On RHEL 6, there have been issues where
yum check-update returns some errors, which might include an error that looks like the following:
https://mirrors.kernel.org/fedora-epel/6/x86_64/repodata/repomd.xml: [Errno -1] repomd.xml does not match metalink for epel
Trying other mirror.
To correct the error, you can try to update 'ca-certificates' with the 'epel' repo disabled:
yum --disablerepo=epel update ca-certificates
yum check-update again.