Subject: Re: ipv4 regression in 2.6.31 ?

From: Stephen Hemminger <[email-masked]>
Date: Mon, 14 Sep 2009 09:31:28 -0700

On Mon, 14 Sep 2009 17:55:05 +0200
Stephan von Krawczynski <[email-masked]> wrote:

> On Mon, 14 Sep 2009 15:57:03 +0200
> Eric Dumazet <[email-masked]> 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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