一、问题：ubuntu16.04.3升级 apt-get update报错
root@setHostName:~# apt-get update
Err:1 http://security.ubuntu.com/ubuntu xenial-security InRelease
Temporary failure resolving 'security.ubuntu.com'
Reading package lists... Done
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/xenial-security/InRelease Temporary failure resolving 'security.ubuntu.com'
W: Some index files failed to download. They have been ignored, or old ones used instead.
root@setHostName:~#
二、根据提示resoving 'security.ubuntu.com',判断是DNS不正确
root@setHostName:~# more /etc/resolv.conf

nameserver 192.168.220.254
nameserver 202.102.224.67 //添加1条外网DNS
root@setHostName:~# service networking restart

root@setHostName:~# ping www.baidu.com
PING www.a.shifen.com (61.135.169.125) 56(84) bytes of data.
64 bytes from 61.135.169.125: icmp_seq=1 ttl=128 time=50.6 ms
64 bytes from 61.135.169.125: icmp_seq=2 ttl=128 time=75.9 ms
^C
--- www.a.shifen.com ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1012ms
rtt min/avg/max/mdev = 50.691/63.317/75.943/12.626 ms
root@setHostName:~#
root@setHostName:~# apt-get update //再次update
Get:1 http://security.ubuntu.com/ubuntu xenial-security InRelease [107 kB]
Fetched 107 kB in 6s (16.5 kB/s)
Reading package lists... Done