TTL ignored when sending Multicast UDP Datagram to IPv4 address via IPv6 socket

[Edward Wang]

Hi Don,

Yes, in recent kernels (2.6.*), both IPv4 and IPv6 socket options can be 
set for an IPv6 socket. And seems setting both of them is the only way 
to make things like TTL/HOP_LIMIT works properly as expected.

Only that I still need to do more testing for different kernel versions. 
  Hope I can integrate the patch soon.

-Edward

Don Coleman wrote:
> -Djava.net.preferIPv4Stack=true works fine for IPv4, but my
> application needs to send both IPv4 and IPv6.
> 
> I'd be interested to see Edward Wang's patch for this issue.
> 
> On 6/1/07, Alan Bateman <Alan.Bateman at sun.com> wrote:
>> Don Coleman wrote:
>> > I'm have a problem setting TTL when sending Multicast packets to an
>> > IPv4 address over an IPv6 socket.
>> >
>> > Setting the TTL in Java has no effect when sending to an IPv4 address.
>> > It is always 1.
>> >
>> > This is only a problem on Linux, it works fine on OS X, Solaris and
>> > Windows.
>> >
>> > This problem exists with JDK5, but it looks like it's also in the JDK7
>> > code.
>> >
>> > 
>> https://openjdk.dev.java.net/source/browse/openjdk/jdk/trunk/j2se/src/solaris/native/java/net/PlainDatagramSocketImpl.c?annotate=237 
>>
>> >
>> >
>> > on line 1873 IPV6_MULTICAST_HOPS is set
>> > for Linux I think we need also need to set IP_MULTICAST_TTL
>> >
>> > I've attached sample Java and C code that demonstrates the problem
>> > here https://bugs.launchpad.net/ubuntu/+bug/112257
>> >
>> > Any thoughts if this is a Java problem or a Linux kernel bug?
>> The bug tracking this one is:
>>     http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6250763
>>
>> At least in older kernels, it wasn't possible to set IPv4 multicast
>> options on IPv6 sockets (the setsockopt would fail). Edward Wang has
>> been testing with recent kernels and has results that suggest that
>> setting the IPv4 options work now. I believe he has a fix/patch in the
>> works to address this (Edward - can you comment?).
>>
>> If you need a workaround then run with -Djava.net.preferIPv4Stack=true
>> so that all sockets are IPv4 (even if the machine has IPv6 enabled).
>>
>> -Alan.
>>
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