[openjdk-x] Bind to a multicast address fails

[David Lloyd]

On Fri, Sep 7, 2018 at 11:08 AM David Lloyd <david.lloyd at redhat.com> wrote:
> On Fri, Sep 7, 2018 at 6:56 AM Andre Naujoks <nautsch2 at gmail.com> wrote:
> > On 9/7/18 1:15 PM, Alan Bateman wrote:
> > > On 07/09/2018 10:49, Decke, Hendrik (K-GERFA/A) wrote:
> > >>
> > >> Hello,
> > >>
> > >> it seems one of our external developers (Andre Naujoks, CC) found a
> > >> bug while binding a IPv6 multicast UDP-socket for one of our projects.
> > >>
> > >>
> > >>
> > >> Since this seems to be a fundamental bug (from our perspective), we
> > >> address this directly to this mailing list.
> > >>
> > >> (reproducible in OpenJDK 8-11, Windows and Linux)
> > >>
> > > This bug was submitted this week on this issue:
> > >   https://bugs.openjdk.java.net/browse/JDK-8210493
> > >
> > > Have you tried joining the mullticast group, specifying the network
> > > interface, rather than binding to the multicast address?
> >
> > Hi Alan.
> >
> > First of all, thank you for the quick reply. I was not aware, that there
> > was actually a bug opened for that issue.
> >
> > The join is not the problem at this point. We need to bind the socket to
> > the address to avoid receiving traffic from all multicast groups, that
> > are joined on the system. Since a join joins the system (not the socket)
> > to the group, all sockets bound to a port, which receive multicast
> > traffic will receive all of that traffic, no matter the destination
> > address. The bind prevents that. IP_MULTICAST_ALL sadly only works for
> > IPv4 and the patch I tried to get IPV6_MULTICAST_ALL upstream into the
> > kernel was even more sadly (almost) ignored. see
> > https://marc.info/?l=linux-netdev&m=152344460530252&w=2
>
> Hi Andre,
>
> I spoke with a colleague about this kernel patch.  They said first of
> all that multicast filtering is pretty complex in the kernel with a
> lot of subtle behaviors.  But, they also said that it may have been
> ignored because of the format of the patch, perhaps even
> accidentally/automatically.  The proposed patch has an "RFC" tag, and
> such patches apparently need to be in "git-format-patch" mode.
> Lastly, they said that since the time that the post was made,
> IP_MULTICAST_ALL (for IPv4 only of course) has changed a little bit in
> that "it only allows receipt of all multicast groups if a specific
> list of multicast groups has not already been set", so it may need to
> be updated accordingly.  FWIW they didn't mention seeing any actual
> problems with the content of the patch, though I'm not sure how
> thoroughly they reviewed it.
>
> So, while I myself am not a Linux kernel contributor, I do suspect
> that if you reposted an updated version of the patch, in the correct
> format, it will enter the patch queue and may be more actively
> discussed.  For more information, see [1].
>
> [1] https://www.kernel.org/doc/html/v4.17/process/submitting-patches.html#the-canonical-patch-format

I should also mention that the recommended approach would be to send a
not-RFC version of the patch and see how that goes.
-- 
- DML