via interface vlan1@eth0 on host B and let > > > host A ping 192.168.2.1 everything works. But if you route 192.168.1.0/24 via > > > interface eth1 on host B and let host A ping 192.168.2.1 you get no reply. > > > With tcpdump we see the icmp packets arrive at vlan1@eth0, but no icmp echo > > > reply being generated neither on vlan1 nor eth1. > > > Kernels 2.6.30.X and below do not show this behaviour. > > > Is this intended? Do we need to reconfigure something to restore the old > > > behaviour? > > > > > > > Asymetric routing ? > > > > Check your rp_filter settings > > > > grep . `find /proc/sys/net -name rp_filter` > > > > rp_filter - INTEGER > > 0 - No source validation. > > 1 - Strict mode as defined in RFC3704 Strict Reverse Path > > Each incoming packet is tested against the FIB and if the interface > > is not the best reverse path the packet check will fail. > > By default failed packets are discarded. > > 2 - Loose mode as defined in RFC3704 Loose Reverse Path > > Each incoming packet's source address is also tested against the FIB > > and if the source address is not reachable via any interface > > the packet check will fail. > > > > Current recommended practice in RFC3704 is to enable strict mode > > to prevent IP spoofing from DDos attacks. If using asymmetric routing > > or other complicated routing, then loose mode is recommended. > > > > conf/all/rp_filter must also be set to non-zero to do source validation > > on the interface > > > > Default value is 0. Note that some distributions enable it > > in startup scripts. > > Ok, here you can see 2.6.31 values from the discussed box: > (remember, no ping reply in this setup) > > /proc/sys/net/ipv4/conf/all/rp_filter:1 > /proc/sys/net/ipv4/conf/default/rp_filter:0 > /proc/sys/net/ipv4/conf/lo/rp_filter:0 > /proc/sys/net/ipv4/conf/eth2/rp_filter:0 > /proc/sys/net/ipv4/conf/eth0/rp_filter:0 > /proc/sys/net/ipv4/conf/eth1/rp_filter:0 > /proc/sys/net/ipv4/conf/vlan1/rp_filter:0 > > > And these are from the same box with 2.6.30.5: > (ping reply works) > > /proc/sys/net/ipv4/conf/all/rp_filter:1 > /proc/sys/net/ipv4/conf/default/rp_filter:0 > /proc/sys/net/ipv4/conf/lo/rp_filter:0 > /proc/sys/net/ipv4/conf/eth2/rp_filter:0 > /proc/sys/net/ipv4/conf/eth0/rp_filter:0 > /proc/sys/net/ipv4/conf/eth1/rp_filter:0 > /proc/sys/net/ipv4/conf/vlan1/rp_filter:0 > > As you can see they're all the same. Does this mean that rp_filter never > really worked as intended before 2.6.31 ? Or does it mean that rp_filter=0 > (eth1 and vlan1) gets overriden by all/rp_filter=1 in 2.6.31 and not before?
RP filter did not work correctly in 2.6.30. The code added to to the loose mode caused a bug; the rp_filter value was being computed as: rp_filter = interface_value & all_value; So in order to get reverse path filter both would have to be set.
In 2.6.31 this was change to: rp_filter = max(interface_value, all_value);
This was the intended behaviour, if user asks all interfaces to have rp filtering turned on, then set /proc/sys/net/ipv4/conf/all/rp_filter = 1 or to turn on just one interface, set it for just that interface.
Sorry for any confusion this caused.
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