netdev - Re: Q: bad routing table cache entries

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Date:	Tue, 12 Jan 2016 23:43:06 +0300
From:	Stas Sergeev <stsp@...t.ru>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	netdev <netdev@...r.kernel.org>,
	Sowmini Varadhan <sowmini.varadhan@...cle.com>
Subject: Re: Q: bad routing table cache entries

12.01.2016 20:47, Hannes Frederic Sowa пишет:
> On 12.01.2016 18:33, Stas Sergeev wrote:
>> 12.01.2016 20:26, Hannes Frederic Sowa пишет:
>>> On 12.01.2016 18:18, Stas Sergeev wrote:
>>>> 12.01.2016 20:06, Hannes Frederic Sowa пишет:
>>>>> On 12.01.2016 17:56, Stas Sergeev wrote:
>>>>>> 12.01.2016 19:42, Stas Sergeev пишет:
>>>>>> Also the rfc1620 you pointed, seems to be saying this:
>>>>>>
>>>>>>                    A Redirect message SHOULD be silently 
>>>>>> discarded if the
>>>>>>                    new router address it specifies is not on the 
>>>>>> same
>>>>>>                    connected (sub-) net through which the 
>>>>>> Redirect arrived,
>>>>>>                    or if the source of the Redirect is not the 
>>>>>> current
>>>>>>                    first-hop router for the specified destination.
>>>>>>
>>>>>> It seems, this is exactly the rule we were trying to find
>>>>>> during the thread. And it seems violated, either. Unless I am
>>>>>> mis-interpreting it, of course.
>>>>>
>>>>> If you read on you will read that with shared_media this exact 
>>>>> clause (the first of those) is not in effect any more.
>>>> OK. But how to get such a redirect to work, if (checked with
>>>> tcpdump) the packets do not even go to eth0, but to "lo"?
>>>
>>> I don't know, the router must be on the same shared medium. I guess 
>>> physical reconfiguration is required?
>> It is same.
>> Router 192.168.8.1 has just one ethernet port.
>> And even on the 192.168.10.202 node I can do:
>> # arp -a |grep "0.1"
>> ? (192.168.0.1) at 14:d6:4d:1c:97:3d [ether] on eth0
>> So even 0.1 is about to be reachable.
>> Still nothing works.
>> Should it work if 192.168.0.1 router, to which 8.1 redirects,
>> has shared_media disabled?
>
> Can you check with tcpdump?
That's what I already did.
I monitored on 8.1 router and on the node itself,
and my conclusion was that the packets do not
even reach the eth0 interface. Instead I captured
them on "lo" interface, so I assumed such route is
completely broken.
If it is not - how can I even see that it exist? How to
list these redirect routes?
I'd like to do some investigations, but this looks no
more than a black magic without a proper support
from tools, proper documentation, etc.

And I suspect that shared_media is disabled on a 0.1
router, so I wonder if this can work at all, even if the node
is cured to do the right thing with those redirects.
In a nearby message David Miller says:
---

2) increasing the chance of successful communication with peers

---
If this can't work right when one of the gateways has
shared_media disabled, then this rule is clearly violated.

> ping requires the router to also find a correct way back, so packet 
> can get stuck at a lot of places. Also uRPF is maybe active which kind 
> of defeats shared_media and please check netfilter.
I am pretty sure the node has a default ubuntu without
any special network tweaks, but I'll double-check.