Ubuntu更新后每次启动需手动执行sudo dhclient -v才能恢复DNS功能的求助

Hey there, sorry to hear you're stuck with this annoying DNS glitch after updating Ubuntu—let’s work through this step by step to get DHCP handling DNS automatically on boot, no more manual commands needed.

• 检查NetworkManager的DHCP配置
  首先确认你的网络连接是完全依赖DHCP获取DNS的：

  1. 打开系统设置里的网络选项，找到你正在使用的有线/无线连接，进入「IPv4设置」
  2. 确保「方法」选择的是「Automatic (DHCP)」，并且「DNS」栏没有手动添加的地址，同时勾选「自动」选项让DHCP全权处理DNS获取
  3. 如果习惯用命令行，也可以用nmcli配置：
     • 先查看当前连接的配置：nmcli con show <你的连接名称>（比如nmcli con show "Wired connection 1"）
     • 重置DHCP相关设置：sudo nmcli con mod <连接名称> ipv4.method auto ipv4.dns "" ipv4.ignore-auto-dns no
     • 重启NetworkManager生效：sudo systemctl restart NetworkManager

• 确保dhclient开机自动运行
  系统更新可能意外禁用了dhclient的开机服务，检查并修复：

  1. 查看dhclient服务状态：sudo systemctl status dhclient.service
  2. 如果显示inactive（未激活），启用并启动服务：sudo systemctl enable --now dhclient.service
  3. 要是你的系统同时装了dhcpcd服务，可能会和dhclient冲突，建议停用：sudo systemctl stop dhcpcd.service && sudo systemctl disable dhcpcd.service

• 排查systemd-resolved的兼容性问题
  现在很多Ubuntu版本用systemd-resolved管理DNS，可能和dhclient存在冲突：

  1. 先查看服务状态：sudo systemctl status systemd-resolved
  2. 编辑/etc/systemd/resolved.conf，确保以下配置（注释掉手动DNS设置）：

     [Resolve]
     #DNS=
     #FallbackDNS=
     DNSSEC=no
     

  3. 重启服务：sudo systemctl restart systemd-resolved
  4. 还可以让NetworkManager对接systemd-resolved，编辑/etc/NetworkManager/NetworkManager.conf，添加或修改：

     [main]
     dns=systemd-resolved
     

     之后重启NetworkManager：sudo systemctl restart NetworkManager

• 清除旧DHCP租约文件
  残留的旧租约可能导致DNS获取异常，尝试清理：

  1. 先备份租约文件：sudo cp /var/lib/dhcp/dhclient.leases /var/lib/dhcp/dhclient.leases.bak
  2. 删除原文件：sudo rm /var/lib/dhcp/dhclient.leases
  3. 重启系统，让DHCP重新获取新的租约和DNS地址

Try these steps one by one, and let me know if any of them resolve your issue—fingers crossed we can get that DNS working seamlessly on boot again!

备注：内容来源于stack exchange，提问作者Apollo