Firewall

Linux

Linux typically uses IPTables for its firewall. On most modern Linux distributions, IPTables is controlled through another program or manager such as firewalld, or CSF.

Firewalld

See whether firewalld is running	firewall-cmd --state
List all rules	firewall-cmd --list-all
List all zones	firewall-cmd --list-all-zones
List all rules in a specific zone	firewall-cmd --list-all --zone=home
Allow a specific port	firewall-cmd --add-port <port-number/port-type> --permanent
Reload the firewall	firewall-cmd --reload

Opening a Port

To allow a specific port access to your machine, you will (most likely) need to make IPTables ACCEPT traffic meeting the port criteria on the INPUT chain.

Simple IPTables:

# iptables -A INPUT -p tcp --dport 80 -j ACCEPT

If using Red Hat's firewalld:

# firewall-cmd --permanent --add-port=80/tcp
# firewall-cmd --permanent --add-service=x
## Reload firewall rules
# firewall-cmd --reload

CSF

If using CSF:

# vi /etc/csf/csf.conf
## Edit 'TCP_IN' to include any TCP traffic IN, or TCP_OUT for traffic out.
## Reload CSF
# csf -r