What prevents a machine from responding to pings?

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http://unix.stackexchange.com – I have a CentOS release 5.4 linux box on Amazon EC2 that I'm trying to set up to be monitored via Nagios. The machine is in the same security group as the nagios server, but it seems to be unresponsive to pings or NRPE checks, although apparently port 22 is open. The CentOS box can ping itself using it's internal IP address, and it can ping the Nagios server, but the server can not ping the CentOS box. I know the CentOS box is using iptables, here are the contents of the /etc/sysconfig/iptables file (some ips changed for security): # Generated by iptables-save v1.3.5 on May 16 11:28:45 2012 (HowTos)