Using Google Container Engine (GKE) with Container Optimized Images the external interfaces (ephemeral IPs) seem to be listening on ports 110, 143, 993, 995, etc. as demonstrated via a port scan with
Nmap scan report for 236.185.xxx.xx.bc.googleusercontent.com (xx.xx.185.236) Host is up (0.0025s latency). Not shown: 65529 filtered ports PORT STATE SERVICE 22/tcp open ssh 110/tcp open pop3 143/tcp open imap 993/tcp open imaps 995/tcp open pop3s 3389/tcp closed ms-wbt-server
I can connect via telnet but ports provide no response:
$telnet 35.192.xxx.xxx 993 Trying 35.192.xxx.xxx... Connected to xxx.xxx.192.35.bc.googleusercontent.com. Escape character is '^]'. USER FOO Connection closed by foreign host.
Finally, if you ssh to a node instance, there's no evidence of anything listening on ports 110, 143, 993, or 995.
Hoping to shed a light on this mystery!