一、临时设置
直接修改 /etc/resolv.conf，例如直接添加nameserver 8.8.8.8。
注意：系统重启或resolvconf重启后会刷新该配置文件。
二、 永久配置
/etc/resolv.conf是一个链接，指向/run/resolvconf/resolv.conf，/run/resolvconf/resolv.conf的内容是由/etc/resolvconf/resolv.conf.d/base加载过去的。
参考：
①Ubuntu通过修改配置文件进行网络配置及ubuntu解决resolv.conf被重写问题和NetworkManager的关系；
②Debian DNS resolv.conf 重启失效/复写；
 
# 进入/etc/resolvconf/resolv.conf.d/文件夹
cd /etc/resolvconf/resolv.conf.d/
# 添加nameserver，也可以修改head或tail
vi /etc/resolvconf/resolv.conf.d/base
nameserver 8.8.8.8
# 刷新或重启
sudo resolvconf -u
sudo systemctl restart resolvconf
 
注：/etc/resolvconf/run/resolv.conf似乎也是指向/run/resolvconf/resolv.conf的链接，并且/etc/resolvconf/run/interface/systemd-resolved文件也会同时添加到/run/resolvconf/resolv.conf中，如果有大佬知道可以指点一下。
 
# 进入/etc/resolvconf/run/interface/文件夹
cd /etc/resolvconf/run/interface/
# 添加nameserver
vi /etc/resolvconf/run/interface/systemd-resolved
nameserver 8.8.8.8
# 刷新或重启
sudo resolvconf -u
sudo systemctl restart resolvconf