You may get CRITICAL error "Connection refused by host" on Nagios monitoring tool after installed and configured Nagios in client system.

Service State Information

Current Status: CRITICAL  (for 0d 0h 6m 15s)
Status Information: Connection refused by host

Login into your nagios server and check the configuration with client IP Address using below command,
bash-3.2# /usr/local/nagios/libexec/check_nrpe -H
Connection refused by host

Note : Assume that Client  IP address is


Check in client system connection configuration,
bash-3.2# /usr/local/nagios/libexec/check_nrpe -H localhost
connect to address port 5666: Connection refused
connect to host localhost port 5666: Connection refused


1. We have to specified Service name, Port number of service  on /etc/services file like below,
nrpe       5666/tcp        # TCP Port for nagios (nrpe)

Also to check below configuration to fix few errors,

2. Check  the Nagios server IP Address added in /etc/xinetd.d/nrpe
disable         =    no
only_from       =
Note: Assume that  the nagios server  IP address is

3. May the Firewall  does'nt allowed the port number 5666, can give accept  permission in IPTABLES  and check the status
bash-3.2#  iptables -I INPUT -p tcp -m tcp --dport 5666 -j ACCEPT
bash-3.2#  /etc/init.d/iptables save

If are you using csf tool you have to allow the port number 5666 in /etc/csf/csf.conf file on TCP_IN field  and restart xinetd
bash-3.2#  /etc/init.d/xinetd restart
Stopping xinetd:                                           [  OK  ]
Starting xinetd:                                             [  OK  ]

Now, can able to check with same command line.
bash-3.2# /usr/local/nagios/libexec/check_nrpe -H
NRPE v2.12