Crie o Repositório dag.repo
#vim /etc/yum.repos.d/dag.repo
Após vamos adicionar a o repositório
[dag]
name=Dag RPM Repository for Red Hat Enterprise Linux
baseurl=http://apt.sw.be/redhat/el$releasever/en/$basearch/dag
gpgcheck=1
gpgkey=http://dag.wieers.com/rpm/packages/RPM-GPG-KEY.dag.txt
enabled=1
Após Salva e Sair do arquivos
#yum install rrdtool