My phone started going crazy with alerts from EM12c, apparently all of my agents had lost communication with OMS. I immediately started checking all of the logs on OMS, not finding anything of interest I decided to try an upload from one of the host:
./emctl upload agent
WARN – Ping communication error
o.s.emSDK.agent.comm.exception.ConnectException [Failure connecting to https://HOST.domain.com:4889/empbs/upload , err host.domain.com]
Can’t find host host.domain.com
Interesting, but makes sense after all the only agent able to communicate to the OMS actually resides on the same host.
After an email exchange with the Linux administration some changes had occurred in DNS that needed to be reverted. Once that had been completed the ping worked and all agents showed as up.