Comment 2 for bug 1149527

> But as soon as I run "exec /sbin/init" from that bash prompt the machine
> once again starts ignoring ipv6, so clearly something that upstart is
> doing is killing the ipv6.

Well, the "something that upstart is doing" is "booting the system and running the configured scripts and jobs". Upstart itself doesn't touch the network stack at all. So even though you say ifupdown isn't configured to manage any of the network interfaces, I'm reassigning the bug there for further analysis.