intfwatchd is one of the worst programs I've ever written. If you don't know what is it and why it's there, here's the story: at some point, the Linux kernel decided that IPv6 addresses are not worth keeping after interface goes down, so they would disappear permanently. What's worse, there was no way around it.
While this behaviour may be somewhat sensible for hosts, it's absolutely unacceptable for routers, so we've had to have a way to add them back. So I wrote a quick and dirty daemon that watches links and restores IPv6 addresses when they go back up after going down.
New kernels still keep this behaviour by default, but luckily, there's no an option to talk some sense back in them:
sudo sysctl net.ipv6.conf.default.keep_addr_on_down=1
So we finally can get rid of that dirty hack.