The following signatures could't be verified because the public key
is not available: NO_PUBKEY XXXX
解决方法是：
以root权限运行：
#gpg --keyserver wwwkeys.eu.pgp.net --recv-keys XXXX
#gpg --armor --export XXXX | apt-key add -
=============
apt-get install debian-keyring debian-archive-keyring
apt-key update
apt-get update
