配置Oracle VPS的防火墙时遇到了问题，所以顺便看一下。用的时间太长了，这种岔开的细事要先不求甚解。
netfilter-persistent.service 做了什么
graph TD;
z[?]-->a
a["/lib/systemd/system/netfilter-persistent.service"] --> b[`/usr/sbin/netfilter-persistent start`
`/usr/sbin/netfilter-persistent stop`] b --> |default config| c1["/etc/default/netfilter-persistent"] b --> c2["/usr/share/netfilter-persistent/plugins.d"] c2 --> d1["./15-ip4tables"] c2 --> d2["./25-ip6tables"] d1 --> e[`iptables-restore < /etc/iptables/rules.v4`
`iptables-save > /etc/iptables/rules.v4`
`iptables -F`
...
] d2 --> e
`/usr/sbin/netfilter-persistent stop`] b --> |default config| c1["/etc/default/netfilter-persistent"] b --> c2["/usr/share/netfilter-persistent/plugins.d"] c2 --> d1["./15-ip4tables"] c2 --> d2["./25-ip6tables"] d1 --> e[`iptables-restore < /etc/iptables/rules.v4`
`iptables-save > /etc/iptables/rules.v4`
`iptables -F`
...
] d2 --> e
netfilter-persistent.service 是如何启动的
tldr systemd-analyze 列出了四个命令，
➜ ~ tldr systemd-analyze
systemd-analyze
Show timing details about the boot process of units (services, mount points, devices, sockets).
- List time of each unit to start up:
systemd-analyze blame
- Print a tree of the time critical chain of units:
systemd-analyze critical-chain
- Create an SVG file showing when each system service started, highlighting the time that they spent on initialization:
systemd-analyze plot > {{path/to/file.svg}}
- Plot a dependency graph and convert it to an SVG file:
systemd-analyze dot | dot -T{{svg}} > {{path/to/file.svg}}
用后两个命令生成图片，用 scp ubuntu@168.138.53.159:/home/ubuntu/dependency.svg .命令复制到本地查看。
依赖图看吐了，图是用 Graphviz 生成的，也可以用Graphviz过滤掉不要的，太晚了不弄了，源头是“*.slice”。至于它是怎么启动的就没再查下去了，有机会的话系统地看下 systemd1。生成依赖图的源文件是2021-05-25-services-dependency.dot。
- 系统地分析 systemd。