环境：Ubuntu 14.04
使用apt-get update出现如下问题：
Fetched 11.8 MB in 26s (448 kB/s)
W: GPG error: http://security.debian.org wheezy/updates InRelease: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 9D6D8F6BC857C906 NO_PUBKEY 8B48AD6246925553
W: GPG error: http://http.debian.net wheezy-updates Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 8B48AD6246925553 NO_PUBKEY 7638D0442B90D010
W: Failed to fetch http://packages.couchbase.com/ubuntu/dists/trusty/InRelease Unable to find expected entry ‘precise/main/binary-amd64/Packages’ in Release file (Wrong sources.list entry or malformed file)
W: Failed to fetch http://http.debian.net/debian/dists/wheezy/Release.gpg Connection failed
解决方法，增加公共的key:
sudo apt-key adv –keyserver keyserver.ubuntu.com –recv-keys 9D6D8F6BC857C906 8B48AD6246925553 7638D0442B90D010 6FB2A1C265FFB764