Wall following is a variation of path following and causes boids to move along the surface of a geometric shape.
At each simulation step boids will look along their current direction and determine if they are likely to collide with any geometry. If a potential collision is detected they will compute a new target a predetermined distance away from the surface of the geometry along the surface normal. They will then alter their steering to aim for that location as their new target.
The same technique can also be used to ensure boids remain inside a geometric surface, and this type of behaviour is known as Containment.