IP_FREEBIND with IPv6 does not work
IP_FREEBIND doesn't work with IPv6 on Linux. Or at least it looks like that.
This is a snippet from the log file when the machine was booting. Knot was started by systemd before network fully was initialized.
host knotd[325]: info: Knot DNS 2.0.0-git-20150701-c7b4d74 starting
host knotd[325]: info: binding to interface '127.0.0.1@5300'
host knotd[325]: info: binding to interface '::1@5300'
host knotd[325]: info: binding to interface 'x.x.x.x@53'
host knotd[325]: warning: address 'x.x.x.x@53' is not available
host knotd[325]: info: binding to interface 'x:x:x:x::1@53'
host knotd[325]: error: cannot bind address 'x:x:x:x::1@53' (address is not available)
Restart of the service made it work.