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

출처 : http://dokumentasifreebsd.blogspot.kr/2015/02/ssh-tunnel-in-30-seconds-mac-osx-linux.html?m=1

Launch an SSH tunnel

To initiate your SSH tunnel, simply [open Mac OSX / Linux Terminal][1] and connect to your remote server via SSH with the following flags:

ssh -D 8080 -C -N username@example.com

This will launch our SSH tunnel on port 8080 and route all traffic (securely) through the server at example.com.

Browse the Web with Your SSH Tunnel (Chrome)

Now, let’s start browsing the web using our new SSH tunnel.

Mac OSX:

1. Open Google Chrome
2. Select ‘Chrome’ up the top left
3. Select ‘Preferences’
4. Select ‘Show advanced settings…’
5. Select ‘Change proxy settings…’
6. Select ‘SOCKS Proxy’
7. Enter ’127.0.0.1′
8. Enter port ’8080′
9. Save changes by selecting ‘OK’

Fedora Linux:

1. Open Google Chrome
2. Select the wrench icon on the top right
3. Select ‘Settings’
4. Select ‘Show advanced settings…’
5. Select ‘Change proxy settings…’
6. Select ‘SOCKS Proxy’
7. Enter ’127.0.0.1′
8. Enter port ’8080′
9. Save changes by selecting ‘OK’

Search Google for ‘my ip’ and take a look at what your IP address is now. Cool right? 

Exiting the SSH Tunnel

To exit the SSH tunnel, simply disable the SOCKS proxy within your browser.

Hope this helps, let me know if you have any suggestions in the comments below!

You can define that you want to keep the connection alive in the ssh configuration. You have to edit (create if it does not exist):

~/.ssh/config

Add these two lines to it:

ServerAliveInterval 300
ServerAliveCountMax 36

The value ServerAliveInterval defines the interval in seconds between two noop operations that keeps to connection alive. It is 5 minutes in this case. 
The value ServerAliveCountMax defines the number of times the noop is sent. In this case the connection will be kept open for 3 hours.