I draw the skybox first thing, with GL_DEPTH_TEST disabled. That works fine, but is probably inefficient. Drawing the skybox last may be better as huge parts of it may be occluded.
The first attempt was to move skybox to the viewing distance, the far plane in the frustum. This was not good as some parts of the skybox would come outside of the far clipping plane when I turn around. But if I move the skybox closer, it will start to occlude other geometry.
I restored the skybox to the viewing distance, and tried to disable GL_DEPTH_CLAMP, but that had no effect. The part of the skybox outside of the frustum far plane was still cut-off.
I get the feeling I am missing something basic here. Any advice would be appreciated.