问题
CentOS 8 yum安装软件时，提示无法从AppStream下载
[root@C8-3 ~]# yum -y install httpd mariadb-server mariadb php php-mysql
Repository AppStream is listed more than once in the configuration
Repository extras is listed more than once in the configuration
Repository PowerTools is listed more than once in the configuration
Repository centosplus is listed more than once in the configuration
Repository fasttrack is listed more than once in the configuration
CentOS-8 - AppStream 0.0 B/s | 0 B 00:08
Failed to download metadata for repo 'AppStream'
Error: Failed to download metadata for repo 'AppStream'
先排除网络问题
检查网通不通，然后确定DNS解析是否正确。
[root@C8-3 ~]# ping baidu.com
PING baidu.com (220.181.38.148) 56(84) bytes of data.
64 bytes from 220.181.38.148 (220.181.38.148): icmp_seq=1 ttl=128 time=12.1 ms
64 bytes from 220.181.38.148 (220.181.38.148): icmp_seq=2 ttl=128 time=12.2 ms
64 bytes from 220.181.38.148 (220.181.38.148): icmp_seq=3 ttl=128 time=11.7 ms
64 bytes from 220.181.38.148 (220.181.38.148): icmp_seq=4 ttl=128 time=13.5 ms
64 bytes from 220.181.38.148 (220.181.38.148): icmp_seq=5 ttl=128 time=12.2 ms
64 bytes from 220.181.38.148 (220.181.38.148): icmp_seq=6 ttl=128 time=12.1 ms
64 bytes from 220.181.38.148 (220.181.38.148): icmp_seq=7 ttl=128 time=13.9 ms
64 bytes from 220.181.38.148 (220.181.38.148): icmp_seq=8 ttl=128 time=11.4 ms
64 bytes from 220.181.38.148 (220.181.38.148): icmp_seq=9 ttl=128 time=13.9 ms
^C
--- baidu.com ping statistics ---
9 packets transmitted, 9 received, 0% packet loss, time 31ms
rtt min/avg/max/mdev = 11.394/12.554/13.949/0.899 ms
有可能是所在网络环境在出口封了相应端口，需用其他方式，比如VPN解决。
更换国内yum源
我们无法使用CentOS原生yum源就是因为网络问题，你懂的。
为了快，更换国内yum源，推荐阿里云yum或者腾讯yum。网易没落了。
阿里yum源： https://developer.aliyun.com/mirror/centos
腾云yum源： https://mirrors.tencent.com/help/centos.html
[root@C8-3 ~]# mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup.3
[root@C8-3 ~]# wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.cloud.tencent.com/repo/centos8_base.repo
--2020-11-02 20:51:42-- http://mirrors.cloud.tencent.com/repo/centos8_base.repo
Resolving mirrors.cloud.tencent.com (mirrors.cloud.tencent.com)... 111.231.36.190
Connecting to mirrors.cloud.tencent.com (mirrors.cloud.tencent.com)|111.231.36.190|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 1792 (1.8K) [application/octet-stream]
Saving to: ‘/etc/yum.repos.d/CentOS-Base.repo’
/etc/yum.repos.d/Ce 100%[==================>] 1.75K --.-KB/s in 0s
2020-11-02 20:51:46 (65.6 MB/s) - ‘/etc/yum.repos.d/CentOS-Base.repo’ saved [1792/1792]
清理之前的yum缓存
[root@C8-3 ~]# yum clean all
Repository AppStream is listed more than once in the configuration
Repository extras is listed more than once in the configuration
Repository PowerTools is listed more than once in the configuration
Repository centosplus is listed more than once in the configuration
Repository fasttrack is listed more than once in the configuration
18 files removed
建立新的缓存
[root@C8-3 ~]# yum makecache
Repository AppStream is listed more than once in the configuration
