When CoreOS are launching they use the virtual box dhcp before setting static addresse. So we have multiple addresse on the card. The one used is 192.168.X.X and not 172.17.8.101 (for the master). It can be solved by disabling dhcp option in virtualbox.
I think it can be solved with something like that:
VBoxManage dhcpserver remove --netname HostInterfaceNetworking-vboxnet0
but i don't really know where can I put it...