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between the wireless LAN cards. Computers may be locked in a storage closet or stuck under a desk. A pile of papers might inhibit transmission, while a sheet of metal might stop it dead. A small antenna that draws the transmission away from the card might be the answer. While most wireless LAN cards don’t require a completely open line-of-sight, an obstacle can certainly slow reception. For example, the Orinoco IEEE range extender antenna can plug directly into an Orinoco Gold or Silver wireless LAN card. A 1.5-meter extension cable can bring the signal out from behind a closed door or out on top of a desk. When you set up the antenna, it is recommended that it be: Placed in a central location. Mounted vertically. Located away from obstructions (in particular, metal surfaces, and to a lesser extent, solid objects such as concrete walls or stacks of papers). Refer to the instructions that come with your antenna for specific guidelines regarding placing and mounting your antenna. Using outdoor antennas Choosing and setting up outdoor antennas for your wireless LAN can be more difficult and expensive than setting them up indoors. Once the outdoor antennas are in place, however, you can save money because you won’t need multiple Internet access accounts (monthly fees, DSL/Cable modems, and so on). Although a complete description of the way to use outside antennas with your wireless LAN is outside the scope of this chapter, here are some tips that will help you choose the type of antennas that might be appropriate for your wireless LAN. Point-to-point or multi-point? If you are creating a point-to-point link between two outdoor locations (for example, to share an Internet connection between two buildings), a directional antenna can help you achieve greater distance and transmission speeds. However, if your antenna is providing multi-point access for several other outdoor antennas or wireless clients (such as students working from laptops on the campus lawn), an omnidirectional antenna may be more appropriate. Clearance The clearer your line-of-sight between your outdoor antennas, the greater the distance and transmission speed you can achieve. Placing antennas at the highest possible points can prevent diminished performance from trees, cars, buildings, and other objects. The measurement of obstacles blocking the coverage area of your wireless transmission is referred to as the clearance factor.
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WECA. Allow you to select transmission rates (11, 5.5, 2, or 1 Mbps), select channels (in the 2.4 GHz range), and use power management features. Because antennas are built into Orinoco PC cards, for indoor office environments you may not need an additional antenna. As with any wireless card, distances that you can achieve between your wireless nodes depend on transmission rate (lower speeds go farther), receiver sensitivity, and the amount and type of obstacles. At the maximum transmission rate (11 Mbps), the Orinoco card gives you an estimated range between 80 feet (25 meters) in a closed area and 525 feet (160 meters) in an open area with no additional antenna. Selecting adapter cards If the computers on your wireless network are all laptops with PCMCIA slots, you only need to plug in your wireless cards to get started. However, if you are using a desktop computer with only ISA and PCI slots available, you will also need an adapter card.. Before you purchase a wireless LAN card for your desktop computer, make sure that you can get a compatible adapter card. Agere Systems offers both ISA and PCI Orinoco adapter cards. If you have an ISA slot available, you should get an ISA adapter. In fact, Agere Systems strongly recommends the ISA adapter if you have a choice between ISA and PCI. The ISA card supports the following I/O addresses: 3E2-3E1 (default) and 3E2-3E3. The PCI adapter works on computers that have the following features: PCI 2.2 (or higher) BIOS support PC99-compliancy PCI slots only The PCI adapter will not work on some older computers that don’t meet the BIOS specifications. To use the PCI adapter in Linux, additional configuration in the operating system may also be required. Note Although using the Orinoco ISA and PCI adapters with standard Type II PC is not specifically supported, many Type II PC cards (besides your wireless cards) will work in those adapters. Selecting antennas If you are setting up your wireless LAN among several computers that are in close proximity to each other, you may not need an additional antenna. To deal with obstructions and longer distances, however, you can add indoor or outdoor antennas to your wireless hardware. Again, because I have been discussing Orinoco wireless PC cards, I will illustrate different types of indoor and outdoor antennas that are compatible with those cards. Using indoor antennas The antennas that are built into wireless LAN cards often work well enough to enable communication between computers in an open area. Additional indoor antennas are useful if direct line-of-sight is blocked
