在yum仓库里新建文件dag.repo，内容如下

# cat /etc/yum.repos.d/dag.repo
[dag]
name=Dag RPM Repository for Red Hat Enterprise Linux
baseurl=http://apt.sw.be/redhat/el$releasever/en/$basearch/dag
gpgcheck=1
enabled=1


# yum repolist
Loaded plugins: fastestmirror, priorities
Loading mirror speeds from cached hostfile
130 packages excluded due to repository priority protections
repo id      repo name                                               status
base         CentOS-5 - Base                                              2,791
dag          Dag RPM Repository for Red Hat Enterprise Linux         11,479+130
extras       CentOS-5 - Extras                                              272
updates      CentOS-5 - Updates                                             342
repolist: 14,884

可以看出，确实增加了好多软件包，安装AVS必备的编码器、解码器都不在话下了

yum -y install mencoder mplayer flvtool2 yamdi ffmpeg --nogpgcheck 

如果提示Header V3 DSA signature: NOKEY, key ID 6b8d79e6

原因：yum安装了旧版本的GPG keys

解决方法：

输入下面命令

rpm --import /etc/pki/rpm-gpg/RPM*

补充：

可以安装官方RPM来增加DAG源

https://wiki.centos.org/AdditionalResources/Repositories/RPMForge/

国内RPMForge源

### Name: RPMforge RPM Repository for RHEL 6 - dag
### URL: http://rpmforge.net/
[rpmforge]
name = RHEL $releasever - RPMforge.net - dag
baseurl = http://mirror.bjtu.edu.cn/repoforge/redhat/el6/en/$basearch/rpmforge
mirrorlist = http://mirrorlist.repoforge.org/el6/mirrors-rpmforge
#mirrorlist = file:///etc/yum.repos.d/mirrors-rpmforge
enabled = 1
protect = 0
gpgkey = file:///etc/pki/rpm-gpg/RPM-GPG-KEY-rpmforge-dag
gpgcheck = 1


[rpmforge-extras]
name = RHEL $releasever - RPMforge.net - extras
baseurl = http://mirror.bjtu.edu.cn/repoforge/redhat/el6/en/$basearch/extras
mirrorlist = http://mirrorlist.repoforge.org/el6/mirrors-rpmforge-extras
#mirrorlist = file:///etc/yum.repos.d/mirrors-rpmforge-extras
enabled = 1
protect = 0
gpgkey = file:///etc/pki/rpm-gpg/RPM-GPG-KEY-rpmforge-dag
gpgcheck = 1


[rpmforge-testing]
name = RHEL $releasever - RPMforge.net - testing
baseurl = http://mirror.bjtu.edu.cn/repoforge/redhat/el6/en/$basearch/testing
mirrorlist = http://mirrorlist.repoforge.org/el6/mirrors-rpmforge-testing
#mirrorlist = file:///etc/yum.repos.d/mirrors-rpmforge-testing
enabled = 0
protect = 0
gpgkey = file:///etc/pki/rpm-gpg/RPM-GPG-KEY-rpmforge-dag
gpgcheck = 1

EPEL详细说明

https://fedoraproject.org/wiki/EPEL/zh-cn

直接说重点：

CentOS 用户可以直接通过 yum install epel-release 安装并启用 EPEL 源

rpm -Uvh http://dl.fedoraproject.org/pub/epel/6/i386/epel-release-6-8.noarch.rpm
rpm -Uvh http://li.nux.ro/download/nux/dextop/el6/x86_64/nux-dextop-release-0-2.el6.nux.noarch.rpm
yum update
yum  install vlc

参考文章：

yum --import 增加资源库 

Centos6.0使用第三方YUM源（EPEL，RPMForge，RPMFusion）