Chapter 18. Firewalls

Information security is commonly thought of as a process and not a product. However, standard security implementations usually employ some form of dedicated mechanism to control access privileges and restrict network resources to users who are authorized, identifiable, and traceable. Red Hat Enterprise Linux includes several tools to assist administrators and security engineers with network-level access control issues.
Firewalls are one of the core components of a network security implementation. Several vendors market firewall solutions catering to all levels of the marketplace: from home users protecting one PC to data center solutions safeguarding vital enterprise information. Firewalls can be stand-alone hardware solutions, such as firewall appliances by Cisco, Nokia, and Sonicwall. Vendors such as Checkpoint, McAfee, and Symantec have also developed proprietary software firewall solutions for home and business markets.
Apart from the differences between hardware and software firewalls, there are also differences in the way firewalls function that separate one solution from another. Table 18.1, “Firewall Types” details three common types of firewalls and how they function:

Table 18.1. Firewall Types

Method Description Advantages Disadvantages
NAT Network Address Translation (NAT) places private IP subnetworks behind one or a small pool of public IP addresses, masquerading all requests to one source rather than several. The Linux kernel has built-in NAT functionality through the Netfilter kernel subsystem.
· Can be configured transparently to machines on a LAN
· Protection of many machines and services behind one or more external IP addresses simplifies administration duties
· Restriction of user access to and from the LAN can be configured by opening and closing ports on the NAT firewall/gateway
· Cannot prevent malicious activity once users connect to a service outside of the firewall
Packet Filter A packet filtering firewall reads each data packet that passes through a LAN. It can read and process packets by header information and filters the packet based on sets of programmable rules implemented by the firewall administrator. The Linux kernel has built-in packet filtering functionality through the Netfilter kernel subsystem.
· Customizable through the iptables front-end utility
· Does not require any customization on the client side, as all network activity is filtered at the router level rather than the application level
· Since packets are not transmitted through a proxy, network performance is faster due to direct connection from client to remote host
· Cannot filter packets for content like proxy firewalls
· Processes packets at the protocol layer, but cannot filter packets at an application layer
· Complex network architectures can make establishing packet filtering rules difficult, especially if coupled with IP masquerading or local subnets and DMZ networks
Proxy Proxy firewalls filter all requests of a certain protocol or type from LAN clients to a proxy machine, which then makes those requests to the Internet on behalf of the local client. A proxy machine acts as a buffer between malicious remote users and the internal network client machines.
· Gives administrators control over what applications and protocols function outside of the LAN
· Some proxy servers can cache frequently-accessed data locally rather than having to use the Internet connection to request it. This helps to reduce bandwidth consumption
· Proxy services can be logged and monitored closely, allowing tighter control over resource utilization on the network
· Proxies are often application-specific (HTTP, Telnet, etc.), or protocol-restricted (most proxies work with TCP-connected services only)
· Application services cannot run behind a proxy, so your application servers must use a separate form of network security
· Proxies can become a network bottleneck, as all requests and transmissions are passed through one source rather than directly from a client to a remote service

18.1. Netfilter and IPTables

The Linux kernel features a powerful networking subsystem called Netfilter. The Netfilter subsystem provides stateful or stateless packet filtering as well as NAT and IP masquerading services. Netfilter also has the ability to mangle IP header information for advanced routing and connection state management. Netfilter is controlled using the iptables tool.

18.1.1. IPTables Overview

The power and flexibility of Netfilter is implemented using the iptables administration tool, a command line tool similar in syntax to its predecessor, ipchains.
A similar syntax does not mean similar implementation, however. ipchains requires intricate rule sets for: filtering source paths; filtering destination paths; and filtering both source and destination connection ports.
By contrast, iptables uses the Netfilter subsystem to enhance network connection, inspection, and processing. iptables features advanced logging, pre- and post-routing actions, network address translation, and port forwarding, all in one command line interface.
This section provides an overview of iptables.