众所周知：

        Ubuntu在域名解析时，最直接使用的是/etc/resolve.conf文件，它是/run/systemd/resolve/

resolve.conf的软链接，而对于刚装完的ubuntu系统，该文件的内容如下：

# This file is managed by man:systemd-resolved(8). Do not edit.
#
# This is a dynamic resolv.conf file for connecting local clients to the
# internal DNS stub resolver of systemd-resolved. This file lists all
# configured search domains.
#
# Run "resolvectl status" to see details about the uplink DNS servers
# currently in use.
#
# 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 127.0.0.53
options edns0 trust-ad

问题分析：

        很显然，我们需要做的就是在/etc/resolve.conf中，配置我们需要的域名解析服务器地址，例如：“nameserver 8.8.8.8”，但是这个文件即使我们修改了，很快又会被覆盖，而且我们注意一个细节——文件的开头就注明了“Do not edit”，到了这里，我们就应该反应过来，直接修改/etc/resolve.conf是不正确的。

解决方案：

        第1步， 修改/etc/systemd/resolved.conf，例如：

#  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=8.8.8.8 114.114.114.114
#FallbackDNS=
#Domains=
#LLMNR=no
#MulticastDNS=no
#DNSSEC=no
#DNSOverTLS=no
#Cache=no-negative
#DNSStubListener=yes
#ReadEtcHosts=yes

       个人猜测，这个时候，系统在往/run/systemd/resolve/resolv.conf里面写域名解析服务器地址的时候，会从/etc/systemd/resolved.conf中取得DNS相关的配置。

        2，重启域名解析服务

                systemctl restart systemd-resolved
                systemctl enable systemd-resolved

        3，备份当前的/etc/resolve.conf，并重新设置/run/systemd/resolve/resolv.conf  到/etc/resolve.conf的软链接

                mv    /etc/resolv.conf    /etc/resolv.conf.bak
                ln  -s   /run/systemd/resolve/resolv.conf    /etc/

验证成果：

# 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 uplink DNS servers. This file lists all configured search domains.
#
# 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 8.8.8.8
nameserver 114.114.114.114

感谢各个论坛大佬的分享，主要参考：

        https://www.cnblogs.com/mouseleo/p/14976527.html