yum contacting www.redhat.com

[Andrew Diamond]

Why does yum contact www.redhat.com each time it is run? I maintain a
local yum repository so there is no requirement to access any external
hosts. I also require all web traffic to go via a non-transparent proxy
and without setting the http_proxy environment variable, yum just sits
there and does nothing. 

How do I turn this off and if anyone can explain why it would need to
talk to www.redhat.com that would also be great.

Thanks,
  Andrew