Firewall Fundamentals

Linksys uses a web-based interface to perform all configuration functions. This interface is accessible by default from any internal host and is accessed using a web browser such as Microsoft Internet Explorer. Upon accessing the web-based interface, you are prompted with a Username/Password dialog box. Refer to the user guide of your appropriate router for the relevant information, but typically the username/password combination of admin/admin is the default user account. You can change the password from the Management screen, which is discussed later in this chapter. In the case of the BEFSR41v4, the interface is separated into five main tabs:

• Setup

• Security

• Applications & Gaming

• Administration

• Status

No configuration settings are accessible from the Status tab. As shown in Figure 5-3, it merely displays the status of the router.

Configuring Basic Setup

The BEFSR41v4 Setup tab consists of four screens:

• Basic Setup

• DDNS

• MAC Address Clone

• Advanced Routing

On the Basic Setup screen, you can configure how the router connects to the service provider (for example, using DHCP or PPPoE). Depending on which connection type you specify, additional options will be made available on the screen. You can also specify the host and domain name as well as the maximum transmission unit (MTU) for the router, if it is required by your service provider.

The Basic Setup screen is also where you configure the local network settings for the router (such as the internal interface IP address, and the DHCP server settings for the router). In the DHCP settings, you can specify the DNS servers to use; if you leave the values empty, the router automatically uses the values that it obtained from the service provider as the DNS server for the internal clients. Figure 5-4 shows the Basic Setup screen.

The DDNS screen is where you can configure the router with the appropriate settings to enable it to dynamically update the DNS settings with either DynDNS or TZO when the external IP address of the router changes. Just enter the username (DynDNS) or e-mail address (TZO) that you registered with, along with the appropriate password and domain name, and the router will automatically update DNS anytime the router's external IP address changes.

On the MAC Address Clone screen, you can configure a specific MAC address (for example, if your ISP requires a specific MAC address to be used by your router).

On the Advanced Routing screen, you can configure whether to use NAT as well as configure RIP or static routes, as shown in Figure 5-5.

NAT configuration is a simple enable/disable toggle. To enable RIP routing, just select Enable and then select the transmit and receive RIP versions from the drop-down boxes. To enter a static route, fill in the appropriate information and specify the interface that the route uses as the exit interface.

Configuring Security

The Security tab consists of two screens, Filter and VPN Passthrough. In both instances, the configuration applies to traffic from the internal network accessing external resources (egress filtering).

The Filter screen is where you can configure IP address, port, and MAC address filtering of internal hosts. For example, if you want to prevent host 192.168.173.115 from accessing the Internet, you can specify that IP address in the Filter IP Address Range fields, and the router will not allow that host to access external resources. Similarly, if you want to prevent certain port numbers from being accessed by internal hosts (for example, instant messenger software or gaming ports), you specify them, too. Keep in mind that the router only supports five entries for either IP address range or port range, so you need to be judicious about what you filter. Figure 5-6 shows the filter screen.

If you want to filter by MAC address, just click the Edit MAC Filter Setting button and specify the MAC addresses that should be denied access. At the bottom of the screen are four radial selections with the default setting in parenthesis:

• Block Anonymous Internet Requests (Enabled) This setting prevents the router from being able to be pinged or otherwise connected to on the external interface, unless you have defined a port-forwarding filter. This should be enabled, but keep in mind that not being able to ping the router can make it more difficult to troubleshoot.

• Filter Multicast (Disabled) This setting allows multicast traffic to be forwarded to the appropriate destination. Multicast traffic is traffic destined to multiple hosts. This allows the traffic to be sent one time, while allowing multiple registered hosts to receive it, which it more efficient than sending the traffic individually to each host (which is a process known as unicast). A host registers to receive this multicast traffic by virtue of the fact that it is running an application that is configured to listen on the corresponding multicast IP address. Multicast is frequently used for the transmission of multimedia and streaming data. Multicast traffic is frequently filtered when it is either unnecessary (for example, because no applications that utilize multicast are running on the network) or to prevent multicast-based attacks from being initiated (for example, to prevent an attack that uses multicast traffic to saturate a network with bogus traffic, thus effecting a DoS on the network). Although somewhat counterintuitive, you want to disable filtering if you want to permit multicast traffic.

• Filter Internet NAT Redirection (Disabled) This setting enables you to configure the router to block access to local resources from other local computers that are attempting to access the local resource via the external (NAT) address.

• Filter IDENT(port 113) (Enabled) IDENT allows hosts to query the device, and thus discover information about the host. Unless applications specifically require this degree of access, you should always filter IDENT traffic.

