安装好Ubuntu之后设置了静态IP地址，再重启后就无法解析域名。想重新设置一下DNS，打开/etc/resolv.conf

cat /etc/resolv.conf
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
#     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN

内容是一段警告：说这个文件是resolvconf程序动态创建的，不要直接手动编辑，修改将被覆盖。

root@Ubuntu14:~# cat /etc/resolv.conf
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
#     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 100.100.2.138
nameserver 100.100.2.136
options timeout:2 attempts:3 rotate single-request-reopen
root@Ubuntu14:~#

ubuntu修改DNS有如下两种方法。

root@Ubuntu14:~# cat /etc/resolv.conf
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
#     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 100.100.2.138
nameserver 100.100.2.136
options timeout:2 attempts:3 rotate single-request-reopen
root@Ubuntu14:~# vim /etc/resolvconf/resolv.conf.d/base
root@Ubuntu14:~# cat !$
cat /etc/resolvconf/resolv.conf.d/base
nameserver 223.5.5.5
nameserver 223.6.6.6
root@Ubuntu14:~# resolvconf -u
root@Ubuntu14:~# cat /etc/resolv.conf
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
#     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 100.100.2.138
nameserver 100.100.2.136
nameserver 223.5.5.5
options timeout:2 attempts:3 rotate single-request-reopen
root@Ubuntu14:~# nslookup www.baidu.com
Server:         100.100.2.138
Address:        100.100.2.138#53
Non-authoritative answer:
www.baidu.com   canonical name = www.a.shifen.com.
Name:   www.a.shifen.com
Address: 220.181.112.244
Name:   www.a.shifen.com
Address: 220.181.111.188
root@Ubuntu14:~#

root@iZbp1b66g5htibj2m7yyv3Z:~# cat /etc/network/interfaces
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet dhcp
dns-nameservers 223.5.5.5   # 在最后添加这一行

linux刷新dns的缓存方法是：　　sudo /etc/init.d/nscd restart

最暴力的方法刷dns，重启网络：　　sudo /etc/init.d/networking restart

查看当前DNS解析情况：

nslookup www.baidu.com

dig | grep SERVER

root@iZbp1b66g5htibj2m7yyv3Z:~# /etc/init.d/nscd restart
 * Restarting Name Service Cache Daemon nscd                                                                                                                                                                                          [ OK ]
root@iZbp1b66g5htibj2m7yyv3Z:~#  /etc/init.d/networking restart
stop: Job failed while stopping
start: Job is already running: networking
root@iZbp1b66g5htibj2m7yyv3Z:~# dig |grep SERVER
;; SERVER: 100.100.2.138#53(100.100.2.138)
root@iZbp1b66g5htibj2m7yyv3Z:~# nslookup www.baidu.com
Server:         100.100.2.138
Address:        100.100.2.138#53
Non-authoritative answer:
www.baidu.com   canonical name = www.a.shifen.com.
Name:   www.a.shifen.com
Address: 220.181.111.188
Name:   www.a.shifen.com
Address: 220.181.112.244
root@iZbp1b66g5htibj2m7yyv3Z:~#

1)　　vim /etc/resolv.conf

2)　　vim /etc/systemd/resolved.conf

3)　　systemctl restart systemd-resolved

4)　　reboot

coreos1 ~ # cat /etc/resolv.conf
# This file is managed by man:systemd-resolved(8). Do not edit.
# This is a dynamic resolv.conf file for connecting local clients directly to
# all known DNS servers.
# Third party programs must not access this file directly, but only through the
# symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a different way,
# replace this symlink by a static file or a different symlink.
# See man:systemd-resolved.service(8) for details about the supported modes of
# operation for /etc/resolv.conf.
nameserver 223.5.5.5
nameserver 223.6.6.6
search openstacklocal
coreos1 ~ # cat /etc/systemd/resolved.conf
#  This file is part of systemd.
#  systemd is free software; you can redistribute it and/or modify it
#  under the terms of the GNU Lesser General Public License as published by
#  the Free Software Foundation; either version 2.1 of the License, or
#  (at your option) any later version.
# Entries in this file show the compile time defaults.
# You can change settings by editing this file.
# Defaults can be restored by simply deleting this file.
# See resolved.conf(5) for details
[Resolve]
DNS=223.5.5.5 223.6.6.6　　　　　　# 用空格分隔多个dns
#FallbackDNS=
#Domains=
LLMNR=no　　　　　　　　　　　　　　　# yes改为no
#DNSSEC=allow-downgrade
#Cache=yes
coreos1 ~ # systemctl restart systemd-resolved
coreos1 ~ # dig |grep SERVER
;; SERVER: 223.5.5.5#53(223.5.5.5)
coreos1 ~ # reboot