ubuntu系统上systemd-resolved不使用127.0.0.53的方法

有些文章说用关闭（删除）systemd-resolved ，而使用 resolvconf 、openresolv、Unbound 替换，测试发现在ubuntu 20.04.2 LTS NetworkManager已经被 systemd-networkd 取代，复杂度（依赖关系）较高。所以研究依然保持使用systemd-resolved而不使用127.0.0.53的配置思路。

后来发现systemd-resolved会产出两个运行配置，如下

/run/systemd/resolve/resolv.conf
/run/systemd/resolve/stub-resolv.conf

默认/etc/resolv.conf 软链接到/run/systemd/resolve/stub-resolv.conf，那么只需将/etc/resolv.conf 软链接到/run/systemd/resolve/resolv.conf

而/run/systemd/resolve/* 生成的配置文件是由/etc/systemd/resolved.conf 决定，于是就写了下边的配置脚本，放到服务器上一执行，确实已经不是指向127.0.0.53了。

mkdir -p /etc/systemd/resolved.conf.d/
cat >/etc/systemd/resolved.conf.d/99-dns.conf << EOF
[Resolve]
DNS=114.114.114.114 8.8.8.8
EOF
ln -s -f /run/systemd/resolve/resolv.conf /etc/resolv.conf
systemctl daemon-reload && systemctl restart systemd-resolved.service && systemctl status -l systemd-resolved.service --no-pager
cat /etc/resolv.conf

主要参考了这篇

下文是讲解了为啥systemd-resolved要搞127.0.0.53:53

How and why (not) to use the 127.0.0.53 nameserver, systemd-resolved and resolvctl Or what is systemd-resolved and how it is integrated on Ubuntu

既然不用127.0.0.53了，那么我们能直接关闭127.0.0.53:53吗？

mkdir -p /etc/systemd/resolved.conf.d/
cat >/etc/systemd/resolved.conf.d/98-disable-127-53.conf << EOF
[Resolve]
DNSStubListener=no
EOF
systemctl daemon-reload && systemctl restart systemd-resolved.service && systemctl status -l systemd-resolved.service --no-pager

执行后再ss -tunlp 查看53端口，127.0.0.53:53监听已经没了。

k8s相关

另发现kubeadm(v1.21.3)安装的kubelet 默认会使用 /run/systemd/resolve/resolv.conf，也可以手工更改，配置( /var/lib/kubelet/config.yaml)的说明在https://kubernetes.io/docs/reference/config-api/kubelet-config.v1beta1/

查看resolvConf字段的文档发现Default: “/etc/resolv.conf”，那为什么kubeadm安装完后会变成/run/systemd/resolve/resolv.conf呢？此处暂留有疑问(20210923)

cat /var/lib/kubelet/config.yaml | grep resolv
resolvConf: /run/systemd/resolve/resolv.conf

查了下kubeadm init 的日志，发现kubeadm-init阶段的各个操作

其中kubelet-start是负责Write a file with KubeletConfiguration and an environment file with node specific kubelet settings, and then (re)start kubelet.

经过查询代码，代码，kubeadm是对环境作了默认的检测，从而得到了一份可用的配置。而二进制的安装就需要在/var/lib/kubelet/config.yaml自己指定下，决不能使用127.0.0.53:53的resolv.conf

cat /var/lib/kubelet/config.yaml
apiVersion: kubelet.config.k8s.io/v1beta1
resolvConf: /run/systemd/resolve/resolv.conf

争夺/etc/resolv.conf控制权

推荐阅读—> 米开杨重新夺回对 /etc/resolv.conf 的控制权

• https://man7.org/linux/man-pages/man5/resolv.conf.5.html