前言
话说Ubuntu的DNS设置，跟centos还不太一样。centos只要把dns写在网络配置项，怎么重启网络都能启用。
然而Ubuntu就不行，因为它有/etc/resolv.conf, 每次一重启网络，这个文件会被覆盖，如下：
root@home:~# 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
# 127.0.0.53 is the systemd-resolved stub resolver.
# run "systemd-resolve --status" to see details about the actual nameservers.
nameserver 127.0.0.53
每次要通外网我还得手动改这个文件，烦求得很。
于是我就想，是不是有办法永久修改这个文件，使得每次重启网络的时候，都可以省去手动修改文件的麻烦。
方法
某位伟人讲过，办法总比困难多，于是我就去找办法。
终于看到一篇文章，他是这么讲的：
修改resolved.conf文件,然后reload服务
我们看看他的要点是什么啊：
1. 停掉NetworkManager
systemctl disable --now NetworkManager
2. 修改resolved.conf文件
# 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=192.168.1.2 #把这个DNS服务器修改为你要改的
#FallbackDNS=
#Domains=
#LLMNR=no
#MulticastDNS=no
#DNSSEC=no
#Cache=yes
#DNSStubListener=yes
3. 重启systemd-resolved服务
systemctl restart systemd-resolved.service
4. 查看是否修改成功
systemd-resolve --status
Global
DNS Servers: 192.168.1.2
DNSSEC NTA: 10.in-addr.arpa
修改成功。
于是我们接下来每次重启网络，它都会使用我们希望的DNS做域名服务器了，就不需要重新改了。