/etx/systemd/resolved.conf是ubuntu下DNS解析服务配置文件，systemd为ubuntu下system and service配置目录
step 1——修改resolved.conf参数
管理员权限打开 /systemd/resolved.conf
sudo nano /etc/systemd/resolved.conf
修改如下：
# 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. Local configuration
# should be created by either modifying this file, or by creating "drop-ins" in
# the resolved.conf.d/ subdirectory. The latter is generally recommended.
# Defaults can be restored by simply deleting this file and all drop-ins.
#
# Use 'systemd-analyze cat-config systemd/resolved.conf' to display the full config.
#
# See resolved.conf(5) for details.[Resolve]
# Some examples of DNS servers which may be used for DNS= and FallbackDNS=:
# Cloudflare: 1.1.1.1#cloudflare-dns.com 1.0.0.1#cloudflare-dns.com 2606:4700:4700::1111#cloudflare-dns.com 2606:4700:4700::1001#cloudflare-dns.com
# Google: 8.8.8.8#dns.google 8.8.4.4#dns.google 2001:4860:4860::8888#dns.google 2001:4860:4860::8844#dns.google
# Quad9: 9.9.9.9#dns.quad9.net 149.112.112.112#dns.quad9.net 2620:fe::fe#dns.quad9.net 2620:fe::9#dns.quad9.net
DNS=1.1.1.1
FallbackDNS=9.9.9.9
#Domains=
DNSSEC=allow-downgrade
DNSOverTLS=opportunistic
#MulticastDNS=no
#LLMNR=no
Cache=no-negative
#CacheFromLocalhost=no
DNSStubListener=yes
#DNSStubListenerExtra=
ReadEtcHosts=yes
#ResolveUnicastSingleLabel=no
参数的具体含义见ubuntu manual
step2——重启DNS服务
systemctl restart systemd-resolved #重置DNS服务
systemctl enable systemd-resolved #开机自启动
resolvectl 为DNS指令，查看状态用status参数
brief1——ubuntu系统调用库resolvectl&systemctl的解析
resolvectl为解析服务调用，只有查看状态和重置参数命令，用python的库可以实现
systemctl为系统调用，
依次有元命令，
元文件命令
机器命令——展示机器指令
作业命令——display作业，取消作业
check，modify环境变量
系统和服务 管理指令——对system manager的配置与执行
系统(调用)命令
step3——小时牛刀
为了更高效地工作，请务必运行以下命令 ～～
服务的操作
启动服务：systemctl start vsftpd.service
关闭服务：systemctl stop vsftpd.service
重启服务：systemctl restart vsftpd.service
显示服务的状态：systemctl status vsftpd.service
在开机时启用服务：systemctl enable vsftpd.service
在开机时禁用服务：systemctl disable vsftpd.service
查看服务是否开机启动：systemctl is-enabled vsftpd.service
查看已启动的服务列表：systemctl list-unit-files|grep enabled
查看启动失败的服务列表：systemctl --failed
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