刚刚装好的虚拟机突然不能上网了，报错很诡异，具体报错如下：
/etc/init.d/network restart
Restarting network (via systemctl): Job for network.service failed because the control process exited with error code. See "systemctl status network.service" and "journalctl -xe" for details.
[失败]
使用systemctl status network.service命令查看结果如下：
network.service - LSB: Bring up/down networking
Loaded: loaded (/etc/rc.d/init.d/network)
Active: failed (Result: exit-code) since 三 2016-08-17 11:34:36 CST; 54s ago
Docs: man:systemd-sysv-generator(8)
Process: 3847 ExecStart=/etc/rc.d/init.d/network start (code=exited, status=1/FAILURE)
8月 17 11:34:36 python.cn network[3847]: RTNETLINK answers: File exists
8月 17 11:34:36 python.cn network[3847]: RTNETLINK answers: File exists
8月 17 11:34:36 python.cn network[3847]: RTNETLINK answers: File exists
8月 17 11:34:36 python.cn network[3847]: RTNETLINK answers: File exists
8月 17 11:34:36 python.cn network[3847]: RTNETLINK answers: File exists
8月 17 11:34:36 python.cn network[3847]: RTNETLINK answers: File exists
8月 17 11:34:36 python.cn systemd[1]: network.service: control process exited, code=exited status=1
8月 17 11:34:36 python.cn systemd[1]: Failed to start LSB: Bring up/down networking.
8月 17 11:34:36 python.cn systemd[1]: Unit network.service entered failed state.
8月 17 11:34:36 python.cn systemd[1]: network.service failed.
查看系统日志，报错如下：
Aug 17 11:34:36 python network: RTNETLINK answers: File exists
Aug 17 11:34:36 python network: RTNETLINK answers: File exists
Aug 17 11:34:36 python network: RTNETLINK answers: File exists
Aug 17 11:34:36 python network: RTNETLINK answers: File exists
Aug 17 11:34:36 python network: RTNETLINK answers: File exists
Aug 17 11:34:36 python network: RTNETLINK answers: File exists
Aug 17 11:34:36 python network: RTNETLINK answers: File exists
Aug 17 11:34:36 python network: RTNETLINK answers: File exists
Aug 17 11:34:36 python network: RTNETLINK answers: File exists
Aug 17 11:34:36 python systemd: network.service: control process exited, code=exited status=1
Aug 17 11:34:36 python systemd: Failed to start LSB: Bring up/down networking.
Aug 17 11:34:36 python systemd: Unit network.service entered failed state.
Aug 17 11:34:36 python systemd: network.service failed.
Aug 17 11:36:28 python systemd: Starting LSB: Bring up/down networking...
Aug 17 11:36:29 python network: 正在打开环回接口： 无法载入文件 '/etc/sysconfig/network-scripts/ifcfg-lo'
Aug 17 11:36:29 python network: 无法载入文件 '/etc/sysconfig/network-scripts/ifcfg-lo'
Aug 17 11:36:29 python network: 无法载入文件 '/etc/sysconfig/network-scripts/ifcfg-lo'
Aug 17 11:36:29 python network: 无法载入文件 '/etc/sysconfig/network-scripts/ifcfg-lo'
Aug 17 11:36:29 python network: [ 确定 ]
Aug 17 11:36:29 python network: 正在打开接口 eno16777736： 错误：激活连接失败：No suitable device found for this connection.
经查，确认/etc/sysconfig/network-scripts/ifcfg-lo文件是存在的。问题出现在哪里呢？无法，只好百度一下吧，百度出来的结果竟然基本一样。
百度答案：
mac地址不对，修改mac地址。
也许有人确实是mac问题，但是莫名其妙怎么可能mac地址更改了，除非你添加新网卡或者克隆的虚拟机。
我特么真笑了，天下百度文章一大抄啊。。。。。。。。
没办法了，Google一下吧，偶然看到一个文章，大意是跟系统自带的NetworkManager这个管理套件有关系，关掉就可以解决。
试一下：
systemctl stop NetworkManager
systemctl disable NetworkManager
Removed symlink /etc/systemd/system/multi-user.target.wants/NetworkManager.service.
