My load balancer (a Rackspace Cloud Load Balancer instance) adds the Via header with these values:
Via 1.1 localhost.localdomain
What does this mean and is it harmful?
Note: According to another question Google also serves this in production.
Load balancers are effectively proxies, so they pass the "Via" header along so your web server can record it for statistics. Otherwise, all traffic would be appearing from the load balancer instead of individual clients.
The "localhost.localdomain" should be configurable in the balancer itself.