Interestingly, if I try a yum update on one of my other boxes I get similar errors. However, it then proceeds to complete the yum update successfully [root at ollie2 ~]# yum update Loaded plugins: fastestmirror, langpacks Determining fastest mirrors Could not get metalink https://mirrors.fedoraproject.org/metalink?repo=epel-7&arch=x86_64 error was 14: curl#60 - "Peer's Certificate issuer is not recognized." Could not retrieve mirrorlist https://mirror.webtatic.com/yum/el7/x86_64/mirrorlist error was 14: curl#60 - "Peer's Certificate issuer is not recognized." * base: mirror.as29550.net * epel: ftp-stud.hs-esslingen.de * extras: mozart.ee.ic.ac.uk * updates: mirror.vorboss.net * webtatic: uk.repo.webtatic.com base | 3.6 kB 00:00:00 http://download.owncloud.org/download/repositories/10.0/CentOS_7/repodata/repomd.xml: [Errno 14] HTTPS Error 302 - Found Trying other mirror. extras | 3.4 kB 00:00:00 google-chrome | 1.3 kB 00:00:00 https://rpm.nodesource.com/pub_6.x/el/7/x86_64/repodata/repomd.xml: [Errno 14] curl#60 - "Peer's Certificate issuer is not recognized." Trying other mirror. It was impossible to connect to the CentOS servers. This could mean a connectivity issue in your environment, such as the requirement to configure a proxy, or a transparent proxy that tampers with TLS security, or an incorrect system clock. You can try to solve this issue by using the instructions on https://wiki.centos.org/yum-errors If above article doesn't help to resolve this issue please use https://bugs.centos.org/. https://download.postgresql.org/pub/repos/yum/9.6/redhat/rhel-7-x86_64/repodata/repomd.xml: [Errno 14] curl#60 - "Peer's Certificate issuer is not recognized." Trying other mirror. updates | 3.4 kB 00:00:00 https://uk.repo.webtatic.com/yum/el7/x86_64/repodata/repomd.xml: [Errno 14] curl#60 - "Peer's Certificate issuer is not recognized." Trying other mirror. https://us-east.repo.webtatic.com/yum/el7/x86_64/repodata/repomd.xml: [Errno 14] curl#60 - "Peer's Certificate issuer is not recognized." Trying other mirror. (1/3): google-chrome/primary | 1.7 kB 00:00:00 (2/3): extras/7/x86_64/primary_db | 215 kB 00:00:00 (3/3): updates/7/x86_64/primary_db | 7.4 MB 00:00:05 google-chrome 3/3 Resolving Dependencies