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Subject: Re: Strange problem with SCTP+IPv6
From:   Michael Tuexen <Michael.Tuexen@lurchi.franken.de>
In-Reply-To: <20200623212143.GR2491@localhost.localdomain>
Date:   Tue, 23 Jun 2020 23:24:59 +0200
Cc:     Corey Minyard <minyard@acm.org>,
        David Laight <David.Laight@aculab.com>,
        Xin Long <lucien.xin@gmail.com>,
        Vlad Yasevich <vyasevich@gmail.com>,
        Neil Horman <nhorman@tuxdriver.com>,
        "linux-sctp@vger.kernel.org" <linux-sctp@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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To:     Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
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> On 23. Jun 2020, at 23:21, Marcelo Ricardo Leitner <marcelo.leitner@gmail.com> wrote:
> 
> On Tue, Jun 23, 2020 at 11:17:56AM -0500, Corey Minyard wrote:
>> On Tue, Jun 23, 2020 at 01:17:28PM +0000, David Laight wrote:
>>> From: Marcelo Ricardo Leitner
>>>> Sent: 22 June 2020 19:33
>>>> On Mon, Jun 22, 2020 at 08:01:24PM +0200, Michael Tuexen wrote:
>>>>>> On 22. Jun 2020, at 18:57, Corey Minyard <minyard@acm.org> wrote:
>>>>>> 
>>>>>> On Mon, Jun 22, 2020 at 08:01:23PM +0800, Xin Long wrote:
>>>>>>> On Sun, Jun 21, 2020 at 11:56 PM Corey Minyard <minyard@acm.org> wrote:
>>>>>>>> 
>>>>>>>> I've stumbled upon a strange problem with SCTP and IPv6.  If I create an
>>>>>>>> sctp listening socket on :: and set the IPV6_V6ONLY socket option on it,
>>>>>>>> then I make a connection to it using ::1, the connection will drop after
>>>>>>>> 2.5 seconds with an ECONNRESET error.
>>>>>>>> 
>>>>>>>> It only happens on SCTP, it doesn't have the issue if you connect to a
>>>>>>>> full IPv6 address instead of ::1, and it doesn't happen if you don't
>>>>>>>> set IPV6_V6ONLY.  I have verified current end of tree kernel.org.
>>>>>>>> I tried on an ARM system and x86_64.
>>>>>>>> 
>>>>>>>> I haven't dug into the kernel to see if I could find anything yet, but I
>>>>>>>> thought I would go ahead and report it.  I am attaching a reproducer.
>>>>>>>> Basically, compile the following code:
>>>>>>> The code only set IPV6_V6ONLY on server side, so the client side will
>>>>>>> still bind all the local ipv4 addresses (as you didn't call bind() to
>>>>>>> bind any specific addresses ). Then after the connection is created,
>>>>>>> the client will send HB on the v4 paths to the server. The server
>>>>>>> will abort the connection, as it can't support v4.
>>>>>>> 
>>>>>>> So you can work around it by either:
>>>>>>> 
>>>>>>> - set IPV6_V6ONLY on client side.
>>>>>>> 
>>>>>>> or
>>>>>>> 
>>>>>>> - bind to the specific v6 addresses on the client side.
>>>>>>> 
>>>>>>> I don't see RFC said something about this.
>>>>>>> So it may not be a good idea to change the current behaviour
>>>>>>> to not establish the connection in this case, which may cause regression.
>>>>>> 
>>>>>> Ok, I understand this.  It's a little strange, but I see why it works
>>>>>> this way.
>>>>> I don't. I would expect it to work as I described in my email.
>>>>> Could someone explain me how and why it is behaving different from
>>>>> my expectation?
>>>> 
>>>> It looks like a bug to me. Testing with this test app here, I can see
>>>> the INIT_ACK being sent with a bunch of ipv4 addresses in it and
>>>> that's unexpected for a v6only socket. As is, it's the server saying
>>>> "I'm available at these other addresses too, but not."
>>> 
>>> Does it even make sense to mix IPv4 and IPv6 addresses on the same
>>> connection?
>>> I don't remember ever seeing both types of address in a message,
>>> but may not have looked.
>> 
>> That's an interesting question.  Do the RFCs say anything?  I would
>> assume it was ok unless ipv6only was set.
>> 
>>> 
>>> I also wonder whether the connection should be dropped for an error
>>> response on a path that has never been validated.
>> 
>> That actually bothered me a bit more.  Shouldn't it stay up if any path
>> is up?  That's kind of the whole point of multihoming.
> 
> Michael explained it on the other email. What he described is what I
> observed in my tests.
> 
>> 
>>> 
>>> OTOH the whole 'multi-homing' part of SCTP sucks.
>> 
>> I don't think so.
>> 
>>> The IP addresses a server needs to bind to depend on where the
>>> incoming connection will come from.
>>> A local connection may be able to use a 192.168.x.x address
>>> but a remote connection must not - as it may be defined locally
>>> at the remote system.
>>> But both connections can come into the public (routable) address.
>>> We have to tell customers to explicitly configure the local IP
>>> addresses - which means the application has to know what they are.
>>> Fortunately these apps are pretty static - usually M3UA.
>> 
>> Umm, no,  If you have a private address, it better be behind a firewall,
>> and the firewall should handle rewriting the packet to fix the addresses.
>> 
>> It doesn't appear that Linux netfilter does this.  There is a TODO in
>> the code for this.  But that's how it *should* work.
> 
> Right, we don't support SCTP aware NAT [1].
> 
> 1.https://tools.ietf.org/html/draft-stewart-behave-sctpnat-04
The current version is: https://tools.ietf.org/html/draft-ietf-tsvwg-natsupp-16

Another possibility for NAT traversal is UDP encapsulation...

Best regards
Michael
> 
>  Marcelo
> 
>> 
>> -corey
>> 
>>> 
>>> 	David
>>> 
>>> -
>>> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
>>> Registration No: 1397386 (Wales)
>>>