甲骨文云实例可以免费开启IPv6，在IPv4逐渐紧张的情况下，必须先做好准备，以防万一甲骨文云像亚马逊一样取消免费的IPv4。甲骨文云在面板操作分配IPv6后（参考），需要登录系统运行dhclient -6获取地址，而且重启后不能自动获取IPv6，需要重新运行该命令。本文旨在解决该问题。

原理

开机自启动功能，debian系已经停用rc.local，切换到 systemd 了，但是对于一些简单的脚本或者懒人来说，添加命令到 /etc/rc.local 文件更方便。自从 Debian 9 开始，Debian 默认不带 /etc/rc.local 文件，而 rc.local 服务却还是自带的：

root@debian ~ # cat /lib/systemd/system/rc-local.service
#  SPDX-License-Identifier: LGPL-2.1-or-later
#
#  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.
# This unit gets pulled automatically into multi-user.target by
# systemd-rc-local-generator if /etc/rc.local is executable.
[Unit]
Description=/etc/rc.local Compatibility
Documentation=man:systemd-rc-local-generator(8)
ConditionFileIsExecutable=/etc/rc.local
After=network.target

[Service]
Type=forking
ExecStart=/etc/rc.local start
TimeoutSec=0
RemainAfterExit=yes
GuessMainPID=no

默认情况下这个服务是关闭的状态：

root@debian ~ # systemctl status rc-local
● rc-local.service - /etc/rc.local Compatibility
     Loaded: loaded (/lib/systemd/system/rc-local.service; static)
    Drop-In: /usr/lib/systemd/system/rc-local.service.d
             └─debian.conf
     Active: inactive (dead)
       Docs: man:systemd-rc-local-generator(8)

为了解决这个问题，我们需要手工添加一个 /etc/rc.local 文件：

cat <<EOF >/etc/rc.local
#!/bin/sh -e
#
# rc.local
#
# This script is executed at the end of each multiuser runlevel.
# Make sure that the script will "exit 0" on success or any other
# value on error.
#
# In order to enable or disable this script just change the execution
# bits.
#
# By default this script does nothing.

exit 0
EOF

然后赋予权限：

chmod +x /etc/rc.local

接着启动 rc-local 服务：

systemctl enable --now rc-local

此时可能会弹出警告：

The unit files have no installation config (WantedBy=, RequiredBy=, Also=,
Alias= settings in the [Install] section, and DefaultInstance= for template
units). This means they are not meant to be enabled using systemctl.
 
Possible reasons for having this kind of units are:
• A unit may be statically enabled by being symlinked from another unit's
  .wants/ or .requires/ directory.
• A unit's purpose may be to act as a helper for some other unit which has
  a requirement dependency on it.
• A unit may be started when needed via activation (socket, path, timer,
  D-Bus, udev, scripted systemctl call, ...).
• In case of template units, the unit is meant to be enabled with some
  instance name specified.

无视警告，因为这个服务没有任何依赖的系统服务，只是开机启动 /etc/rc.local 脚本而已。

再次查看状态：

root@debian ~ # systemctl status rc-local.service 
● rc-local.service - /etc/rc.local Compatibility
     Loaded: loaded (/lib/systemd/system/rc-local.service; enabled-runtime; vendor preset: enabled)
    Drop-In: /usr/lib/systemd/system/rc-local.service.d
             └─debian.conf
     Active: active (exited) since Thu 2022-01-27 18:52:43 UTC; 10s ago
       Docs: man:systemd-rc-local-generator(8)
    Process: 541 ExecStart=/etc/rc.local start (code=exited, status=0/SUCCESS)
        CPU: 3ms

Jan 27 18:52:43 debian systemd[1]: Starting /etc/rc.local Compatibility...
Jan 27 18:52:43 debian systemd[1]: Started /etc/rc.local Compatibility.

至此可以使用rc.local开机自启动命令或脚本。

自动获取IPv6地址

现在就可以把获取IPv6的命令 sudo dhclient -6添加到 /etc/rc.local 文件，丢在 exit 0 前面即可，并尝试reboot后运行ip a可见：

opc@debian:~$ ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host noprefixroute
       valid_lft forever preferred_lft forever
2: ens3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq state UP group default qlen 1000
    link/ether 02:00:17:01:7f:e0 brd ff:ff:ff:ff:ff:ff
    altname enp0s3
    inet 10.0.0.65/24 brd 10.0.0.255 scope global ens3
       valid_lft forever preferred_lft forever
    inet6 2***:****:****:****:****:****:****:abcd/128 scope global dynamic
       valid_lft 87767sec preferred_lft 84167sec
    inet6 fe80::17ff:fe01:7fe0/64 scope link
       valid_lft forever preferred_lft forever

实现了开机自动获取IPv6地址。