Today we will walk through iptables and firewalld and we will learn about the history of these two along with installation & how we can configure these for our Linux distributions.

Let’s begin wihtout wasting further more time.

What is iptables?

First, we need to know what is iptables. Most of senior IT professionals knows about it and used to work with it as well. Iptables is an application / program that allows a user to configure the security or firewall security tables provided by the Linux kernel firewall and the chains so that a user can add / remove firewall rules to it accordingly to meet his / her security requirements. Iptables uses different kernel modules and different protocols so that user can take the best out of it. As for example, iptables is used for IPv4 ( IP version 4/32 bit ) and ip6tables for IPv6 ( IP version 6/64 bit ) for both tcp and udp. Normally, iptables rules are configured by System Administrator or System Analyst or IT Manager.  You must have root privileges to execute each iptables rules. Linux Kernel uses the Netfilter framework so that it can provide various networking-related operations which can be performed by using iptables. Previously, ipchains was used in most of the Linux distributions for the same purpose. Every iptables rules are directly handled by the Linux Kernel itself and it is known as kernel duty. Whatever GUI tools or other security tools you are using to configure your server’s firewall security, at the end of the day, it is converted into iptables rules and supplied to the kernel to perform the operation.

History of iptables

The rise of the iptables begin with netfilter. Paul Rusty Russell was the initial author and the head think tank behind netfilter / iptables. Later he was joined by many other tech people then form and build the Netfilter core team and develop & maintain the netfilter/iptables project as a joint effort like many other open source projects. Harald Welte was the former leader until 2007 and then Patrick McHardy was the head until 2013. Currently, netfilter core team head is Pablo Neira Ayuso.

To know more about netfilter, please visit this link. To know more about the histrity of netfilter, please visit this link.

To know more about iptables history, please visit this link.

How to install iptables

Now a days, every Linux Kernel comes with iptables and can be found pre build or pre installed on every famous modern Linux distributions. On most Linux systems, iptables is installed in this /usr/sbin/iptables directory. It can be also  found in /sbin/iptables, but since iptables is more like a service rather than an “essential binary”, the preferred location remains in /usr/sbin directory.

For Ubuntu or Debian

sudo apt-get install iptables

For CentOS

sudo yum install iptables-services

For RHEL

sudo yum install iptables

Iptables version

To know your iptables version, type the following command in your terminal.

sudo iptables --version

Start & Stopping your iptables firewall

For OpenSUSE 42.1, type the following to stop.

sudo /sbin/rcSuSEfirewall2 stop

To start it again

sudo /sbin/rcSuSEfirewall2 start

For Ubuntu, type the following to stop.

sudo service ufw stop

To start it again

sudo service ufw start

For Debian & RHEL , type the following to stop.

sudo /etc/init.d/iptables stop

To start it again

sudo /etc/init.d/iptables start

For CentOS, type the following to stop.

sudo service iptables stop

To start it again

sudo service iptables start

Getting all iptables rules lists

To know all the rules that is currently present & active in your iprables, simply open a terminal and type the following.

sudo iptables -L

If there are no rules exits on the iptables means if there are no rules added so far in your iptables firewall, you will see something like the below image.

In this above picture, you can see that , there are three (3) chains and they are INPUT, FORWARD, OUTPUT and there are no rules exists. Actually I haven’t add one yet.

Type the following to know the status of the chains of your iptables firewall.

sudo iptables -S

With the above command, you can learn whether your chains are accepting or not.

Clear all iptables rules

To clear all the rules from your iptables firewall, please type the following. This is normally known as flushing your iptables rules.

sudo iptables -F

If you want to flush the INPUT chain only, or any individual chains, issue the below commands as per your requirements.

sudo iptables -F INPUT

sudo iptables -F OUTPUT

sudo iptables -F FORWARD

ACCEPT or DROP Chains

To accept or drop a particular chain, issue any of the following command on your terminal to meet your requirements.

iptables --policy INPUT DROP

The above rule will not accept anything that is incoming to that server. To revert it again back to ACCEPT, do the following

iptables --policy INPUT ACCEPT

Same goes for other chains as well like

iptables --policy OUTPUT DROP

iptables --policy FORWARD DROP

Note: By default, all chains of iptables ( INPUT, OUTPUT, FORWARD ) are in ACCEPT mode. This is known as Policy Chain Default Behavior.

