No login page for intranet users, require login for internet/external users

[Egils]

Greetings,

I have an application that needs to be accessed by intranet and
internet users.

Login screen needs to be displayed when application is accessed by
Internet users.

Login screen needs to be bypassed when application is accessed by
valid intranet users. Security checking cannot be bypassed for
intranet users. Not every intranet user will have access to the
system.

In another words - user code and password needs to be prompted for
outside users but user code for inside users needs to be picked up
from network. Password for outside users will be validated against
user code table. Password for inside users will not be validated
however their user code will be validated against user code table to
allow them access to application.

IIS must allow anonymous login for the web-site for two reasons:
- majority of internet users will not have domain account
- when anonymous login is disabled browser shows pop-up login screen;
application login screen should be displayed instead

Any suggestions how to pick up network user code without disabeling
anonymous login at IIS level?

Egils