问题：
VMWare虚拟机安装了CentOS7 ，突然发现连接不上，进入系统使用ifconfig命令查看网络设备，发现无外网网卡。
（推荐教程：centos教程）
如下：
[root@localhost ~]# ifconfig
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10<host>
loop txqueuelen 1000 (Local Loopback)
RX packets 12 bytes 1068 (1.0 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 12 bytes 1068 (1.0 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
virbr0: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
inet 192.168.122.1 netmask 255.255.255.0 broadcast 192.168.122.255
ether 52:54:00:f6:af:41 txqueuelen 1000 (Ethernet)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0登录后复制
重启网卡试试看。结果报错了！报错信息如下：
[root@localhost ~]# systemctl status network
● 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 Mon 2020-09-28 09:59:00 CST; 9s ago
Docs: man:systemd-sysv-generator(8)
Process: 13344 ExecStart=/etc/rc.d/init.d/network start (code=exited, status=1/FAILURE)
Sep 28 09:59:00 localhost.localdomain network[13344]: RTNETLINK answers: File...
Sep 28 09:59:00 localhost.localdomain network[13344]: RTNETLINK answers: File...
Sep 28 09:59:00 localhost.localdomain network[13344]: RTNETLINK answers: File...
Sep 28 09:59:00 localhost.localdomain network[13344]: RTNETLINK answers: File...
Sep 28 09:59:00 localhost.localdomain network[13344]: RTNETLINK answers: File...
Sep 28 09:59:00 localhost.localdomain network[13344]: RTNETLINK answers: File...
Sep 28 09:59:00 localhost.localdomain systemd[1]: network.service: control pr...
Sep 28 09:59:00 localhost.localdomain systemd[1]: Failed to start LSB: Bring ...
Sep 28 09:59:00 localhost.localdomain systemd[1]: Unit network.service entere...
Sep 28 09:59:00 localhost.localdomain systemd[1]: network.service failed.
Hint: Some lines were ellipsized, use -l to show in full.登录后复制
出现此现象的原因一般是系统的 NetworkManager 管理套件出现了问题
Sep 28 09:59:00 localhost.localdomain systemd[1]: Failed to start LSB: Bring ...
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所以我们要停止这个套件的服务
[root@localhost ~]# systemctl stop NetworkManager
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然后重启网卡试试
[root@localhost ~]# systemctl restart network.service
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没有报错，是不是可以了呢？我们查看网络设备
[root@localhost ~]# ifconfig
ens33: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.100.15 netmask 255.255.255.0 broadcast 192.168.100.255
inet6 fe80::20c:29ff:fe35:e8d7 prefixlen 64 scopeid 0x20<link>
ether 00:0c:29:35:e8:d7 txqueuelen 1000 (Ethernet)
RX packets 9 bytes 626 (626.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 32 bytes 4309 (4.2 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10<host>
loop txqueuelen 1000 (Local Loopback)
RX packets 12 bytes 1068 (1.0 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 12 bytes 1068 (1.0 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
virbr0: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
inet 192.168.122.1 netmask 255.255.255.0 broadcast 192.168.122.255
ether 52:54:00:f6:af:41 txqueuelen 1000 (Ethernet)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0登录后复制
出现外网网卡了，为了防止重启后 NetworkManager 再次开启，我们这是不让它开机启动
systemctl disable NetworkManager
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完成！
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