1、编辑 /etc/systemd/system/rc-local.service，没有这个文件就创建一个：

sudo touch /etc/systemd/system/rc-local.service

2、然后编辑文件：

sudo vi /etc/systemd/system/rc-local.service

[Unit]
Description=/etc/rc.local Compatibility
ConditionPathExists=/etc/rc.local

[Service]
Type=forking
ExecStart=/etc/rc.local start
TimeoutSec=0
StandardOutput=tty
RemainAfterExit=yes
SysVStartPriority=99

[Install]
WantedBy=multi-user.target

3、创建/etc/rc.local文件：

sudo touch /etc/rc.local

4、编辑/etc/rc.local文件：

sudo vi /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.
/bin/su -c '/usr/bin/autossh -M 1234 -NfR 12345:localhost:22 seven@xxx.xxx.xxx.xxx -p22' - seven
exit 0

5、添加权限，使能服务：

sudo chmod +x /etc/rc.local
sudo systemctl enable rc-local
sudo systemctl start rc-local.service
sudo systemctl status rc-local.service