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    SubjectRE: Strange problem with SCTP+IPv6
    Date
    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.

    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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    Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
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