| Documentation / Chapter 28. Wireless Communication | ||||
|---|---|---|---|---|
 | Part VI. Mobility | Chapter 29. Using Tablet PCs |  | |
| Documentation / Chapter 28. Wireless Communication | ||||
|---|---|---|---|---|
| Part VI. Mobility | Chapter 29. Using Tablet PCs | | |
Contents
Abstract
There are several possibilities for using your Linux system to communicate with other computers, cellular phones, or peripheral devices. WLAN (wireless LAN) can be used to network laptops.
Wireless LANs have become an indispensable aspect of mobile computing. Today, most laptops have built-in WLAN cards. The 802.11 standard for the wireless communication of WLAN cards was prepared by the IEEE organization. Originally, this standard provided for a maximum transmission rate of 2 Mbit/s. Meanwhile, several supplements have been added to increase the data rate. These supplements define details such as the modulation, transmission output, and transmission rates (see Table 28.1, “Overview of Various WLAN Standards”). Additionally, a lot of companies implement hardware with proprietary or draft features.
Table 28.1. Overview of Various WLAN Standards
|
Name |
Band (GHz) |
Maximum Transmission Rate (Mbit/s) |
Note |
|---|---|---|---|
|
802.11 Legacy |
2.4 |
2 |
Outdated; virtually no end devices available |
|
802.11a |
5 |
54 |
Less interference-prone |
|
802.11b |
2.4 |
11 |
Less common |
|
802.11g |
2.4 |
54 |
Widespread, backwards-compatible with 11b |
|
802.11n draft |
2.4 and/or 5 |
300 |
Common |
802.11 Legacy cards are not supported by openSUSE®. Most cards using 802.11a, 802.11b, 802.11g and 802.11n draft are supported. New cards usually comply with the 802.11n draft standard, but cards using 802.11g are still available.
In wireless networking, various techniques and configurations are used to ensure fast, high-quality, and secure connections. Different operating types suit different setups. It can be difficult to choose the right authentication method. The available encryption methods have different advantages and pitfalls.
Basically, wireless networks can be classified as managed networks and ad-hoc networks. Managed networks have a managing element: the access point. In this mode (also referred to as infrastructure mode), all connections of the WLAN stations in the network run over the access point, which may also serve as a connection to an ethernet. Ad-hoc networks do not have an access point. The stations communicate directly with each other, therefore an ad-hoc network is usually faster than a managed network. However, the transmission range and number of participating stations are greatly limited in ad-hoc networks. They also do not support WPA authentication. Therefore, an access point is usually used. It is even possible to use a WLAN card as an access point. Some cards support this functionality.
Because a wireless network is much easier to intercept and compromise than a wired network, the various standards include authentication and encryption methods. In the original version of the IEEE 802.11 standard, these are described under the term WEP. However, because WEP has proven to be insecure (see Section 28.1.4.2, “Security”), the WLAN industry (joined under the name Wi-Fi Alliance) has defined a new extension called WPA, which is supposed to eliminate the weaknesses of WEP. The later IEEE 802.11i standard (also referred to as WPA2, because WPA is based on a draft version 802.11i) includes WPA and some other authentication and encryption methods.
To make sure that only authorized stations can connect, various authentication mechanisms are used in managed networks:
An open system is a system that does not require authentication. Any station can join the network. Nevertheless, WEP encryption (see Section 28.1.1.2, “Encryption”) can be used.
In this procedure, the WEP key is used for the authentication. However, this procedure is not recommended, because it makes the WEP key more susceptible to attacks. All an attacker needs to do is to listen long enough to the communication between the station and the access point. During the authentication process, both sides exchange the same information, once in encrypted form and once in unencrypted form. This makes it possible for the key to be reconstructed with suitable tools. Because this method makes use of the WEP key for the authentication and for the encryption, it does not enhance the security of the network. A station that has the correct WEP key can authenticate, encrypt, and decrypt. A station that does not have the key cannot decrypt received packets. Accordingly, it cannot communicate, regardless of whether it had to authenticate itself.
WPA-PSK (PSK stands for preshared key) works similarly to the Shared Key procedure. All participating stations as well as the access point need the same key. The key is 256 bits in length and is usually entered as a passphrase. This system does not need a complex key management like WPA-EAP and is more suitable for private use. Therefore, WPA-PSK is sometimes referred to as WPA “Home”.
Actually, WPA-EAP is not an authentication system but a protocol for transporting authentication information. WPA-EAP is used to protect wireless networks in enterprises. In private networks, it is scarcely used. For this reason, WPA-EAP is sometimes referred to as WPA “Enterprise”.
