Ubuntu 18.04 LTSを使っていたらネットワーク設定の方法がifup/downからnetplanというものへと置き換わり、サービスの管理もnetworkingからsystemd-networkd, systemd-resolvedというものに変わっていた。
厳密にいつから変わったのか追ってみるのと、Ubuntu 17.04 でDNSの設定でsystemd-resolvedを使うようになっており、17.10でネットワーク設定で netplan を使うように変わっていた。
Networking The default DNS resolver is now systemd-resolved.
Network configuration ifupdown has been deprecated in favor of netplan and is no longer present on new installs. The installer will generate a configuration file for netplan in /etc/netplan, which will set up the system to configure the network via systemd-networkd or NetworkManager. Desktop users will see their system fully managed via NetworkManager as it has been the case in previous releases, but Server users now have their network devices managed via systemd-networkd on new installs. This only applies to new installations. Given that ifupdown is no longer installed by default, its commands will not be present: ifup and ifdown are thus unavailable, replaced by ip link set $device up and ip link set $device down. The networkctl command is also available for users to see a summary of the network devices. networkctl status will display the current global state of IP addresses on the system; and networkctl status $device can display the details specific to a network device. For more information about netplan, please refer to the manual page using the man 5 netplan command.