Repository extras is listed more than once in the configuration
Repository PowerTools is listed more than once in the configuration
Repository centosplus is listed more than once in the configuration
Repository fasttrack is listed more than once in the configuration
CentOS-8 - AppStream 0.0 B/s | 0 B 00:04
Failed to download metadata for repo 'AppStream'
Error: Failed to download metadata for repo 'AppStream'
makecache的时候报错了！
解决makecahe报错
Failed to download metadata for repo 'AppStream'
Error: Failed to download metadata for repo 'AppStream' # 下载meta信息失败
问题的原因
先看看CentOS7的yum.repos.d
[21:19:17 root@C7-1 ~]#ll /etc/yum.repos.d/
total 44
-rw-r--r--. 1 root root 2523 Oct 27 22:47 CentOS-Base.repo
-rw-r--r--. 1 root root 1664 Sep 5 2019 CentOS-Base.repo.bak
-rw-r--r--. 1 root root 1309 Sep 5 2019 CentOS-CR.repo
-rw-r--r--. 1 root root 649 Sep 5 2019 CentOS-Debuginfo.repo
-rw-r--r--. 1 root root 314 Sep 5 2019 CentOS-fasttrack.repo
-rw-r--r--. 1 root root 630 Sep 5 2019 CentOS-Media.repo
-rw-r--r--. 1 root root 1331 Sep 5 2019 CentOS-Sources.repo
-rw-r--r--. 1 root root 6639 Sep 5 2019 CentOS-Vault.repo
-rw-r--r--. 1 root root 951 Oct 2 2017 epel.repo
-rw-r--r--. 1 root root 1050 Oct 2 2017 epel-testing.repo
再看看CentOS8的yum.repos.d
[root@C8-3 ~]# ll /etc/yum.repos.d/
total 56
-rw-r--r--. 1 root root 815 Oct 28 02:33 CentOS-AppStream.repo
-rw-r--r--. 1 root root 1792 Mar 25 2020 CentOS-Base.repo
-rw-r--r--. 1 root root 712 Jan 2 2020 CentOS-Base.repo.backup
-rw-r--r--. 1 root root 1792 Mar 25 2020 CentOS-Base.repo.backup.3
-rw-r--r--. 1 root root 798 Jan 2 2020 CentOS-centosplus.repo
-rw-r--r--. 1 root root 1043 Jan 2 2020 CentOS-CR.repo
-rw-r--r--. 1 root root 668 Jan 2 2020 CentOS-Debuginfo.repo
-rw-r--r--. 1 root root 756 Jan 2 2020 CentOS-Extras.repo
-rw-r--r--. 1 root root 338 Jan 2 2020 CentOS-fasttrack.repo
-rw-r--r--. 1 root root 738 Jan 2 2020 CentOS-HA.repo
-rw-r--r--. 1 root root 928 Jan 2 2020 CentOS-Media.repo
-rw-r--r--. 1 root root 736 Jan 2 2020 CentOS-PowerTools.repo
-rw-r--r--. 1 root root 1382 Jan 2 2020 CentOS-Sources.repo
-rw-r--r--. 1 root root 74 Jan 2 2020 CentOS-Vault.repo
CentOS 8 多了一个 CentOS-AppStream.repo
这是出问题的关键。
查看一下CentOS-AppStream.repo
[root@C8-3 ~]# cat /etc/yum.repos.d/CentOS-AppStream.repo.bac
# CentOS-AppStream.repo
#
# The mirror system uses the connecting IP address of the client and the
# update status of each mirror to pick mirrors that are updated to and
# geographically close to the client. You should use this for CentOS updates
# unless you are manually picking other mirrors.
#
# If the mirrorlist= does not work for you, as a fall back you can try the
# remarked out baseurl= line instead.
#
#
[AppStream]
name=CentOS-$releasever - AppStream
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=AppStream&infra=$infra
baseurl=http://mirror.centos.org/$contentdir/$releasever/AppStream/$basearch/os/
#mirrorlist=http://mirror.centos.org/$contentdir/$releasever/AppStream/$basearch/os/
