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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

ubuntu@ip-172-31-36-184:/etc$ cat 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 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 172.31.0.2
search ap-northeast-1.compute.internal

第1步， 修改/etc/systemd/resolved.conf，例如：
谷歌的dns=8.8.8.8 cloudflare的dns: 1.1.1.1

ubuntu@ip-172-31-36-184:/etc/systemd$ ls
journald.conf  logind.conf  network  networkd.conf  pstore.conf  resolved.conf  sleep.conf  system  system.conf  timesyncd.conf  user  user.conf
ubuntu@ip-172-31-36-184:/etc/systemd$ cat 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=8.8.8.8 1.1.1.1
#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-resolvedsystemctl enable systemd-resolved3，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.bakln  -s   /run/systemd/resolve/resolv.conf    /etc/

验证：

ubuntu@ip-172-31-36-184:/etc$ cat 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 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 1.1.1.1
nameserver 172.31.0.2
search ap-northeast-1.compute.internal

参考：
1-华为云开发者论坛
2-csdn https://blog.csdn.net/booklijian/article/details/116491288