WPA-EAP needs a Radius server to authenticate users. EAP offers three different methods for connecting and authenticating to the server: TLS (Transport Layer Security), TTLS (Tunneled Transport Layer Security), and PEAP (Protected Extensible Authentication Protocol). In a nutshell, these options work as follows:
TLS authentication relies on the mutual exchange of certificates both for server and client. First, the server presents its certificate to the client where it is evaluated. If the certificate is considered valid, the client in turn presents its certificate to the server. While TLS is secure, it requires a working certification management infrastructure in your network. This infrastructure is rarely found in private networks.
Both TTLS and PEAP are two-stage protocols. In the first stage, a secure connection is established and in the second one the client authentication data is exchanged. They require far less certification management overhead than TLS, if any.
There are various encryption methods to ensure that no unauthorized person can read the data packets that are exchanged in a wireless network or gain access to the network:
This standard makes use of the RC4 encryption algorithm, originally with a key length of 40 bits, later also with 104 bits. Often, the length is declared as 64 bits or 128 bits, depending on whether the 24 bits of the initialization vector are included. However, this standard has some weaknesses. Attacks against the keys generated by this system may be successful. Nevertheless, it is better to use WEP than not encrypt the network at all.
Some vendors have implemented the non-standard “Dynamic WEP”. It works exactly as WEP and shares the same weaknesses, except the fact that the key is periodically changed by a key management service.
This key management protocol defined in the WPA standard uses the same encryption algorithm as WEP, but eliminates its weakness. Because a new key is generated for every data packet, attacks against these keys are in vain. TKIP is used together with WPA-PSK.
CCMP describes the key management. Usually, it is used in connection with WPA-EAP, but it can also be used with WPA-PSK. The encryption takes place according to AES and is stronger than the RC4 encryption of the WEP standard.
To configure the wireless network card, select + in the YaST control center. The Network Settings dialog where you can configure general network settings opens. Please refer to Section 19.4, “Configuring a Network Connection with YaST” for more information about the general network configuration. All network cards that have been detected by the system are listed under the tab.
Choose your wireless card from the list and click to open the Network Card Setup dialog. Configure whether to use a dynamic or a static IP address under the tab . You can also adjust and settings such as or and driver settings. In most cases there is no need to change the preconfigured values.
Click to proceed to the wireless network card specific configuration dialog. If you are using NetworkManager (refer to Section 19.5, “NetworkManager” for more information), there is no need to adjust the wireless device settings, since these will be set by NetworkManager on demand—proceed with and to finish the configuration. If you are using your computer only in a specific wireless network, make the basic settings for WLAN operation here.
A station can be integrated in a WLAN in three different modes. The suitable mode depends on the network in which to communicate: (peer-to-peer network without access point), (network is managed by an access point), or (your network card should be used as the access point). To use any of the WPA-PSK or WPA-EAP modes, the operating mode must be set to .
All stations in a wireless network need the same ESSID for communicating with each other. If nothing is specified, the card may automatically selects an access point, which may not be the one you intended to use. Use for a list of available wireless networks.
Select a suitable authentication method for your network: , , , , or . If you select WPA authentication, a network name (ESSID) must be set.
WEP and WPA-PSK authentication methods require to input a key. The key has to be entered as either a , as an string, or string.
Either enter the default key here or click to enter the advanced key configuration dialog. Set the length of the key to or . The default setting is . In the list area at the bottom of the dialog, up to four different keys can be specified for your station to use for the encryption. Press to define one of them as the default key. Unless you change this, YaST uses the first entered key as the default key. If the standard key is deleted, one of the other keys must be marked manually as the default key. Click to modify existing list entries or create new keys. In this case, a pop-up window prompts you to select an input type (, , or ). If you select , enter a word or a character string from which a key is generated according to the length previously specified. requests an input of 5 characters for a 64-bit key and 13 characters for a 128-bit key. For , enter 10 characters for a 64-bit key or 26 characters for a 128-bit key in hexadecimal notation.
To enter a key for WPA-PSK, select the input method or . In the mode, the input must be 8 to 63 characters. In the mode, enter 64 characters.
This button opens a dialog for the detailed configuration of your WLAN connection. Usually there should be no need to change the preconfigured settings.
The specification of a channel on which the WLAN station should work is only needed in and modes. In mode, the card automatically searches the available channels for access points. In mode, select one of the offered channels (11 to 14, depending on your country) for the communication of your station with the other stations. In mode, determine on which channel your card should offer access point functionality. The default setting for this option is .
Depending on the performance of your network, you may want to set a certain bit rate for the transmission from one point to another. In the default setting , the system tries to use the highest possible data transmission rate. Some WLAN cards do not support the setting of bit rates.
