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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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modem connection is made, the NAS prompts the user for a name <strong>and</strong><br />

password. The NAS then creates the so-called authentication request from the<br />

supplied data packet, which consists of information identifying the specific NAS<br />

device sending the authentication request, the port that is being used for the<br />

modem connection, <strong>and</strong> the user name <strong>and</strong> password.<br />

For protection against eavesdropping by hackers, the NAS, acting as the<br />

RADIUS or TACACS client, encrypts the password before it sends it to the<br />

authentication server. If the primary security server cannot be reached, the<br />

security client or NAS device can route the request to an alternate server. When<br />

an authentication request is received, the authentication server validates the<br />

request <strong>and</strong> then decrypts the data packet to access the user name <strong>and</strong><br />

password information. If the user name <strong>and</strong> password are correct, the server<br />

sends an authentication acknowledgment packet. This acknowledgement packet<br />

can include additional filters, such as information on the user's network resource<br />

requirements <strong>and</strong> authorization levels. The security server can, for instance,<br />

inform the NAS that a user needs <strong>TCP</strong>/<strong>IP</strong> or <strong>IP</strong>X using PPP, or that the user<br />

needs SL<strong>IP</strong> to connect to the network. It can include information about the<br />

specific network resource that the user is allowed to access.<br />

To circumvent snooping in the network, the security server sends an<br />

authentication key, or signature, identifying itself to the security client. After the<br />

NAS receives this information, it enables the necessary configuration to allow the<br />

user the necessary access rights to network services <strong>and</strong> resources. If at any<br />

point in this log-in process all necessary authentication conditions are not met,<br />

the security database server sends an authentication reject message to the NAS<br />

device <strong>and</strong> the user is denied access to the network.<br />

22.13 Extensible Authentication Protocol (EAP)<br />

Extensible Authentication Protocol (EAP) is used for the exchange of<br />

authentication information. EAP is defined in RFC 2284 <strong>and</strong> is an extension to<br />

the Point-to-Point Protocol (PPP). EAP supports multiple authentication vehicles<br />

such as:<br />

► Kerberos<br />

► Public key authentication<br />

► Key tokens<br />

► One time passwords<br />

874 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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