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WECA. Allow you to select transmission rates (11, 5.5, 2, or 1 Mbps), select channels (in the 2.4 GHz range), and use power management features. Because antennas are built into Orinoco PC cards, for indoor office environments you may not need an additional antenna. As with any wireless card, distances that you can achieve between your wireless nodes depend on transmission rate (lower speeds go farther), receiver sensitivity, and the amount and type of obstacles. At the maximum transmission rate (11 Mbps), the Orinoco card gives you an estimated range between 80 feet (25 meters) in a closed area and 525 feet (160 meters) in an open area with no additional antenna. Selecting adapter cards If the computers on your wireless network are all laptops with PCMCIA slots, you only need to plug in your wireless cards to get started. However, if you are using a desktop computer with only ISA and PCI slots available, you will also need an adapter card.. Before you purchase a wireless LAN card for your desktop computer, make sure that you can get a compatible adapter card. Agere Systems offers both ISA and PCI Orinoco adapter cards. If you have an ISA slot available, you should get an ISA adapter. In fact, Agere Systems strongly recommends the ISA adapter if you have a choice between ISA and PCI. The ISA card supports the following I/O addresses: 3E2-3E1 (default) and 3E2-3E3. The PCI adapter works on computers that have the following features: PCI 2.2 (or higher) BIOS support PC99-compliancy PCI slots only The PCI adapter will not work on some older computers that don’t meet the BIOS specifications. To use the PCI adapter in Linux, additional configuration in the operating system may also be required. Note Although using the Orinoco ISA and PCI adapters with standard Type II PC is not specifically supported, many Type II PC cards (besides your wireless cards) will work in those adapters. Selecting antennas If you are setting up your wireless LAN among several computers that are in close proximity to each other, you may not need an additional antenna. To deal with obstructions and longer distances, however, you can add indoor or outdoor antennas to your wireless hardware. Again, because I have been discussing Orinoco wireless PC cards, I will illustrate different types of indoor and outdoor antennas that are compatible with those cards. Using indoor antennas The antennas that are built into wireless LAN cards often work well enough to enable communication between computers in an open area. Additional indoor antennas are useful if direct line-of-sight is blocked
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Other wireless cards that are supported in Red Hat Linux are listed in the PCMCIA Card Configuration Database file (/etc/pcmcia/config). Table 25-1 shows the wireless network adapters listed in this file, along with the module needed for each item. Note Just because you don t see your wireless card listed in Table 25-1 doesn t mean that it won’t work. The same technology is often referred to by different names. Rather than try to track all the various name changes and acquired companies in the wireless industry, I refer you to the Linux Wireless LAN HOWTO (http://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux). The Drivers section provides insight into which drivers work with which cards. Table 25-1: Supported Wireless Network Adapters and Modules Wireless Network Adapter Module 350 Series Wireless LAN Adapter (Cisco Systems) airo_cs Aironet PC4500 (Cisco Systems) airo_cs Aironet PC4800 (Cisco Systems) airo_cs AT&T WaveLAN Adapter wavelan_cs Cabletron RoamAbout 802.11 DS wvlan_cs Compaq WL100 11 Mbps Wireless Adapter orinoco_cs Digital RoamAbout/DS wavelan_cs ELSA AirLancer MC-11 wvlan_cs Lucent Technologies WaveLAN Adapter wavelan_cs Intersil PRISM2 11 Mbps Wireless Adapter wvlan_cs Lucent Technologies WaveLAN/IEEE Adapter wvlan_cs MELCO WLI-PCM-L11 wvlan_cs NCR WaveLAN Adapter wavelan_cs NCR WaveLAN/IEEE Adapter wvlan_cs RayLink PC Card WLAN Adapter ray_cs Xircom CreditCard Netwave netwave_cs The Orinoco PC cards I purchased use the wvlan_cs module. The cards come in two types: Gold Label, which offers 128-bit WEP RC4 encryption, and Silver Label, which only offers 64-bit WEP RC4 encryption. They also differ in cost by about $10. Both cards: Offer compatibility with earlier WaveLAN/IEEE products. Can communicate with other 802.11b wireless LAN equipment that has been Wi-Fi-certified by
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Note To see a complete list of Wi-Fi-certified products, visit the WECA Web site (www.wirelessethernet.org/certified_products.asp). Although these products should be able to communicate with each other, they do not all have drivers that allow them to be used on a computer driven by a Linux system. After your wireless network has been configured, you can use the wireless connections as you would a regular wired Ethernet connection. For example, you can configure TCP/IP on top of your wireless network so that it acts as a gateway to your network’s Internet connection. If you are using Linux as a wireless network client as well, you can take full advantage of firewall, masquerading, network proxy, or other networking features to protect and make full use of your wireless network. Choosing Wireless Hardware To get started with a wireless Linux LAN, you need at least two computers and two wireless LAN cards. The wireless LAN cards described in this