以前安装gcc好像都很顺畅，但是今天不知道咋回事在一个centos系统的虚拟机上使用yum命令安装gcc的时候一直报错。
我执行的安装指令是：yum -y install gcc gcc-c++ kernel-devel
报错内容（部分）：
Error Downloading Packages:
mpfr-2.4.1-6.el6.x86_64: failure: Packages/mpfr-2.4.1-6.el6.x86_64.rpm from base: [Errno 256] No more mirrors to try.
libgomp-4.4.7-17.el6.x86_64: failure: Packages/libgomp-4.4.7-17.el6.x86_64.rpm from base: [Errno 256] No more mirrors to try.
gcc-c++-4.4.7-17.el6.x86_64: failure: Packages/gcc-c++-4.4.7-17.el6.x86_64.rpm from base: [Errno 256] No more mirrors to try.
libgcc-4.4.7-17.el6.x86_64: failure: Packages/libgcc-4.4.7-17.el6.x86_64.rpm from base: [Errno 256] No more mirrors to try.
cpp-4.4.7-17.el6.x86_64: failure: Packages/cpp-4.4.7-17.el6.x86_64.rpm from base: [Errno 256] No more mirrors to try.
libstdc++-4.4.7-17.el6.x86_64: failure: Packages/libstdc++-4.4.7-17.el6.x86_64.rpm from base: [Errno 256] No more mirrors to try.
gcc-4.4.7-17.el6.x86_64: failure: Packages/gcc-4.4.7-17.el6.x86_64.rpm from base: [Errno 256] No more mirrors to try.
cloog-ppl-0.15.7-1.2.el6.x86_64: failure: Packages/cloog-ppl-0.15.7-1.2.el6.x86_64.rpm from base: [Errno 256] No more mirrors to try.
ppl-0.10.2-11.el6.x86_64: failure: Packages/ppl-0.10.2-11.el6.x86_64.rpm from base: [Errno 256] No more mirrors to try.
libstdc++-devel-4.4.7-17.el6.x86_64: failure: Packages/libstdc++-devel-4.4.7-17.el6.x86_64.rpm from base: [Errno 256] No more mirrors to try.
看报错的大意就是说，镜像找不到，即源没有找到；然后我就到网上搜了下，看可以用下面方法解决：
[Errno 256] No more mirrors to try. 得知这可能是错误的缓存源导致，直接两个命令解决：
yum clean all
yum makecache
但是我这里执行到yum makecache 这个指令的时候还是又报错如下：
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=6&arch=x86_64&repo=os error was
14: PYCURL ERROR 6 - "Couldn't resolve host 'mirrorlist.centos.org'"
Error: Cannot retrieve repository Metadata (repomd.xml) for repository: base. Please verify its path and try again
[root@app softback]# yum -y install gcc gcc-c++ kernel-devel
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=6&arch=x86_64&repo=os error was
14: PYCURL ERROR 6 - "Couldn't resolve host 'mirrorlist.centos.org'"
Error: Cannot retrieve repository Metadata (repomd.xml) for repository: base. Please verify its path and try again
[root@app softback]# yum install gcc-c++ libstdc++-devel
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=6&arch=x86_64&repo=os error was
14: PYCURL ERROR 6 - "Couldn't resolve host 'mirrorlist.centos.org'"
Error: Cannot retrieve repository Metadata (repomd.xml) for repository: base. Please verify its path and try again
国内服务器在运行命令yum -y install wget的时候会出现这种问题，
这种错误，是因为没有配置/etc/resolv.conf。
解决方法：编辑resolv.conf文件，添加：nameserver 8.8.8.8
然后保存退出即可。
然后在执行yum makecache 指令，等该指令结束后再执行安装gcc指令，就可以安装成功了!
