Hi kyang
* Yes restarting the nagios fixing the issue ..
Now i am getting the issue every 5 hours once .
OS verision :CentOS release 6.4 (Final)
Nagios core version : Nagios® Core™ 3.5.1
Lot of --- " Unable to establish communication with Agent" errors
and finally none of client getting pinged from the nagios server .. all the Host unreachable
Some notable errors from nagios.log
[1510147925] livestatus: Cannot set FD_CLOEXEC on client socket: Bad file descriptor
[1510147925] livestatus: Cannot set FD_CLOEXEC on client socket: Bad file descriptor
LOT of ---- fork() error: 'Resource temporarily unavailable
[1510147857] Warning: The check of service 'EVENTLOG-System' on host 'XXXX' could not be performed due to a fork() error: 'Resource temporarily unavailable'. The check will be rescheduled.
[1510147857] Warning: The check of service 'Eventlog-system' on host 'XXXX' could not be performed due to a fork() error: 'Resource temporarily unavailable'. The check will be rescheduled.
[1510147857] Warning: The check of service 'EVENTLOG-System' on host 'XXXX' could not be performed due to a fork() error: 'Resource temporarily unavailable'. The check will be rescheduled.
[1510147857] Warning: The check of service 'EVENTLOG-System' on host 'XXXX' could not be performed due to a fork() error: 'Resource temporarily unavailable'. The check will be rescheduled.
[1510147857] Warning: The check of service 'EVENTLOG-System' on host 'XXXX' could not be performed due to a fork() error: 'Resource temporarily unavailable'. The check will be rescheduled.
Please help me to fix this issue .