Allowing any port

If you are running any web server on your host, then you must allow your iptables firewall so that your server listen or respond to port 80. By default web server runs on port 80. Let’s do that then.

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

On the above line, A stands for append means we are adding a new rule to the iptables list. INPUT stands for the INPUT chain. P stands for protocol and dport stands for destination port. By default any web server runs on port 80. Similarly, you can allow SSH port as well.

sudo iptables -A INPUT -p tcp --dport 22 -j ACCEPT

By default, SSH runs on port 22. But it’s good practise not to run SSH on port 22. Always run SSH on a different port. To run SSH on a different port, open /etc/ssh/sshd_config file on your favorite editor and change the port 22 to a other port.

Blocking any port

Say we want to block port 135. We can do it by

sudo iptables -A INPUT -p tcp --dport 135 -j DROP

if you want to block your server to initiate any SSH connection from the server to another host/server, issue the following command

sudo iptables -A OUTPUT -p tcp --dport 22 -j DROP

By doing so, no one can use your sever to initiate a SSH connection from the server. The OUPUT chain will filter and DROP any outgoing tcp connection towards another hosts.

Allowing specific IP with Port

sudo iptables -A INPUT -p tcp -s 0/0 --dport 22  -j ACCEPT

Here -s 0/0 stand for any incoming source with any IP addresses. So, there is no way your server is going to respond for a tcp packet which destination port is 22. If you want to allow only any particular IP then use the following one.

sudo iptables -A INPUT -p tcp -s 12.12.12.12/32 --dport 22  -j ACCEPT

On the above example, you are only allowing 12.12.12.12 IP address to connect to port SSH. Rest IP addresses will not be able to connect to port 22. Similarly you can allow by using CIDR values. Such as

sudo iptables -A INPUT -p tcp -s 12.12.12.0/24 --dport 22  -j ACCEPT

The above example show how you can allow a whole IP block for accepting connection on port 22. It will accept IP starting from 12.12.12.1 to 12.12.12.255.

If you want to block such IP addresses range, do the reverse by replacing ACCEPT by DROP like the following

sudo iptables -A INPUT -p tcp -s 12.12.12.0/24 --dport 22  -j DROP

So, it will not allow to get a connection on port 22 from from 12.12.12.1 to 12.12.12.255 IP addresses.

Blocking ICMP

If you want to block ICMP (ping) request to and from on your server, you can try the following. The first one will block not to send ICMP ping echo request to another host.

sudo iptables -A OUTPUT -p icmp --icmp-type 8 -j DROP

Now, try to ping google.com. Your OpenSUSE server will not be able to ping google.com.

If you want block the incoming ICMP (ping) echo request for your server, just type the following on your terminal.

sudo iptables -I INPUT -p icmp --icmp-type 8 -j DROP

Now, It will not reply to any ICMP ping echo request. Say, your server IP address is 13.13.13.13. And if you ping ping that IP of your server then you will see that your server is not responding for that ping request.

Blocking MySql / MariaDB Port

As Mysql is holding your database so you must protect your database from outside attach. Allow your trusted application server IP addresses only to connect with your MySQL server. To block other

sudo iptables -A INPUT -p tcp -s 192.168.1.0/24 --dport 3306 -m state --state NEW,ESTABLISHED -j ACCEPT

So, it will not take any MySql connection except 192.168.1.0/24 IP block. By default MySql runs on 3306 port.

Blocking SMTP

If you not running any mail server on your host server or if your server is not configured to act like a mail server, you must block SMTP so that your server is not sending any spam or any mail towards any domain. You must do this to block any outgoing mail from your server. To do so,

sudo iptables -A OUTPUT -p tcp --dport 25 -j DROP

Block DDoS

We all are familiar with the term DDoS. To get rid of it, issue the following command in your terminal.

iptables -A INPUT -p tcp --dport 80 -m limit --limit 20/minute --limit-burst 100 -j ACCEPT

You need to configure the numerical value to meet your requirements. This is just a standard to maintain.

