Linux SSH Tunnel

This an example of the correct syntax

ssh -l username_ssh ip_firewall -L local_port:remote_pc:remote_port

For example: if you want to connect from your linux client to a remote pc via VNC through a linux firewall
you can use this command

firewall ip is: 72.35.34.2
firewall username is : vncext
remote pc ip is: 192.168.0.23
vnc port is: 5900
local port is: 5900

ssh -l vncext 72.35.34.2 -L 5900:192.168.0.23:5900

after this command you must insert the vncext password for 72.35.34.2

and now you can connect to remote vnc simply use this command

xvnc4viewer 127.0.0.1