apt-cacher is an APT proxy, i.e. a HTTP proxy intended for Debian packages. It is one of n APT proxies in the archive, but benefits include being easy to set up, the ability to be used as a proxy (rather than having to specify it as a mirror) and having the possibility to import packages, e.g. from a previously installed system.

1) Install apt-cacher, i.e. aptitude install apt-cacher

2) Configure apt-cacher, unless you have an upstream HTTP proxy or you want to restrict access to apt-cacher, you only need to make the following change to /etc/apt-cacher/apt-cacher.conf:

Find the index_files_regexp line, uncomment it and add |\.key|\.msg before the closing parenthesis, e.g.:

This is necessary because the installer downloads a .key and a .msg file using the proxy. Apt-cacher is only intended to be used as an APT proxy, and so rejects non-APT URLs.

3) Boot the netinstall CD. Choose the menu option you want, but don't press enter; instead press TAB and then add the text "mirror/http/proxy=" to the kernel command line. Now press enter. The installer will boot and use your apt-cacher proxy. Substitute for the IP of your apt-cacher proxy.