yum update输出地址unresolvable，无法解析，修改/etc/resolv.conf
nameserver 114.114.114.114
过了，但是仍然报下面的问题：
Loaded plugins: product-id, search-disabled-repos, subscription-manager
This system is not registered with an entitlement server. You can use subscription-manager to register.
http://mirror.centos.org/centos/7Server/os/x86_64/repodata/repomd.xml: [Errno 14] HTTP Error 404 - Not Found
Trying other mirror.
To address this issue please refer to the below knowledge base article
https://access.redhat.com/articles/1320623
If above article doesn't help to resolve this issue please open a ticket with Red Hat Support.
One of the configured repositories failed (CentOS-7Server - Base),
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. Run the command with the repository temporarily disabled
yum --disablerepo=base ...
4. Disable the repository permanently, 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 base
or
subscription-manager repos --disable=base
5. 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=base.skip_if_unavailable=true
failure: repodata/repomd.xml from base: [Errno 256] No more mirrors to try.
http://mirror.centos.org/centos/7Server/os/x86_64/repodata/repomd.xml: [Errno 14] HTTP Error 404 - Not Found
yum repolist all
发现有个disable的，删除之，继续yum update仍然碰壁，
继续，
一般可以认为是repo的问题了，repo内容如下：
[base]
name=CentOS-$releasever - Base
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
#released updates
[updates]
name=CentOS-$releasever - Updates
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/extras/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
去掉baseurl的注释：
[base]
name=CentOS-$releasever - Base
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra
baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
#released updates
[updates]
name=CentOS-$releasever - Updates
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates&infra=$infra
baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras&infra=$infra
baseurl=http://mirror.centos.org/centos/$releasever/extras/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus&infra=$infra
baseurl=http://mirror.centos.org/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
仍然不行，但是隐约有种预感，快解决了
go on
网页上打开这个baseurl发现打不开，404 not found，仔细一看url明显有问题，里边怎么还有环境变量呢？
去掉环境变量，basearch=x86_64
[base]
name=CentOS-7 - Base
mirrorlist=http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=$infra
baseurl=http://mirror.centos.org/centos/7/os/x86_64/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
#released updates
[updates]
name=CentOS-7 - Updates
mirrorlist=http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=updates&infra=$infra
baseurl=http://mirror.centos.org/centos/7/updates/x86_64/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
#additional packages that may be useful
[extras]
name=CentOS-7 - Extras
mirrorlist=http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=extras&infra=$infra
baseurl=http://mirror.centos.org/centos/7/extras/x86_64/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
#additional packages that extend functionality of existing packages
ok了，不过又出其他错误：
Retrieving key from file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
GPG key retrieval failed: [Errno 14] curl#37 - "Couldn't open file /etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7"
看来是一些私钥认证啥的没过，
这个问题是因为我的repo里边配置了gpgkey=file，这样yum会到本地去找，这样肯定找不到啊，所以打开官网，拷贝url链接，修改repo
[base]
name=CentOS-7 - Base
mirrorlist=http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=$infra
baseurl=http://mirror.centos.org/centos/7/os/x86_64/
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/7/os/x86_64/RPM-GPG-KEY-CentOS-7
#released updates
[updates]
name=CentOS-7 - Updates
mirrorlist=http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=updates&infra=$infra
baseurl=http://mirror.centos.org/centos/7/updates/x86_64/
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/7/os/x86_64/RPM-GPG-KEY-CentOS-7
#additional packages that may be useful
[extras]
name=CentOS-7 - Extras
mirrorlist=http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=extras&infra=$infra
baseurl=http://mirror.centos.org/centos/7/extras/x86_64/
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/7/os/x86_64/RPM-GPG-KEY-CentOS-7
#additional packages that extend functionality of existing packages
成功。哎，不能给我个能用的repo么，累死...
竟然还没有结束。。。。，又报了这么个鬼：
Running transaction check
ERROR You need to update rpm to handle:
rpmlib(PayloadIsZstd) <= 5.4.18-1 is needed by tinyproxy-1.10.0-4.fc31.x86_64
RPM needs to be updated
You could try running: rpm -Va --nofiles --nodigest
Your transaction was saved, rerun it with:
yum load-transaction /tmp/yum_save_tx.2019-09-05.04-18.pJXiGd.yumtx
说我rpm比较老，安装一下epel然后安装的时候会自动给我升级rpm
rpm -ivh https://dl.fedoraproject.org/pub/epel/7/x86_64/Packages/e/epel-release-7-11.noarch.rpm
这次是真的搞定了