Step 1: Install UFW

To install UFW on your Ubuntu or Debian-based system, run the following command:
sudo apt-get update
sudo apt-get install ufw

Step 2: Enable UFW

To enable the UFW firewall, run:
sudo ufw enable

Step 3: Set default rule to allow incoming traffic

By default, UFW denies all incoming traffic. To change this behavior (not recommended, see below), run:
sudo ufw default allow incoming

Step 1: Install firewalld

To install firewalld on CentOS 7, run:
sudo yum install firewalld

Step 2: Enable and start firewalld

To enable and start the firewalld service, run:
sudo systemctl enable firewalld
sudo systemctl start firewalld

Step 3: Set default rule to allow incoming traffic

By default, firewalld denies all incoming traffic. To change this behavior (not recommended, see below), run:
sudo firewall-cmd --set-default-zone=trusted
sudo firewall-cmd --runtime-to-permanent

Step 1: Install firewalld

To install firewalld on CentOS Stream 8, run:
sudo dnf install firewalld

Step 2: Enable and start firewalld

To enable and start the firewalld service, run:
sudo systemctl enable firewalld
sudo systemctl start firewalld

Step 3: Set default rule to allow incoming traffic

By default, firewalld denies all incoming traffic. To change this behavior (not recommended, see below), run:
sudo firewall-cmd --set-default-zone=trusted
sudo firewall-cmd --runtime-to-permanent

Step 1: Install firewalld

To install firewalld on Fedora & Red Hat Enterprise Linux (RHEL), run:
sudo dnf install firewalld

Step 2: Enable and start firewalld

To enable and start the firewalld service, run:
sudo systemctl enable firewalld
sudo systemctl start firewalld

Step 3: Set default rule to allow incoming traffic

By default, firewalld denies all incoming traffic. To change this behavior (not recommended, see below), run:
sudo firewall-cmd --set-default-zone=trusted
sudo firewall-cmd --runtime-to-permanent

Step 1: Install firewalld

To install firewalld on openSUSE, run:
sudo zypper install firewalld

Step 2: Enable and start firewalld

To enable and start the firewalld service, run:
sudo systemctl enable firewalld
sudo systemctl start firewalld

Step 3: Set default rule to allow incoming traffic

By default, firewalld denies all incoming traffic. To change this behavior (not recommended, see below), run:
sudo firewall-cmd --set-default-zone=trusted
sudo firewall-cmd --runtime-to-permanent

Enable/disable UFW:

sudo ufw enable
sudo ufw disable

Check UFW status and rules:

sudo ufw status verbose

Set default policies (allow, deny, or reject):

sudo ufw default allow incoming
sudo ufw default deny incoming
sudo ufw default reject incoming

Allow/deny specific ports and protocols:

sudo ufw allow 22/tcp
sudo ufw deny 22/tcp

Allow/deny specific IP addresses or subnets:

sudo ufw allow from 192.168.1.0/24
sudo ufw deny from 192.168.1.0/24

Delete a rule:

sudo ufw delete allow 22/tcp

Reset UFW to default settings:

sudo ufw reset

In the context of firewalld, a "zone" is a predefined set of rules that define the level of trust for network connections and interfaces. Each zone has its own rules, services, and ports that are allowed or denied based on the trust level. By setting a default zone, you are determining the default set of rules that will be applied to network connections that do not explicitly belong to any other zone.

Firewalld comes with several predefined zones, such as:

• drop: All incoming connections are dropped without any reply. Only outgoing connections are allowed.
• block: Similar to 'drop', but incoming connections are rejected with an icmp-host-prohibited or icmp6-adm-prohibited message.
• public: Represents public, untrusted networks. Only selected incoming connections are allowed.
• external: Used for external networks with masquerading enabled, especially for routers. Only selected incoming connections are allowed.
• dmz: Used for computers in your demilitarized zone (DMZ). Only selected incoming connections are allowed.
• work: Used for work machines. Trust most of the machines in the network. Only selected incoming connections are allowed.
• home: Used for home machines. Trust most of the machines in the network. Only selected incoming connections are allowed.
• internal: Used for internal networks. Trust all machines in the network. Only selected incoming connections are allowed.
• trusted: All network connections are accepted.

When you "set default zone" in firewalld, you are defining which zone's rules should be applied to network connections that have not been explicitly assigned to another zone. For example, if you set the default zone to 'public', any connections not specified to be in another zone will be treated according to the rules and policies of the 'public' zone.

Enable/disable firewalld:

sudo systemctl enable firewalld
sudo systemctl disable firewalld

Start/stop firewalld:

sudo systemctl start firewalld
sudo systemctl stop firewalld

Check firewalld status and rules:

sudo firewall-cmd --state
sudo firewall-cmd --list-all

Set default zone:

sudo firewall-cmd --set-default-zone=public

Allow/deny specific ports and protocols:

sudo firewall-cmd --zone=public --add-port=22/tcp --permanent
sudo firewall-cmd --zone=public --remove-port=22/tcp --permanent

Allow/deny specific services:

sudo firewall-cmd --zone=public --add-service=http --permanent
sudo firewall-cmd --zone=public --remove-service=http --permanent

Allow/deny specific IP addresses or subnets:

sudo firewall-cmd --zone=public --add-source=192.168.1.0/24 --permanent
sudo firewall-cmd --zone=public --remove-source=192.168.1.0/24 --permanent

Reload firewalld configuration:

sudo firewall-cmd --reload