But if I use a terminal on the xymon server and ping the clients
"manually" there is no timeout.
How many times did you test it manually? And how often does it fail in
Xymon for each host being tested? Does the graph on the "conn" page show
any pattern in the failures?
The main thing here is to determine if the ping failures are due to a comms
outage or a failure within the probe (ie, inside the monitoring server or
outside it).
Perhaps you could do a packet capture of ICMP packets long enough to
include a failure and then check if a packet was sent and received.
J