用YUM安装程序出现错误~

bijmh

安装任何文件都出现下列错误，请高手们帮帮，没有YUM太不方便。谢谢~~
Repository at-stable already added, not adding again
Repository dag already added, not adding again
Repository dries already added, not adding again
Repository extras already added, not adding again
Repository updates-testing already added, not adding again
Repository base already added, not adding again
Repository livna already added, not adding again
Setting up Install Process
Setting up repositories
dries                     100% |=========================|  951 B    00:00
kde-redhat-stable-all     100% |=========================|  951 B    00:00
ftp://mirrors.ftp.sciences.univ- ... epodata/repomd.xml: [Errno 4] IOError: [Errno ftp error] 550 at-good: No such file or directory
Trying other mirror.
ftp://ftp-stud.fht-esslingen.de/ ... epodata/repomd.xml: [Errno 4] IOError: [Errno ftp error] 550 Failed to change directory.
Trying other mirror.
Cannot open/read repomd.xml file for repository: at-good
failure: repodata/repomd.xml from at-good: [Errno 256] No more mirrors to try.