Iptables

Here is an example firewall that blocks out everything except ssh, http, and vnc ports 5901 to 5920 on the local subnet (this file would be found at /etc/sysconfig/iptables on Red Hat):

# Firewall configuration written by redhat-config-securitylevel
# Manual customization of this file is not recommended (but has been performed!)
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:RH-Firewall-1-INPUT - [0:0]
-A INPUT -j RH-Firewall-1-INPUT
-A FORWARD -j RH-Firewall-1-INPUT
-A RH-Firewall-1-INPUT -i lo -j ACCEPT
-A RH-Firewall-1-INPUT -p icmp --icmp-type any -j ACCEPT
-A RH-Firewall-1-INPUT -p 50 -j ACCEPT
-A RH-Firewall-1-INPUT -p 51 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 80 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT
-A RH-Firewall-1-INPUT -s 123.45.67.0/24 -m state --state NEW -m tcp -p tcp --dport 5901:5920 -j ACCEPT
-A RH-Firewall-1-INPUT -j REJECT --reject-with icmp-host-prohibited
COMMIT

Changes to this file won't be reflected until the iptables service is restarted:

/etc/init.d/iptables restart

check with

iptables -L -n