Introduction

In this tutorial, we will teach you how to use a Iptables with DigitalOcean Private Networking. We will also cover why you would want to do this, and provide an example of how to implement this in your own environment. The example given in this tutorial will explain the concept in such a way that you will be able to adapt the configuration to your own needs.

DigitalOcean's private networking option grants a second networking interface to a VPS, which is only accessible to other servers provisioned on the same account that are also located in the same data center. This means that data sent over a Droplet's private interface is only accessible to those on your team or with access to your DO password.

Note: This tutorial covers IPv4 security. In Linux, IPv6 security is maintained separately from IPv4. For example, iptables only maintains firewall rules for IPv4 addresses but it has an IPv6 counterpart called ip6tables, which can be used to maintain firewall rules for IPv6 network addresses.

If your VPS is configured for IPv6, please remember to secure both your IPv4 and IPv6 network interfaces with the appropriate tools. For more information about IPv6 tools, refer to this guide: How To Configure Tools to Use IPv6 on a Linux VPS
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Example Scenario

For our example, we will use the environment created by the following tutorial: How To Optimize WordPress Performance With MySQL Replication On Ubuntu 14.04
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Here is a diagram of what the environment looks like:

The example environment uses five VPSs (and iptables are not configured):

• haproxy-www: Reverse proxy load balancer
• wordpress-1: First application server
• wordpress-2: Second application server
• mysql-1: Master MySQL database server
• mysql-2: Slave MySQL database server

If your setup doesn't look like this, you should still be able to follow along. Also, if you would like to read up on setting up a VPS with private networking or iptables basics, here are a few links that you might find to be useful (this tutorial assumes you know the basics of iptables):

• How To Set Up And Use DigitalOcean Private Networking
• How To Enable DigitalOcean Private Networking on Existing Droplets
• How the Iptables Firewall Works
• How To Set Up a Firewall Using Iptables on Ubuntu 14.04

If you are already familiar with the concepts, and would like to see the iptables setup, feel free to skip to the Overview of Iptables Configuration section.

Our Goal

When we are finished with this tutorial, we should have an environment that looks something like the following diagram:

All of the servers in the private network area can only be communicated with by other servers within this private network (the orange box). The load balancer will be accessible via the Internet and also be linked to the private network. The enforcement of this policy will be implemented via iptables on each server.

Note: To block traffic to your public interface, you can either disable your public interface or set up firewall rules to achieve a similar effect with Iptables. We will go with the firewall option because we can configure it block unwanted network traffic, while allowing our server to access the Internet when it initiates the connection (this is useful for things like downloading updates on the server).

Ways To Access Your VPS

Before we get into how to lock down your own private network, we will discuss different the methods to access your servers (especially the command line). Knowing all the methods to connect to your server is particularly important because you can lock yourself out of your own server if you're not careful.

If you have private networking setup on your DigitalOcean VPS, you have three ways to access it:

• Public Interface
• Private Interface
• Control Panel Console Access

Public Interface

The public interface is accessible via the global Internet. This means that you, or anyone else on the Internet, can access this interface unless it is locked down.

A public interface is necessary on servers that need to be accessible via the Internet, so your customers or users can connect to whatever service you are providing (e.g. a web page or application). If it needs to be accessible by a user, a public interface's IP address is typically mapped to a domain name (e.g. example.com) via DNS.

Every VPS has a public interface enabled by default. In this tutorial, we will use iptables to restrict the public interface to only accept network traffic that is necessary for our application to function properly (i.e. HTTP).

Private Interface

The private interface is accessible only to other VPSs on the same private network. In the case of DigitalOcean, this means that only other VPSs provisioned under the same account can access the private interface.

If you are connected to multiple VPSs under the same account, you can connect to the private interface of another VPS via SSH. For example, you can SSH to haproxy-www's public interface, and then SSH to mysql-1's private interface from there. This is useful if you use iptables to drop SSH connections from the public interface of some of your servers.

In this tutorial, we will restrict the network traffic on the private interfaces to only the VPSs within our defined "private network" (the orange box in the diagram above) and some other necessary network traffic (between the load balancer and the application servers).

Control Panel Console Access

If you lose access to both your public and private interfaces, you can connect to your VPS via console access. In the real world, this is analogous to connecting a keyboard, mouse, and monitor directly to your server. Remember that you can always access your VPS this way, if you accidentally disable both of your interfaces or SSH service.

Note: If your VPS logins are all authenticated with SSH keys, you will need to reset the root password via the control panel to login via the console.

Identify Interface/Port Access Requirements

Before continuing, it is important to determine your interface and port access requirements. Many applications use default ports or can be configured to bind to specific interfaces and ports. Do not change your iptables configuration until you are certain about your policy needs because a misconfigured firewall can potentially break your application.

