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在ubuntu更新时，出现错误E: Some index files failed to download.They have been ignored, or old ones used instead.
在sudo apt-get update的时候出现了下列问题：

root@0cc8c10d217a:# sudo apt-get update

sudo: unable to resolve host 0cc8c10d217a

Err http://archive.ubuntu.com trusty InRelease

  

Err http://archive.ubuntu.com trusty-security InRelease

  

Err http://archive.ubuntu.com trusty-updates InRelease

  

Err http://archive.ubuntu.com trusty-proposed InRelease

  

Err http://archive.ubuntu.com trusty-backports InRelease

  

Err http://archive.ubuntu.com trusty Release.gpg

  Could not resolve ‘archive.ubuntu.com’

Err http://archive.ubuntu.com trusty-security Release.gpg

  Could not resolve ‘archive.ubuntu.com’

Err http://archive.ubuntu.com trusty-updates Release.gpg

  Could not resolve ‘archive.ubuntu.com’

Err http://archive.ubuntu.com trusty-proposed Release.gpg

  Could not resolve ‘archive.ubuntu.com’

Err http://archive.ubuntu.com trusty-backports Release.gpg

  Could not resolve ‘archive.ubuntu.com’

Reading package lists… Done

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty/InRelease  

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W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty-security/InRelease  

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty-updates/InRelease  

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty-proposed/InRelease  

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty-backports/InRelease  

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty/Release.gpg  Could not resolve ‘archive.ubuntu.com’

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty-security/Release.gpg  Could not resolve ‘archive.ubuntu.com’

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty-updates/Release.gpg  Could not resolve ‘archive.ubuntu.com’

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty-proposed/Release.gpg  Could not resolve ‘archive.ubuntu.com’

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty-backports/Release.gpg  Could not resolve ‘archive.ubuntu.com’

W: Some index files failed to download. They have been ignored, or old ones used instead.

root@0cc8c10d217a:# apt-get install  nginx

Reading package lists… Done

Building dependency tree       

Reading state information… Done

The following extra packages will be installed:

  nginx-common nginx-core

Suggested packages:

  fcgiwrap nginx-doc

The following NEW packages will be installed:

  nginx nginx-common nginx-core

0 upgraded, 3 newly installed, 0 to remove and 241 not upgraded.

Need to get 349 kB of archives.

After this operation, 1,300 kB of additional disk space will be used.

Do you want to continue? [Y/n] Y

WARNING: The following packages cannot be authenticated!

  nginx-common nginx-core nginx

Install these packages without verification? [y/N] Y

Err http://archive.ubuntu.com/ubuntu/ trusty-updates/main nginx-common all 1.4.6-1ubuntu3.7

  Could not resolve ‘archive.ubuntu.com’

Err http://archive.ubuntu.com/ubuntu/ trusty-updates/main nginx-core amd64 1.4.6-1ubuntu3.7

  Could not resolve ‘archive.ubuntu.com’

Err http://archive.ubuntu.com/ubuntu/ trusty-updates/main nginx all 1.4.6-1ubuntu3.7

  Could not resolve ‘archive.ubuntu.com’

E: Failed to fetch http://archive.ubuntu.com/ubuntu/pool/main/n/nginx/nginx-common_1.4.6-1ubuntu3.7_all.deb  Could not resolve ‘archive.ubuntu.com’

E: Failed to fetch http://archive.ubuntu.com/ubuntu/pool/main/n/nginx/nginx-core_1.4.6-1ubuntu3.7_amd64.deb  Could not resolve ‘archive.ubuntu.com’

E: Failed to fetch http://archive.ubuntu.com/ubuntu/pool/main/n/nginx/nginx_1.4.6-1ubuntu3.7_all.deb  Could not resolve ‘archive.ubuntu.com’

E: Unable to fetch some archives, maybe run apt-get update or try with –fix-missing?

因为刚刚换过apt-get源 ，所以百度了一下知道大概的做法是DNS出现了问题

应着重查看以下方面：
cat /etc/resolv.conf查看里面的domain, search和nameserver配置是否正确。
然后一看是DNS出现了问题，所以将nameserver修改成8.8.8.8后，问题得到了解决。