On the VPN Passthrough screen, you can configure the router to transparently pass IPSec, PPPoE, and PPTP traffic from internal hosts to external resources. All three settings are enabled by default, and if you are going to use NAT and need to access remote resources using any of the three protocols, you should enable these settings.

Configuring Applications & Gaming

The name of the Applications & Gaming tab is somewhat misleading because although the settings are typically going to be implemented by home users to support their gaming applications, in function the Applications & Gaming tab is where the configuration of filtering from external sources to internal resources is performed. This tab has five screens:

• Port Range Forwarding

• Port Triggering

• UPnP Forwarding

• DMZ

• QoS

On the Port Range Forwarding screen, you can configure the router to permit certain types of traffic from all external hosts over the specified ports to the specified internal destination. Thus, you can protect servers behind the router/firewall, while still allowing access to the applications and resources on the server from external hosts. For example, if you were running an SMTP server on the internal server located at 192.168.173.115, you would configure the router as shown in Figure 5-7.

On the Port Triggering screen, you can define a port or range of ports that, when the router detects an internal host attempting to connect to, causes the router to dynamically permit a port or range of ports to be forwarded to the internal host. In this fashion, applications that are running on external servers and that attempt to connect to the internal host over ports other than the one the internal host originally used can be configured to be permitted. This is typically done to support gaming applications, which frequently work by having a computer initiate a connection to a central server on one port and then communicate with any number of other servers using a different set of ports. For example, to support Unreal Tournament, you configure the router as shown in Figure 5-8. In this case, when the router detects an internal host attempting to connect to an external resource using TCP or UDP port 27900, the router automatically configures a forwarding rule to allow all external hosts to connect to the internal host over TCP or UDP ports 7777 through 7779.

On the UPnP Forwarding screen, you can configure port forwarding to UPnP-based devices. Unless you require UPnP, it is recommended to use basic port forwarding, which is more secure because it cannot be manipulated by hosts running the UPnP protocol.

On the DMZ screen, you can identify a single host that will be treated as a completely unfiltered and unprotected host by the router. Although the internal host still uses NAT for communications with external resources, the router/firewall allows all solicited and unsolicited traffic from external sources to the server specified as being in the DMZ.

On the QoS screen, you can define specific levels of service and priority for different types of network traffic. Such distinctions are typically done to ensure that latency-sensitive applications such as videoconferencing and Voice over IP (VoIP) are given priority and preferential treatment by the router. On this screen, you can configure the router to essentially place the defined traffic in front of any other traffic, to ensure that the specified traffic is allowed to pass instead of being delayed by other less-important traffic.

You can specify quality of service (QoS) priority by either the device MAC address, the Ethernet switch port that the traffic came from, or the application port in question. For example, if you are running an Internet-based phone, you can specify the MAC address to ensure that all traffic coming from the phone is given preferential treatment by the router. There are two priorities, low and high, allowing you to decide how the traffic should be treated.

Configuring Administration

On the Administration tab, you can define how the router will be managed and how logging should be configured. You can also perform software upgrades and reset the router to the factory defaults.

The Management screen is used to specify what the router password is. Keep in mind that all users will access the router web-based interface using the same password, so you should consider using a unique password for the router and sharing the password with as few people as possible. In addition, you can configure the router to allow remote management access, which can prove handy if your company has distributed the routers to remote locations, but expects the routers to be managed by a central entity. As a word of caution, however, permitting remote access allows anyone on the remote network who knows the password to potentially be able to access and configure the router; therefore, unless you really need this functionality, you should disable it. Also, keep in mind that because the router uses HTTP as the access protocol, all the data being transmittedincluding passwordsis sent in an unencrypted format, which means anyone with a network sniffer can capture and obtain that information. As a general rule, remote management access should not be permitted, and you should ensure that Block Anonymous Internet Requests is enabled on the Security|Filter screen.

The Management screen is also where you can configure the router to use UPnP to automatically configure the router to open ports and permit traffic. This is used in conjunction with the UPnP Forwarding screen of the Applications & Gaming tab that was previously mentioned in this chapter. Because UPnP allows for the automatic configuration of the router from UPnP hosts, unless you require UPnP it should be disabled, which is the default setting. Figure 5-9 shows the Management screen.

On the Log screen, you can specify the IP address of a syslog server and enable logging from the router. The Factory Defaults screen contains a simple toggle selection that enables you to reset the router to the factory defaults.

If you need to upgrade the software on the router, you can do so on the Firmware Upgrade screen. You can browse for an upgrade file on the local computer that is managing the router and click the button to upgrade. When the router has been upgraded, it reboots to begin running the new code.