Linux系统只能连上内网不能够连上外网。Unit network.service entered failed state.，外网访问内网服务器
我的系统是centos7.0系统，所以有些服务的命令与之前的版本稍微有点小差异。理解就行了。
首先，查看network的状态，
#systemctl status network
显示有错误信息。
$ systemctl status network
network.service - LSB: Bring up/down networking
Loaded: loaded (/etc/rc.d/init.d/network)
Active: failed (Result: exit-code) since Sun 2015-05-17 21:57:22 EDT; 11s ago
Process: 25051 ExecStart=/etc/rc.d/init.d/network start (code=exited, status=1/FAILURE)
May 17 21:57:22 localhost.localdomain network[25051]: RTNETLINK answers: File exists
May 17 21:57:22 localhost.localdomain network[25051]: RTNETLINK answers: File exists
May 17 21:57:22 localhost.localdomain network[25051]: RTNETLINK answers: File exists
May 17 21:57:22 localhost.localdomain network[25051]: RTNETLINK answers: File exists
May 17 21:57:22 localhost.localdomain network[25051]: RTNETLINK answers: File exists
May 17 21:57:22 localhost.localdomain network[25051]: RTNETLINK answers: File exists
May 17 21:57:22 localhost.localdomain network[25051]: RTNETLINK answers: File exists
May 17 21:57:22 localhost.localdomain systemd[1]: network.service: control process exite...=1
May 17 21:57:22 localhost.localdomain systemd[1]: Failed to start LSB: Bring up/down net...g.
May 17 21:57:22 localhost.localdomain systemd[1]: Unit network.service entered failed state.
Hint: Some lines were ellipsized, use -l to show in full.
之后尝试打开：
#systemctl enable NetworkManager-wait-online.service
仍然不成功，此时查看配置文件中的MAC地址与本机的物理MAC地址是否匹配。
#cat /sys/class/net/eth0/address 查看eth0的物理MAC地址。
#cat /etc/sysconfig/network-scripts/ifcfg-eth0 中的IPADDR0的值是否与本台的一致。
不一致尝试修改配置文件。
总结：网络连接不正常，一般为IP配置错误，DNS配置错误，或者是MAC地址配置不争取导致的，可以检查下一般就好了。如果配置都正常了尝试下重启就好了。