Potential Cause of "ping unknown hosts"

A common error when pinging some websites, say google.com, is "unknown hosts". Several potential misconfigurations, as listed below, might partially account for this error.

1. Wrong routing configuration:
# route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface * U 0 0 0 eth2
link-local * U 1004 0 0 eth2
default wiles UG 0 0 0 eth2
Make sure the gateway is correctly configured for the default destination

2. Wrong DNS configuration:
# nslookup google.com
# dig google.com
# cat /etc/resolv.conf

3. Wrong firewall configuration:
# cat /etc/sysconfig/iptables