Setting Up IEEE 802.11b Wireless Ethernet Networks

Understanding Wireless Ethernet Networks

There are two popular IEEE standards-802.11a and 802.11b-for wireless Ethernet networks, also commonly referred to Wi-Fi (for Wireless Fidelity) networks. These two standards were finalized in 1999, and they specify how the wireless Ethernet network works at the physical level.

Secret

The two wireless Ethernet standards have the following characteristics:

• 802.11b-Operates in the 2.4-GHz radio band (2.4 GHz to 2.4835 GHz) in up to three nonoverlapping frequency bands or channels. Supports a maximum bit rate of 11 Mbps per channel. One of the disadvantages of 802.11b is that the 2.4-GHz frequency band is crowded-many devices such as microwave ovens, cordless phones, medical and scientific equipment, as well as Bluetooth devices, all work within the 2.4-GHz frequency band. Nevertheless, 802.11b is very popular in corporate as well as home networks.

• 802.11a-Operates in the 5-GHz radio band (5.725 GHz to 5.850 GHz) in up to eight nonoverlapping channels. Supports a maximum bit rate of 54 Mbps per channel. The 5-GHz band is not as crowded as the 2.4-GHz band, but the 5-GHz band is not approved for use in Europe. Products conforming to 802.11a standard are now beginning to appear, and there are wireless access points that are designed to handle both 802.11a and 802.11b connections.

  In both cases, the maximum data throughput that a user sees is much less because all users of that radio channel share the capacity of the channel. Also, the data transfer rate decreases as the distance between the user's PC and the wireless access point increases.

  There is a third standard-802.11g-under development that would support 54-Mbps data rates in the 2.4-GHz band (the same band that 802.11b uses). 802.11g achieves the higher bit rate by using a technology called OFDM (orthogonal frequency division multiplexing), which is also used by 802.11a. 802.11g-compliant equipment is currently becoming available on the market.

Understanding Wired Equivalent Privacy (WEP)

The 802.11 standard includes the Wired Equivalent Privacy (WEP) for protecting wireless communications from eavesdropping. WEP relies on a 40-bit or 104-bit secret key that is shared between a mobile station (such as a laptop with a wireless Ethernet card) and an access point (also called a base station). The secret key is used to encrypt packets before they are transmitted and an integrity check is performed to ensure that packets are not modified in transit. The 802.11 standard does not explain how the shared key is established. In practice, most wireless LANs use a single key that is shared between all mobile stations and access points. Such an approach, however, does not scale well to an environment such as a college campus due to the fact that the keys are shared with all users. Therefore, WEP is typically not used on large wireless networks such as the ones at universities. In such wireless networks, users should other security approaches such as using SSH (Secure Shell) to log in to remote systems.

Secret

WEP uses the RC4 encryption algorithm, which is known as a stream cipher. Such a stream cipher works by taking a short secret key and generating an infinite stream of pseudorandom bits. The sending station performs an exclusive-OR operation between the pseudorandom bits and the bits representing the data packet. The receiver has a copy of the same secret key, so it generates an identical stream of pseudo-random bits. Performing an exclusive-OR between this pseudorandom stream and the received bits regenerates the original, unencrypted data packet.

Such a method of stream cipher has a few problems. If a bit is flipped (from a 0 to 1 or vice versa) in the encrypted data stream, then the corresponding is flipped in the decrypted output. Also, if an eavesdropper intercepts two encoded messages that were encoded with the same stream, then it is possible to generate the exclusive-OR of the original messages. That knowledge is enough to mount attacks that can eventually break the encryption.

To counter against these weaknesses, WEP uses some defenses:

• Integrity Check (IC) Field-To make sure that data packets are not modified in transit, WEP uses an Integrity Check field in each packet.

• Initialization Vector (IV)-To avoid encrypting two messages with the same key stream, WEP uses a 24-bit initialization vector (IV) that augments the shared secret key to produce a different RC4 key for each packet. The IV itself is also included in the packet.

Experts say that both of these defenses are poorly implemented, making WEP ineffective. The problems with IC and IV are the following:

• The Integrity Check field is implemented using a checksum algorithm called 32-bit cyclic redundancy code (CRC-32), and that checksum is then included as part of the data packet. Unfortunately, it is possible for an attacker to flip arbitrary bits in an encrypted message and correctly adjust the checksum so that the resulting message appears valid.

• The 24-bit IV is sent in the clear. This means that there are only 2²⁴ possible initialization vectors, and they have to be reused after running through all of them. In other words, after sending 2^24, or 16,777,216 packets, the IV has to be repeated. The number might sound like a lot, but consider the case of a busy access point that sends 1,500-byte packets at a rate of 11 Mbps. Each packet has 8x1,500 = 12,000 bits. That means each second the access point sends 11,000,000/12,000 = 916 packets. At that rate, the access point sends 16,777,216 packets in 16,777,216/916 = 18,315 seconds or 5 hours. That means the IV is reused after 5 hours, and the time may be less than that because many message are smaller than 1,500 bytes. Thus, there are ample opportunities for an attacker to collect two encrypted messages that are encrypted with the same key stream and perform statistical attacks to decrypt the message.

Thus, WEP has its weaknesses, but it's better than nothing. You can use it in smaller wireless LANs where sharing the same key among all wireless stations is not an onerous task.

