I looked at a lot of resources, but I couldn't find any useful information. Either repos are broken or prepug problems...
I/O warning : failed to load external entity "/usr/share/openscap/xsl/security-guide.xsl" compilation error: file /usr/share/preupgrade/xsl/preup.xsl line 40 element import xsl:import : unable to load /usr/share/openscap/xsl/security-guide.xsl I/O warning : failed to load external entity "/usr/share/openscap/xsl/oval-report.xsl" compilation error: file /usr/share/preupgrade/xsl/preup.xsl line 41 element import xsl:import : unable to load /usr/share/openscap/xsl/oval-report.xsl I/O warning : failed to load external entity "/usr/share/openscap/xsl/sce-report.xsl" compilation error: file /usr/share/preupgrade/xsl/preup.xsl line 42 element import xsl:import : unable to load /usr/share/openscap/xsl/sce-report.xsl OpenSCAP Error:: Could not parse XSLT file '/usr/share/preupgrade/xsl/preup.xsl' [oscapxml.c:416] Unable to open file /root/preupgrade/result.html Usage: preupg [options]
preupg: error: [Errno 2] No such file or directory: '/root/preupgrade/result.html' [root@localhost upgrade]# yum localinstall redhat-upgrade-tool-0.7.22-3.el6.centos.noarch.rpm Failed to set locale, defaulting to C Loaded plugins: fastestmirror Setting up Local Package Process Examining redhat-upgrade-tool-0.7.22-3.el6.centos.noarch.rpm: 1:redhat-upgrade-tool-0.7.22-3.el6.centos.noarch redhat-upgrade-tool-0.7.22-3.el6.centos.noarch.rpm: does not update installed package. Nothing to do [root@localhost upgrade]# rpm --import http://ftp.plusline.de/centos/7.0.1406/os/x86_64/RPM-GPG-KEY-CentOS-7 curl: (22) The requested URL returned error: 404 Not Found error: http://ftp.plusline.de/centos/7.0.1406/os/x86_64/RPM-GPG-KEY-CentOS-7: import read failed(2). [root@localhost upgrade]# [root@localhost upgrade]# rpm --import http://isoredirect.centos.org/centos/7/os/x86_64/RPM-GPG-KEY-CentOS-7 curl: (22) The requested URL returned error: 404 Not Found error: http://isoredirect.centos.org/centos/7/os/x86_64/RPM-GPG-KEY-CentOS-7: import read failed(2).
Sadly, the upgrade path from Centos 6.x to 7.x has been broken since shortly after 7.x was released, with no fixes to preupg in sight - and at this point, it seems unlikely the official route will work. Further, I don't know of ANY unofficial route that is proven to work well, either.
The only real and trusted upgrade path from Centos 6x to 7x is to install 7 onto fresh hardware/vm and migrate services over.
This surprises folk coming from other distros where, whilst not being trivial, it is usually achievable to upgrade between major versions in-situ, but this does not seem to be the Centos way.