You can protect more by

echo 1 > /proc/sys/net/ipv4/ip_forward
echo 1 > /proc/sys/net/ipv4/tcp_syncookies
echo 0 > /proc/sys/net/ipv4/conf/all/accept_redirects
echo 0 > /proc/sys/net/ipv4/conf/all/accept_source_route
echo 1 > /proc/sys/net/ipv4/conf/all/rp_filter
echo 1 > /proc/sys/net/ipv4/conf/lo/rp_filter
echo 1 > /proc/sys/net/ipv4/conf/lo/arp_ignore
echo 2 > /proc/sys/net/ipv4/conf/lo/arp_announce
echo 1 > /proc/sys/net/ipv4/conf/all/arp_ignore
echo 2 > /proc/sys/net/ipv4/conf/all/arp_announce
echo 0 > /proc/sys/net/ipv4/icmp_echo_ignore_all
echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts
echo 30 > /proc/sys/net/ipv4/tcp_fin_timeout
echo 1800 > /proc/sys/net/ipv4/tcp_keepalive_time
echo 1 > /proc/sys/net/ipv4/tcp_window_scaling 
echo 0 > /proc/sys/net/ipv4/tcp_sack
echo 1280 > /proc/sys/net/ipv4/tcp_max_syn_backlog

Blocking Port Scanning

There are hundred of people out there to scan your open ports of your server and try to break down your server security. To block it

sudo iptables -N block-scan

sudo iptables -A block-scan -p tcp —tcp-flags SYN,ACK,FIN,RST RST -m limit —limit 1/s -j RETURN

sudo iptables -A block-scan -j DROP

Here, block-scan is a name of a new chain.

Blocking Bad Ports

You may need to block some bad ports for your server as well. Here is how you can do this.

badport="135,136,137,138,139,445"
sudo iptables -A INPUT -p tcp -m multiport --dport $badport -j DROP
sudo iptables -A INPUT -p udp -m multiport --dport $badport -j DROP

You can add more ports according to your needs.

What is firewalld?

Firewalld provides a dynamically managed firewall with support for network/firewall zones that defines the trust level of network connections or interfaces. It has support for IPv4, IPv6 firewall settings, ethernet bridges and IP sets. There is a separation of runtime and permanent configuration options. It also provides an interface for services or applications to add firewall rules directly.

The former firewall model with system-config-firewall/lokkit was static and every change required a complete firewall restart. This included also to unload the firewall netfilter kernel modules and to load the modules that are needed for the new configuration. The unload of the modules was breaking stateful firewalling and established connections. The firewall daemon on the other hand manages the firewall dynamically and applies changes without restarting the whole firewall. Therefore there is no need to reload all firewall kernel modules. But using a firewall daemon requires that all firewall modifications are done with that daemon to make sure that the state in the daemon and the firewall in kernel are in sync. The firewall daemon can not parse firewall rules added by the iptables and ebtables command line tools. The daemon provides information about the current active firewall settings via D-BUS and also accepts changes via D-BUS using PolicyKit authentication methods.

So, firewalld uses zones and services instead of chain and rules for performing the operations and it can manages rule(s) dynamically allowing updates & modification without breaking existing sessions and connections.

It has following features.

• D-Bus API.
• Timed firewall rules.
• Rich Language for specific firewall rules.
• IPv4 and IPv6 NAT support.
• Firewall zones.
• IP set support.
• Simple log of denied packets.
• Direct interface.
• Lockdown: Whitelisting of applications that may modify the firewall.
• Support for iptables, ip6tables, ebtables and ipset firewall backends.
• Automatic loading of Linux kernel modules.
• Integration with Puppet.

To know more about firewalld, please visit this link.

How to install firewalld

Before installing firewalld, please make sure you stop iptables and also make sure that iptables are not using or working anymore. To do so,

sudo systemctl stop iptables

This will stop iptables form your system.

