Network and firewall filtering using Netfilter and NFtables

1. Introduction

In this post, I will give you how to install minikube on Ubuntu 18.04, first let’s start with an introduction before diving to the installation steps.

2. What is iptables/netfilter ?

netfilter is a set of hooks inside the Linux kernel that allows kernel modules to register callback functions with the network stack.

iptables is the userspace command line program used to configure the Linux 2.4.x and later packet filtering ruleset. It is targeted towards system administrators.

The iptables package also includes ip6tables. ip6tables is used for configuring the IPv6 packet filte

3. Adience

This article intended for people who want to secure their environment and control network traffic using iptables tool

4. What can I do with netfilter/iptables?

• build internet firewalls based on stateless and stateful packet filtering
• deploy highly available stateless and stateful firewall clusters
• use NAT and masquerading for sharing internet access if you don’t have enough public IP addresses
• use NAT to implement transparent proxies
• aid the tc and iproute2 systems used to build sophisticated QoS and policy routers
• do further packet manipulation (mangling) like altering the TOS/DSCP/ECN bits of the IP header

5. Prerequisites

• An SSH key pair on your local Linux/Mac OS/BSD machine.
• linux environment khnowledge

6. Basic iptables Parameters and Syntax

Before we begin creating rules, let’s review the syntax of an iptables rule.

For example, the following :

This command adds a rule to the beginning of the chain that will drop all packets from the address 198.192.100.0

The sample command above:

1. Calls the iptables program
2. Uses the -I option for insertion. Using a rule with the insertion option will add it to the beginning of a chain and will be applied first. To indicate a specific placement in the chain, you may also use a number with the -I option.
3. The -s parameter, along with the IP address (198.192.100.0), indicates the source.
4. The -j parameter stands for jump. It specifies the target of the rule and what action will be performed if the packet is a match.

Parameter	Description
-p, –protocol	The protocol, such as TCP, UDP, etc.
-s, –source	Can be an address, network name, hostname, etc.
-d, –destination	An address, hostname, network name, etc.
-j, –jump	Specifies the target of the rule; i.e. what to do if the packet matches.

7. Installing Iptables

Iptables is pre-installed in most Linux distributions. However, if you don’t have it in Ubuntu/Debian system by default, follow the steps below:

• Connect to your server via SSH.

• Execute the following command one by one

• Check the status of your current iptables configuration by running

the -L option is used to list all the rules, and -v is for showing the info in a more detailed format.

8. Defining Chain Rules

Defining a rule means appending it to the chain. To do this, you need to insert the -A option (Append) right after the iptables command, like so:

It will alert iptables that you are adding new rules to a chain. Then, you can combine the command with other options, such as:

• -i (interface) — the network interface whose traffic you want to filter, such as eth0, lo, ppp0, etc.
• -p (protocol) — the network protocol where your filtering process takes place. It can be either tcp, udp, udplite, icmp, sctp, icmpv6, and so on. Alternatively, you can type all to choose every protocol.
• -s (source) — the address from which traffic comes from. You can add a hostname or IP address.
• –dport (destination port) — the destination port number of a protocol, such as 22 (SSH), 443 (https), etc.
• -j (target) — the target name (ACCEPT, DROP, RETURN). You need to insert this every time you make a new rule.

If you want to use all of them, you must write the command in this order:

Enabling Traffic on Localhost

To allow traffic on localhost, type this command:

Enabling Connections on HTTP, SSH, and SSL Port

Next, we want http (port 80), https (port 443), and ssh (port 22) connections to work as usual. To do this, we need to specify the protocol (-p) and the corresponding port (–dport). You can execute these commands one by one:

It’s time to check if the rules have been appended in iptables:

Filtering Packets Based on Source

Iptables allows you to filter packets based on an IP address or a range of IP addresses. You need to specify it after the -s option. For example, to accept packets from 192.168.1.3, the command would be:

You can also reject packets from a specific IP address by replacing the ACCEPT target with DROP.

If you want to drop packets from a range of IP addresses, you have to use the -m option and iprange module. Then, specify the IP address range with –src-range. Remember, a hyphen should separate the range of ip addresses without space, like this:

Dropping all Other Traffic

It is crucial to use the DROP target for all other traffic after defining –dport rules. This will prevent an unauthorized connection from accessing the server via other open ports. To achieve this, simply type:

Now, the connection outside the specified port will be dropped.

Deleting Rules

If you want to remove all rules and start with a clean slate, you can use the -F option (flush):

This command erases all current rules. However, to delete a specific rule, you must use the -D option. First, you need to see all the available rules by entering the following command:

You will get a list of rules with numbers:

Chain INPUT (policy ACCEPT)

num  target        prot     opt source                  destination

1      ACCEPT     all —  192.168.0.4           anywhere

2      ACCEPT     tcp — anywhere             anywhere tcp dpt:https

3      ACCEPT     tcp — anywhere             anywhere tcp dpt:http

4      ACCEPT     tcp — anywhere             anywhere tcp dpt:ssh

To delete a rule, insert the corresponding chain and the number from the list. Let’s say for this iptables tutorial, we want to get rid of rule number three of the INPUT chain. The command should be:

9. Persisting Changes

The iptables rules that we have created are saved in memory. That means we have to redefine them on reboot. To make these changes persistent after restarting the server, you can use this command:

It will save the current rules on the system configuration file, which will be used to reconfigure the tables every time the server reboots.

Note that you should always run this command every time you make changes to the rules. For example, if you want to disable iptables, you need to execute these two lines:

10. Conclusion

In this post, i have explained how to install and use the iptable tool
If you have any questions or feedback, feel free to leave a comment.
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