gpgcheck=1
enabled=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial
解决问题的方法
我们在替换yum源的时候，按照镜像网站的操作方式，只替换了CentOS-Base.repo。
可是在CentOS8中，CentOS-AppStream.repo的内容没有更新。
解决方法有两种
直接删除掉CentOS-AppStream.repo这个文件。（不推荐）
将CentOS-AppStream.repo文件中的链接更新
解决问题
查看跟新后的CentOS-Base.repo文件
[root@C8-3 ~]# cat /etc/yum.repos.d/CentOS-Base.repo
# CentOS-Base.repo
#
# The mirror system uses the connecting IP address of the client and the
# update status of each mirror to pick mirrors that are updated to and
# geographically close to the client. You should use this for CentOS updates
# unless you are manually picking other mirrors.
#
# If the mirrorlist= does not work for you, as a fall back you can try the
# remarked out baseurl= line instead.
#
#
[BaseOS]
name=Qcloud centos OS - $basearch
baseurl=http://mirrors.cloud.tencent.com/centos/$releasever/BaseOS/$basearch/os/
enabled=1
gpgcheck=1
gpgkey=http://mirrors.cloud.tencent.com/centos/RPM-GPG-KEY-CentOS-Official
[centosplus]
name=Qcloud centosplus - $basearch
baseurl=http://mirrors.cloud.tencent.com/centos/$releasever/centosplus/$basearch/os/
enabled=0
gpgcheck=1
gpgkey=http://mirrors.cloud.tencent.com/centos/RPM-GPG-KEY-CentOS-Official
[extras]
name=Qcloud centos extras - $basearch
baseurl=http://mirrors.cloud.tencent.com/centos/$releasever/extras/$basearch/os/
enabled=1
gpgcheck=1
gpgkey=http://mirrors.cloud.tencent.com/centos/RPM-GPG-KEY-CentOS-Official
[fasttrack]
name=Qcloud centos fasttrack - $basearch
baseurl=http://mirrors.cloud.tencent.com/centos/$releasever/fasttrack/$basearch/os/
enabled=0
gpgcheck=1
gpgkey=http://mirrors.cloud.tencent.com/centos/RPM-GPG-KEY-CentOS-Official
[AppStream]
name=Qcloud centos AppStream - $basearch
baseurl=http://mirrors.cloud.tencent.com/centos/$releasever/AppStream/$basearch/os/
enabled=0
gpgcheck=1
gpgkey=http://mirrors.cloud.tencent.com/centos/RPM-GPG-KEY-CentOS-Official
[PowerTools]
name=Qcloud centos PowerTools - $basearch
baseurl=http://mirrors.cloud.tencent.com/centos/$releasever/PowerTools/$basearch/os/
enabled=0
gpgcheck=1
gpgkey=http://mirrors.cloud.tencent.com/centos/RPM-GPG-KEY-CentOS-Official
替换更新
找到跟新后CentOS-Base.repo中[AppStream]标签内的内容并复制
[AppStream]
name=Qcloud centos AppStream - $basearch
baseurl=http://mirrors.cloud.tencent.com/centos/$releasever/AppStream/$basearch/os/
enabled=0
gpgcheck=1
gpgkey=http://mirrors.cloud.tencent.com/centos/RPM-GPG-KEY-CentOS-Official
打开CentOS-AppStream.repo，注释掉原有内容，并插入新内容
[root@C8-3 ~]# vim /etc/yum.repos.d/CentOS-AppStream.repo
#[AppStream]
#name=CentOS-$releasever - AppStream
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=AppStream&infra=$infra
#baseurl=http://mirror.centos.org/$contentdir/$releasever/AppStream/$basearch/os/
#mirrorlist=http://mirror.centos.org/$contentdir/$releasever/AppStream/$basearch/os/
#gpgcheck=1
#enabled=1
#pgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial
[AppStream]
name=Qcloud centos AppStream - $basearch
baseurl=http://mirrors.cloud.tencent.com/centos/$releasever/AppStream/$basearch/os/
enabled=0
gpgcheck=1
gpgkey=http://mirrors.cloud.tencent.com/centos/RPM-GPG-KEY-CentOS-Official
我们要养成修改内容时使用注释而不是删除的操作方法，用来对比前后差异，出问题了取消掉注释就可以了。
问题解决
重新运行yum makecache，操作成功
[root@C8-3 ~]# yum makecache
Repository AppStream is listed more than once in the configuration
Repository extras is listed more than once in the configuration
Repository PowerTools is listed more than once in the configuration
Repository centosplus is listed more than once in the configuration
Repository fasttrack is listed more than once in the configuration
Qcloud centos OS - x86_64 119 kB/s | 2.2 MB 00:19
Qcloud centos extras - x86_64 477 B/s | 8.1 kB 00:17
Metadata cache created.
