ORTE_ERROR_LOG: A message is attempting to be sent to a process whose contact information is unknown in file ../../../../../ompi/orte/mca/rml/oob/rml_oob_send.c
attempted to send to [[21341,0],2]: tag 15
ORTE_ERROR_LOG: A message is attempting to be sent to a process whose contact information is unknown in file ../../../../ompi/orte/mca/grpcomm/base/grpcomm_base_xcast.c

The file etc/hosts is composed by ipaddress hostname, I have exchange ssh keys among the machines and ssh login works without requiring authentication password. Surprisingly if I try to run my program with at most 2 hosts, and so the file hosts contains only two hosts, it works but if i try to run my program with more than two hosts i have this error; mpi works well on each machine and I also tried to run my program with different couple of machines in order to be sure that no machine could be the problem.