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Message-ID: <58351b57-23a1-44ae-cb27-76eef35de419@kernel.org>
Date: Fri, 5 May 2023 09:22:14 -0600
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Subject: Re: [BUG] Dependence of routing cache entries on the ignore-df flag
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To: Stephen Hemminger <stephen@networkplumber.org>
Cc: Aleksey Shumnik <ashumnik9@gmail.com>, netdev@vger.kernel.org,
 waltje@uwalt.nl.mugnet.org, Jakub Kicinski <kuba@kernel.org>,
 gw4pts@gw4pts.ampr.org, kuznet@ms2.inr.ac.ru,
 "willemdebruijn.kernel@gmail.com" <willemdebruijn.kernel@gmail.com>,
 gnault@redhat.com
References: <CAJGXZLjLXrUzz4S9C7SqeyszMMyjR6RRu52y1fyh_d6gRqFHdA@mail.gmail.com>
 <20230503113528.315485f1@hermes.local> <20230505031043.GA4009@u2004-local>
 <20230505080014.67bdd6cb@hermes.local>
From: David Ahern <dsahern@kernel.org>
In-Reply-To: <20230505080014.67bdd6cb@hermes.local>
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On 5/5/23 9:00 AM, Stephen Hemminger wrote:
> On Thu, 4 May 2023 21:10:43 -0600
> David Ahern <dsahern@kernel.org> wrote:
> 
>> On Wed, May 03, 2023 at 11:35:28AM -0700, Stephen Hemminger wrote:
>>> On Wed, 3 May 2023 18:01:03 +0300
>>> Aleksey Shumnik <ashumnik9@gmail.com> wrote:
>>>   
>>>> Might you answer the questions:
>>>> 1. How the ignore-df flag and adding entries to the routing cache is
>>>> connected? In which kernel files may I look to find this connection?
>>>> 2. Is this behavior wrong?
>>>> 3. Is there any way to completely disable the use of the routing
>>>> cache? (as far as I understand, it used to be possible to set the
>>>> rhash_entries parameter to 0, but now there is no such parameter)
>>>> 4. Why is an entry added to the routing cache if a suitable entry was
>>>> eventually found in the arp table (it is added directly, without being
>>>> temporarily added to the routing table)?  
>>>
>>> What kernel version. The route cache has been completely removed from
>>> the kernel for a long time.  
>>
>> These are exceptions (fib_nh_exception), not the legacy routing cache.
> 
> I tried to reproduce your example, and did not see anything.
> Could it be the multicast routing daemon (pimd) is watching and adding
> the entry?

That looks like a pmtu based entry - added by the kernel processing an ICMP.