Q. I’ve Redhat Enterprise Linux 5 server with Iptabeles firewall enabled. By default it is only allowing port 22. How do I open port 80 / 143 / 110 on Linux? I’ve started all services but don’t know how to open port using iptables.

A. By default iptables firewall stores its configuration at /etc/sysconfig/iptables file. You need to edit this file and add rules to open port number. This file only avilable under
Red Hat Enterprise Linux 3 / 4 / 5 and above
=> Old Red hat Linux version
=> CentOS 4 and above
=> Fedora Linux

Open port 80

Open flle /etc/sysconfig/iptables:
# vi /etc/sysconfig/iptables
Append rule as follows:
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 80 -j ACCEPT
Save and close the file. Restart iptables:
# /etc/init.d/iptables restart

Open port 110

Append rule as follows:
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 110 -j ACCEPT

Open port 143

Append rule as follows:
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 143 -j ACCEPT

Restart iptables service

Type the following command:
# service iptables restart

Verify that port is open

Run following command:
netstat -tulpn | less
Make sure iptables is allowing port 80 / 110 / 143 connections:
iptables -L -n

Refer to iptables man page for more information about iptables usage and syntax:
man iptables