Insider Insight

There is work underway to provide better security than WEP for wireless networks. The 802.11i standard, which is under development, would use public key encryption with digital certificates and an an authentication, authorization, and accounting RADIUS (Remote Authentication Dial-In User Service) server to provide better security for wireless Ethernet networks. While the 802.11i standard is in progress, the Wi-Fi Alliance-a multivendor consortium that supports Wi-Fi-is publishing an interim specification called Wi-Fi Protected Access (WPA) that's a precursor to 11i. WPA will replace the existing WEP standard and improve security by making some changes. For example, unlike WEP, which uses fixed keys, the WPA standard uses Temporal Key Integrity Protocol (TKIP), which generates new keys for every 10K of data transmitted over the network. This makes WPA more difficult to break.

Setting Up the Wireless Hardware

To set up the wireless connection, you need a wireless access point and a wireless network card in each PC. You can also set up an ad hoc wireless network among two or more PCs with wireless network cards, but that would be a wireless LAN among those PCs only. In this section, I focus on the scenario where you want to set up a wireless connection to an established LAN that has a wired Internet connection through a cable modem or DSL.

In addition to the wireless access point, you would also need a cable modem or DSL connection to the Internet, along with a NAT router/hub, as described in the previous sections. Figure 13-7 shows a typical setup for wireless Internet access through an existing cable modem or DSL connection.

Figure 13-7: Connecting a Mixed Wired and Wireless Ethernet LAN to the Internet.

As Figure 13-7 shows, the LAN has both wired and wireless PCs. In this example, a cable or DSL modem connect the LAN to the Internet through a NAT router/hub. A wireless access point is attached to one of the RJ-45 ports on the hub. Laptops with wireless network cards connect the LAN through the wireless access point. To connect desktop PCs to the wireless network, you can use a USB wireless network card-a wireless network card that connects to a USB port.

Configuring the Wireless Access Point

Configuring the wireless access point involves the following:

• Setting the frequency or channel on which the wireless access point communicates with the wireless network cards. Both the access point and cards must use the same channel.

• Whether encryption is to be used or not.

• If encryption is to be used, the number of bits in the encryption key and the value of the encryption key. Twenty-four bits of the encryption key are internal to the access point; you only specify the remaining bits of the encryption key. Thus, for 64-bit encryption, you have to specify a 40-bit key, which comes to 10 hexadecimal digits (a hexadecimal digit is one of 0 through 9 and the letters A through F). For a 124-bit encryption key, you specify 104 bits, or 26 hexadecimal digits.

• Setting the access method that wireless network cards would use when connecting to the access point. The choices are open access and shared key. You typically use the open access method (even when you use encryption).

• Setting the wireless access point to operate in infrastructure mode because that's the way you should connect wireless network cards to an existing Ethernet LAN.

  Insider Insight

  The exact method of configuring the wireless access point depends on the make and model of the access point. The vendor provides the instructions to configure the wireless access point. You would typically work through a graphical client application to configure the wireless access point. If you enable encryption, make a note of the encryption key because you have to specify the same key for each wireless network card.

Configuring Wireless Networking

On your Red Hat Linux laptop, the PCMCIA or PC Card manager should recognize the wireless network card and load the appropriate driver for the card. The wireless network card is treated like another Ethernet device and assigned a device name such as eth0 or eth1. If you already have an Ethernet card in the laptop, that card gets the eth0 device name, and the wireless PC card is named the eth1 device.

Secret

You do have to configure certain parameters to enable the wireless network card to communicate with the wireless access point. For example, the channel selection must be the same as that used by the wireless access point, and the encryption settings must match those on the access point.

Follow these steps to configure and activate the wireless network connection on your Red Hat Linux system:

1. Log in as root and type the iwconfig command (this command is similar to ifconfig, but meant for configuring a wireless network interface). Note the Ethernet device name for the wireless interface.

2. Assuming that the wireless network device name is eth0, edit the text file /etc/sysconfig/network-scripts/ifcfg-eth0. If the wireless device name is eth1, you have to edit /etc/sysconfig/network-scripts/ifcfg-eth1. This file contains various settings for the wireless network card. Table 13-2 explains the meaning of the settings. Here is a slightly edited version of the /etc/sysconfig/network-scripts/ifcfg-eth0 file from my laptop PC:

   USERCTL=no
   PEERDNS=yes
   TYPE=Wireless
   DEVICE=eth0
   HWADDR=00:02:2d:8c:f8:c5
   BOOTPROTO=dhcp
   ONBOOT=no
   DHCP_HOSTNAME=
   NAME=
   ESSID='UNKNOWN!!'
   CHANNEL=6
   MODE=Managed
   KEY=aecf-a00f-03
   RATE=auto
   

3. Edit the text file /etc/pcmcia/network and locate the following line:

   'start')

   Right after this line, add the following lines of text:

   [ -f /etc/sysconfig/network-scripts/ifcfg-${device} ] && \
        /etc/sysconfig/network-scripts/ifup ifcfg-${device}

   Save the file and exit.

4. Type the following command to reactivate the wireless network interface after making changes to the configuration file:

   service pcmcia restart
   

Now, the wireless interface should be working properly. To check the interface status, type the following command:

iwconfig

Here's a typical output on my Red Hat Linux laptop:

lo        no wireless extensions.

eth0      IEEE 802.11-DS  ESSID:"Test-Network"  Nickname:"localhost.localdomain"
          Mode:Managed  Frequency:2.437GHz  Access Point: 00:30:AB:06:2F:4E
          Bit Rate:11Mb/s   Tx-Power=15 dBm   Sensitivity:1/3
          RTS thr:off   Fragment thr:off
          Encryption key:AABB-CCDD-EE
          Power Management:off
          Link Quality:40/92  Signal level:-57 dBm  Noise level:-97 dBm
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:0  Invalid misc:4221   Missed beacon:0

Here the eth0 interface refers to the wireless network card. I have edited the encryption key and some other parameters, but the sample output shows what you should see when the wireless link is working.