And then make sure iptables are not used by your system any more by issuing the below command in the terminal.

sudo systemctl mask iptables

Now, check the status of iptables.

sudo systemctl status iptables

Now, we are ready to install firewalld on to our system.

For Ubuntu

To install it on Ubuntu, you must remove UFW first and then you can install Firewalld. To remove UFW, issue the below command on the terminal.

sudo apt-get remove ufw

After removing UFW, issue the below command in the terminal

sudo apt-get install firewall-applet

Or

You can open Ubuntu Software Center and look or seacrh  for “firewall-applet” then install it on to your Ubuntu system.

For RHEL, CentOS & Fedora

Type the below command to install firewalld on your CentOS system.

sudo yum install firewalld firewall-config -y

How to configure firewalld

Before configuring firewalld, we must know the status of firewalld after the installation. To know that, type the following.

sudo systemctl status firewalld

As firewalld works on zones basis, we need to check all the zones and services though we haven’t done any configuring yet.

For Zones

sudo firewall-cmd --get-active-zones

or

sudo firewall-cmd --get-zones

To know the default zone, issue the below command

sudo firewall-cmd --get-default-zone

And, For Services

sudo firewall-cmd --get-services

Here, you can see those services covered under firewalld.

Setting Default Zone

An important note is, after each modification, you need to reload firewalld so that your changes can take place.

To set the default zone

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

or

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

After changing the zone, check whether it changes or not.

sudo firewall-cmd --get-default-zone

Adding Port in Public Zone

sudo firewall-cmd --permanent --zone=public --add-port=80/tcp

This will add tcp port 80 in the public zone of firewalld. You can add your desired port as well by replacing 80 by your’s.

Now reload the firewalld.

sudo firewall-cmd --reload

Now, check the status to see whether tcp 80 port has been added or not.

sudo firewall-cmd --zone=public --list-ports

Here, you can see that tcp port 80 has been added.

Or even you can try something like this.

sudo firewall-cmd --zone=public --list-all

Removing Port from Public Zone

To remove Tcp 80 port from the public zone, type the following.

sudo firewall-cmd --zone=public --remove-port=80/tcp

You will see a “success” text echoing in your terminal.

You can put your desired port as well by replacing 80 by your’s own port.

Adding Services in Firewalld

To add ftp service in firewalld, issue the below command

sudo firewall-cmd --zone=public --add-service=ftp

You will see a “success” text echoing in your terminal.

Similarly for adding smtp service, issue the below command

sudo firewall-cmd --zone=public --add-service=smtp

Replace ftp and smtp by your’s own service that you want to add in the firewalld.

Removing Services from Firewalld

For removing ftp & smtp services from firewalld, issue the below command in the terminal.

sudo firewall-cmd --zone=public --remove-service=ftp

sudo firewall-cmd --zone=public --remove-service=smtp

Block Any Incoming and Any Outgoing Packet(s)

If you wish, you can block any incoming or outgoing packets / connections by using firewalld. This is known as “panic-on” of firewalld. To do so, issue the below command.

sudo firewall-cmd --panic-on

You will see a “success” text echoing in your terminal.

After doing this, you will not be able to ping a host or even browse any websites.

To turn this off, issue the below command in your terminal.

sudo firewall-cmd --panic-off

Adding IP Address in Firewalld

sudo firewall-cmd --zone=public --add-rich-rule='rule family="ipv4" source address="192.168.1.4" accept'

By doing so, firewalld will accept IP v4 packets from the source IP 192.168.1.4.

Blocking IP Address From Firewalld

Similarly, to block any IP address

sudo firewall-cmd --zone=public --add-rich-rule='rule family="ipv4" source address="192.168.1.4" reject'

By doing so, firewalld will drop / discards every IP v4 packets from the source IP 192.168.1.4.

I stuck with the very basic of Firewalld over here so that you can easily understand the working methodology of it and the differences of it with iptables.

That’s all for today. Hope you enjoy reading this article.

Take care.