Why does one ping work, and the same ping doesn’t?

I was trying to check connectivity from z/OS running on my laptop. For some remote sites I could issue ping and get a response back. For some other sites I issue the ping and did not get a response back.

When I issued the pings from Linux – they both worked.

I noticed that for the pings from z/OS the field Timestamp from icmp data (relative): was 27 seconds behind. This was caused by z/OS adding leap seconds. It was not the problem.

By comparing all the fields in a successful and an unsuccessful ping, I could see that z/OS send 256 bytes of data, and Linux sent only 40 bytes of data.

On Linux, when I used

ping …. -s 256

it failed. When I used

ping …. -s 20

it worked.

Similarly on z/OS.

ping .... (length 20

The defaults lengths are different between z/OS and Linux.

The moral of this tale is

If ping does not return any data – try a very short ping.