Removed symlink /etc/systemd/system/dbus-org.freedesktop.NetworkManager.service.
Removed symlink /etc/systemd/system/dbus-org.freedesktop.nm-dispatcher.service.
重新启动网络：
systemctl start network.service
ifconfig |grep eno
eno16777736: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.1.192 netmask 255.255.255.0 broadcast 192.168.1.255
inet6 fe80::20c:29ff:fe3d:b305 prefixlen 64 scopeid 0x20<link>
ether 00:0c:29:3d:b3:05 txqueuelen 1000 (Ethernet)
RX packets 2 bytes 120 (120.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 22 bytes 3551 (3.4 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
网络正常启动，访问正常。
还有其他两种可以尝试的方法虽然这两种都没解决我的问题,但是也许能解决其他人的问题:
第一种方法查看错误:
[root@localhost ~]# systemctl status network.service
● network.service - LSB: Bring up/down networking
Loaded: loaded (/etc/rc.d/init.d/network; bad; vendor preset: disabled)
Active: failed (Result: exit-code) since Wed 2017-04-26 06:22:44 EDT; 12s ago
Docs: man:systemd-sysv-generator(8)
Process: 2664 ExecStart=/etc/rc.d/init.d/network start (code=exited, status=1/FAILURE)
Apr 26 06:22:44 localhost.localdomain network[2664]: RTNETLINK answers: File exists
Apr 26 06:22:44 localhost.localdomain network[2664]: RTNETLINK answers: File exists
Apr 26 06:22:44 localhost.localdomain network[2664]: RTNETLINK answers: File exists
Apr 26 06:22:44 localhost.localdomain network[2664]: RTNETLINK answers: File exists
Apr 26 06:22:44 localhost.localdomain network[2664]: RTNETLINK answers: File exists
Apr 26 06:22:44 localhost.localdomain network[2664]: RTNETLINK answers: File exists
Apr 26 06:22:44 localhost.localdomain systemd[1]: network.service: control process exited, code=exited status=1
Apr 26 06:22:44 localhost.localdomain systemd[1]: Failed to start LSB: Bring up/down networking.
Apr 26 06:22:44 localhost.localdomain systemd[1]: Unit network.service entered failed state.
Apr 26 06:22:44 localhost.localdomain systemd[1]: network.service failed.
[root@localhost ~]# vi /etc/sysconfig/network-scripts/ifcfg-ens33
[root@localhost ~]# systemctl status network.service
● network.service - LSB: Bring up/down networking
Loaded: loaded (/etc/rc.d/init.d/network; bad; vendor preset: disabled)
Active: failed (Result: exit-code) since Wed 2017-04-26 06:22:44 EDT; 1min 11s ago
Docs: man:systemd-sysv-generator(8)
Process: 2664 ExecStart=/etc/rc.d/init.d/network start (code=exited, status=1/FAILURE)
[root@localhost ~]# ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN qlen 1
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: ens33: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
link/ether 00:0c:29:54:ed:b7 brd ff:ff:ff:ff:ff:ff
inet 192.168.218.100/24 brd 192.168.218.255 scope global ens33
valid_lft forever preferred_lft forever
inet6 fe80::20c:29ff:fe54:edb7/64 scope link
valid_lft forever preferred_lft forever
将上面的网卡地址00:0c:29:54:ed:b7添加到网卡的配置中:
[root@bogon ~]# vi /etc/sysconfig/network-scripts/ifcfg-ens33
加入:
HWADDR=00:0c:29:54:ed:b7
方法二:
通过查找相关文档，使用如下方法解决了问题：设定开机启动一个名为NetworkManager-wait-online服务，命令为：
systemctl enable NetworkManager-wait-online.service
