centos7 无法启动网络(service network restart)错误解决办法:
(以下方法均为网上COPY,同时感谢原博主分享)
systemctl status network.service 出现以下错误
“rtnetlink answers file exists” 的解决方法
第一种： 和 NetworkManager 服务有冲突，这个好解决，直接关闭 NetworkManger 服务就好了， service
NetworkManager stop，并且禁止开机启动 chkconfig NetworkManager off 。之后重启就好了。(我使用第一种成功)
第二种：和配置文件的MAC地址不匹配，这个也好解决，直接修改 /etc/udev/rules.d/70-persistent-net.rules文件的MAC
地址和 /etc/sysconfig/network-scripts/ifcfg-eth0一样就好了。
(两种都不行的话可以试试这个)
ip addr flush dev enoXXXXX
(copy地址:http://www.2cto.com/os/201411/350848.html)
三种方法解决 Failed to start LSB: Bring up/down networking 问题
《—》
1、执行 service network restart 出现以下错误
Restarting network (via systemctl): Job for network.service failed. See 'systemctl status network.service'
and 'journalctl -xn' for details.
2、根据上面的提示，执行“systemctl status network.service”输出以下的类似信息：
[root@localhost ~]# systemctl status network.service
network.service - LSB: Bring up/down networking
Loaded: loaded (/etc/rc.d/init.d/network)
Active: failed (Result: exit-code) since三 2014-11-05 15:30:10 CST; 1min 5s ago
11月 05 15:30:10 localhost.localdomain network[2920]: RTNETLINK answers: File exists
11月 05 15:30:10 localhost.localdomain network[2920]: RTNETLINK answers: File exists
11月 05 15:30:10 localhost.localdomain network[2920]: RTNETLINK answers: File exists
11月 05 15:30:10 localhost.localdomain network[2920]: RTNETLINK answers: File exists
11月 05 15:30:10 localhost.localdomain network[2920]: RTNETLINK answers: File exists
11月 05 15:30:10 localhost.localdomain network[2920]: RTNETLINK answers: File exists
11月 05 15:30:10 localhost.localdomain network[2920]: RTNETLINK answers: File exists
11月 05 15:30:10 localhost.localdomain systemd[1]: network.service: control process exited, code=exited
status=1
11月 05 15:30:10 localhost.localdomain systemd[1]: Failed to start LSB: Bring up/down networking.
11月 05 15:30:10 localhost.localdomain systemd[1]: Unit network.service entered failed state.
3、解决的办法（由于centos7中没有70-persistent-net.rules这个文件，复制出来的虚拟机(vmware）需要修改mac地址）
，就是修改mac地址。
4、如何查看centos7复制出来的虚拟机的mac地址，需要执行 “ip addr”，输出如下的类似信息：
[root@localhost ~]# ip addr
1: lo:
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eno16777736:
link/ether00:0f:19:10:12:f1 brd ff:ff:ff:ff:ff:ff
inet 192.168.38.135/24 brd 192.168.38.255 scope global dynamic eno16777736
valid_lft 1681sec preferred_lft 1681sec
inet6 fe80::20c:11aa:ae11:4fe4/64 scope link
valid_lft forever preferred_lft forever
注意上面加粗加红的字，这就是新的mac地址。
接下来就剩下修改ifcfg-XXX文件了，只修改HWADDR就可以了。接下来执行service network restart就正常了。
《二》
如果还是启动失败 查看是否有如下错误信息
Nov 23 22:09:07 hdcoe02 network[597]: Bringing up loopback interface: Could not load file
'/etc/sysconfig/network-scripts/ifcfg-lo'
Nov 23 22:09:07 hdcoe02 network[597]: Could not load file '/etc/sysconfig/network-scripts/ifcfg-lo'
Nov 23 22:09:08 hdcoe02 network[597]: Could not load file '/etc/sysconfig/network-scripts/ifcfg-lo'
Nov 23 22:09:08 hdcoe02 network[597]: Could not load file '/etc/sysconfig/network-scripts/ifcfg-lo' Nov 23
22:09:08 hdcoe02 network[597]: [ OK ]
Nov 23 22:09:08 hdcoe02 network[597]: Bringing up interface enp0s25: Error: Connection activation failed:
Connection 'enp0s25' is not available on the device enp0s25 at this time.
设定开机启动一个名为NetworkManager-wait-online服务，命令为：
systemctl enable NetworkManager-wait-online.service
《三》
我也是上面方法都测试完了 一直不成功，最后发现时因为在VMware 虚拟机里面的系统 右上角显示的连接都没有连接上，
虽然报错都一样，最后我在虚拟机设置里面将原来的网络适配器删掉，重新建了一个新的网络适配器，这次进去，显示连接
成功，再用上面的方法，问题解决。
(copy地址:http://blog.csdn.net/qq_21398167/article/details/46694179)