恢复Ubuntu的DHCP默认设置，通常有以下两种情况：
/etc/dhcp/dhclient.conf，然后删除或重命名该文件，最后重启网络服务。具体命令如下：sudo cp /etc/dhcp/dhclient.conf /etc/dhcp/dhclient.conf.bak
sudo mv /etc/dhcp/dhclient.conf /etc/dhcp/dhclient.conf.old
sudo systemctl restart networking
nmcli命令重新启用DHCP。先查看网络连接nmcli connection show，再对指定连接执行sudo nmcli connection modify <connection-name> ipv4.method auto和sudo nmcli connection up <connection-name>。/etc/dhcp/dhcpd.conf，然后重启服务。命令如下：sudo systemctl stop isc-dhcp-server
sudo cp /etc/dhcp/dhcpd.conf /etc/dhcp/dhcpd.conf.bak
sudo rm /etc/dhcp/dhcpd.conf
sudo systemctl start isc-dhcp-server
netplan工具，编辑/etc/netplan/*.yaml文件，确保其中包含正确的DHCP设置，如dhcp4: true，然后应用配置并重启网络服务。