chapter are PCMCIA-type cards that you insert into those credit card sized slots on laptop computers. If ISA or PCI slots alone are available on your computers, you will need to add an adapter card. You may also want (or need) to add indoor or outdoor antennas to your wireless network. Selecting wireless cards Not all wireless LAN cards that you can purchase today will work with Linux. When you select a card, make sure that a Linux driver has been created for the card. Beyond that, you want to look for several other features: Card type Most wireless LAN cards are PCMCIA (PC card) form. To use these cards in desktop computers, you probably need an ISA or PCI adapter card. Cost If you shop around, you can find wireless LAN cards that cost between $90 and $150 (and the prices will probably go down by the time you read this). More expensive cards might include external antennas or better encryption (128-bit as opposed to 64-bit). External connector Different wireless LAN cards have different types of connectors, which can lock you into buying antennas from the same manufacturer if you are communicating outside of a small area. People who know such things tell me that it is possible to take apart the cards and hack together your own antenna. However, because I don’t want to be responsible for wrecking your card, and because there are legal issues related to antenna usage, I don’t recommend this course of action. Configurable If you really want to fine-tune your wireless LAN, find out how much control you have over configuring a card before you choose it. Descriptions of the iwconfig command later in this chapter will help you understand what wireless extensions can be manipulated. Refer to man pages for individual wireless card drivers (for example, type man wvlan_cs), for information on specific parameters that you can change. For this chapter, I chose to use Orinoco wireless PC cards from Agere Systems to illustrate how to set up a wireless Linux LAN. Although other wireless cards are supported in Linux, Orinoco cards seem to have particularly good Linux drivers, and many people have reported success using them. The cards were also relatively inexpensive (about $90 to $100). Previously, Orinoco cards were referred to as WaveLAN cards and were produced by Lucent Technologies (as well as AT&T and NCR) before Agere Systems was spun off as a separate company.
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Understanding Wireless Networks Wireless LANs are most appropriate in environments where wires are impractical. Despite some challenges relating to security and interference, a wireless LAN provides several advantages: You don’t have to run wires in places that are hard to reach. In many cases, a single wireless LAN can extend your network throughout a building or to another building without the need for wires between all nodes. For the price of a wireless card, you can save the expense of wires, hubs (the air is your hub), and wall repairs (to fix the holes from pulling through wires). You can freely move computers around within the transmission range that your environment allows (with distances limited by such variables as antenna power, obstacles, and rates of transmission). Although several different wireless networking standards exist, this chapter focuses on the installation of relatively low-cost, standard IEEE 802.11b wireless networking equipment. An 802.11b wireless network uses space in the spectrum available to the public (in other words, you use space in the air for which no special license is required). The 802.11b standard is often referred to as the Wi-Fi, or Wireless Fidelity, standard. An 802.11b network is characterized by the following: Provides transmission rates of up to 11 Mbps. Transmission rates can also be set (or auto-detected) to 5.5, 2, and 1 Mbps. Uses the 2.4-GHz band of the spectrum. Microwave ovens and some high-end mobile phones also use the 2.4-GHz band. (Check local regulations if you are setting up an 802.11b network outside the United States.) To reduce congestion, 14 separate channels have been made available within the 2.4-GHz range. Allows transmission over distances as short as a desktop away to as long as several miles away (using special antennas). Greater distances can be gained at lower transmission speeds. Makes connections between multiple clients or clients and a base station (usually referred to as an access point). On the clients, the wireless LAN cards to run in Ad hoc mode, while the base station uses Managed mode. Note The Orinoco card, as well as other wireless network cards supported in Linux, cannot act as an access point because it does not do bridging. Bridging allows a node to receive a frame from one node and forward it to another node without changing the first node’s MAC address. A wireless LAN card in Linux, however, can communicate with an access point by running in Managed mode and indicating the MAC address of the access point. Other 802.11 standards exist (such as 802.11a which can operate at higher speeds), but for the most part, wireless equipment manufacturers have rallied around the 802.11b standard. Wireless cards and other equipment certified Wi-Fi (802.11b) compatible by the Wireless Ethernet Compatibility Alliance (WECA) should be able to communicate with each other.
