netdev - Re: [patch net] ipv6: do not create neighbor entries for local delivery

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Date:	Thu, 8 Aug 2013 21:47:02 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Debabrata Banerjee <dbavatar@...il.com>
Cc:	Jiri Pirko <jiri@...nulli.us>, mleitner@...hat.com,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	"jmorris@...ei.org" <jmorris@...ei.org>,
	"yoshfuji@...ux-ipv6.org" <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>,
	"Banerjee, Debabrata" <dbanerje@...mai.com>,
	Joshua Hunt <johunt@...mai.com>
Subject: Re: [patch net] ipv6: do not create neighbor entries for local delivery

On Thu, Aug 08, 2013 at 02:45:40PM -0400, Debabrata Banerjee wrote:
> On Wed, Jan 30, 2013 at 3:26 AM, Jiri Pirko <jiri@...nulli.us> wrote:
> > From: Marcelo Ricardo Leitner <mleitner@...hat.com>
> >
> > They will be created at output, if ever needed. This avoids creating
> > empty neighbor entries when TPROXYing/Forwarding packets for addresses
> > that are not even directly reachable.
> >
> > Note that IPv4 already handles it this way. No neighbor entries are
> > created for local input.
> >
> > Tested by myself and customer.
> >
> > Signed-off-by: Jiri Pirko <jiri@...nulli.us>
> > Signed-off-by: Marcelo Ricardo Leitner <mleitner@...hat.com>
> > ---
> >  net/ipv6/route.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> > index e229a3b..363d8b7 100644
> > --- a/net/ipv6/route.c
> > +++ b/net/ipv6/route.c
> > @@ -928,7 +928,7 @@ restart:
> >         dst_hold(&rt->dst);
> >         read_unlock_bh(&table->tb6_lock);
> >
> > -       if (!rt->n && !(rt->rt6i_flags & RTF_NONEXTHOP))
> > +       if (!rt->n && !(rt->rt6i_flags & (RTF_NONEXTHOP | RTF_LOCAL)))
> >                 nrt = rt6_alloc_cow(rt, &fl6->daddr, &fl6->saddr);
> >         else if (!(rt->dst.flags & DST_HOST))
> >                 nrt = rt6_alloc_clone(rt, &fl6->daddr);
> 
> 
> 
> I'm not sure this patch is doing the right thing. It seems to break
> IPv6 loopback functionality, it is no longer equivalent to IPv4, as
> stated above. It doesn't just stop neighbor creation but it stops
> cached route creation. Seems like a scary change for a stable tree.
> See below:
> 
> $ ip -4 route show local
> local 127.0.0.0/8 dev lo  proto kernel  scope host  src 127.0.0.1
> 
> This local route enables us to use the whole loopback network, any
> address inside 127.0.0.0/8 will work.
> 
> $ ping -c1 127.0.0.9
> PING 127.0.0.9 (127.0.0.9) 56(84) bytes of data.
> 64 bytes from 127.0.0.9: icmp_seq=1 ttl=64 time=0.012 ms
> 
> --- 127.0.0.9 ping statistics ---
> 1 packets transmitted, 1 received, 0% packet loss, time 0ms
> rtt min/avg/max/mdev = 0.012/0.012/0.012/0.000 ms
> 
> This also used to work equivalently for IPv6 local loopback routes:
> 
> $ ip -6 route add local 2001:::/64 dev lo
> $ ping6 -c1 2001::9
> PING 2001::9(2001::9) 56 data bytes
> 64 bytes from 2001::9: icmp_seq=1 ttl=64 time=0.010 ms
> 
> --- 2001::9 ping statistics ---
> 1 packets transmitted, 1 received, 0% packet loss, time 0ms
> rtt min/avg/max/mdev = 0.010/0.010/0.010/0.000 ms
> 
> However with this patch, this is very broken:
> 
> $ ip -6 route add local 2001::/64 dev lo
> $ ping6 -c1 2001::9
> PING 2001::9(2001::9) 56 data bytes
> ping: sendmsg: Invalid argument

I do think that the patch above is fine. I wonder why you get a blackhole
route back here. Maybe backtracking in ip6_pol_route or in fib6_lookup_1 was
way too aggressive?

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