CentOS 7 修改源

One of the configured repositories failed (Unknown),
and yum doesn't have enough cached data to continue. At this point the only
safe thing yum can do is fail. There are a few ways to work "fix" this:

1. Contact the upstream for the repository and get them to fix the problem.

2. Reconfigure the baseurl/etc. for the repository, to point to a working
upstream. This is most often useful if you are using a newer
distribution release than is supported by the repository (and the
packages for the previous distribution release still work).

3. Disable the repository, so yum won't use it by default. Yum will then
just ignore the repository until you permanently enable it again or use
--enablerepo for temporary usage:

yum-config-manager --disable <repoid>

4. Configure the failing repository to be skipped, if it is unavailable.
Note that yum will try to contact the repo. when it runs most commands,
so will have to try and fail each time (and thus. yum will be be much
slower). If it is a very temporary problem though, this is often a nice
compromise:

yum-config-manager --save --setopt=<repoid>.skip_if_unavailable=true

1.备份原有yum源：

 mv /etc/yum.repos.d /etc/yum.repos.d.bak

2.创建yum源目录

 mkdir /etc/yum.repos.d

3.下载阿里云源配置

 wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo

4.重建缓存

 #yum clean all          

#yum makecache

　　

如果 yum  update  出现 Loaded plugins: fastestmirror解决办法

1.修改插件配置文件

# vim  /etc/yum/pluginconf.d/fastestmirror.conf

[main]
enabled=0    //由 1 改成0 ，禁用该插件
verbose=0
always_print_best_host = true
socket_timeout=3
#  Relative paths are relative to the cachedir (and so works for users as well
# as root).
hostfilepath=timedhosts.txt
maxhostfileage=10
maxthreads=15
#exclude=.gov, facebook
#include_only=.nl,.de,.uk,.ie

2.修改yum 配置文件　　

# vim /etc/yum.conf
9 plugins=0  //不使用插件

3.清理缓存并重新构建

# yum clean all 
# yum makecache