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Figure 25-2: In Managed mode, wireless communication can go through an access point. Roaming Each wireless network can be configured as a separate cell. Multiple cells can be joined together in what is called a Managed Wireless LAN. In this arrangement, each cell’s access point acts as a bridge (in fact, its actions are referred to as bridging) by passing all data from the cell on to other cells without changing any node’s MAC address. In other words, the Managed WLAN makes the fact that there are multiple wireless cells invisible to the clients on those cells. This arrangement allows users to roam among cells as they come in and out of range without losing continuity of communication. Note Although there are no supported wireless LAN cards in Linux that can act as a bridging wireless access point, you can have the Linux system act as a client to a bridging access point. Set your card to Managed mode and identify the MAC address for your wireless LAN’s access point. Although you can use wireless LAN cards with other computer systems, you may want to use Linux systems for one or more nodes in your wireless network. For example, the features in Linux can eliminate the need to buy other types of equipment. Some additional Linux features that make it a valuable asset on a wireless LAN include: Internet access You don’t need a separate router or gateway machine to attach your wireless LAN to the Internet. Having wired and wireless Ethernet LAN cards on a Linux system enables your wireless clients to access the Internet through your Linux system. Firewall Due to some inherent security weaknesses with wireless encryption protocols, you may want to add an extra measure of security to your network by configuring firewalls. With a Red Hat Linux firewall (ipchains or iptables) on the boundaries between your wireless LAN and your larger network, if someone were to hack into your wireless LAN, you would still have a measure of protection for your larger network. Monitoring and logging All the tools that you use for monitoring and logging activity on your wired networks in Red Hat Linux are also available for your wireless network. This chapter describes how to use wireless LAN equipment on computers running Red Hat Linux to create a wireless Ethernet LAN. It focuses on configuring two Red Hat Linux systems for wireless communication; however, once you configure these nodes, you can add Windows, Linux, or other types of systems to your wireless LAN by installing compatible wireless cards on each system.
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Chapter 25: Building a Wireless Linux LAN Overview by Christopher Negus Sometimes it s not convenient to run wires to all the computers on your network. Pulling Ethernet cables through existing walls can be a pain. Dragging wires into your garden so you can sit in a lounge chair and surf the Internet can ruin the ambiance. In many cases, a wireless LAN is an economical solution. A wireless Ethernet LAN can be configured in several different modes, depending on how you want to use the LAN. Here are some examples: Peer-to-peer To directly access other nodes on the wireless Ethernet LAN, you can set your wireless card to operate in Ad hoc mode. In this mode, communication doesn’t have to go through a wireless access point to communicate with peer computers. Instead, frames of data are broadcast to all nodes within range, but are consumed only by the computers for which they are intended. This arrangement is useful if you are sharing file and print services among a group of client computers. Figure 25-1 shows an example of a peer-to-peer wireless LAN. Figure 25-1: In Ad hoc mode, wireless LANs can communicate as peers. Access point By setting your wireless LAN card to Managed mode, your wireless interface can act as an access point for one or more wireless clients. Clients can be configured to communicate directly with the access point, instead of every client that is within range. This arrangement is useful for point-to-point connections between two buildings, where the access point is acting as a gateway to the Internet or campus Intranet. Figure 25-2 depicts a point-to-point wireless LAN operating in Managed mode.
