现象
centOS 7.5 yum安装提示找不到yum源，更换yum源为163依然不生效，报错
failure: repodata/repomd.xml from base: [Errno 256] No more mirrors to try.
http://mirrors.163.com/centos/7/os/x86_64/repodata/repomd.xml: [Errno 14] curl#7 - "Failed connect to 127.0.0.1:8118; 拒绝连接"
http://mirrors.163.com/centos/7/os/x86_64/repodata/repomd.xml: [Errno 14] curl#7 - "Failed connect to 127.0.0.1:8118; 拒绝连接"
http://mirrors.163.com/centos/7/os/x86_64/repodata/repomd.xml: [Errno 14] curl#7 - "Failed connect to 127.0.0.1:8118; 拒绝连接"
http://mirrors.163.com/centos/7/os/x86_64/repodata/repomd.xml: [Errno 14] curl#7 - "Failed connect to 127.0.0.1:8118; 拒绝连接"
http://mirrors.163.com/centos/7/os/x86_64/repodata/repomd.xml: [Errno 14] curl#7 - "Failed connect to 127.0.0.1:8118; 拒绝连接"
http://mirrors.163.com/centos/7/os/x86_64/repodata/repomd.xml: [Errno 14] curl#7 - "Failed connect to 127.0.0.1:8118; 拒绝连接"
http://mirrors.163.com/centos/7/os/x86_64/repodata/repomd.xml: [Errno 14] curl#7 - "Failed connect to 127.0.0.1:8118; 拒绝连接"
http://mirrors.163.com/centos/7/os/x86_64/repodata/repomd.xml: [Errno 14] curl#7 - "Failed connect to 127.0.0.1:8118; 拒绝连接"
http://mirrors.163.com/centos/7/os/x86_64/repodata/repomd.xml: [Errno 14] curl#7 - "Failed connect to 127.0.0.1:8118; 拒绝连接"
http://mirrors.163.com/centos/7/os/x86_64/repodata/repomd.xml: [Errno 14] curl#7 - "Failed connect to 127.0.0.1:8118; 拒绝连接"
观察127.0.0.1:8118，推测走了本地的代理
想起之前配置ss的时候设置了代理， /etc/profile
export http_proxy=http://127.0.0.1:8118
export https_proxy=http://127.0.0.1:8118
注释掉，source /etc/profile后
yum makecacke
依然报错
查看当前环境中的代理，发现代理依然存在
env | grep -i proxy
http_proxy=http://127.0.0.1:8118
https_proxy=http://127.0.0.1:8118
执行下面命令后生效
unset http_proxy
unset https_proxy
yum安装恢复正常
单纯注释/etc/profile并source之后，并不会把当前登陆用户的环境变量删除，必须手动unset掉，或者重新登陆。
