How to fix yum errors on CentOS, RHEL or Fedora

The post explains that yum errors on Red Hat-based systems often come from stale metadata, broken cache files, network connection problems, or another process holding the yum lock. For each case, it gives a concrete command or setting change to restore package installs and searches.

Reading notes

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• 404 repository errors can appear when downloaded yum metadata has become obsolete; the fix is to clean metadata or clear the whole yum cache.
• Connection failures such as “network is unreachable” or “couldn’t connect to host” may happen when yum runs behind a proxy that is not configured in yum.conf.
• Metadata checksum errors are also linked to outdated metadata, and the suggested fix is to clean yum metadata.
• Yum lock errors on Fedora are caused by PackageKit automatically starting and holding the yum lock; the page says to disable automatic update checks in the software update preferences.
• Repository database read errors can happen when yum is interrupted while downloading a repository database, leaving an incomplete cached file; the fix is to clean yum metadata.
• Repository metadata read errors such as “Cannot find a valid baseurl for repo” can be fixed by running yum clean all.