How to Forward Ports With Iptables in Linux | phoenixNAP KB

introduction

Port forwarding is a NAT technique that allows proxy firewalls to redirect communication requests from one IP address and port to another. On Linux systems, port forwarding is frequently configured with Iptables, a utility for configuring IP packet filter rules.

This tutorial teaches you how to forward ports using Iptables.

Prerequisites

  • Two Linux systems with Internet access and connected to the same private network
  • .

  • Administrative privileges on both systems

.

Iptables Port Forwarding

The proxy firewall plays an essential role in protecting web application infrastructure. The application is installed on a proxy server with a dedicated public IP and acts as a gateway that protects the internal network from external threats.

The following sections describe the procedure for configuring a simple Iptables-based firewall that handles network traffic to and from a Web server.

Step 1: Configure

the web server

The first step in configuring firewall-based network access is to ensure that the web server accepts only connections made through the private network. Follow the steps below to create a sample Nginx web server that only allows access from a private IP address.

Collect

web server network interface details

Open the terminal on the web server and follow these steps:

1. Type the following command to list available IPv4 connections

ip -4 addr display global scope The output of the

ip command lists

the available network interfaces and assigned IP addresses.

The output displays the private (bond0.10) and public (bond0.3) network interfaces and IP addresses of the system. 3

.

Note the interface names and their respective IP addresses

.

Configure

Nginx

Follow these steps on your web server to install and configure Nginx:

1. Update the repository information on the web server using the package manager of your Linux distribution. The tutorial uses APT.

Sudo apt update

2. Install the Nginx web server package.

sudo apt install nginx

Type Y, press Enter and wait for the installation to finish

.

3. Use a text editor such as Nano or Vim to open the configuration file for the default Nginx server block.

sudo nano /etc/nginx/sites-enabled/default

4. Locate the server section in the file. The content should resemble the following example

: server { listen 80 default_server; listen [::]:80 default_server ipv6only=on;

5. Add the private IP address of the server before the port number in the first line of the section.

listen [web-server-private-ip]:80 default_server;

Delete the second line as it relates to the IPv6 address not covered in this tutorial. The following example shows the server section after editing.

Save the file and exit.

6. Test the syntax of the Nginx configuration

by typing:

sudo nginx -t

Nginx displays syntax errors if any. When there are no errors, Nginx displays the following output:

7. Restart Nginx to apply the new settings.

sudo systemctl restart nginx

Test Web Server Configuration

Confirm that the Nginx server is working as expected with the curl command. Run this command from another computer on the same

private network: curl [web-server-private-ip]

The output shows the HTML data of the

Nginx welcome page.

To confirm that the web server does not accept public network connections, use curl with the server’s public IP address. curl [web-server-public-ip

]

Nginx rejects the connection because it is configured to accept requests only from the private network

.

Step 2: Configure

firewall

After configuring the web server, create a proxy firewall on another machine. The following example shows how to configure a firewall with basic Iptables rules.

Collect the details

of the firewall network interface on the other machine that will act as a firewall:

1. View the available IPv4 network interfaces with this command:

ip -4 addr display global scope

2. Identify and note the public and private network interfaces and corresponding IP addresses. In our case, the public interface is bond0.2 with IP 131.153.158, and the private interface is bond0.10 with IP 10.3.0.11. Install Persistent Firewall Package 1.

Update the repository information on the firewall system.

Sudo apt update

2. Install the iptables-persist package.

sudo apt install iptables-persistent

Type Y and press Enter to start the installation

.

3. When prompted, choose Yes to save the current iptables rules.

Configure

basic IPv4 rules

After you install the persistent firewall, edit the firewall server settings to configure basic IPv4 rules

.

