Subject: Re: [BUG] net/ipv4: inconsistent routing table

From: Alexander Duyck <[email-masked]>
Date: Wed, 5 Aug 2015 10:45:18 -0700

On 08/05/2015 02:06 AM, Daniel Borkmann wrote:
> [ please cc netdev ]
>
> On 08/05/2015 10:56 AM, Zang MingJie wrote:
>> Hi:
>>
>> I found a bug when remove an ip address which is referenced by a 
>> routing entry.
>>
>> step to reproduce:
>>
>> ip li add type dummy
>> ip li set dummy0 up
>> ip ad add 10.0.0.1/24 dev dummy0
>> ip ad add 10.0.0.2/24 dev dummy0

Okay, so up to this point you have 2 addresses on the same subnet that 
are now on dummy0.

>> ip ro add default via 10.0.0.2/24

This makes the default route go through 10.0.0.2.

>> ip ad del 10.0.0.2/24 dev dummy0

Then you remove 10.0.0.2 from the local system, however since 10.0.0.1 
is on the same subnet dummy0 would still be the correct interface to 
access 10.0.0.2 it is just no longer local to the system.

>> after deleting the secondary ip address, the routing entry still
>> pointing to 10.0.0.2

You didn't delete the default routing entry so why would you expect it 
to change?  All you did is remove 10.0.0.2 from the local system.  I 
believe the assumption is that 10.0.0.2 is still out there somewhere, it 
just isn't on the local system anymore.

>> # ip ro
>> default via 10.0.0.2 dev dummy0
>> 10.0.0.0/24 dev dummy0  proto kernel  scope link  src 10.0.0.1

This matches up with what I would expect.  10.0.0.2 is the default 
gateway and it is accessible from dummy0 since 10.0.0.0/24 is accessible 
from dummy0.

>> but actually, kernel considers the default route is directly connected.
>>
>> # ip ro get 1.1.1.1
>> 1.1.1.1 dev dummy0  src 10.0.0.1
>>      cache

I'm not sure how you came to the "directly connected" conclusion. It is 
still routing things out through 10.0.0.2 from 10.0.0.1.

Maybe your example would work better if you used 10.0.0.1 and 10.0.1.1 
instead.  Then I think you might be able to better see that when you 
delete the second address the route would be broken.

- Alex