Let A and B be two distinct points on the line $L:\frac{{x - 6}}{3} = \frac{{y - 7}}{2} = \frac{{z - 7}}{{ - 2}}$. Both A and B are at a distance $2\sqrt {17}$ from the foot of perpendicular drawn from the point (1,2,3) on the line L. If O is the origin, then $\overrightarrow {OA} \cdot \overrightarrow {OB}$ is equal to