Linux Firewalls Using iptables
Introduction
Network security is a primary consideration in any decision to host a website as the threats are becoming more widespread and persistent every day. One means of providing additional protection is to invest in a firewall. Though prices are always falling, in some cases you may be able to create a comparable unit using the Linux iptables package on an existing server for little or no additional expenditure.
This chapter shows how to convert a Linux server into:
• A firewall while simultaneously being your home website's mail, web and DNS server.
• A router that will use NAT and port forwarding to both protect your home network and have another web server on your home network while sharing the public IP address of your firewall.
Creating an iptables firewall script requires many steps, but with the aid of the sample tutorials, you should be able to complete a configuration relatively quickly.
What Is iptables?
Originally, the most popular firewall/NAT package running on Linux was ipchains, but it had a number of shortcomings. To rectify this, the Netfilter organization decided to create a new product called iptables, giving it such improvements as:
• Better integration with the Linux kernel with the capability of loading iptables-specific kernel modules designed for improved speed and reliability.
• Stateful packet inspection. This means that the firewall keeps track of each connection passing through it and in certain cases will view the contents of data flows in an attempt to anticipate the next action of certain protocols. This is an important feature in the support of active FTP and DNS, as well as many other network services.
• Filtering packets based on a MAC address and the values of the flags in the TCP header. This is helpful in preventing attacks using malformed packets and in restricting access from locally attached servers to other networks in spite of their IP addresses.
• System logging that provides the option of adjusting the level of detail of the reporting.
• Better network address translation.
• Support for transparent integration with such Web proxy programs as Squid.
• A rate limiting feature that helps iptables block some types of denial of service (DoS) attacks.
Considered a faster and more secure alternative to ipchains, iptables has become the default firewall package installed under RedHat and Fedora Linux.
Download And Install The Iptables Package
Before you begin, you need to make sure that the iptables software RPM is installed. (See Chapter 6, "Installing Linux Software", if you need a refresher.) When searching for the RPMs, remember that the filename usually starts with the software package name by a version number, as in iptables-1.2.9-1.0.i386.rpm.
How To Start iptables
You can start, stop, and restart iptables after booting by using the commands:
[root@bigboy tmp]# service iptables start
[root@bigboy tmp]# service iptables stop
[root@bigboy tmp]# service iptables restart
To get iptables configured to start at boot, use the chkconfig command:.
[root@bigboy tmp]# chkconfig iptables on
Determining The Status of iptables
You can determine whether iptables is running or not via the service iptables status command. Fedora Core will give a simple status message. For example
[root@bigboy tmp]# service iptables status
Firewall is stopped.
[root@bigboy tmp]#
Packet Processing In iptables
All packets inspected by iptables pass through a sequence of built-in tables (queues) for processing. Each of these queues is dedicated to a particular type of packet activity and is controlled by an associated packet transformation/filtering chain.
There are three tables in total. The first is the mangle table which is responsible for the alteration of quality of service bits in the TCP header. This is hardly used in a home or SOHO environment.
The second table is the filter queue which is responsible for packet filtering. It has three built-in chains in which you can place your firewall policy rules. These are the:
• Forward chain: Filters packets to servers protected by the firewall.
• Input chain: Filters packets destined for the firewall.
• Output chain: Filters packets originating from the firewall.
The third table is the nat queue which is responsible for network address translation. It has two built-in chains; these are:
• Pre-routing chain: NATs packets when the destination address of the packet needs to be changed.
• Post-routing chain: NATs packets when the source address of the packet needs to be changed
Table 14-1 Processing For Packets Routed By The Firewall
Queue
Type Queue
Function Packet transformation chain in Queue Chain Function
Filter Packet filtering FORWARD Filters packets to servers accessible by another NIC on the firewall.
INPUT Filters packets destined to the firewall.
OUTPUT Filters packets originating from the firewall
Nat Network Address Translation PREROUTING Address translation occurs before routing. Facilitates the transformation of the destination IP address to be compatible with the firewall's routing table. Used with NAT of the destination IP address, also known as destination NAT or DNAT.
POSTROUTING Address translation occurs after routing. This implies that there was no need to modify the destination IP address of the packet as in pre-routing. Used with NAT of the source IP address using either one-to-one or many-to-one NAT. This is known as source NAT, or SNAT.
OUTPUT Network address translation for packets generated by the firewall. (Rarely used in SOHO environments)
Mangle TCP header modification PREROUTING POSTROUTING OUTPUT INPUT FORWARD Modification of the TCP packet quality of service bits before routing occurs
(Rarely used in SOHO environments)
You need to specify the table and the chain for each firewall rule you create. There is an exception: Most rules are related to filtering, so iptables assumes that any chain that's defined without an associated table will be a part of the filter table. The filter table is therefore the default.
To help make this clearer, take a look at the way packets are handled by iptables. In Figure 14.1 a TCP packet from the Internet arrives at the firewall's interface on Network A to create a data connection.
The packet is first examined by your rules in the mangle table's PREROUTING chain, if any. It is then inspected by the rules in the nat table's PREROUTING chain to see whether the packet requires DNAT. It is then routed.
If the packet is destined for a protected network, then it is filtered by the rules in the FORWARD chain of the filter table and, if necessary, the packet undergoes SNAT in the POSTROUTING chain before arriving at Network B. When the destination server decides to reply, the packet undergoes the same sequence of steps. Both the FORWARD and POSTROUTING chains may be configured to implement quality of service (QoS) features in their mangle tables, but this is not usually done in SOHO environments.
If the packet is destined for the firewall itself, then it passes through the mangle table of the INPUT chain, if configured, before being filtered by the rules in the INPUT chain of the filter table before. If it successfully passes these tests then it is processed by the intended application on the firewall.
At some point, the firewall needs to reply. This reply is routed and inspected by the rules in the OUTPUT chain of the mangle table, if any. Next, the rules in the OUTPUT chain of the nat table determine whether DNAT is required and the rules in the OUTPUT chain of the filter table are then inspected to help restrict unauthorized packets. Finally, before the packet is sent back to the Internet, SNAT and QoS mangling is done by the POSTROUTING chain
Figure 14-1 Iptables Packet Flow Diagram
