itojun@itojun.org said:
> i would suggest specifying interface
> 	explicitly, always, even with IPV6_JOIN_GROUP

There is another problem with IPV6_JOIN_GROUP and no explitely
specified interface: it doesn't work if no more global route was
assigned.

Looking at the code, I believe I understand how this happens:
The rtalloc() in ip6_setmoptions() fails unless a global route
is present because no sensible scope_id can be passed at this
point.

While the ntp4 code can be easily changed to specify the if id
on IPV6_JOIN_GROUP, that case should still be handled somehow...

best regards
Matthias