linux-kernel - RE: Strange problem with SCTP+IPv6

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Date:   Tue, 23 Jun 2020 13:17:28 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Marcelo Ricardo Leitner' <marcelo.leitner@...il.com>,
        Michael Tuexen <Michael.Tuexen@...chi.franken.de>
CC:     "minyard@....org" <minyard@....org>,
        Xin Long <lucien.xin@...il.com>,
        "Vlad Yasevich" <vyasevich@...il.com>,
        Neil Horman <nhorman@...driver.com>,
        "linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: RE: Strange problem with SCTP+IPv6

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@....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@....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.

I also wonder whether the connection should be dropped for an error
response on a path that has never been validated.

OTOH the whole 'multi-homing' part of SCTP sucks.
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.

	David

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