在对一台 Debian 服务器执行更新时，发生如下错误：
# apt-get update
Hit http://security.debian.org wheezy/updates Release.gpg
Hit http://security.debian.org wheezy/updates Release
Hit http://security.debian.org wheezy/updates/main i386 Packages
Hit http://security.debian.org wheezy/updates/contrib i386 Packages
Hit http://security.debian.org wheezy/updates/non-free i386 Packages
Hit http://security.debian.org wheezy/updates/contrib Translation-en
Hit http://security.debian.org wheezy/updates/main Translation-en
Hit http://security.debian.org wheezy/updates/non-free Translation-en
Hit http://ftp.debian.org wheezy Release.gpg
Hit http://ftp.debian.org wheezy Release
Hit http://ftp.debian.org wheezy/main i386 Packages
Hit http://ftp.debian.org wheezy/contrib i386 Packages
Hit http://ftp.debian.org wheezy/non-free i386 Packages
Hit http://ftp.debian.org wheezy/contrib Translation-en
Hit http://ftp.debian.org wheezy/main Translation-en
Hit http://ftp.debian.org wheezy/non-free Translation-en
Reading package lists... Done
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 在网上搜索了一下，找到相关文档：
http://www.linuxquestions.org/questions/debian-26/there-is-no-public-key…
其中介绍了二种方法，一种方法如下：
# gpg --recv-keys 4D270D06F42584E6
# gpg --export 4D270D06F42584E6 | apt-key add -另一种方法似乎更为简单：
# apt-get install debian-keyring debian-archive-keyring
# apt-key update尝试了第二种方法，成功解决了 public key 的问题。
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