Hit http://ftp.us.debian.org squeeze/contrib amd64 Packages
Hit http://download.proxmox.com squeeze/pve amd64 Packages
Fetched 1,672 B in 0s (2,041 B/s)
W: GPG error: http://ftp.us.debian.org squeeze Release: The following signatures
were invalid: BADSIG AED4B06F473041FA Debian Archive Automatic Signing Key (6.0/squeeze) <ftpmaster@debian.org>
这个GPG erro之前经常遇到。先找到解决方法：
为了解决这个问题，你需要从文件夹/var/lib/apt/lists删除ftp.us.debian.org 文件。
#rm -f /var/lib/apt/lists/ftp.us.debian.org*
#aptitude update