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Date:   Tue, 23 Jun 2020 18:21:43 -0300
From:   'Marcelo Ricardo Leitner' <marcelo.leitner@gmail.com>
To:     Corey Minyard <minyard@acm.org>
Cc:     David Laight <David.Laight@aculab.com>,
        Michael Tuexen <Michael.Tuexen@lurchi.franken.de>,
        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>
Subject: Re: Strange problem with SCTP+IPv6
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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

  Marcelo

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