[Bug 240137] Multicast source blocking using IP_BLOCK_SOURCE not working?

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=240137

            Bug ID: 240137
           Summary: Multicast source blocking using IP_BLOCK_SOURCE not
                    working?
           Product: Base System
           Version: 12.0-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: net at FreeBSD.org
          Reporter: glewis at FreeBSD.org
 Attachment #206944 text/plain
         mime type:

Created attachment 206944
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=206944&action=edit
Test program

While testing Java on FreeBSD, I noticed a test failure due to multicast source
blocking not working.  I can supply the Java code if that would be helpful, but
I've boiled down the test into some C code which does the same thing which I
think will be more useful.

The test code will attempt to first send and receive a multicast packet.  It
will then block the source and try it again.  This should result in the second
receive just blocking since no packet should come through.  However, on FreeBSD
it does.  The same test program on Linux behaves as expected (i.e., the second
receive blocks).

Please let me know if the test program is doing something incorrectly on
FreeBSD.  This is based on my reading of the section of ip(4) regarding
IP_BLOCK_SOURCE and testing on Linux.

Note that I've tested the code on both 11.3 and 12.0, with the same result.

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