1. Open the rules.v4 file in a text editor to add the rules.

sudo nano /etc/iptables/rules.v4

2. Below is an example configuration, with an explanation of each section provided in the comments.

*filter # Delete incoming and forwarded packets; allow outgoing packets :INPUT DROP [0:0] :FORWARD DROP [0:0] :OUTPUT ACCEPT [0:0] # Custom protocol-specific rules :UDP – [0:0] # Write UDP rules here :TCP – [0:0] # Write TCP rules here :ICMP – [0:0] # Write ICMP rules here # Accept traffic SSH TCP -A TCP -p tcp -dport 22 -j ACCEPT # Acceptance Policy -A INPUT -m conntrack -ctstate ESTABLISHED, RELATED -j ACCEPT -A INPUT -i lo -j ACCEPT # Drop a packet if it is invalid -A INPUT -m conntrack -ctstate INVALID – j DROP # Pass traffic to protocol-specific strings # Allow only new TCP connections established with new packets SYN -A INPUT -p udp -m conntrack -ctstate NEW -j UDP -A INPUT -p tcp -syn -m conntrack -ctstate NEW -j TCP -A INPUT -p icmp -m conntrack -ctstate NEW -j ICMP # Reject anything who has reached this point -A INPUT -p udp -j REJECT -reject-with icmp-port-unreachable -A INPUT -p tcp -j REJECT -reject-with tcp-reset -A INPUT -j REJECT -reject-with icmp-proto-unreachable # Commit changes COMMIT *raw 😛 REROUTING ACCEPT [0:0] :OUTPUT ACCEPT [0:0] COMMIT *nat 😛 REROUTING ACCEPT [0:0] :INPUT ACCEPT [0:0] :OUTPUT ACCEPT [0:0] 😛 OSTROUTING ACCEPT [0:0] COMMIT *security :INPUT ACCEPT [0:0] : FORWARD ACCEPT [0:0] :OUTPUT ACCEPT [0:0] COMMIT *mangle 😛 REROUTING ACCEPT [0:0] :INPUT ACCEPT [0:0] :FORWARD ACCEPT [0:0] :OUTPUT ACCEPT [0:0] 😛 OSTROUTING ACCEPT [0:0] COMMIT

Save the file and exit

. 3. Check the syntax of the rules.v4

file by typing:

sudo iptables-restore -t /etc/iptables/rules.v4 If

there are no syntax errors, the command returns no results

.

4. Apply the firewall settings with the following command:

sudo service netfilter-persistent reload

5. Print the currently active rules to confirm that the changes have been applied correctly.

sudo iptables -S

The output shows the rules configured in the rules.v4 file.

Configure basic IPv6 rules Iptables has an attached tool called

Ip6tables for configuring IPv6 packet rules. Since this tutorial covers only creating an IPv4 firewall with Iptables, the following section shows how to block all traffic over IPv6.

1. Open the rules.v6 file in a text editor.

sudo nano /etc/iptables/rules.v6

2. Instruct Ip6tables to delete all connections.

*filter :INPUT DROP [0:0] :FORWARD DROP [0:0] :OUTPUT DROP [0:0] COMMIT *raw 😛 REROUTING DROP [0:0] :OUTPUT DROP [0:0] COMMIT *nat 😛 REROUTING DROP [0:0] :INPUT DROP [0:0] :OUTPUT DROP [0:0] 😛 OSTROUTING DROP [0:0] COMMIT *security :INPUT DROP [0:0] :FORWARD DROP [0:0] :OUTPUT DROP [0:0] COMMIT *mangle 😛 REROUTING DROP [0:0] :INPUT DROP [0:0] :FORWARD DROP [0:0] :OUTPUT DROP [0:0] : POSTROUTING DROP [0:0] COMMIT

Save the file and exit

.

3. Check the configuration syntax.

sudo ip6tables-restore -t /etc/iptables/rules.v6

4. Apply the new IPv6 rules.

sudo service netfilter-persistent reload

5. List the rules with the following ip6tables command to confirm that the new configuration is active.

sudo ip6tables -S

Step 3: Configure

port forwarding

Once you configure your web server and proxy firewall, you can create specific forwarding rules that:

Accept traffic requests through the firewall’s public IP address. Forward the packets to the

  • firewall’s
  • private interface.

