These configurations are typically called
???pinholes?????”a very restrictive peek from outside to inside.
Many routers offer preconfigured pinholes??”such as one for pcAnywhere, a
remote-control program that lets someone ???drive??? the keyboard and mouse of a computer
and see what is happening on the screen, which is very useful for technical
support. Similar preconfigured pinholes may exist for Internet phone, or you can configure
customized pinholes for webcams, radio communications services, and almost
anything else you can imagine.
One critical aspect of using pinholes instead of opening your firewall to general
application use that lets traffic flow to many computers (described earlier in the project,
and typical for multiplayer games) is that for a pinhole, the request coming from
outside will be directed to one and only one computer on the inside.
For a pinhole to be configured and work, you must know the IP address of the
specific computer that you want to allow the data to get to, and you must set a fixed
IP address on that computer instead of using dynamic addressing. You must also
know the TCP or UDP data ports the pinhole will move data through. Applications
that require pinholes typically document all of these factors for you??”all you need to
do is type them in.
Figure 11-5
Configuring a pinhole
for a specific application
to a specific computer
inside your network
80 Project 11
Step 5: Allow Unfiltered Access to One Computer
There may be a rare occasion when you want to expose one computer to the whole
unfiltered, unprotected Internet without exposing your entire local network??”this is
called creating a ???demilitarized zone,??? or DMZ.
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