wiki:UpgradingChroots

Build chroots will auto-upgrade themselves when doing a build, but due to the ephemeral nature of build chroots, they won't retain those changes. The source chroots should be periodically upgraded manually to avoid needless upgrades of build-essential and other core packages for each package build. This can be done as root on the build server with all-schroots upgrade-schroot.

Note: The chroots talk to an Approx cache on the build server (localhost:9999) for their APT repositories. This cache uses mirrors.mit.edu as its source for Debian and Ubuntu (modulo security updates). If mirrors.mit.edu breaks, you can edit /etc/approx/approx.conf appropriately on the build server, and run approx-update (as root). If approx breaks, build chroots can be re-created by setting the MIRRORHOST variabel prior to running make-chroot. http://$MIRRORHOST/ubuntu (or /debian) must be a valid APT repo.