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Broadband Routers and Firewalls

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Nov 17, 2006.

Chapter Description

Want to take advantage of the increased speed and better functionality that a broadband connection provides, but need to ensure that your system is as secure as possible? This sample chapter, from the book Firewall Fundamentals, explains how routers and firewalls work and offers specific tips for setting up Linksys equipment.

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Firewall Fundamentals

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Linksys Broadband Routers/Firewalls

Linksys makes a number of broadband routers (with basic firewall functionality) and broadband firewalls (with advanced firewall functionality) for both wired and wireless networks. Most of the wired products begin with a model number of BEF; most of the wireless products begin with a model number of WRT. The Linksys broadband routers/firewalls are designed with the home user in mind, and therefore are designed with simplicity of implementation in mind. All function as NAT routers, and some models and versions also provide stateful packet inspection in addition to NAT; unfortunately, Linksys does not do a good job of specifying which models and versions of firmware have this functionality. This difficulty is compounded by the fact that SPI was removed from some versions of firmware, so literally the same hardware with different versions of firmware may or may not support SPI.

This chapter examines the Linksys BEFSR41v4 EtherFast Cable/DSL Router with 4-Port Switch. The BEFSR41v4 is designed primarily for the home and small office user, and as a result has a relatively basic and simple-to-implement feature set. For ease of review, the features have been categorized as follows for the discussion that follows:

  • Security and filtering features
  • Routing features
  • Management and administration features
  • Miscellaneous features

Security and Filtering Features

The BEFSR41v4 is a basic NAT router (with firewall functionality) that can perform basic port filtering to allow traffic both coming into and going out of the protected network to be filtered. Unlike many firewalls that take a "block all, permit only" minimalist approach to filtering outbound traffic, the Linksys is just the opposite, instead taking the approach of "permit all outbound, block only." The idea is that it is easier to block a couple of ports or IP addresses than it is to identify the ports or IP addresses that should be permitted.

Inbound traffic still adheres to the minimalist filtering policy, blocking all traffic to all ports unless you otherwise configure the router to permit the traffic. Unfortunately, filtering incoming traffic can only be done based on the destination port number, so it is not possible to permit only certain external hosts to access the protected resources. Either the entire Internet can access the resources or none of the Internet can.

The BEFSR41v4 also supports the concept of a demilitarized zone (DMZ) system. The DMZ functions by effectively taking a host from the internal network and using NAT to expose it in an unfiltered fashion to the Internet. This exposure allows any Internet host to fully connect to and access the host in an unrestricted and nonfirewalled manner. In general, a DMZ is a bad idea; however, some circumstances, particularly when attempting to run gaming applications and such, require connectivity to the system that the Linksys filtering rules are not capable of easily or properly supporting. Consequently, a DMZ provides a simple, albeit entirely insecure method of making sure that the host can be accessed by Internet hosts.

Because Linksys routers utilize NAT, some protocols such as IPSec, PPP over Ethernet (PPPoE) passthrough, and Point-to-Point Tunneling Protocol (PPTP) fail to function properly. This failure results because NAT changes the source address of packets that are translated through the router, causing the destination host for those packets to believe that the data has been compromised (which strictly speaking, it has). To facilitate using these protocols through a Linksys router/firewall, Linksys supports what is known as virtual private network (VPN) passthrough. VPN passthrough allows traffic in a VPN tunnel to pass through the router/firewall by essentially encapsulating the entire VPN packet in another packet, typically User Datagram Protocol (UDP). The router can then perform the NAT translation on that UDP packet, never actually changing the contents of the VPN packet. If you want to allow VPN traffic to pass through the router, you must enable VPN passthrough.

Routing Features

Because the BEFSR41v4 is targeted at the small office as well as the home user market, it supports some basic routing capabilities to allow it to be deployed in an environment with multiple internal subnets. In addition to being able to configure static routes, the router also supports RIP versions 1 and 2. Although RIP can prove adequate for small environments, the implementation of RIP on the router is extremely basic and lacks any kind of security functions; therefore, you should strongly consider whether this router is the appropriate firewall solution for you if you need the firewall to provide advanced routing functionality. In such cases, a more robust firewall such as the Cisco Secure PIX Firewall might be a better solution.

Management and Administration Features

Most Linksys network devices use a web-based management interface that uses HTTP as the transport protocol. Unfortunately, HTTP does not provide for encryption or security of the data being transported, so you should use caution with regard to the passwords you configure for the router, because they can relatively easily be captured using a network sniffer. By default, the router does not allow management access to the external interface, and although it can be permitted, it is generally a bad idea to do so.

The security model employed by Linksys is a simple shared password security model. All users log in using the same username and password to perform any management functions, and all authenticated users have the same rights.

The Linksys routers also typically provide basic syslog functionality, allowing the router to send events to a syslog server on the same subnet as the internal interface, as well as their own internal log-viewing software known as Log Viewer (which you can find at ftp://ftp.linksys.com/pub/befsr41/).

Miscellaneous Features

Because most home users do not have a Dynamic Host Configuration Protocol (DHCP) server on their home network, most Linksys routers feature DHCP server functionality built in to the router and enabled by default. This functionality allows a user to simply plug a computer into one of the router's switch ports, obtain an IP address that is valid for the router (typically on the subnet), and then connect to the router using a web browser on the computer to configure the router accordingly (typically, the router internal interface IP address is

Another feature of newer Linksys routers that can be enabled but is typically disabled by default is Universal Plug-and-Play (UPnP). UPnP allows hosts on the internal network that are using UPnP-capable operating systems to automatically configure the router to allow traffic from the external network to access the corresponding internal network resource. As a general rule, unless this functionality is required, you should disable UPnP on your router.

To facilitate connectivity to various broadband providers, most Linksys routers support multiple Internet connection types. The default setting is just to use DHCP to obtain an external IP address from the service provider, but static assigned IP addresses and PPPoE are supported, as well as solutions specific to certain areas of the world, such as Remote Access Service (RAS), PPTP, and Heart Beat Signal. Because many service providers provide only a dynamic IP address for use on the external interface, most Linksys routers also support dynamic Domain Name System (DNS) through either DynDNS (http://www.dyndns.org) or TZO (http://www.tzo.com). This support allows the router to automatically update the DNS entries for hosts that are protected by the router but need to be Internet accessible (such as websites). In both cases, you need to have a valid account with either DynDNS or TZO for this functionality to work properly.

3. Linksys Requirements | Next Section Previous Section

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