ICMP echo request via the tunnel interface


I am experimenting with IP tunneling over GNU radio links. This question
has more to do with tunneling and the tun interface under linux than it
does with gnu radio.

The situation I have is as follows:

I send a ping (ICMP echo request) over the sending side of an OFDM
send/receive pair. The receiver gets the ICMP echo request and writes it
the tun interface. I would have expected the kernel to respond with an
echo reply and for the receiver thread that is reading the tunnel device
via a socket to just relay that reply back. However, this ICMP echo
from the kernel is not forthcoming.

Any clues as to what might be happening here?

Thanks in advance for your help.