  The boundary conditions for the Fokker-Planck equations, forward and backward
ones are directly derived from the Chapman-Kolmogorov equation for
M-dimensional region with boundaries. The boundaries are assumed, in addition,
to be able to absorb wandering particles or to give rise to fast surface
transport. It is demonstrated that the boundaries break down the symmetry of
random walks in their vicinity, leading to the boundary singularities in the
corresponding kinetic coefficients. Eliminating these singularities we get the
desired boundary conditions. As it must be the boundary condition for the
forward Fokker-Planck equation matches the mass conservation.
