Ubuntu – A start job is running for wait for network to be configured. Ubuntu server 17.10

If:

em1 = ethernet
192.168.1.254 = IP address of router
192.168.0.120 = IP address of server

Test this /etc/network/interfaces file:

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto em1
iface em1 inet static
address 192.168.1.120
gateway 192.168.1.254
netmask 255.255.255.0
network 192.168.1.0
broadcast 192.168.1.255

open /etc/network/interfaces using

sudo vi  /etc/network/interfaces

add line

auto eth0 
iface eth0 inet dhcp 

then do

sudo ifdown eth0
sudo ifup eth0

you are saying sudo dhclient eth0 working so do above change it will search for DHCP when you start system also make sure you have proper proxy setting in your system .current ifconfig output shows your system getting IP from DHCP.

EDIT: For device not managed you have to open /etc/NetworkManager/NetworkManager.conf using command :

sudo vi /etc/NetworkManager/NetworkManager.conf

and change the line managed=false to managed=true and restart NetworkManager

sudo service network-manager restart

For more info related this problem you can take help from Askubuntu another question network manager says “device not managed”