Ping is used to check how many systems are up. Most of the hackers are using ping for finding the hackable machine. The hacking chances can be reduced by turning off ping response.
For RHEL/Cent OS/Fedora Linux, open terminal as a root and execute,
# sysctl -a | grep icmp
net.netfilter.nf_conntrack_icmp_timeout = 30
net.ipv4.icmp_echo_ignore_all = 0
net.ipv4.icmp_echo_ignore_broadcasts = 1
net.ipv4.icmp_ignore_bogus_error_responses = 1
net.ipv4.icmp_errors_use_inbound_ifaddr = 0
net.ipv4.icmp_ratelimit = 1000
net.ipv4.icmp_ratemask = 6168
net.ipv6.icmp.ratelimit = 1000
To turn off ping, net.ipv4.icmp_echo_ignore_all = 0 is useful.
# sysctl -w net.ipv4.icmp_echo_ignore_all=1
Now try to ping your system from another system. It will not work. To enable it again,
# sysctl -w net.ipv4.icmp_echo_ignore_all=0
For Ubuntu, ping can be disable by adding following line to /etc/init.d/networking
# echo 1 >/proc/sys/net/ipv4/icmp_echo_ignore_all
to re-enable
# echo 0 >/proc/sys/net/ipv4/icmp_echo_ignore_all
Note: This is not permanent, to make it permanent, add to /etc/sysctl.conf.
# vim /etc/sysctl.conf
net.ipv4.icmp_echo_ignore_all=1
Or
net.ipv4.icmp_echo_ignore_all=0

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