I was wondering if you could do in through Apache by <Listen>ing on both the external and internal IP then set up a virtual host for each ip address. Add Apache auth to the external host. 
Listen xxx.xxx.xxx.xxx:80
Listen 192.168.xxx.xxx:80

<VirtualHost xxx.xxx.xxx.xxx:80>
   <Location />
      AuthType Basic
      ...
   </Location>
</VirtualHost>

<VirtualHost 192.168.xxx.xxx:80>
   <Location />
      #AuthType Basic
      ...
   </Location>
</VirtualHost>

[download]
(untested and refers to Apache 2.0)

In reply to Re^2: Discriminating between local and remote IP's (Apache ACLs) by mbeast
in thread Discriminating between local and remote IP's by Avitar

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