refs:
http://unix.stackexchange.com/questions/2544/how-to-work-around-release-file-expired-problem-on-a-local-mirror
apt-get update 过期,
E: Release file for http://ftp.at.debian.org/debian-backports/dists/squeeze-backports/Release is expired (invalid since 1d 4h 40min 18s). Updates for this repository will not be applied.
等待源修复，或者暂时不检查有效性.
sudo apt-get -o Acquire::Check-Valid-Until=false update