VPN Authentication Services

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If you’re planning to take the Security+ exam, you should have a basic understanding of the function and purpose of VPN authentication services. For example, can you answer this question?

Q. Which of the following choices provide authentication services for remote users and devices? (Select TWO.)

A. Kerberos

B. RADIUS

C. Secure LDAP

D. Diameter

More, do you know why the correct answer is correct and the incorrect answers are incorrect? The answer and explanation is available at the end of this post.

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RADIUS

Remote Authentication Dial-In User Service (RADIUS) is a centralized authentication service. Instead of each individual RAS server needing a separate database to identify who can authenticate, authentication requests are forwarded to a central RADIUS server.

One of the ways to visualize this is to think of a dial-up ISP, such as AOL. AOL provides dial-up digital subscriber line (DSL) services for many cities in the United States. Although anyone can dial in to an AOL server, the servers only allow access to users with accounts in good standing.

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Suppose you live in Virginia Beach, Virginia. You could sign up for AOL, pay the required fee, and access the AOL service via a server in Virginia Beach. AOL could maintain a database on the server in Virginia Beach showing your account is in good standing.

However, what if you travel to Atlanta, Chicago, San Francisco, or somewhere else? Because you paid your fees, you would reasonably expect to be able to access AOL no matter where you traveled. If the databases were stored on individual servers, AOL would need to update every single server in the United States. Similarly, if you canceled your subscription, AOL would need to update every single server again. Clearly, this would take a lot of work.

Instead, AOL could use a centralized RADIUS server, as shown in the following figure. Each AOL server is configured with a shared secret (similar to a password) and the RADIUS server is configured with a matching shared secret for each of the AOL servers.

Remote Users Authentication Services

Remote Authentication Dial-In User Service (RADIUS) configuration

This centralized RADIUS server would hold a centralized database of user accounts. Now if you tried to access AOL from any city, that AOL server would contact the RADIUS server to check your account.

Although this example with AOL works well to illustrate how RADIUS works, you don’t need servers all over the country to take advantage of RADIUS. You could have as few as two or three RAS servers. Instead of having authentication databases on each server, you could configure a centralized RADIUS server.

RADIUS uses the User Datagram Protocol (UDP), which uses a best-effort delivery mechanism. In contrast, TACACS+ uses Transmission Control Protocol (TCP), which provides guaranteed delivery. In addition, RADIUS only encrypts the password, whereas TACACS+ encrypts the entire authentication process.

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Diameter

Diameter is an extension of RADIUS and many organizations have switched to Diameter as a replacement for RADIUS due to its extra capabilities. One significant improvement is the support of the Extensible Authentication Protocol (EAP), which significantly enhances the security of Diameter. Additionally, Diameter adds several other commands beyond the capabilities of RADIUS. Diameter uses TCP instead of UDP used by RADIUS.

In geometry, the diameter of a circle is a straight line between the two edges of a circle, whereas the radius is a straight line from the center to an edge. In other words, the diameter of a circle is twice as long as the radius. The designers considered this when naming Diameter to indicate indirectly that it is twice as good as RADIUS.

Remember this

RADIUS provides centralized authentication. Diameter is an improvement over RADIUS, and it supports many additional capabilities, including securing transmissions with EAP.


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Q. Which of the following choices provide authentication services for remote users and devices? (Select TWO.)

A. Kerberos

B. RADIUS

C. Secure LDAP

D. Diameter

Answers are B, D. Both Remote Authentication Dial-In User Service (RADIUS) and Diameter are authentication services for remote users and devices. Diameter is more secure than RADIUS.

Kerberos is an authentication service used with a domain or realm and Secure Lightweight Directory Access Protocol (LDAP) uses Transport Layer Security (TLS) for encryption and is used to query directories.

See Chapter 1 of the CompTIA Security+: Get Certified Get Ahead: SY0-401 Study Guide for more information on authentication concepts.

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