错误信息

 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
 
Cannot retrieve metalink for repository: epel/x86_64. Please verify its path and try again

解决方法

1. 将/etc/yum.repos.d/epel.repo（若此文件不存在，修改对应路径下 CenteOS-Base.repo 文件）中的 mirrorlist 行注释（存在多处），baseurl 行取消注释（存在多处）。

[root@ec-cache ~]# /usr/bin/vi /etc/yum.repos.d/epel.repo
 
[epel]
name=Extra Packages for Enterprise Linux 7 - $basearch
baseurl=http://download.fedoraproject.org/pub/epel/7/$basearch
#mirrorlist=https://mirrors.fedoraproject.org/metalink?repo=epel-7&arch=$basearch
failovermethod=priority
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7
 
[epel-debuginfo]
name=Extra Packages for Enterprise Linux 7 - $basearch - Debug
baseurl=http://download.fedoraproject.org/pub/epel/7/$basearch/debug
#mirrorlist=https://mirrors.fedoraproject.org/metalink?repo=epel-debug-7&arch=$basearch
failovermethod=priority
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7
gpgcheck=1
 
[epel-source]
name=Extra Packages for Enterprise Linux 7 - $basearch - Source
baseurl=http://download.fedoraproject.org/pub/epel/7/SRPMS
#mirrorlist=https://mirrors.fedoraproject.org/metalink?repo=epel-source-7&arch=$basearch
failovermethod=priority
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7
gpgcheck=1

2. /etc/resolv.conf  文件中增加两行：（主要作用：网络配置，可以联通外网）

nameserver 8.8.8.8
nameserver 114.114.114.114

说明：

nameserver 114.114.114.114 是国内移动、电信和联通通用的 DNS 服务系统，解析成功率相对来说更高，国内用户使用的比较多，速度相对快、稳定，是国内用户上网常用的DNS；

nameserver 8.8.8.8 是谷歌公司提供的 DNS，该地址是全球通用的，相对来说，更适合国外以及访问国外网站的用户使用。

3.此时 ping www.baidu.com 可以连接外网，先通过 yum update 命令更新下，然后使用 yum install 继续安装软件。