Voxel engine lighting

Hey all,

I'm currently working on a voxel shader and I have chunks with meshes that can be renderen already. I want to use a custom shader. The shader has to render the vertices with the colors set for them. This works. But I also want to have some shadows cast by higher blocks. For example:

 []      <- This block has to cast a shadow on the top of
   []    <- this block

Does anyone know how I can achieve this effect?

This is the shader:

 Shader "VoxelShader" {
 Properties {
     _Color ("Main Color", COLOR) = (1,1,1,1)
 }
 SubShader {
     Pass {
         Fog { Mode Off }
         CGPROGRAM
         // Upgrade NOTE: excluded shader from DX11 and Xbox360; has structs without semantics (struct appdata members vertex,color)
         #pragma exclude_renderers d3d11 xbox360
         // Upgrade NOTE: excluded shader from OpenGL ES 2.0 because it does not contain a surface program or both vertex and fragment programs.
         #pragma exclude_renderers gles
         // Upgrade NOTE: excluded shader from Xbox360; has structs without semantics (struct appdata members vertex,color)
         #pragma exclude_renderers xbox360
         #pragma vertex vert
         
         // vertex input: position, color
         struct appdata {
             float4 vertex;
             float4 color;
         };
         
         struct v2f {
             float4 pos : POSITION;
             float4 color : COLOR;
         };
         v2f vert (appdata v) {
             v2f o;
             o.pos = mul( UNITY_MATRIX_MVP, v.vertex );
             o.color = v.color;
             return o;
         }
         ENDCG
     }
 }
 }