Our neighbour came by yesterday and told us our dog barks non-stop when we're not home. This is the first we've heard of it (have had him for 6 months), and were a little shocked to find out. We appreciate him telling us, and we obviously want to fix the problem.

Our dog is a mainly outside dog, and alone for about 6.5 hrs during the day. I walk him before I go to work, and DH walks him when gets home from work.
Has anyone had this problem and found a solution that works?