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http://repo.varnish-cache.org/redhat/varnish-3.0/el6/x86_64/repodata/repomd.xml
Diz que este repositório não está mais em uso.
Deb and RPM repositories now at Packagecloud The official packages for Debian, Ubuntu, CentOS and Red Hat are now located in repositories at Packagecloud.io. Users of repo.varnish-cache.org repositories will have to change their hosts over to the new repository configuration. For details see the below sections for the distribution in question.