1) modify '/etc/ssh/sshd_config' as follows

TCPKeepAlive yes

GatewayPorts yes

ClientAliveInterval 60

ClientAliveCountMax 999999999

2) based on "external_server -- firewall -- internal_server" structure,

we can run following command on internal_server

$ sudo ssh -fN -R *:7777:localhost:22 id@external_server_ip -p 2222

where sshd port for external_server_ip is 2222 and port 7777 can be any available ports on external_server. 

3) connection.

on external_server, we can run following command to connect internal_server

$ ssh localhost -p 7777