env
A 10.21.32.106 gdut728 目标服务器，处于内网
B 123.123.123.123 root 外网服务器，相当于桥梁的作用
step:
on A:
ssh -fCNR 7280:localhost:22 root@123.123.123.123
on B:
ssh -fCNL *:1234:localhost:7280 localhost
on any:
ssh -p1234 gdut728@123.123.123.123
auto ssh (depri)
autossh -M 7281 -fCNR 7280:localhost:22 root@123.123.123.123
auto ssh service
mv autossh.service autossh-ssh.service
vim autossh-ssh.service
systemctl enable autossh-ssh.service
service autossh-ssh start
service autossh-ssh status
Refer: