Near/far clipping plane


I am trying out this tutorial: Infinite ground plane using GLSL shaders · martin-pr/possumwood Wiki · GitHub

But I really cannot figure out how to get the “vec3 iNearPositionVert” and “vec3 iFarPositionVert”.
I guess it is the near and far plane of the camera frustum, but how can I find it with openGL or directly in the shader?