Ubuntu Server apt-get出现'Failed to fetch'问题

在安装cloudera的过程中，遇到下面问题，原先网络是通的，后来才发现原来是网络不通了。
问题导读
1.使用apt-update失败,会出现什么现象？
2.什么情况下 sudo apt-key add -会失败？





问题1：Failed to fetch


使用apt-update失败

W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise/multiverse/binary-amd64/Packages  Something wicked happened resolving 'us.archive.ubuntu.com:http' (-5 - No address associated with hostname)

W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise/main/binary-i386/Packages  Something wicked happened resolving 'us.archive.ubuntu.com:http' (-5 - No address associated with hostname)

W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise/restricted/binary-i386/Packages  Something wicked happened resolving 'us.archive.ubuntu.com:http' (-5 - No address associated with hostname)

W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise/universe/binary-i386/Packages  Something wicked happened resolving 'us.archive.ubuntu.com:http' (-5 - No address associated with hostname)
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问题2：gpg: no valid OpenPGP data found.

$ curl -s http://archive.cloudera.com/cm5/ubuntu/precise/amd64/cm/archive.key | sudo apt-key add -
gpg: no valid OpenPGP data found.
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以上两个问题需要检查网络，不同的话则会发生上面两个问题