I was recently installing Oracle 19c RAC on a pair of servers running OEL8 UEK.

During the install of Grid Infrastructure the installer repeatedly failed the verification check stating that:

“[INS-06006] Passwordless SSH connectivity not set up between the following nodes(s)”

It is worth remembering that Oracle offers a relatively simple method to set up passwordless ssh connections between machines, which I had used prior to trying to install RAC:

In the Grid Infrastructure directory there is a script called sshUserSetup.sh