由于网关地址改变没有及时更新配置，造成无法下载
failure: repodata/repomd.xml from base: [Errno 256] No more mirrors to try
http://mirrors.neusoft.edu.cn/centos/7.6.1810/os/x86_64/Packages/tftp-5.2-22.el7.x86_64.rpm: [Errno -1] Package does not match intended download. Suggestion: run yum --enablerepo=base clean metadata
Trying other mirror.
当时不知道原因，执行 yum clean all 后 出现以下错误
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:
Cannot find a valid baseurl for repo: base/7/x86_64
当出现以上问题的时候，可以检查一下网关配置