Can't start CentOS 7 "network" service

In Centos7.0 disabling NetworkManager will leave a dhcp client running configured for NetworkManager. This causes the error message RTNETLINK answers: File exists when the network service is started.

The stale dhclient process has the additional "benefit" that when the lease expires your dhclient will choke, since it cannot reach NetWorkManager, thus removing your IP address.

If you grep for it, you will see that it points to a NetWorkManager configuration file.

[root@host ~]# ps -ef | grep dhc
root      1865   792  0 Apr28 ?        00:00:00 /sbin/dhclient -d -sf \
 /usr/libexec/nm-dhcp-helper -pf /var/run/dhclient-eno1.pid -lf\
 /var/lib/NetworkManager/dhclient-c96e56d3-a4c9-4a87-85ca-93dc0ca268f2-eno1.lease\
 -cf /var/lib/NetworkManager/dhclient-eno1.conf eno1

So what you can do is kill the dhclient and only then start your network service.

An IP conflict will also cause this error. Try: systemctl stop network, then ifup eth0.

It seems this happens also if you manually configure an interface that isn't connected to the network.