In an environment with several access points, one of them can be preselected by specifying the MAC address.
When you are on the road, use power saving technologies to maximize the operating time of your battery. Using power management may affect the connection quality and increase the network latency.
Click next to finish the setup. If you have chosen WPA-EAP
authentication, another configuration step is needed before your station
is ready for deployment in the WLAN. Enter the credentials you have been
given by your network administrator. For TLS, provide
, ,
, and .
TTLS and PEAP require and
. and
are optional. YaST searches for
any certificate under /etc/cert. Therefore, save the
certificates given to you to this location and restrict access to these
files to 0600 (owner read and write). Click
to enter the advanced authentication dialog
for your WPA-EAP setup. Select the authentication method for the second
stage of EAP-TTLS or EAP-PEAP communication. If you selected TTLS in the
previous dialog, choose any, MD5,
GTC, CHAP, PAP,
MSCHAPv1, or MSCHAPv2. If you
selected PEAP, choose any, MD5,
GTC, or MSCHAPv2. can be used to force the use of a certain PEAP
implementation if the automatically-determined setting does not work for
you.
![[Important]](admon/important.png) | Security in Wireless Networks |
|---|---|
Be sure to use one of the supported authentication and encryption methods to protect your network traffic. Unencrypted WLAN connections allow third parties to intercept all network data. Even a weak encryption (WEP) is better than none at all. Refer to Section 28.1.1.2, “Encryption” and Section 28.1.4.2, “Security” for information. | |
The package wireless-tools
contains utilities that allow to set wireless LAN specific parameters and
get statistics. See
http://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Tools.html
for more information.
kismet (package kismet) is a
network diagnosis tool with which to listen to the WLAN packet traffic.
In this way, you can also detect any intrusion attempts in your network.
More information is available at
http://www.kismetwireless.net/ and in the manual page.
These tips can help tweak speed and stability as well as security aspects of your WLAN.
The performance and reliability of a wireless network mainly depend on
whether the participating stations receive a clean signal from the other
stations. Obstructions like walls greatly weaken the signal. The more
the signal strength sinks, the more the transmission slows down. During
operation, check the signal strength with the iwconfig utility on the
command line (Link Quality field) or with NetworkManager or
KNetworkManager. If you have problems with the signal quality, try to set up the
devices somewhere else or adjust the position of the antennas of your
access points. Auxiliary antennas that substantially improve the
reception are available for a number of PCMCIA WLAN cards. The rate
specified by the manufacturer, such as 54 Mbit/s, is a nominal
value that represents the theoretical maximum. In practice, the maximum
data throughput is no more than half this value.
If you want to set up a wireless network, remember that anybody within the transmission range can easily access it if no security measures are implemented. Therefore, be sure to activate an encryption method. All WLAN cards and access points support WEP encryption. Although this is not entirely safe, it does present an obstacle for a potential attacker. WEP is usually adequate for private use. WPA-PSK would be even better, but it is not implemented in older access points or routers with WLAN functionality. On some devices, WPA can be implemented by means of a firmware update. Furthermore, although Linux supports WPA on most hardware components, some drivers do not offer WPA support. If WPA is not available, WEP is better than no encryption. In enterprises with advanced security requirements, wireless networks should only be operated with WPA.
If your WLAN card is not automatically detected, check whether it is supported by openSUSE. A list of supported WLAN network cards is available under http://en.opensuse.org/HCL/Network_Adapters_(Wireless). If your card is not supported, it may be possible to make it work using the Microsoft Windows drivers with Ndiswrapper. Please refer to http://en.opensuse.org/Ndiswrapper for detailed information.
If your WLAN card fails to respond, check if you have downloaded the
needed firmware. Refer to
/usr/share/doc/packages/wireless-tools/README.firmware
for more information.
Modern laptops usually have a network card and a WLAN card. If you configured both devices with DHCP (automatic address assignment), you may encounter problems with the name resolution and the default gateway. This is evident from the fact that you can ping the router but cannot surf the Internet. The Support Database features an article on this subject at http://en.opensuse.org/SDB:Name_Resolution_Does_Not_Work_with_Several_Concurrent_DHCP_Clients.
Several drivers are available for devices with
Prism2 chips. The various cards work more or
less smoothly with the various drivers. With these cards, WPA is only
possible with the hostap driver. If such a card does not work properly
or not at all or you want to use WPA, read
/usr/share/doc/packages/wireless-tools/README.prism2.
The Internet pages of Jean Tourrilhes, who developed the Wireless Tools for Linux, present a wealth of useful information about wireless networks. See http://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Wireless.html.
