-p tcp - Apply the rule to the tcp protocol.-A INPUT - Append a rule to the 'input' chain.This will allow incoming SSH (port 22) traffic: sudo iptables -A INPUT -p tcp -dport ssh -j ACCEPT The first rule that matches the type of traffic will be used. Since each rule in a chain followed in order, it's important that the rules are setup in proper order. You can manage firewall rules in each chain by using the commands to append, insert or remove rules. RedHat/CentOS servers tend to DENY all traffic in each chain by default. Debian/Ubuntu servers tend to come with all chains open to all traffic - set to ACCEPT. You can define a default behaviour for each chain - either to ACCEPT all traffic, or DENY all traffic. FORWARD - rules to determine which traffic to be forwarded will be accepted or denied.OUTPUT - rules to determine which outbound traffic will be accepted or denied.INPUT - rules to determine which inbound traffic will be accepted or denied.Each chain is a list of rules for packets (traffic) that are followed in order. There are a few things to know about firewall rules with iptables before diving in and creating rules. This article goes into depth with configuring your iptables firewall. In previous articles I listed how to set up a firewall without getting into any detail. Your firewall is an important first line of defense on any publicly-accessible server.

Sudo iptables