Q. I run both RHEL / CentOS Linux server and by default firewall blocked out everything including telnet / ssh access. How do I allow telnet – port 23 and ssh port 22 thought Linux iptables firewall ?

A.By default firewall rules stored at /etc/sysconfig/iptables location / file under CentOS / RHEL. All you have to do is modify this file to add rules to open port 22 or 23.

Login as the root user.

Open /etc/sysconfig/iptables file, enter:
# vi /etc/sysconfig/iptables
Find line that read as follows:
COMMIT
To open port 22 (ssh), enter (before COMMIT line):

-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT

To open port 23 (telnet), enter (before COMMIT line):

-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 23 -j ACCEPT

Save and close the file. Restart the firewall:
# /etc/init.d/iptables restart