/etc/network/if-post-down.d/avahi-daemon is in avahi-daemon 0.6.32~rc+dfsg-1ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /etc/network/if-post-down.d/../if-up.d/avahi-daemon
/etc/network/if-post-down.d/../if-up.d/avahi-daemon