Re: [RFC PATCH net-next] sctp: fix src address selection if using secondary addresses

[Vlad Yasevich <vyasevich@gmail.com>]

On 07/10/2015 12:17 PM, Marcelo Ricardo Leitner wrote:
> On Fri, Jul 10, 2015 at 11:35:28AM -0400, Vlad Yasevich wrote:
>> On 07/10/2015 07:53 AM, Marcelo Ricardo Leitner wrote:
>>> On Thu, Jul 09, 2015 at 09:55:19PM +0200, Michael Tuexen wrote:
>>>>> On 09 Jul 2015, at 18:54, Marcelo Ricardo Leitner <mleitner@redhat.com> wrote:
>>>>>
>>>>> Cc'ing Michael too.
>>>> I'm not familiar with the Linux kernel code, so I can't comment on it.
>>>> But making sure to use a source address belonging to the emitting
>>>> interface makes sense for me.
>>>>
>>>> Best regards
>>>> Michael
>>>
>>> That's pretty much what I was looking for, in case I missed something on
>>> SCTP RFCs. 
>>>
>>
>> Well, the RFCs do not really specify what the source address should be, and there
> 
> That's why I was afraid of having missed something ;)
> 
>> have been numerous times where I've seen weak host model in use on the wire
>> even with a BSD peer.
>>
>> This also puts a very big nail through many suggestions we've had over the years
>> to allow source based path multihoming in addition to destination based multihoming
>> we currently support.
>>
>> It might be a good idea to make rp-filter like behavior best effort, and have
>> the old behavior as fallback.  I am still trying to think up different scenarios
>> where rp-filter behavior will cause things to fail prematurely...
> 
> The old behavior is like "if we don't have a src yet and can't find a
> preferred src for this dst, use the 1st bound address". We can add it
> but as I said, I'm afraid it is just doing wrong and not worth. If such
> randomly src addressed packet is meant to be routed, the router will
> likely drop it as it is seen as a spoof. And if it reaches the peer, it
> will probably come back through a different path.
> 
> I'm tempted to say that current usual use cases are handled by the first
> check on this function, which returns the preferred/primary address for
> the interface and checks against bound addresses. Whenever you reach the
> second check, it just allows you to use that 1st bound address that is
> checked. I mean, I can't see use cases that we would be breaking with
> this change. 

Yes,  the secondary check didn't amount to much, but we've kept it since 2.5
days (when sctp was introduced).  I've made attempts over the years to
try to make it stricter, but that never amounted to anything that worked well.

> 
> But yeah, it impacts source based routing, and I'm not aware of previous
> discussions on it. I'll try to dig some up but if possible, please share
> some pointers on it.

It's been suggested a few times that we should support source based multihoming
particularly for the case where one peer has only 1 address.
We've always punted on this, but people still ask every now and then.

I do have a question about the code though.. Have you tried with mutlipath routing
enabled.  I see rp_filter checks have special code to handle that.  Seem like we
might get false negatives in sctp.

-vlad

> 
>   Marcelo
>