There are several scenarios when telnet could be a useful tool on Exadata or ODA.
For example, when there is a need to see why a specific server in the environment isn't responding to Exadata as expected.
More specific examples could be related to troubleshooting the email alerts on Exadata compute nodes or the storage cells or
to validating the ports needed for OEM setup or even X11.
One immediate thought could be: "Let's just run # telnet <hostname> <port> command and be done"
That is easier said than done on Exadata though as telnet is not installed as part of standard Exadata image.
Also, in many cases you may not be permitted to install either telnet or any additional tools on Exadata due to the policies and restrictions in place.
Luckily though the following command could be used in place of Telnet with ease:
# echo > /dev/tcp/<hostname>/<port>; echo $?;
Since the method above is using the basic Linux "echo" command that technique works equally well on the compute nodes as well as the storage cells.
The result should be 0 (zero) when the requested host responds on the port number provided.
Coming back to our more specific example of testing SMTP relay and whether or not it is accepting requests from our Exadata machine's compute nodes or the storage cells the syntax could be as following:
# echo > /dev/tcp/smtp.acme.com/25; echo $?;
If you see 0 (zero) for the output things are looking up and your SMTP relay smpt.acme.com is indeed responding on the port 25, as expected.