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You could also check tables for all your databases at once as follows: # myisamchk /var/lib/mysql/*/*.MYI The example above shows a simple, 5-record database where no errors were encountered. If instead of the output shown above, you see output like the following you may need to repair the database: Checking MyISAM file: names.MYI Data records: 5 Deleted blocks: 0 - check file-size myisamchk: warning: Size of datafile is: 689 Should be: 204 - check key delete-chain - check record delete-chain - check index reference - check record links myisamchk: error: Found wrong record at 0 MyISAM-table ‘names.MYI’ is corrupted Fix it using switch “-r” or “-o” 3. To fix a corrupted database, you could run the following command # myisamchk -r /var/lib/allusers/names.MYI - recovering (with keycache) MyISAM-table ‘names.MYI’ Data records: 5 Found wrong stored record at 0 Data records: 4 4. If for some reason the -r options doesn’t work, you can try running the myisamchk command with the -o option. This is a slower, older method of repair, but it can handle a few problems that the -r option cannot. Here is an example: # myisamchk -o /var/lib/allusers/names.MYI Tip If your computer has a lot of memory, raise the key buffer size value on the myisamchk command line, which will lessen the time it takes to check the databases. For example, you could use the following command line: # myisamchk -r -O –key_buffer_size=64M *.MYI This would set the key buffer size to 64 Megabytes. Summary MySQL is a Structured Query Language (SQL) server that runs on Red Hat Linux, as well as other operating systems. Using a start-up script (/etc/init.d/mysqld) and a configuration file (/etc/my.cnf), you can quickly get a MySQL server up and running. With tools such as the mysqladmin and mysql commands, you can administer the MySQL server and create databases and tables that are as simple or complex as you need. During mysql sessions, you can modify the structure of your database tables or add, update, or delete database records. You have a variety of options for querying data and sorting the output. You also have a lot of control over who can access your database tables and what privileges users have to modify, add to, or delete from the databases you control.
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Found wrong stored record at 0 Data records: 4 4. If for some reason the -r options doesn’t work, you can try running the myisamchk command with the -o option. This is a slower, older method of repair, but it can handle a few problems that the -r option cannot. Here is an example: # myisamchk -o /var/lib/allusers/names.MYI Tip If your computer has a lot of memory, raise the key buffer size value on the myisamchk command line, which will lessen the time it takes to check the databases. For example, you could use the following command line: # myisamchk -r -O –key_buffer_size=64M *.MYI This would set the key buffer size to 64 Megabytes. Checking and Fixing Databases Over time, databases can become corrupted or store information inefficiently. MySQL comes with commands that you can use to check and repair your databases. The myisamchk and isamchk commands are available to check MyISAM and ISAM database tables, respectively. MyISAM tables are used by default with MySQL. The tables are stored in the /var/lib/mysql/dbname directory by default, where dbname is replaced by the name of the database you are using. For each table, there are three files in this directory. Each file begins with the table name and ends with one of the following three suffixes: .frm Contains the definition (or form) or the table .MYI Contains the table’s index. .MYD Contains the table’s data. The following procedure describes how to use the myisamchk command to check your MyISAM tables. (The procedure is the same for checking ISAM tables, except that you use the isamchk command instead.) Note Do a backup of your database tables before running a repair with myisamchk. Though myisamchk is unlikely to damage your data, backups are still a good precaution. 1. Stop your MySQL server temporarily by typing the following from a Terminal window as the root user: # /etc/init.d/mysqld stop 2. You can check all or some of your database tables at once. The first example shows how to check a table called “names” in the “allusers” database. # myisamchk /var/lib/mysql/allusers/names.MYI Checking MyISAM file: /var/lib/mysql/allusers/names.MYI Data records: 5 Deleted blocks: 0 - check file-size - check key delete-chain - check record delete-chain - check index reference - check record links
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