Date:   Mon, 15 Jul 2019 12:55:48 -0600
From:   David Ahern <dsahern@...il.com>
To:     Paul Donohue <linux-kernel@...lSD.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        netdev@...r.kernel.org
Subject: Re: IPv6 L2TP issues related to 93531c67

Hi Paul:

As an FYI, gmail thinks your emails are spam.

On 7/15/19 10:18 AM, Paul Donohue wrote:
> I have a system that establishes four L2TP over IPv6 tunnels using site-local addresses via the following:
...

> 
> These tunnels worked fine on kernel 4.4.  On kernel 4.15, there was a bug that caused intermittent L2TP packet errors, but everything worked fine after applying 4522a70db7aa5e77526a4079628578599821b193.
> 
> However, after upgrading to kernel 4.18 with 4522a70d (or upgrading to kernel 5.0 which includes 4522a70d, or upgrading to the current master kernel branch), two of the four tunnels always fail to work properly after a reboot, although it appears random which two work and which two fail.
> 
> When I say "fail to work properly", the problem is that packets generated by the l2tp kernel modules (in response to a packet being sent to the associated net_l2tpX interface) are silently dropped.  The l2tp_debugfs kernel module reports that L2TP packets are being transmitted with no errors, iptables counters and nflog rules can be used to confirm that well-formed packets are generated and sent, but tcpdump does not see the packets being sent on any interface on the system.  iptables reports that the destination interface of the lost packets is "lo" (which is clearly incorrect and probably an indicator of the underlying issue), but `tcpdump -nnn -i lo` doesn't show any packets.  Incoming L2TP packets appear to be processed correctly, only outgoing L2TP packets appear affected.
> 
> Reverting commit 93531c6743157d7e8c5792f8ed1a57641149d62c (identified by bisection) fixes this issue.

That commit can not be reverted. It is a foundational piece for a lot of
other changes. Did you mean the commit before it works and this commit
fails?

> 
> IPv4 L2TP tunnels do not appear affected by this issue.  Based on a few quick tests, it appears that switching to publicly-routable IPv6 addresses instead of site-local addresses seems to prevent this issue, although I haven't done sufficient testing of this, and it is not clear to me how the code in 93531c67 might be affected by the type of IPv6 address, so this observation may be a red herring.  Manually deleting and re-creating a broken interface seems to make it work again, although I have not thoroughly experimented with making changes after boot time to see if the problem is entirely random, if it is based on the number of existing interfaces, if it is based on a boot-time timing issue, etc.
> 
> It is not obvious to me how commit 93531c6743157d7e8c5792f8ed1a57641149d62c causes this issue, or how it should be fixed.  Could someone take a look and point me in the right direction for further troubleshooting?
> 

Let's get a complete example that demonstrates the problem, and I can go
from there. Can you take the attached script and update it so that it
reflects the problem you are reporting? That script works on latest
kernel as well as 4.14.133. It uses network namespaces for 2 hosts with
a router between them.

Also, check the return of the fib lookups using:
    perf record -e fib6:* -a
    <run test, ctrl-c on the record>
    perf script

Checkout the fib lookup parameters and result. Do they look correct to
you for your setup?

Download attachment "l2tp.sh" of type "application/x-sh" (3751 bytes)

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