  • Forward the packets to the web server using the private network
  • .

  • Accept and forward web server traffic to the Internet.

Enable forwarding

in the kernel

Before you use packet forwarding, you must instruct the system to allow it. To enable forwarding for the current session, type:

echo 1 | sudo tee /proc/sys/net/ipv4/ip_forward

Follow the procedure below to enable packet forwarding permanently

.

1. Open the sysctl.conf file in a text editor.

sudo nano /etc/sysctl.conf

2. Find the line shown below:

# net.ipv4.ip_forward=1

3. Uncomment the line by removing the leading # symbol.

Save the file and exit.

4. Load the new file settings with sysctl: sudo sysctl

-p

5. Apply all system changes.

sudo sysctl -system Packet forwarding

now works on the system

. Provide forwarding rules Specify

forwarding rules by adding them to the rules.v4 file. Alternatively, use the following command line and syntax

: sudo iptables [rule]

Use the following rules to configure the firewall to forward packets to and from the web server successfully:

1. Allow public interface connections to be established to port 80 and forward them to the private interface: sudo iptables -A FORWARD -i [firewall-public-interface] -o [firewall-private-interface

] -p tcp -syn -dport 80 -m conntrack -ctstate NEW -j ACCEPT

With the parameters in our example, the rule looks like this:

sudo iptables -A FORWARD -i bond0.2 -o bond0.10 -p tcp -syn -dport 80 -m conntrack -ctstate NEW -j ACCEPT

2. Allow packet traffic marked as ESTABLISHED and RELATED to travel from the public to private interface.

sudo iptables -A FORWARD -i [firewall-public-interface] -o [firewall-private-interface] -m conntrack -ctstate ESTABLISHED,RELATED -j ACCEPT

For example:

sudo iptables -A FORWARD -i bond0.2 -o bond0.10 -m conntrack -ctstate SET,RELATED -j ACCEPT

3. Allow the same traffic to return from the private to the public interface.

sudo iptables -A FORWARD -i [firewall-private-interface] -o [firewall-public-interface] -m conntrack -ctstate ESTABLISHED,RELATED -j OK

Using the example parameters:

sudo iptables -A FORWARD -i bond0.10 -o bond0.2 -m conntrack -ctstate ESTABLISHED,RELATED -j ACCEPT Provide NAT rules

NAT rules

tell Iptables how to alter packets to allow proper routing between networks. Simple port forwarding can be achieved with two NAT rules.

1. Modify the destination address of the packet, i.e. change it from the public interface of the firewall to the private interface of the web server.

sudo iptables -t nat -A PREROUTING -i [firewall-public-interface] -p tcp -dport 80 -j DNAT -to-destination [web-server-private-ip]

The following shows what the command looks like using the sample web server data from the article

. sudo iptables -t nat -A PREROUTING -i bond0.2 -p tcp -dport 80 -j DNAT -to-destination 10.3.0.12

2. Change the source address of the packet to the private IP address of the firewall. In this way, the web server sends the packet to the firewall, which forwards it to the source.

sudo iptables -t nat -A POSTROUTING -o [firewall-private-interface] -p tcp -dport 80 -d [web-server-private-ip] -j SNAT -to-source [firewall-private-ip]

The command looks like this with our example parameters:

sudo iptables -t nat -A POSTROUTING -o bond0.10 -p tcp -dport 80 -d 10.3.0.12 -j SNAT -to-source 10.3.0.11

3. Type the following command to save all iptables rules.

sudo service netfilter-persistent save

Test

firewall

settings To test the final firewall configuration, use curl on another machine to send a request to the public IP address of the proxy firewall. curl

[firewall-public-ip]

The output shows that the firewall successfully forwarded the Nginx page served by the web server.

Conclusion

After reading this article, you should know how to create a simple firewall based on Iptables and use port forwarding to forward traffic to and from a web server

.

If you are using Ubuntu, you may be interested in learning how to set up a firewall with UFW, an easy-to-use Iptables interface.

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