Tip: use curl to test network port and DNS service in the Pod

curl is installed in most of docker images by default. Most of pods have it
We can use curl to test if network ports are open and DNS service is working

Example:  To test DB service port 1521
curl -v telnet://mydb.testdb.com:1521

*   Trying 1.1.1.1:1521...
*   TCP_NODELAY set
*   connect to 1.1.1.1:1521 port 1521 failed: Connection timed out
*   Failed to connect to port 1521: Connection timed out
*  Closing connection 0curl: (7) Failed to connect to  Connection timed out

It tells us DNS is working as we see the IP address 1.1.1.1

But port is not open.