[UPHPU] Bi-directional Communication Through a Firewall

[Kyle Waters]

It's important to understand Firefox, Quake, etc are NOT doing 
anything.   The issue is who's initiating the connection.  Your router 
is working the magic.

When you pull up Firefox on your computer the request is then sent the 
router which then repackages in a special way so it will be able to tell 
which computer sent the reply when it gets a response back.  This is 
called Network Address Translation(NAT).  So you pull up Firefox on your 
computer it sends the request to the router.  The router repackages it 
and sends it on to "the interenet"  it reaches google, google replies 
your router.  Your router than repackages the reply and send it to the 
appropriate computer based off of what information in the reply.

So let's say want to set up a web app on your side of the router, and 
you want computers on the other side of the router to be able to use 
it.  What ip address do you give to people on the other side of the 
router?  After all the ips your computer is using is only valid with in 
that network(It's all part of the whole nat thing).  If you go to 
whatismyip.com you will see a completely different ip then if you run 
ifconfig.  So if you give people the ip address of the router(which is 
the one you will get from whatismyip.com) then when they type the IP 
address into their browsers they are sent to your router.  Your router 
doesn't know what to do since this is not a reply to a packet it sent.  
So it has no idea which computer to send the packet to.

I hope this helps you understand what is going on.


Kyle