apt-get update丢失key

一些主机商的源码包并非标准版本，在运行apt-get update时候会提示
W: There is no public key available for the following key IDs:
9D6D8F6BC857C906
W: There is no public key available for the following key IDs:
7638D0442B90D010

之类的错误
解决办法：
运行apt-get install debian-keyring debian-archive-keyring
apt-get update
即可。