Install APT caching proxy

apt-get install -y apt-cacher-ng

# config
/etc/apt-cacher-ng/acng.conf

# restart service
/etc/init.d/apt-cacher-ng restart

# configure offline mode
echo "Offlinemode: 1" >> /etc/apt-cacher-ng/acng.conf
service apt-cacher-ng restart

# web interface
http://YOUR_IP:3142

# add to preseed config
d-i mirror/http/proxy string http://apt-cacher:3142/

# configure APT to use proxy
echo 'Acquire::http::Proxy "http://YOUR_IP:3142/";' >> /etc/apt/apt.conf.d/01proxy
apt-get update

# clear ?
/usr/lib/apt-cacher-ng/distkill.pl

# fix hash sum mismatch
rm -r /var/cache/apt-cacher-ng/