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# Install Net - Tools
sudo apt-get install net-tools 
# Show active Ports
sudo netstat -tunlp

# Installing iptables
sudo apt-get install iptables

# Permission Evelation
sudo su

# Showing active rules
iptables -L -v

# Adding specific rules
sudo iptables -A INPUT -p tcp --dport 22 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 514 -j ACCEPT
sudo iptables -A INPUT -p udp --dport 514 -j ACCEPT

# Modifying the default output policy
sudo iptables -A OUTPUT -j ACCEPT
sudo iptables -I INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

# Modifying the default input policy
sudo iptables -A INPUT -j DROP

# Showing active rules
iptables -L -v

# Saving iptables config
sudo /sbin/iptables-save

# Reset Config
iptables -P INPUT ACCEPT
iptables -P OUTPUT ACCEPT
iptables -P FORWARD ACCEPT
# Flush & Delete all Rules
iptables -F
# Delete all Custom Chainss
iptables -X

Use of simplified administration by ufw (uncomplicated firewall)

# Install ufw
sudo apt-get update
sudo apt-get install ufw

# Disable ufw
sudo ufw --force disable

# Reset ufw
sudo ufw reset
sudo ufw --force reset

# Check ufw Status
sudo ufw status

# Allow All Outgoing Connections
sudo ufw default allow outgoing

# Deny All Incoming Connections
sudo ufw default deny incoming

# Allow SSH
sudo ufw allow ssh

# Allow SYSLOG
sudo ufw allow 514/tcp
sudo ufw allow 514/udp

# Enable ufw
sudo ufw enable
sudo ufw --force enable

# Show rules (with index) 
sudo ufw status numbered

# Delete a specific rule
sudo ufw delete 2

# Reload Config
sudo ufw reload

# Clean up (ufw Deinstallation)
sudo apt-get purge ufw

References:

https://www.cyberciti.biz/tips/linux-iptables-4-block-all-incoming-traffic-but-allow-ssh.html