Here is a breakdown of the network access needs of our example scenario:

• haproxy-www:
  • Internet (public) / port 80
  • WordPress servers (private) / port 80
• wordpress-1: (all private)
  • haproxy_www / port 80
  • wordpress-2 / various glusterFS ports
  • MySQL servers / port 3306
• wordpress-2: (all private)
  • haproxy_www / port 80
  • wordpress-1 / various glusterFS ports
  • MySQL servers / port 3306
• mysql-1: (all private)
  • WordPress servers / port 3306
  • mysql-2 / port 3306
• mysql-2: Slave MySQL database server
  • WordPress servers / port 3306
  • mysql-1 / port 3306

You will also want to allow SSH to at least one of the public interfaces, and SSH between all of the servers on the private network area. In the example, we will allow public SSH to another VPS called tunnel-1, and only private SSH on the other servers--this means that we will need to SSH tunnel through the tunnel-1 to SSH to any of the other servers. Technically, you may use any (or all) your VPSs for this purpose.

Now that we know what we need our firewall to accept, and potentially drop, let's get into configuring it.

Overview of Iptables Configuration

Here is an overview of how we will configure iptables to fulfill our needs:

• Drop by default
• Allow SSH from private network interface to tunnel-1 VPS
• Allow Internet traffic that is initiated from your server
• Explicitly allow specific private network traffic (by IP address and/or port)

Let's start with haproxy-www, our only public-facing server. Note that in the iptables commands, eth0 refers to a VPS's public interface, and eth1 refers to a VPS's private interface--if your interface names differ, please substitute them when appropriate.

Configure Public Server (haproxy-www)

SSH to tunnel-1:

ssh user@tunnel_1_public_IP

From here, SSH to haproxy-www's private interface:

ssh user@haproxy_www_private_IP

On haproxy-www, set all chains defaults to ACCEPT and delete any existing rules:

sudo iptables -P INPUT ACCEPT
sudo iptables -P OUTPUT ACCEPT
sudo iptables -P FORWARD ACCEPT
sudo iptables -F

Allow tunnel-1 to SSH via its private interface to haproxy-www:

sudo iptables -A INPUT -p tcp -s tunnel_1_private_IP --dport 22 -i eth1 -j ACCEPT
sudo iptables -A OUTPUT -p tcp -d tunnel_1_private_IP --sport 22 -o eth1 -m state --state ESTABLISHED -j ACCEPT

Allow loopback traffic on your server. This allows your server to use 127.0.0.1 or localhost:

sudo iptables -A INPUT -i lo -j ACCEPT
sudo iptables -A OUTPUT -o lo -j ACCEPT

Allow public and private traffic that is initiated from your server. This will allow your server to access the Internet to do things like download updates or software:

sudo iptables -I OUTPUT -o eth0 -d 0.0.0.0/0 -j ACCEPT
sudo iptables -I INPUT -i eth0 -m state --state ESTABLISHED,RELATED -j ACCEPT

Allow all HTTP traffic (port 80) on the public interface. This is necessary so users can access our site via http://www.example.com/:

sudo iptables -A INPUT -i eth0 -p tcp --dport 80 -m state --state NEW,ESTABLISHED -j ACCEPT
sudo iptables -A OUTPUT -o eth0 -p tcp --sport 80 -m state --state ESTABLISHED -j ACCEPT

Allow both WordPress servers access to port 80 via their private IP addresses:

sudo iptables -A INPUT -p tcp -s wordpress_1_private_IP --sport 80 -j ACCEPT
sudo iptables -A OUTPUT -p tcp -d wordpress_1_private_IP --dport 80 -j ACCEPT
sudo iptables -A INPUT -p tcp -s wordpress_2_private_IP --sport 80 -j ACCEPT
sudo iptables -A OUTPUT -p tcp -d wordpress_2_private_IP --dport 80 -j ACCEPT

Now that we have allowed all of the necessary network traffic, we can drop all other traffic by setting DROP as the default behavior for each iptables chain:

sudo iptables -P INPUT DROP
sudo iptables -P OUTPUT DROP
sudo iptables -P FORWARD DROP

Now that you are done configuring haproxy-www's firewall, you will want to make sure that everything works properly. If you are happy with your configuration, you can save it by installing the iptables-persistent package with the following apt commands:

sudo apt-get update
sudo apt-get install iptables-persistent

During the installation of iptables-persistent, it will ask you if you would like to save your current firewall settings. Answer yes.

