winforms gs1 128 Controlling Wireless Access Using Group Policies in VB.NET

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Controlling Wireless Access Using Group Policies
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Windows Server 2003 provides security capabilities for wireless networking in the form of group policies that you can use to restrict users wireless access to the network. In the Group Policy Object Editor console, you can create a policy in the Com puter Configuration\Windows Settings\Security Settings\Wireless Network (IEEE 802.11) Policies subheading that enables you to specify whether wireless-equipped computers can connect to ad hoc networks only, infrastructure networks only, or both (see Figure 13-7).
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Designing a Security Infrastructure
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Figure 13-7 The New Wireless Network Policy Properties dialog box
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In the Preferred Networks tab, you can specify the networks to which users can con nect and set properties for the IEEE 802.1X security protocol, such as which authenti cation protocol to use (see Figure 13-8). Using these group policy settings, you can configure the wireless networking properties for all the computers on your WLAN.
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Figure 13-8 The New Preferred Setting Properties dialog box
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Securing a Wireless Network
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Authenticating Users
You can use several methods to authenticate users attempting to connect to your WLAN and to prevent unauthorized access by outsiders. The IEEE 802.11 standard itself defines two methods: Open System authentication and Shared Key authentication, and Windows Server 2003 supports a third method, based on another standard called IEEE 802.1X.
Open System Authentication
Open System authentication is the default authentication method used by IEEE 802.11 devices, and it actually provides no authentication at all. Open System authentication is simply an exchange of messages in which one system identifies itself to another and the other system replies. There is no exchange of passwords, keys, or any other type of credential, and there is no way for a device configured to use Open System authen tication to refuse authentication to another.
Shared Key Authentication
Shared Key authentication is a system by which wireless devices authenticate each other using a secret key that both possess. The key is assumed to have been shared before authentication using a secure channel independent of 802.11 communications to prevent it from being compromised during transmission. Shared Key authentication is not a particularly secure method because all the computers in the same BSS must possess the same key. Compromising the key on one system nullifies the authentica tion security for the entire BSS.
Important Shared Key authentication requires the use of the Wired Equivalent Privacy (WEP) algorithm. If WEP is not implemented, Shared Key authentication is not available.
During a Shared Key authentication, messages are exchanged between the requester and the responder as follows: 1. The system requesting authentication asserts its identity to the other system, using a message that contains a value that identifies the shared key (not the shared key itself) that the system is using. 2. The system receiving the authentication request responds with a message contain ing the authentication result. If the authentication is successful, the response mes sage includes a 128-byte block of challenge text generated by the WEP pseudorandom number generator. 3. The requester copies the challenge text from the response message to a new mes sage and encrypts it with WEP, using the shared key as an encryption key.
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Designing a Security Infrastructure
4. The responder decrypts the message and compares the decrypted challenge text with the text the system transmitted in step 2. If the values match, the responder grants the authentication.
IEEE 802.1X Authentication
The IEEE 802.1X standard, Port Based Network Access Control, defines a method of authenticating and authorizing users connecting to an IEEE 802 LAN, and blocking those users access to the LAN should the authentication fail. IEEE 802.1X can authen ticate users connecting to any type of LAN, such as Ethernet or Token Ring, but in this case, it is particularly valuable in the case of IEEE 802.11 wireless LANs. Most IEEE 802.1X implementations function as clients of a server running a Remote Authentication Dial-In User Service (RADIUS), such as the Internet Authentication Service (IAS) included with Windows Server 2003. The RADIUS server provides centralized authentication and authorization services for the entire network; for WLAN authentica tion, RADIUS typically uses one of the following two authentication protocols:
Extensible Authentication Protocol-Transport Level Security (EAP-TLS) EAP is an authentication protocol that is designed to be adaptable, so that it can carry a vari ety of authentication mechanisms within a given packet framework. TLS is an authentication mechanism that transports its messages within EAP packets and provides mutual authentication, integrity-protected negotiation of cryptographic service providers, and secret key exchange between two systems that use public key cryptography. The networks that use EAP-TLS typically have a public key infrastructure (PKI) in place and use certificates for authentication, that are stored on the computer or on smart cards. Protected EAP-Microsoft Challenge Handshake Authentication Protocol, version 2 (PEAP-MS-CHAP v2) PEAP is a variation on EAP that is designed for use on wireless networks that do not have a PKI in place. With PEAP, you can use a password-based authentication method, such as MS-CHAP, to securely authenticate wireless connections. PEAP creates an encrypted channel before the passwordbased authentication occurs. Therefore, password-based authentication exchanges such as those that occur in MS-CHAP v2 are not subject to offline dictionary attacks. (Put simply, an offline dictionary attack uses a brute force dictionary attack to make repeated attempts to decrypt captured packets that use an encryp tion key derived from a user s password. This process is made easier for the intruder when the encryption key is derived from a weak password.)
Important To use PEAP-MS-CHAP v2 for wireless network authentication, the wireless cli ent must be running either Windows Server 2003 or Windows XP with SP1 installed.
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