Raise Network Interfaces failed... Boot.Wait.Online - howto fix it

Raise Network Interfaces failed
(& how-to fix solution)

:~$ systemctl status networking.service 

* networking.service - Raise network interfaces
    Loaded: loaded (/lib/systemd/system/networking.service; enabled; vendor preset: enabled)
   Drop-In: /run/systemd/generator/networking.service.d
            `-50-insserv.conf-$network.conf
    Active: failed (Result: exit-code) since Sun 2016-08-14 17:02:50 PDT; 39min ago
    Docs: man:interfaces(5)
  Main PID: 893 (code=exited, status=1/FAILURE)

 Aug 14 17:02:41 sheeva systemd[1]: Starting Raise network interfaces...
 Aug 14 17:02:46 sheeva ifup[893]: /sbin/ifup: waiting for lock on /run/network/ifstate.eth0
 Aug 14 17:02:49 sheeva ifup[893]: RTNETLINK answers: File exists
 Aug 14 17:02:49 sheeva ifup[893]: Failed to bring up eth0.
 Aug 14 17:02:50 sheeva systemd[1]: networking.service: Main process exited, code=exited, status=1/FAILURE
 Aug 14 17:02:50 sheeva systemd[1]: Failed to start Raise network interfaces.
 Aug 14 17:02:50 sheeva systemd[1]: networking.service: Unit entered failed state.
 Aug 14 17:02:50 sheeva systemd[1]: networking.service: Failed with result 'exit-code'.

However the interface *is* up:

 :~$ ifconfig
 eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500

*****************************************************

Here is a possibly relevant config file

#Original Line
auto ens34
iface ens34 inet dhcp

#New line
allow-hotplug ens34
iface ens34 inet dhcp 

 --------- /etc/network/interfaces ------------
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo eth0
iface lo inet loopback

# The primary network interface
allow-hotplug eth0
iface eth0 inet dhcp

(iface eth0 inet static)
address 192.168.3.111
 netmask 255.255.240.000
 network 192.168.0.0
 broadcast 192.168.15.255
 gateway 192.168.1.1

iface eth0 inet6 static
       address 2001:1234:2d2:1:f2ad:4eff:fe00:3077
       netmask 64
--------- /etc/network/interfaces ------------ 

systemctl status <systemd or NetworkManager below>

systemd-networkd-wait-online.service

NetworkManager-wait-online.service

 
sudo systemctl disable systemd-networkd-wait-online.service

 sudo systemctl disable etworkManager-wait-online.service

...or you can also reduce the timeout

sudo nano /etc/systemd/system/network-online.targets.wants/networking.service 

 TimeoutStartSec=10sec

save (CTRL + x, y, Enter) and  

sudo systemctl daemon-reload 

systemctl status networking.service

networkctl

ifconfig enp5s0f1 up  

networkctl

ifconfig enp5s0f1 down

 ip link set enp5s0f1 name eth0

systemctl restart networking

*** ABOVE IS ONLY TEMP SOLUTION ***

 it won't persist after reboot. Create a 

 sudo touch /etc/udev/rules.d/70-persistent-net.rules 

(should contain smthng like:
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="smsc95xx", ATTR{address}=="*", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="eth0"