How to set up Apt caching server on Ubuntu or Debian – Xmodulo

After following this guide (which was easy) there look to be a few caveats

stick to the stable release as well as stable repos or you may start getting lots of 403 errors while trying to find local mirrors.

it took me a while to realise because i thought it was just a volatile repo but disabling the proxy on custom installs or weird builds fixed that problem.