Now the haproxy-www firewall allows the following:

• SSH from tunnel-1 via the private network
• Loopback traffic
• Internet activity that haproxy-www initiates
• HTTP on public Internet
• HTTP between itself and the WordPress application servers
• No incoming traffic from other sources

Let's move on to securing our remaining servers.

Configure Private Network Servers

Note: Do all these steps for all remaining servers: wordpress-1, wordpress-2, mysql-1, and mysql-2. We will refer to these servers, generally, as private-VPS in this section.

Because of the large number of network interfaces and ports that require communication within the private network, we will simplify things by whitelisting the necessary IP addresses instead of only allowing specific IP address and port combinations. Also, we will allow outgoing traffic by default, and just restrict incoming traffic.

SSH to tunnel-1:

ssh user@tunnel_1_public_IP

From here, SSH to private-VPS's private interface:

ssh user@private_VPS_private_IP

On private-VPS, set all chains defaults to ACCEPT and delete any existing rules:

sudo iptables -P INPUT ACCEPT
sudo iptables -P OUTPUT ACCEPT
sudo iptables -P FORWARD ACCEPT
sudo iptables -F

Allow tunnel-1 to SSH via its private interface to private-VPS:

sudo iptables -A INPUT -p tcp -s tunnel_1_private_IP --dport 22 -i eth1 -j ACCEPT

Allow loopback traffic on your server. This allows your server to use 127.0.0.1 or localhost:

sudo iptables -A INPUT -i lo -j ACCEPT

Allow public and private traffic that is initiated from your server. This will allow your server to access the Internet to do things like download updates or software:

sudo iptables -I INPUT -i eth0 -m state --state ESTABLISHED,RELATED -j ACCEPT

Whitelist all of the servers that only need access to the private network area (you may omit the entry for the server you are working on):

sudo iptables -A INPUT -p tcp -s wordpress_1_private_IP -j ACCEPT
sudo iptables -A INPUT -p tcp -s wordpress_2_private_IP -j ACCEPT
sudo iptables -A INPUT -p tcp -s mysql_1_private_IP -j ACCEPT
sudo iptables -A INPUT -p tcp -s mysql_2_private_IP -j ACCEPT

Only on both WordPress servers, allow haproxy-www HTTP access (port 80), so it can retrieve pages:

sudo iptables -A INPUT -p tcp -s haproxy_www_private_IP --sport 80 -j ACCEPT
sudo iptables -A OUTPUT -p tcp -d haproxy_www_private_IP --dport 80 -j ACCEPT

Drop INPUT and FORWARD chains by default. Note that we are leaving OUTPUT's default as ACCEPT, as we trust the servers on our private network:

sudo iptables -P INPUT DROP
sudo iptables -P FORWARD DROP

Now that you are done configuring private-VPS's firewall, you will want to make sure that everything works properly. If you are happy with your configuration, you can save it by installing the iptables-persistent package with the following apt commands:

sudo apt-get update
sudo apt-get install iptables-persistent

During the installation of iptables-persistent, it will ask you if you would like to save your current firewall settings. Answer yes.

Now the private-VPS firewall allows the following:

• SSH from tunnel-1 via the private network
• Loopback traffic
• Internet activity that the private-VPS initiates
• All outgoing network traffic
• All incoming network traffic amongst the servers that were whitelisted (i.e. all of the servers in the private network area)
• No incoming traffic from other sources

Troubleshooting: Iptables List and Logging

If you configure your firewall and find that your application stops working, the best way to troubleshoot it is to look at the policy list and the logs.

Show Iptables Configuration

To view the iptables configuration, or policy list, run the following command:

sudo iptables -vL --line-numbers

This will display all of the chains and rules that you set up, with line numbers. Also, it shows the number of packets dropped. If you are expecting no dropped packets, check the logs.

Read Iptables Logs

Create a new chain called LOG:

iptables -N LOG

Route INPUT/OUTPUT/FORWARD to LOG chain (substitute CHAIN with the chain you want to monitor, such as "INPUT"):

iptables -A INPUT -j LOG

Now log the packets with this command:

iptables -A LOG -m limit --limit 60/min -j LOG --log-prefix "Iptables DROP: " --log-level 7

Now you can monitor your system messages to see which packets are being dropped.

In Ubuntu, the messages can be read in real-time with the following command:

sudo tail -f /var/log/syslog

In CentOS, the messages can be read in real-time with the following command:

sudo tail -f /var/log/messages

The logs will list the interface, source port, destination port, and some other information about each packet dropped. This should help you figure out any issues that you may have.

Conclusion

After following this tutorial, you should have a good foundation for using iptables to protect your VPSs from the public Internet and from other VPSs within the same shared private network (i.e. the same datacenter). Remember that you will need to update your firewall as you add new servers or change your server setup.