[L] Cannot retrieve repository metadata (repomd.xml) for repository: Dropbox.

这个问题比较蛋疼，因为是中国特色。。解决方法：

1. sudo gedit /etc/yum.repos.d/dropbox.repo
2.
original:

[Dropbox]
name=Dropbox Repository
baseurl=http://linux.dropbox.com/fedora/$releasever/
gpgkey=http://linux.dropbox.com/fedora/rpm-public-key.asc

updated:

[Dropbox]
name=Dropbox Repository
baseurl=https://linux.dropbox.com/fedora/15/
gpgkey=https://linux.dropbox.com/fedora/rpm-public-key.asc

还不行的话，只能怪自己了。。
BY THE WAY, you should have dropbox-dist first. If you don't, download it.

就第一次需要repo,之后记得去yum repo 那里删掉，不然每次yum 有你好受的。

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Any way, there is another way.

http_proxy=http://.....
export http_proxy