Drawing camera on material

Hi.
I’m attempting to create something simulator to a portal gun but I have not found a way to draw a camera on a object.
I want a camera to be drawn on the portal so that you can look throw it. I don’t know if that is the best way to do it but if you know how to do it I would appreciate if you told me.
If you know of better way to do this than please help me.

if you have Unity Pro than you should use Render Textures. If not you can chack out FXLab on the Asset Store or check this thread out http://forum.unity3d.com/threads/225075-Volumetric-scope-scripting